People And Nature  Part 8 of Decarbonising the Built Environment: a Global Overview, by Tom Ackers.

There are two main ways to cut greenhouse gas emissions from construction, that will be covered in this part. These are:

a. Demand reduction: mainly, reducing the quantity of unnecessary new construction. This means extending the lifetime of buildings and infrastructure, and reducing waste.

Spraying hempcrete. Photo by fiberfort.com

b. Decarbonising construction: mostly, reducing the embodied emissions in construction materials. The prime targets here are the embodied emissions of cement and steel.

One way of decarbonising construction is to replace those existing construction materials wholesale, with alternative, lower carbon alternatives. I will consider some of those alternatives here, including so-called bio-based materials.

A second way to decarbonise construction is to engineer down the use of cement and steel – for example, by using them in conjunction with low carbon alternatives, and by using them more efficiently through changes in design and through waste reduction. A third method is recycling.

Another approach to reducing embodied emissions in construction – unsurprisingly the dominant focus for capital-intensive industry – is to try to decarbonise the production of existing construction materials. The focus is on cement and steel, but other high-energy materials, such as glass, also need to be decarbonised.

Reducing the carbon intensity of cement and steel is challenging: their production emissions are considered “hard to abate” – especially process emissions that comprise two-thirds of cement production’s footprint. This area is full of technological innovators seeking to insert themselves into essential supply lines of construction.

In this part, I begin with demand reduction: the avoidance of unnecessary construction and demolition, and the potential for re-using materials (section 8.1); then the importance of material efficiencies (section 8.2). Then I look at ways of decarbonising construction: some low carbon materials (section 8.3) and controversies around plant-based materials and their potential benefits in terms of carbon sequestration (section 8.4). After that, I provide an overview of steps to decarbonise cement and concrete (section 8.5) and steel (section 8.6), and the projected role of carbon capture and storage (CCS) in achieving those aims. Then I turn to some benchmarking initiatives by architects (Section 8.7). I have commented critically on the IEA’s approach to decarbonisation in Appendix 5, in the PDF version.

All in all, construction should in future have a qualitatively different material relation to the world. Not only does it need to be less energy- and material-intensive, but also those same principles need to extend to the operational use designed into buildings at the commissioning stage (see parts 9 and 10). And construction should proceed on the basis of enhancing ecological reconstruction and wellbeing.

8.1. Avoiding unnecessary new construction and demolition, and re-using materials

Avoiding unnecessary new construction and demolition needs to be the starting point for decarbonising the built environment. This applies to buildings and to infrastructure.

Reducing construction means that, as far as possible, existing structures should be retained for use – retrofitted and updated as necessary, in order to extend their lifetime of use. This means avoiding demolitions, which create needless demand for new construction, and waste already-existing embodied materials and previously imposed environmental burdens. (This is to say nothing of deliberate destruction of buildings and infrastructure in wars.)

In the words of Carl Elefante, former president of the American Institute of Architects: The greenest building is the one that is already built. The same is true of infrastructure, so long as it works as it should. Instead of new-build, the emphasis needs to be on retrofit, and “adaptive reuse”.

According to research by the Royal Institute of British Architects (RIBA) in collaboration with Architects Declare, a 20% reduction in demand for new buildings globally could be achieved just by (re)using existing structures better – and this could save “up to 12% of global emissions in the building and infrastructure sector”.

Carry the same principle of reuse into construction materials. Avoid construction waste, in favour of deconstruction, reusing materials and construction elements as much as possible (“design for disassembly”). Tap waste streams. Aim for a zero waste, or circular, construction economy.

There are many vanguard initiatives in this vein (for profit, and not) that seek to adapt the principle of reuse to the convergence of ecological urgency and computational possibility. For example, the Dutch group Metabolic, and the cooperative RotorDC, with its waste stream database-cum-marketplace, an eBay of sorts for recycled materials.

The architects’ firm Orms has been one leader in the practical development of “materials passports” – intended “to gather and organise data about materials contained within a building”, so that they can be effectively harvested for components in the future. Such data would include precise inventory and location data, engineering and performance specifications, and the embodied and operational material and environmental footprints associated with a given component.

At least in theory, this also means that material footprints, and wholelife assessments through to waste disposal and resale, can enter into the design process as active parameters, alongside engineering issues such as building physics, or questions of cost and availability of materials. To this end, architectural and engineering design platforms seem to be trending towards high-level parametric integration, mediating the space between markets and physics.

Of course all of this leans heavily on the quite possibly spurious idea that effective technological and market-based interventions can contribute meaningfully to decarbonising construction – especially outside the “value-added”-rich markets of rich countries. Questions remain around the practical extent of such schemes, and the politics of access to markets, platform data and modelling streams.

In any case, these sorts of initiatives indicate possible directions that a green capitalism might take. They are consonant with trends towards “eco-labelling” in certain segments of the global food industry. But they also suggest one aspect of what an eco-socialist transitionary programme might look like, in terms of steering local and global economies in a sustainable direction.

8.2. Material efficiencies in construction

When new construction is necessary – which includes retrofit – embodied and operational emissions need to be factored in as parameters from the start.[1]

The dominant high-carbon construction materials – steel, other metals, concrete and glass – need to be used in moderation, and sidelined wherever possible.

The carbon intensity of construction has come down in recent years. However, decarbonisation of existing styles of construction, to anything like the necessary extent, is likely to be a long time coming.

So long as established construction methods and economies prevail, whatever shifts do occur in the relative carbon-intensity of construction will most likely continue to be swamped in absolute terms by the volume of construction, at least in the short-to-medium term.

Priority must be given to three things: “build light”, aim for materials efficiencies, and use low-carbon construction materials and processes.

“Build light” is an overall ethos. It involves constructing only what is necessary. Building elements can be specified to be literally less heavy, and require less structural support – since structural strength often comes with more embodied emissions.

Materials efficiencies: embodied carbon can be reduced by engineering down the use of construction materials, and especially high-energy materials. Using standardised material sizes can help reduce waste.

So-called “lean” design and construction techniques combine both of these approaches – achieved through changes in design, so that buildings and infrastructure are not needlessly over-designed or over-specified; and through minimising waste during manufacturing and construction.

In theory, off-site fabrication and short-term “flying factories” at construction sites can help to improve material efficiencies and reduce waste. However, designing manufacturing processes and supply lines on a project-by-project basis comes with its own challenges.

Materials like timber that do not require extra finishing treatment – they are “self-finishing” – are also good, because they are more easily deconstructed and recycled in the future. It is also preferable to avoid adhesives for the same reason.

8.3. Low-carbon materials

Worldwide, almost all of the embodied emissions of buildings come from the manufacture of building materials, as described in part 7. Some of the best low-carbon materials are “traditional” ones such as wood, stone and rammed earth. These options are far more practical than they may seem, and more viable in engineering terms than a habituated fetish for fossil-fuel-heavy products might allow.

Traditional construction materials can be a route by which construction and design work can incorporate and learn from local sources of knowledge, about what materials work well locally in relation to factors like climate, and what is available. Engaging and elevating local craft skills is of value in its own right.

Traditional construction materials and methods are usually less capital-intensive than industrial ones. This may well make them less profitable and less worthwhile inputs for capital. They may or may not require higher absolute inputs of labour.

By contrast, the type of capitalist housebuilding dominant in the UK is very labour-intensive. Here, profitability for capital is assured by monopolies on land, poor construction standards, favourable access to low interest rates and political favouritism.

But at the level of social use-values in the locales of construction sites, plentiful labour-content can provide a source of employment. In many places, employing and/or training people in traditional construction methods, updated as necessary, can furthermore be a valuable source of local worker-led autonomy.

Local materials can give construction a degree of local texture and specificity it might otherwise lack. Where a small emissions footprint is the aim, less transport can also be a plus. Transportation is usually a comparatively small component of the embodied carbon of high-emissions industrial materials, but when the embodied carbon of manufacture is cut back, so the transport component becomes relatively more important.

Traditional construction methods can also be used to produce prefabricated components, that have a much smaller carbon footprint than prevailing carbon-intensive materials. Flying factories, close to one or more construction sites, could integrate the benefits of traditional modes of construction with the flexibility of prefabrication.

As with agroecological approaches to farming, the point here is to concentrate the “knowledge intensity” of production, rather than the capital- or carbon-intensity. Ways of providing materials can then be found that are environmentally friendly, but also potentially transferable away from the immediate site of production – and, again, also job-creating, in environmentally sustainable industries.

These are some of the important low-carbon materials (not including substitutes for cement, concrete and steel, that are covered in sections 8.5 and 8.6 below.)

🔥 Rammed earth is traditionally used with earth excavated at, or close to, the construction site. It is a good insulating material, possessing tremendous thermal inertia, i.e. its thermal massing retains warmth, or insulates against it, very effectively. It is also available as a prefab, prepackaged, low-carbon means for load-bearing construction.

🔥 Stone is another very effective, and ancient, structural material. A good example of its use is the limestone used in the load-bearing exoskeleton of the Stirling Prize-shortlisted 15 Clerkenwell Close in London. Its embodied carbon footprint is apparently 10% of what it would have been if steel and concrete were used. The cost, too, was a small fraction. The declared embodied emissions for the whole building are 335 kilogrammes of carbon dioxide equivalent per square metre (kgCO2e/m2).

🔥 Hemp is perhaps the most exciting traditional construction material – one that could be hugely beneficial to bringing down levels of embodied carbon, for example as insulation or in the form of hempcrete (see below). It grows quickly, locking up biogenic carbon, making hemp itself carbon negative.

Also in hemp’s favour is that it grows in a wide diversity of temperate climates; it can be grown in untilled soils; and it is harvestable within just a few weeks. As a plant-based material it has high carbon-sequestering capacities through the rapid growth phase, and a rapid rate of throughput from atmospheric CO2 to a stock of biogenic carbon.

According to a recent study by the Biorenewables Development Centre at the University of York in the UK, industrially-grown hemp crops can sequester up to 22 tonnes of CO2 per hectare annually – “more than any other crop or woodland”. (Although note that the crop’s net emissions will be a bit less than that, due to emissions associated with cultivation and processing.)

A serious concern about the widespread use of hemp might be the risk, as with biofuels, of hemp cultivation displacing food agriculture. This is a disadvantage, compared to non-biological, non-renewable materials such as limestone, which are relatively abundant and relatively independent from food markets.

However, also on the plus side is that hemp’s root system is extensive, and it tends to suppress weed growth, which means it can help improve degraded soil structures, remediate polluted soils, and boost subsequent crop yields. As a rotation crop it can be used as a “break crop” for all these reasons, to suppress weeds and pests, and additionally support growers’ economic sustainability.

🔥  Hempcrete, made by combining hemp stalks’ inner core with water and lime, is a bit of a wonder material. Like rammed earth, it has great thermal inertia. It is highly flame-resistant, resistant to pests and mould, biodegradable and breathable – meaning that it naturally regulates a building’s temperature and humidity, which improves thermal comfort.

Mixed on-site, hempcrete is readily mouldable into large, lightweight bricks, or applied as a surface render. It can also be manufactured into prefab wall panels with very little expenditure of energy.

Hempcrete is comparatively strong, but has only about 5% of the compressive strength of residential grade concrete – so walls made of it require the addition of another loadbearing material such as timber or CLT (see below). But it weighs only about one-seventh as much as concrete. It is well suited to construction in areas at risk of seismic activity, because it is resistant to fracture under movement. Hempcrete can also be reused if milled and then rehydrated.

On the downside, like cement, hempcrete produces process emissions in the lime binding stage. These far outweigh other emissions from hempcrete production, such as energy-related emissions. The process emissions tend to be about equal to the carbon sequestered during the hemp’s growth phase, so cradle-to-gate hempcrete is roughly carbon neutral. Additionally, hempcrete, also like cement, re-absorbs CO2 during the remainder of its lifecycle, potentially around 40% of what was produced when it was made – so that cradle-to-grave emissions are carbon negative.[2]

🔥 Bamboo can have a tensile strength similar to steel, and its compressive strength is double that of concrete! Bamboo is also incredibly fast-growing, making it great for carbon sequestration. (See section 8.4 below.) It is also lightweight – and cheaper than concrete, a big advantage, especially in poor countries.

Indeed, bamboo is already widely used as a building material globally. According to a recent (2023) Global ABC report on building materials, bamboo (like hemp) is a promising material for use in areas prone to earthquakes, and also durable during floods. All round, it is ideal for designing climate-resilient buildings. It can be used directly, in its raw, unfinished state.

🔥 Engineered timbers such as cross-laminated timber (CLT) (also termed “mass timber”) are a recent and important innovation. Like hemp, bamboo, or other timber products, CLT contains whatever biogenic carbon was fixed into it during its lifetime as a plant.

Like bamboo, CLT is capable in many instances of replacing structural steel and reinforced concrete. It has a high strength-to-mass ratio, and can be used for walls, floors, and ceilings. It can be deployed alongside hempcrete to give load-bearing strength, or alongside steel and reinforced concrete as a partial replacement.[3] CLT can also be made out of bamboo.

CLT has been used to impressive engineering effect in recent prestige buildings in the UK: for example, the Stirling-shortlisted Cambridge Central Mosque (embodied emissions, 844 kgCO2e/m2) – and also here and here. You can even construct highrise buildings out of CLT (albeit with a concrete base, elevator shaft and stair wells).

As with hemp, the sequestration potential of CLT can be increased dramatically by using high-yield, fast-growing crops.

By-products of production, such as off-cuts, can also be used for long-term carbon sequestration as small-scale timber products.

It is also important that petrochemical-based glues, chemicals and coatings used in wood products are phased out and replaced with bio-based alternatives. According to Global ABC, as well as reducing embodied emissions, this could enhance mechanical performance and reduce the unintended movement of heat and damp through structures. i.e. improve hydrothermal properties.

8.4. Plant-based materials and carbon sequestration

Bio-based materials such as hemp and timber can provide very good alternatives to classical industrial building materials like including steel and concrete. They have the additional benefit of storing the carbon accumulated in them during their growth phase in the form of biogenic carbon. This allows for what the Global ABC, the UNEP-IEA building strategy document, calls “carbon pool replenishment”.

However, the advantages here can also be overstated and misleading. Much depends on what type of biological growth is involved, the rate of growth – and, most importantly of all, whether the materials are sustainably sourced.

In the case of timber, global rates of deforestation presently exceed rates of regrowth. Moreover, the international trade in timber shows a marked flow of wood resources from poor to rich countries. Rich countries such as Germany and the USA import wood from the global south for reasons of cost efficiency, even in the face of ample, comparatively untapped wood resources within their own borders.

Hemp, as I have said, grows very quickly. It sequesters carbon rapidly, and provides all sorts of other agricultural benefits. Bamboo too grows very fast indeed. As cultivated crops, so long as the cultivation is otherwise ecologically sustainable, those can be win-win. However, most trees grow slowly.

Dead timber used in construction obviously also stops absorbing CO2. A living tree would have continued to grow and absorb more CO2 from the atmosphere during the remainder of its life, and the only net gain in carbon sequestration occurs when a new tree grows in the place of the felled one. Young tree saplings take a while to get going with their carbon fixing. However, older trees also slow down. (See the illustration.)

Source: excerpted from Michael T. Ter-Mikaelian et al (2015). “Forest carbon”
here means the total level of sequestered biogenic carbon in a forest

“Don’t worry, we’ll grow trees to suck up the carbon” is an increasingly common theme from politicians seeking to delay effective action on climate change. And this has led to disputes about how to measure the real effect of afforestation.[4]

The point, in my view, is that carbon sequestration has an important time efficiency component, depending on the speed at which farmed trees, hemp, or other materials grow, and the resultant rate of throughput from atmospheric CO2 to biogenic carbon to a biogenic end-product. Again, one of the many attractions of bamboo and hemp is that they grows fast.

The Global ABC cites evidence that in China bamboo sequesters carbon 30-40% faster per hectare per year than a tropical mountain rainforest, or a fast-growing variety of fir, and 2-6 times faster than average sequestration rates for forests in China and globally.

But of course forests also have tremendous ecological value in and of themselves: they are complex ecosystems and carbon sinks that should be allowed to thrive. They are not just a “resource” for wood products or for burning, or a “dividend”. In my view, there should be no farming of existing old forests.

Timber products should only come from farmed timber. And to be truly sustainable, planted forests cannot simply be monocultures, but must be thriving, if transient, ecosystems in their own right.

But Interpol estimated in 2021 that 15-30% of internationally traded timber was harvested illegally. And, according to the UNEP, illegal logging is responsible for up to 90% of tropical timbers felled in the world’s main tropical forest regions – the Amazon basin, Congo basin and south-east Asia.

There is presently a big push globally towards bioenergy. In my opinion, there is no defensible reason to burn wood industrially: cultivated timber should only go into durable products. Household wood combustion should also be phased-out.

Moreover, there needs to be a massive global effort at rewilding, including reforestation and mangrove restoration, to re-establish ecologies and carbon sinks lost to economic development. Much of that needs to happen in rich countries. Globally, rewilding should centre the needs of land-based and indigenous peoples, and not become a further tool of land-grabbing and greenwash.

Meanwhile, the aim for timber products and other bio-based materials should be to maximise their useful life, as with all useful materials. That is why bio-based materials are good for construction: buildings (and infrastructure) tend to be around a long time – and a long lifecycle of use should also be the norm, in order to minimise new construction.

The counter example would be timber used for cheap, non-durable furniture. Moreover, in the really existing market of timber products, Ikea can get away with being complicit in illegal logging of Romania’s old growth forests.

The sequestration potential of most CLT is clear. It will be improved if the logging and CLT industries decarbonise their sources of energy. They also need to clear forest residues – a large source of greenhouse gas emissions. Crucially, however, bamboo-based CLT is at present a net producer of emissions, due to the exceptionally high emissions associated with its manufacture.[5]

Optimistic claims about wood, bamboo and CLT also often fail to confront wider issues around land use.

The authors of one paper claim that 90% of the world’s new urban populations to 2100 could be housed “in newly built urban mid-rise buildings with wooden constructions”, instead of using steel and concrete, saving 106 Gt of CO2 during those 80 years.

On their calculation, that would require more than doubling farmed forest plantations – adding 1.49 million km2 to the current 1.32 million km2 by 2100. That would also mean encroaching on other forested areas that are presently unfarmed – in order to ensure no “major repercussions on agricultural production”.

However, a Greenpeace representative quoted in the Guardian points out that additional forested area for construction timber would better come at the expense of meat and dairy farmland, instead of further trampling old and biodiverse forests with monocrop plantations. That criticism is on-point: for the authors, unfarmed forests seem to be available for new plantations, whereas agriculture and other forms of land-use can only ever be ratcheted outwards.

The Global ABC notes that areas available for bamboo cultivation are already scarce, given the existing land-use pressures from agriculture and (to a lesser extent) housing.

Wherever and however bamboo or CLT are farmed, what happens to their biogenic carbon at end-of-life is also crucial: forget the gate, where is the grave? Is the wood re-used, keeping the carbon locked away (100% sequestration)? Is it burned, releasing the CO2 back into the atmosphere (0% sequestration)? Or does it rot in land-fill, releasing methane?

Bamboo, CLT, and similar bio-based construction materials – and timber furniture – should at least impose no net ecological harm. They should only be considered viable as mass-market goods when the bio-based materials they are constructed from are harvested in an environmentally positive way. They should also be built to last, within a holistically planned human ecology – not (as with Ikea furniture) produced as a throwaway quick fix.

Forestry residues should be removed, and/or harmful emissions suitably minimised.

In short, carbon-intensive construction materials need to be supplanted with ones that are as close to zero-carbon as possible, and construction materials as a whole used in the most efficient ways possible. But equally, the virtues of bio-based products, and their time-horizons, should not be exaggerated, or abstracted away from the full breadth of social and ecological concerns. Nor should ecologies and biologies simply be regarded as physical materials to be taken at will.

In any case, the entire rationale on which societies “resource-ise” materials from the natural world needs in time to be retrofitted, if not demolished and rebuilt from scratch.

8.5. Decarbonising cement & concrete

In the case of decarbonising cement and concrete, there are possibilities to change the way cement is made. One way is to change the recipe. Brimstone Energy, a US-based start-up, replaces limestone with calcium silicate – which they say avoids the process emissions associated with limestone, and reduces kiln temperatures, making electric kilns feasible.

Another way is through material efficiencies. For example, you can make cement with a lower proportion of clinker, the most carbon-intensive component. Up to 50% of the clinker can be replaced with limestone and calcinated clays. Another alternative is to introduce graphene into the cement mix, to make it stronger and permit a reduced clinker component.

Seratech, a start-up spun out of research at Imperial College London, combines carbon capture with cement manufacture to produce a nominally “net-zero” cement. Their technology removes industrial CO2 emissions from flues, and uses that to produce carbon-negative silica, which can be used in place of cement. When that is mixed with Portland cement, the negative emissions in the silica are said to balance the positive emissions from the Portland cement, to make an overall “net zero” cement mix.

In the long term, direct electrification of cement kilns should also be possible. That will not reduce process emissions, but would allow the most energy-intensive part of production to be decarbonised.

There are also ways to grow cement-like materials biologically, that seem to be free of many of the sustainability and land-use problems that arise with plant-based products.

Biomason, a US company, makes a product called Biocement, or biogenic cement. The company says it uses marine microorganisms to mimic the process of calcination that makes coral. In this way the company “grows” limestone in an effective reversal of cement production. They can do this either in-situ, or to produce pre-cast units.

The House in Bordeaux (1999), created by Anne Lacaton and Jean-Philippe Vassal
from a former biscuit factory. Photo from 
dezeen.com

When used underwater, this process sources its ingredients from seawater, “for propagative calcium carbonate precipitation”, which gives underwater structures “self-healing abilities”. That sounds great.

Biomason partners with other companies, licensing Biocement for particular uses – for example, BioBasedTile, a Netherlands-based company making pre-cast concrete tiles from recycled waste and biocement. The tiles “grow” in less than three days, and are intended for use on facades, interior walls and flooring. The end product consists of ~85% recycled granite and 15% biologically grown limestone. The company claim their tiles are three times as strong as traditional concrete tiles, weigh 20% less, and have just 5% of the embodied emissions.

Unfortunately, it looks as though Biomason is funded with more socially destructive uses in mind: according to their website, the company works “with support from the US Department of Defense” towards the obvious marine applications of this technology, and for land-based uses in “forward operating positions”, “where native, non-engineered surfaces prevent safe vertical take-off and landing”.

All such uses should be non-proprietary, and available for use in civil engineering projects.

A similar but seemingly inferior approach, pursued by scientists at the University of Colorado Boulder, uses microalgae to grow “biogenic limestone”. The microalgae capture CO2 via photosynthesis in the growth phase; this is released again during the calcination process: net-zero as far as the process emissions are concerned.

Apart from cement manufacture, there are some indications that cementitious materials, such as concrete, could be artificially and rapidly recarbonated with CO2 after manufacture. Part of the natural curing process of cement involves it reabsorbing CO2 over its lifetime, as mentioned in Part 7. However, that process is slow when it occurs naturally, and can lead to structural problems when it occurs in an uncontrolled fashion.

Research recently reported in the architectural press, by Tunley Environmental (a sustainability consultancy) suggests two methods of controlled recarbonation: (1) injection of CO2 into precast concrete; and (2) embedding CO2-rich materials into the concrete mix. They also propose that controlled recarbonation could be performed on waste concrete, before recycling it into fresh concrete as recycled concrete aggregate (RCA). The controlled nature of these processes, they say, would avoid the structural degradation that can result from natural recarbonation.

Noting that the theoretical maximum carbonation capacity of cement is 50%, Tunley point out that, were all the world’s production of concrete recarbonised in this way, it would sequester around 77% of CO2 emissions from cement manufacture.[6] Setting aside that theoretical figure, they suggest a more modest 17.3% of cement-related CO2 emissions re-sequestered annually through method (2) above. Again, though, that is if all concrete produced globally were recarbonised in this way. So there are plainly practical constraints of scale, and of effectiveness.

In short, controlled recarbonation can presumably play some role. However, it seems hugely unlikely that such processes could be applied at sufficient scale to make a meaningful impact, within the necessary timeframe for decarbonising the built environment. Indeed, stories like this risk encouraging complacency.

There are also options for changing the way concrete is made. Again, a very simple approach is to reduce the amount of cement used to what is structurally necessary. For example, concrete roadside kerbs have very different structural requirements to the high strength grades of concrete used in bridges and skyscrapers.

CarbonCure, a company using similar technology to Seratech, injects captured CO2 into concrete during mixing. This takes advantage of the usually very slow natural process of cement carbonation (recarbonation), producing nano-sized particles of calcium carbonate in the concrete that help to strengthen it.

Perhaps more promisingly, it is possible to make concrete without cement at all, by using a different binder. One example of that is “Earth Friendly Concrete”, produced by the Australian materials firm Wagner, which apparently has up to 70% less embodied carbon than regular concrete made with cement.

Earth Friendly Cement has been licensed for use in the new Silvertown Tunnel under the Thames in London. Needless to say, reducing the embodied emissions in the construction of that road tunnel will do nothing to mitigate the much larger energy-based emissions arising from greater induced traffic flow.

And note the word “licensed”. While some of these techniques are now operational or close to maturity, they all seem to be based on closed proprietary systems.

That is fine, perhaps, for decarbonising cement and concrete production where the technology is available, and where the costs are deemed commensurate to the benefits. But it is highly doubtful that a broader rollout of such technologies internationally could work on that “cost premium” basis.

In my view, the technologies to decarbonise cement and concrete manufacture should not – and must not – be subject to intellectual property restrictions.

If it is technically available and economical, then it should be actually available to all. As with essential drugs, off-brand “generic” versions must be produced, and made available at a price competitive with, or cheaper than, dirt-cheap bog-standard materials. By subsidy or regulation, that seems to me the only real solution to decarbonising cement and concrete manufacture on a world scale.

The IEA says that these emergent technologies are most unlikely, on their own, to be competitive on price against old-school cement, with its process emissions unabated. For them to become price-competitive, carbon pricing would have to be introduced globally. However, cement is mostly produced locally to where it is used (see part 3), making it hard to police.
 
8.6. Decarbonising steel

The steel industry now relies almost entirely on thermochemical processes that use coal. Recycling is already a big thing, and producing steel from scrap only requires about one-eighth of the energy used to produce steel from iron ore. But there is not nearly enough scrap to meet present market demand for steel products. Recycled steel only comprises 30% of total global supply.

The international steel industry is focused on reducing CO2 emissions within the thermochemical paradigm, by using “green” hydrogen or biomass, instead of coal, to produce pig iron. The industry talks a good talk about decarbonisation along this route, and so do supportive states.

In Europe there’s the HYBRIT demonstration project in Sweden, intended to “demonstrate a complete industrial value chain for hydrogen-based iron and steelmaking”, and produce 1.2 Mt of crude steel a year.

China is developing Zhangjiakou as a “hydrogen energy pilot city”, with a plan to open a zero-carbon steel plant there. But as a whole, China’s steel decarbonisation plans do not go far or fast enough to meet the 1.5°C Paris target.

However, most industrial hydrogen is presently made using fossil fuels, and that is unlikely to change anytime soon. Hydrogen production itself is also incredibly energy intensive. “Green” hydrogen at any scale – or kindred alternative “flavours” of hydrogen – seems a long way off, if indeed they can ever be a practical eco-friendly reality.

Biomass combustion, meanwhile, carries its own problems: entailing significant on-site emissions, potentially colonising valuable agricultural land, and encouraging deforestation.

Innovations in steel manufacturing also face large economic barriers – from the capital intensity, long life, and sunk costs of industrial steel-making facilities; and from the low margin, fully globalised, highly competitive international market in steel products. Outside of boutique prestige projects, a universally-applied and enforced carbon price would seem to be necessary to encourage the necessary shifts in the industry at large.

Greening the international steel industry requires not just billions of dollars of investment, but trillions, according to Nathaniel Bullard, writing for Bloomberg. That’s thousands of billions of dollars, just to get to low-emissions steel, globally.

The main future promise for low-emissions steel manufacture, however, lies further from the status quo: switching from thermochemical processes to electrochemical processes – and in particular, low-temperature electrolysis. This is less capital-intensive than the alternatives, is suited to intermittent power, and produces steel in a single process, so that it does not require further refining.

Crucially, electrochemical processes use electricity, so they can be run wholesale from renewable sources of energy.

However, electrochemical steel production at scale is also still a long way away. Such a widespread shift in technology would seem to require severe economic sanctions, or universally-applied regulations, to become economically justified.

Another issue is the centrality, in proposals for decarbonising both the cement and steel industries, of carbon capture, utilisation and storage (CCUS) and/or carbon capture and storage (CCS).

Reaching “net zero” will be “virtually impossible” without the mass roll-out of CCUS, according to the IEA, especially in the case of “hard to abate” emissions, i.e. those of heavy industry and long-distance aviation.

However, CCUS at sufficient scale and energy-efficiency to course-correct existing levels of greenhouse gas emissions, let alone draw down historic emissions, in anything like the requisite timeframe, is a total pipe dream.

In the minds of corporate and state high-ups captured by fossil capital, as the IPCC’s Working Group III point out, “CCS can allow fossil fuels to be used longer”, and this reduces the degree and speed at which fossil reserves become de facto stranded assets –a necessary economic lever for limiting warming to 1.5°C or even 2°C.

The Working Group also notes that, regardless of its supposed promise, the “global rates of CCS deployment are far below those in modelled pathways limiting global warming to 1.5°C or 2°C”.

A highly critical report on CCS, published by the Institute for Energy Economics and Financial Analysis (IEEFA) in September 2022, specifically takes aim at the IEA’s advocacy for the technology.

I have commented in more detail on the IEA’s approach in Appendix 5, in the PDF version.

8.7 Actions by architects

I began this series with reference to some worker-led groups and professional bodies across architecture, design and engineering that have become prominent advocates for decarbonising the built environment. In the UK, these include the Architects Climate Action Network (ACAN), Low Energy Transformation Initiative (LETI), Architects Declare (AD) and the Royal Institute of British Architects (RIBA).

Each of those has published a stream of pamphlets, petitions, guidance documents, case studies, regulatory and policy advice, and benchmarking objectives, all aiming to shift the built environment professions internally, and shift external norms and expectations.

They consider not just emissions, but also the built environment’s direct impacts on other aspects of the natural environment, such as wildlife habitats, biodiversity, water systems, soil health, and people’s health and wellbeing.

15 Clerkenwell Close, London. Photo by Chris Wood / Wikimedia Commons

The intellectual momentum behind these proposals is significant. What they amount to practically and at scale remains to be seen.

In architecture, there are political tensions that need to be worked out. One of these is that large architecture firms usually get paid to design new buildings and shepherd them through construction. But what if one of the climate-aware architect’s main tasks is to halt most new construction? What if the only way to be a climate-responsible architect is to discourage, not encourage, large projects with high rates of material use and large carbon footprints?

What if the world needs architects and engineers to divest from surplus projects in rich countries, and be redeployed delivering services to the global poor?

In 2020, the famous architect Norman Foster, an early signatory to Architects Declare, withdrew his firm’s support, because of his enthusiasm for airports.

The prestige buildings mentioned above for their novel use of low-carbon materials were all new builds, with stated tonnages of embodied carbon. Few people actively want to see an end to construction – and indeed new construction is necessary to meet urgent social needs.

But the onus needs to be on those new buildings causing emissions in the built environment proving their social worth, both in terms of redressing inequalities, and in an international context of limited and shrinking carbon budgets. Certainly this is the case in rich countries, but also wherever capital pours into construction.

In this respect, it was nice to see Anne Lacaton and Jean-Philippe Vassal win the prestigious Pritzker architecture prize in 2021, for their spatially generous, and beautiful, retrofitted expansions of existing social housing blocks in France. Retrofits like that, which improve buildings’ operational performance, at minimal cost in terms of embodied emissions – should be the model.

Most buildings construction worldwide is designed with little regard for the climate, and with more of an interest in catering to architects’ clients – whose agendas will not often be led by environmental concerns. For that reason, compulsory regulations on embodied and operational carbon are essential, internationally.

Voluntary benchmarks set by RIBA and LETI give a sense of scale, when thinking about what reasonable legal limits on embodied carbon might look like, on a per-project basis.

However, legally-binding hard limits on the carbon footprints of new construction as a whole are needed, on a per-country, or regional basis. Plus tough benchmarks for the carbon intensity of the delivery of services, with legally-binding obligations prioritising the collective provision of essential and universal social goods, such as housing.

In its 2030 Climate Challenge (version 2, 2021), RIBA proposes to almost halve embodied carbon for new buildings in the UK by 2030. The target is for an embodied carbon performance of less than 625 kgCO2e/m2 for domestic buildings, and less than 750 kgCO2e/m2 for non-domestic office buildings. There are interim targets for 2025, and additional targets for school buildings (see below).

LETI has published more stringent targets for 2030: less than 300 kgCO2e/m2 for domestic buildings (over 6 storeys), schools, and retail; and less than 350 kgCO2e/m2 for offices.

For comparison, 15 Clerkenwell Close, the (deluxe) residential and office building mentioned above, had declared embodied emissions of 335 kgCO2e/m2.

Source: adapted from RIBA (2021), 2030 Climate Challenge

For a new home of 70 m2, according to the RIBA 2030 benchmark, the embodied carbon would have to be under 43.75 tonnes CO2e. For the LETI 2030 benchmark, it would be under 21 tonnes CO2e.

But in the UK, “business as usual” residential construction, under current regulations, produces an embodied carbon footprint of 84 tonnes CO2e for the same floor area.

The architects’ associations are saying this footprint needs to be slashed by nearly half (RIBA), or more than three quarters (LETI).

🔴 Go to part 9

🔴 Go to Contents and Introduction

Download the whole series as a PDF here

[1] See Feilden Clegg Bradley Studios’ whole life carbon review tool

[2] For a 1m2 area of wall made of hempcrete, offering a typical level of insulation (R-20 grade, with a heat transfer coefficient of 0.27 Watts per meter-square Kelvin), process emissions come to about 36 kg CO2e. Typically, this is roughly equal to the carbon sequestered during the hemp’s growth phase.

Over 60-100 years of use, hempcrete re-carbonation can re-absorb up to around 40% of the original process emissions, according to the study just cited. So hempcrete’s “cradle-to-grave” carbon footprint ranges up to, potentially, the sequestration of about 15 kg CO2e per m2 for the same R-20 piece of wall.

Different lifecycle emissions of hempcrete are associated with different densities of “mix” and different “model” estimates for process emissions. See here for a useful comparison.

[3] See pages 34-35 in the 2023 Global ABC report.

[4] I said that the only extra net gain in carbon sequestration occurs when a new tree grows in the place of the felled one. This is seeing things according to a “debt-then-dividend” (or “carbon repayment”) model. However, the other way to look at it is that you start with the dividend; then once you fell a tree you are simply back to zero.

These are two different carbon accounting conventions – and they are most salient when applied burning biomass, and releasing biogenic carbon into the atmosphere. However, they are also relevant to the stock and flow of timber-based products. Sustainable forestry – amongst other things – is the practice of managing the “flux” in the stock of carbon in a forest over time. In carbon accounting terms, the gains versus losses are recorded under the category “Land Use, Land Use-Change and Forestry” (LULUCF).

But whichever way you look at it, the “flux” of biomass growth is hardly instantaneous. Bamboo produces wood much quicker than most other plants: for most timber products, you might expect forest regrowth to return to pre-harvest levels (“carbon sequestration parity”) after around 40 years.

Crucially, the forest by-products of logging also need to be removed, in such a way as to avoid decay on land, which itself releases methane – and to reduce the risk of forest fires. (Note that both living and dead trees transport methane into and out of the atmosphere.)

[5] A recent literature review compiled the reported cradle-to-gate emissions of some CLT manufacturers, based in Europe, North America, and Australia (that is, the emissions flux from growth through manufacture, to point of sale). The average declared sequestration was -643.6 kgCO2e/m3 of CLT. That is, the CLT has a negative carbon footprint – when also taking account of the other, energy-based emissions that arise during manufacture.

However, a better way to assess the sequestration potential of bio-based products like CLT, is to look at manufacture in the context of land-use; and to consider the whole life-cycle, cradle-to-grave. In particular, it is important to consider what happens to forest residues after logging, and what happens to timber products at the end of their life – forest residue decay and land-fill decay are both a significant source of greenhouse gases, including methane.

One recent study cited in the Global ABC report does both of these things. It modelled a 100-year lifecyle for CLT produced in the Southeastern United States, from high-productivity pine plantations, and found that net greenhouse gas emissions could be as low as -1,445 tonnes CO2e per hectare (100m x 100m). This modelling is based on current modes of CLT production: it assumes in this instance that the drying kiln is powered by gas, that all vehicles are powered by diesel, that forests are “clear cut”, and that all harvest residues are left on the forest floor to rot. CLT recycling rates are presently low, and the study models 50% of manufactured CLT going to land-fill at the end of a 60-year lifetime of use.

(It is also common – because economical – for timber mills and processing plants to generate heat by burning wood residues and cut-offs. This releases more greenhouse gases than burning fossil fuels – combustion of dry wood produces more CO2 combustion emissions per unit energy out than coal does. Use of wood residues increases emissions compared to the use of gas.)

All of which indicates that the sequestration potential of CLT could be improved yet further by reducing emissions at all those stages in the lifecyle – and that timber plantations for CLT should be cultivated in more ecologically beneficial ways.

In the case of bamboo CLT, the high rate of carbon sequestration tends to be offset, for the time being, by a very emissions-intensive process of fabrication, when compared to other forms of engineered timber. The same Global ABC report outlines some of what is required in the case of bamboo CLT – carbonisation, high-temperature air drying, synthetic glues and anti-mould treatments. According to recent modelling, that can mean that the CO2 emissions during the manufacturing stage for bamboo CLT may end up comparable to those of steel on a per kilogram basis. Those emissions produced during the manufacturing stage are only offset ~35% by the biogenic carbon sequestered in the wood.

However, bamboo is also lightweight. I cannot find any data comparing the lifetime emissions of bamboo CLT and steel on an end-use basis. For example, if a lower mass of bamboo than steel imparts similar tensile strength, then that would reduce its real-world embodied emissions.

Another recent study calculated that, on average, CLT manufacture requires felling about 12m2 of forest for every m2 of a finished CLT building. However, in that study, the length of the trees’ growth phase and the time to harvest is unspecified. The same study reckoned that, if all new buildings worldwide to 2060 were built of CLT, supplying the timber would require 310,000 km2 of forest – about 0.8% of the world’s “available” forested area of 40.6 million km2. (The volume of construction required is based on international agencies’ forecasts, discussed below.)

[6] Based on the reported 2011 emissions, the authors estimate ~2 Gt CO2 sequestered annually.

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Decarbonising Embodied Emissions

People And Nature  Part 8 of Decarbonising the Built Environment: a Global Overview, by Tom Ackers.

There are two main ways to cut greenhouse gas emissions from construction, that will be covered in this part. These are:

a. Demand reduction: mainly, reducing the quantity of unnecessary new construction. This means extending the lifetime of buildings and infrastructure, and reducing waste.

Spraying hempcrete. Photo by fiberfort.com

b. Decarbonising construction: mostly, reducing the embodied emissions in construction materials. The prime targets here are the embodied emissions of cement and steel.

One way of decarbonising construction is to replace those existing construction materials wholesale, with alternative, lower carbon alternatives. I will consider some of those alternatives here, including so-called bio-based materials.

A second way to decarbonise construction is to engineer down the use of cement and steel – for example, by using them in conjunction with low carbon alternatives, and by using them more efficiently through changes in design and through waste reduction. A third method is recycling.

Another approach to reducing embodied emissions in construction – unsurprisingly the dominant focus for capital-intensive industry – is to try to decarbonise the production of existing construction materials. The focus is on cement and steel, but other high-energy materials, such as glass, also need to be decarbonised.

Reducing the carbon intensity of cement and steel is challenging: their production emissions are considered “hard to abate” – especially process emissions that comprise two-thirds of cement production’s footprint. This area is full of technological innovators seeking to insert themselves into essential supply lines of construction.

In this part, I begin with demand reduction: the avoidance of unnecessary construction and demolition, and the potential for re-using materials (section 8.1); then the importance of material efficiencies (section 8.2). Then I look at ways of decarbonising construction: some low carbon materials (section 8.3) and controversies around plant-based materials and their potential benefits in terms of carbon sequestration (section 8.4). After that, I provide an overview of steps to decarbonise cement and concrete (section 8.5) and steel (section 8.6), and the projected role of carbon capture and storage (CCS) in achieving those aims. Then I turn to some benchmarking initiatives by architects (Section 8.7). I have commented critically on the IEA’s approach to decarbonisation in Appendix 5, in the PDF version.

All in all, construction should in future have a qualitatively different material relation to the world. Not only does it need to be less energy- and material-intensive, but also those same principles need to extend to the operational use designed into buildings at the commissioning stage (see parts 9 and 10). And construction should proceed on the basis of enhancing ecological reconstruction and wellbeing.

8.1. Avoiding unnecessary new construction and demolition, and re-using materials

Avoiding unnecessary new construction and demolition needs to be the starting point for decarbonising the built environment. This applies to buildings and to infrastructure.

Reducing construction means that, as far as possible, existing structures should be retained for use – retrofitted and updated as necessary, in order to extend their lifetime of use. This means avoiding demolitions, which create needless demand for new construction, and waste already-existing embodied materials and previously imposed environmental burdens. (This is to say nothing of deliberate destruction of buildings and infrastructure in wars.)

In the words of Carl Elefante, former president of the American Institute of Architects: The greenest building is the one that is already built. The same is true of infrastructure, so long as it works as it should. Instead of new-build, the emphasis needs to be on retrofit, and “adaptive reuse”.

According to research by the Royal Institute of British Architects (RIBA) in collaboration with Architects Declare, a 20% reduction in demand for new buildings globally could be achieved just by (re)using existing structures better – and this could save “up to 12% of global emissions in the building and infrastructure sector”.

Carry the same principle of reuse into construction materials. Avoid construction waste, in favour of deconstruction, reusing materials and construction elements as much as possible (“design for disassembly”). Tap waste streams. Aim for a zero waste, or circular, construction economy.

There are many vanguard initiatives in this vein (for profit, and not) that seek to adapt the principle of reuse to the convergence of ecological urgency and computational possibility. For example, the Dutch group Metabolic, and the cooperative RotorDC, with its waste stream database-cum-marketplace, an eBay of sorts for recycled materials.

The architects’ firm Orms has been one leader in the practical development of “materials passports” – intended “to gather and organise data about materials contained within a building”, so that they can be effectively harvested for components in the future. Such data would include precise inventory and location data, engineering and performance specifications, and the embodied and operational material and environmental footprints associated with a given component.

At least in theory, this also means that material footprints, and wholelife assessments through to waste disposal and resale, can enter into the design process as active parameters, alongside engineering issues such as building physics, or questions of cost and availability of materials. To this end, architectural and engineering design platforms seem to be trending towards high-level parametric integration, mediating the space between markets and physics.

Of course all of this leans heavily on the quite possibly spurious idea that effective technological and market-based interventions can contribute meaningfully to decarbonising construction – especially outside the “value-added”-rich markets of rich countries. Questions remain around the practical extent of such schemes, and the politics of access to markets, platform data and modelling streams.

In any case, these sorts of initiatives indicate possible directions that a green capitalism might take. They are consonant with trends towards “eco-labelling” in certain segments of the global food industry. But they also suggest one aspect of what an eco-socialist transitionary programme might look like, in terms of steering local and global economies in a sustainable direction.

8.2. Material efficiencies in construction

When new construction is necessary – which includes retrofit – embodied and operational emissions need to be factored in as parameters from the start.[1]

The dominant high-carbon construction materials – steel, other metals, concrete and glass – need to be used in moderation, and sidelined wherever possible.

The carbon intensity of construction has come down in recent years. However, decarbonisation of existing styles of construction, to anything like the necessary extent, is likely to be a long time coming.

So long as established construction methods and economies prevail, whatever shifts do occur in the relative carbon-intensity of construction will most likely continue to be swamped in absolute terms by the volume of construction, at least in the short-to-medium term.

Priority must be given to three things: “build light”, aim for materials efficiencies, and use low-carbon construction materials and processes.

“Build light” is an overall ethos. It involves constructing only what is necessary. Building elements can be specified to be literally less heavy, and require less structural support – since structural strength often comes with more embodied emissions.

Materials efficiencies: embodied carbon can be reduced by engineering down the use of construction materials, and especially high-energy materials. Using standardised material sizes can help reduce waste.

So-called “lean” design and construction techniques combine both of these approaches – achieved through changes in design, so that buildings and infrastructure are not needlessly over-designed or over-specified; and through minimising waste during manufacturing and construction.

In theory, off-site fabrication and short-term “flying factories” at construction sites can help to improve material efficiencies and reduce waste. However, designing manufacturing processes and supply lines on a project-by-project basis comes with its own challenges.

Materials like timber that do not require extra finishing treatment – they are “self-finishing” – are also good, because they are more easily deconstructed and recycled in the future. It is also preferable to avoid adhesives for the same reason.

8.3. Low-carbon materials

Worldwide, almost all of the embodied emissions of buildings come from the manufacture of building materials, as described in part 7. Some of the best low-carbon materials are “traditional” ones such as wood, stone and rammed earth. These options are far more practical than they may seem, and more viable in engineering terms than a habituated fetish for fossil-fuel-heavy products might allow.

Traditional construction materials can be a route by which construction and design work can incorporate and learn from local sources of knowledge, about what materials work well locally in relation to factors like climate, and what is available. Engaging and elevating local craft skills is of value in its own right.

Traditional construction materials and methods are usually less capital-intensive than industrial ones. This may well make them less profitable and less worthwhile inputs for capital. They may or may not require higher absolute inputs of labour.

By contrast, the type of capitalist housebuilding dominant in the UK is very labour-intensive. Here, profitability for capital is assured by monopolies on land, poor construction standards, favourable access to low interest rates and political favouritism.

But at the level of social use-values in the locales of construction sites, plentiful labour-content can provide a source of employment. In many places, employing and/or training people in traditional construction methods, updated as necessary, can furthermore be a valuable source of local worker-led autonomy.

Local materials can give construction a degree of local texture and specificity it might otherwise lack. Where a small emissions footprint is the aim, less transport can also be a plus. Transportation is usually a comparatively small component of the embodied carbon of high-emissions industrial materials, but when the embodied carbon of manufacture is cut back, so the transport component becomes relatively more important.

Traditional construction methods can also be used to produce prefabricated components, that have a much smaller carbon footprint than prevailing carbon-intensive materials. Flying factories, close to one or more construction sites, could integrate the benefits of traditional modes of construction with the flexibility of prefabrication.

As with agroecological approaches to farming, the point here is to concentrate the “knowledge intensity” of production, rather than the capital- or carbon-intensity. Ways of providing materials can then be found that are environmentally friendly, but also potentially transferable away from the immediate site of production – and, again, also job-creating, in environmentally sustainable industries.

These are some of the important low-carbon materials (not including substitutes for cement, concrete and steel, that are covered in sections 8.5 and 8.6 below.)

🔥 Rammed earth is traditionally used with earth excavated at, or close to, the construction site. It is a good insulating material, possessing tremendous thermal inertia, i.e. its thermal massing retains warmth, or insulates against it, very effectively. It is also available as a prefab, prepackaged, low-carbon means for load-bearing construction.

🔥 Stone is another very effective, and ancient, structural material. A good example of its use is the limestone used in the load-bearing exoskeleton of the Stirling Prize-shortlisted 15 Clerkenwell Close in London. Its embodied carbon footprint is apparently 10% of what it would have been if steel and concrete were used. The cost, too, was a small fraction. The declared embodied emissions for the whole building are 335 kilogrammes of carbon dioxide equivalent per square metre (kgCO2e/m2).

🔥 Hemp is perhaps the most exciting traditional construction material – one that could be hugely beneficial to bringing down levels of embodied carbon, for example as insulation or in the form of hempcrete (see below). It grows quickly, locking up biogenic carbon, making hemp itself carbon negative.

Also in hemp’s favour is that it grows in a wide diversity of temperate climates; it can be grown in untilled soils; and it is harvestable within just a few weeks. As a plant-based material it has high carbon-sequestering capacities through the rapid growth phase, and a rapid rate of throughput from atmospheric CO2 to a stock of biogenic carbon.

According to a recent study by the Biorenewables Development Centre at the University of York in the UK, industrially-grown hemp crops can sequester up to 22 tonnes of CO2 per hectare annually – “more than any other crop or woodland”. (Although note that the crop’s net emissions will be a bit less than that, due to emissions associated with cultivation and processing.)

A serious concern about the widespread use of hemp might be the risk, as with biofuels, of hemp cultivation displacing food agriculture. This is a disadvantage, compared to non-biological, non-renewable materials such as limestone, which are relatively abundant and relatively independent from food markets.

However, also on the plus side is that hemp’s root system is extensive, and it tends to suppress weed growth, which means it can help improve degraded soil structures, remediate polluted soils, and boost subsequent crop yields. As a rotation crop it can be used as a “break crop” for all these reasons, to suppress weeds and pests, and additionally support growers’ economic sustainability.

🔥  Hempcrete, made by combining hemp stalks’ inner core with water and lime, is a bit of a wonder material. Like rammed earth, it has great thermal inertia. It is highly flame-resistant, resistant to pests and mould, biodegradable and breathable – meaning that it naturally regulates a building’s temperature and humidity, which improves thermal comfort.

Mixed on-site, hempcrete is readily mouldable into large, lightweight bricks, or applied as a surface render. It can also be manufactured into prefab wall panels with very little expenditure of energy.

Hempcrete is comparatively strong, but has only about 5% of the compressive strength of residential grade concrete – so walls made of it require the addition of another loadbearing material such as timber or CLT (see below). But it weighs only about one-seventh as much as concrete. It is well suited to construction in areas at risk of seismic activity, because it is resistant to fracture under movement. Hempcrete can also be reused if milled and then rehydrated.

On the downside, like cement, hempcrete produces process emissions in the lime binding stage. These far outweigh other emissions from hempcrete production, such as energy-related emissions. The process emissions tend to be about equal to the carbon sequestered during the hemp’s growth phase, so cradle-to-gate hempcrete is roughly carbon neutral. Additionally, hempcrete, also like cement, re-absorbs CO2 during the remainder of its lifecycle, potentially around 40% of what was produced when it was made – so that cradle-to-grave emissions are carbon negative.[2]

🔥 Bamboo can have a tensile strength similar to steel, and its compressive strength is double that of concrete! Bamboo is also incredibly fast-growing, making it great for carbon sequestration. (See section 8.4 below.) It is also lightweight – and cheaper than concrete, a big advantage, especially in poor countries.

Indeed, bamboo is already widely used as a building material globally. According to a recent (2023) Global ABC report on building materials, bamboo (like hemp) is a promising material for use in areas prone to earthquakes, and also durable during floods. All round, it is ideal for designing climate-resilient buildings. It can be used directly, in its raw, unfinished state.

🔥 Engineered timbers such as cross-laminated timber (CLT) (also termed “mass timber”) are a recent and important innovation. Like hemp, bamboo, or other timber products, CLT contains whatever biogenic carbon was fixed into it during its lifetime as a plant.

Like bamboo, CLT is capable in many instances of replacing structural steel and reinforced concrete. It has a high strength-to-mass ratio, and can be used for walls, floors, and ceilings. It can be deployed alongside hempcrete to give load-bearing strength, or alongside steel and reinforced concrete as a partial replacement.[3] CLT can also be made out of bamboo.

CLT has been used to impressive engineering effect in recent prestige buildings in the UK: for example, the Stirling-shortlisted Cambridge Central Mosque (embodied emissions, 844 kgCO2e/m2) – and also here and here. You can even construct highrise buildings out of CLT (albeit with a concrete base, elevator shaft and stair wells).

As with hemp, the sequestration potential of CLT can be increased dramatically by using high-yield, fast-growing crops.

By-products of production, such as off-cuts, can also be used for long-term carbon sequestration as small-scale timber products.

It is also important that petrochemical-based glues, chemicals and coatings used in wood products are phased out and replaced with bio-based alternatives. According to Global ABC, as well as reducing embodied emissions, this could enhance mechanical performance and reduce the unintended movement of heat and damp through structures. i.e. improve hydrothermal properties.

8.4. Plant-based materials and carbon sequestration

Bio-based materials such as hemp and timber can provide very good alternatives to classical industrial building materials like including steel and concrete. They have the additional benefit of storing the carbon accumulated in them during their growth phase in the form of biogenic carbon. This allows for what the Global ABC, the UNEP-IEA building strategy document, calls “carbon pool replenishment”.

However, the advantages here can also be overstated and misleading. Much depends on what type of biological growth is involved, the rate of growth – and, most importantly of all, whether the materials are sustainably sourced.

In the case of timber, global rates of deforestation presently exceed rates of regrowth. Moreover, the international trade in timber shows a marked flow of wood resources from poor to rich countries. Rich countries such as Germany and the USA import wood from the global south for reasons of cost efficiency, even in the face of ample, comparatively untapped wood resources within their own borders.

Hemp, as I have said, grows very quickly. It sequesters carbon rapidly, and provides all sorts of other agricultural benefits. Bamboo too grows very fast indeed. As cultivated crops, so long as the cultivation is otherwise ecologically sustainable, those can be win-win. However, most trees grow slowly.

Dead timber used in construction obviously also stops absorbing CO2. A living tree would have continued to grow and absorb more CO2 from the atmosphere during the remainder of its life, and the only net gain in carbon sequestration occurs when a new tree grows in the place of the felled one. Young tree saplings take a while to get going with their carbon fixing. However, older trees also slow down. (See the illustration.)

Source: excerpted from Michael T. Ter-Mikaelian et al (2015). “Forest carbon”
here means the total level of sequestered biogenic carbon in a forest

“Don’t worry, we’ll grow trees to suck up the carbon” is an increasingly common theme from politicians seeking to delay effective action on climate change. And this has led to disputes about how to measure the real effect of afforestation.[4]

The point, in my view, is that carbon sequestration has an important time efficiency component, depending on the speed at which farmed trees, hemp, or other materials grow, and the resultant rate of throughput from atmospheric CO2 to biogenic carbon to a biogenic end-product. Again, one of the many attractions of bamboo and hemp is that they grows fast.

The Global ABC cites evidence that in China bamboo sequesters carbon 30-40% faster per hectare per year than a tropical mountain rainforest, or a fast-growing variety of fir, and 2-6 times faster than average sequestration rates for forests in China and globally.

But of course forests also have tremendous ecological value in and of themselves: they are complex ecosystems and carbon sinks that should be allowed to thrive. They are not just a “resource” for wood products or for burning, or a “dividend”. In my view, there should be no farming of existing old forests.

Timber products should only come from farmed timber. And to be truly sustainable, planted forests cannot simply be monocultures, but must be thriving, if transient, ecosystems in their own right.

But Interpol estimated in 2021 that 15-30% of internationally traded timber was harvested illegally. And, according to the UNEP, illegal logging is responsible for up to 90% of tropical timbers felled in the world’s main tropical forest regions – the Amazon basin, Congo basin and south-east Asia.

There is presently a big push globally towards bioenergy. In my opinion, there is no defensible reason to burn wood industrially: cultivated timber should only go into durable products. Household wood combustion should also be phased-out.

Moreover, there needs to be a massive global effort at rewilding, including reforestation and mangrove restoration, to re-establish ecologies and carbon sinks lost to economic development. Much of that needs to happen in rich countries. Globally, rewilding should centre the needs of land-based and indigenous peoples, and not become a further tool of land-grabbing and greenwash.

Meanwhile, the aim for timber products and other bio-based materials should be to maximise their useful life, as with all useful materials. That is why bio-based materials are good for construction: buildings (and infrastructure) tend to be around a long time – and a long lifecycle of use should also be the norm, in order to minimise new construction.

The counter example would be timber used for cheap, non-durable furniture. Moreover, in the really existing market of timber products, Ikea can get away with being complicit in illegal logging of Romania’s old growth forests.

The sequestration potential of most CLT is clear. It will be improved if the logging and CLT industries decarbonise their sources of energy. They also need to clear forest residues – a large source of greenhouse gas emissions. Crucially, however, bamboo-based CLT is at present a net producer of emissions, due to the exceptionally high emissions associated with its manufacture.[5]

Optimistic claims about wood, bamboo and CLT also often fail to confront wider issues around land use.

The authors of one paper claim that 90% of the world’s new urban populations to 2100 could be housed “in newly built urban mid-rise buildings with wooden constructions”, instead of using steel and concrete, saving 106 Gt of CO2 during those 80 years.

On their calculation, that would require more than doubling farmed forest plantations – adding 1.49 million km2 to the current 1.32 million km2 by 2100. That would also mean encroaching on other forested areas that are presently unfarmed – in order to ensure no “major repercussions on agricultural production”.

However, a Greenpeace representative quoted in the Guardian points out that additional forested area for construction timber would better come at the expense of meat and dairy farmland, instead of further trampling old and biodiverse forests with monocrop plantations. That criticism is on-point: for the authors, unfarmed forests seem to be available for new plantations, whereas agriculture and other forms of land-use can only ever be ratcheted outwards.

The Global ABC notes that areas available for bamboo cultivation are already scarce, given the existing land-use pressures from agriculture and (to a lesser extent) housing.

Wherever and however bamboo or CLT are farmed, what happens to their biogenic carbon at end-of-life is also crucial: forget the gate, where is the grave? Is the wood re-used, keeping the carbon locked away (100% sequestration)? Is it burned, releasing the CO2 back into the atmosphere (0% sequestration)? Or does it rot in land-fill, releasing methane?

Bamboo, CLT, and similar bio-based construction materials – and timber furniture – should at least impose no net ecological harm. They should only be considered viable as mass-market goods when the bio-based materials they are constructed from are harvested in an environmentally positive way. They should also be built to last, within a holistically planned human ecology – not (as with Ikea furniture) produced as a throwaway quick fix.

Forestry residues should be removed, and/or harmful emissions suitably minimised.

In short, carbon-intensive construction materials need to be supplanted with ones that are as close to zero-carbon as possible, and construction materials as a whole used in the most efficient ways possible. But equally, the virtues of bio-based products, and their time-horizons, should not be exaggerated, or abstracted away from the full breadth of social and ecological concerns. Nor should ecologies and biologies simply be regarded as physical materials to be taken at will.

In any case, the entire rationale on which societies “resource-ise” materials from the natural world needs in time to be retrofitted, if not demolished and rebuilt from scratch.

8.5. Decarbonising cement & concrete

In the case of decarbonising cement and concrete, there are possibilities to change the way cement is made. One way is to change the recipe. Brimstone Energy, a US-based start-up, replaces limestone with calcium silicate – which they say avoids the process emissions associated with limestone, and reduces kiln temperatures, making electric kilns feasible.

Another way is through material efficiencies. For example, you can make cement with a lower proportion of clinker, the most carbon-intensive component. Up to 50% of the clinker can be replaced with limestone and calcinated clays. Another alternative is to introduce graphene into the cement mix, to make it stronger and permit a reduced clinker component.

Seratech, a start-up spun out of research at Imperial College London, combines carbon capture with cement manufacture to produce a nominally “net-zero” cement. Their technology removes industrial CO2 emissions from flues, and uses that to produce carbon-negative silica, which can be used in place of cement. When that is mixed with Portland cement, the negative emissions in the silica are said to balance the positive emissions from the Portland cement, to make an overall “net zero” cement mix.

In the long term, direct electrification of cement kilns should also be possible. That will not reduce process emissions, but would allow the most energy-intensive part of production to be decarbonised.

There are also ways to grow cement-like materials biologically, that seem to be free of many of the sustainability and land-use problems that arise with plant-based products.

Biomason, a US company, makes a product called Biocement, or biogenic cement. The company says it uses marine microorganisms to mimic the process of calcination that makes coral. In this way the company “grows” limestone in an effective reversal of cement production. They can do this either in-situ, or to produce pre-cast units.

The House in Bordeaux (1999), created by Anne Lacaton and Jean-Philippe Vassal
from a former biscuit factory. Photo from 
dezeen.com

When used underwater, this process sources its ingredients from seawater, “for propagative calcium carbonate precipitation”, which gives underwater structures “self-healing abilities”. That sounds great.

Biomason partners with other companies, licensing Biocement for particular uses – for example, BioBasedTile, a Netherlands-based company making pre-cast concrete tiles from recycled waste and biocement. The tiles “grow” in less than three days, and are intended for use on facades, interior walls and flooring. The end product consists of ~85% recycled granite and 15% biologically grown limestone. The company claim their tiles are three times as strong as traditional concrete tiles, weigh 20% less, and have just 5% of the embodied emissions.

Unfortunately, it looks as though Biomason is funded with more socially destructive uses in mind: according to their website, the company works “with support from the US Department of Defense” towards the obvious marine applications of this technology, and for land-based uses in “forward operating positions”, “where native, non-engineered surfaces prevent safe vertical take-off and landing”.

All such uses should be non-proprietary, and available for use in civil engineering projects.

A similar but seemingly inferior approach, pursued by scientists at the University of Colorado Boulder, uses microalgae to grow “biogenic limestone”. The microalgae capture CO2 via photosynthesis in the growth phase; this is released again during the calcination process: net-zero as far as the process emissions are concerned.

Apart from cement manufacture, there are some indications that cementitious materials, such as concrete, could be artificially and rapidly recarbonated with CO2 after manufacture. Part of the natural curing process of cement involves it reabsorbing CO2 over its lifetime, as mentioned in Part 7. However, that process is slow when it occurs naturally, and can lead to structural problems when it occurs in an uncontrolled fashion.

Research recently reported in the architectural press, by Tunley Environmental (a sustainability consultancy) suggests two methods of controlled recarbonation: (1) injection of CO2 into precast concrete; and (2) embedding CO2-rich materials into the concrete mix. They also propose that controlled recarbonation could be performed on waste concrete, before recycling it into fresh concrete as recycled concrete aggregate (RCA). The controlled nature of these processes, they say, would avoid the structural degradation that can result from natural recarbonation.

Noting that the theoretical maximum carbonation capacity of cement is 50%, Tunley point out that, were all the world’s production of concrete recarbonised in this way, it would sequester around 77% of CO2 emissions from cement manufacture.[6] Setting aside that theoretical figure, they suggest a more modest 17.3% of cement-related CO2 emissions re-sequestered annually through method (2) above. Again, though, that is if all concrete produced globally were recarbonised in this way. So there are plainly practical constraints of scale, and of effectiveness.

In short, controlled recarbonation can presumably play some role. However, it seems hugely unlikely that such processes could be applied at sufficient scale to make a meaningful impact, within the necessary timeframe for decarbonising the built environment. Indeed, stories like this risk encouraging complacency.

There are also options for changing the way concrete is made. Again, a very simple approach is to reduce the amount of cement used to what is structurally necessary. For example, concrete roadside kerbs have very different structural requirements to the high strength grades of concrete used in bridges and skyscrapers.

CarbonCure, a company using similar technology to Seratech, injects captured CO2 into concrete during mixing. This takes advantage of the usually very slow natural process of cement carbonation (recarbonation), producing nano-sized particles of calcium carbonate in the concrete that help to strengthen it.

Perhaps more promisingly, it is possible to make concrete without cement at all, by using a different binder. One example of that is “Earth Friendly Concrete”, produced by the Australian materials firm Wagner, which apparently has up to 70% less embodied carbon than regular concrete made with cement.

Earth Friendly Cement has been licensed for use in the new Silvertown Tunnel under the Thames in London. Needless to say, reducing the embodied emissions in the construction of that road tunnel will do nothing to mitigate the much larger energy-based emissions arising from greater induced traffic flow.

And note the word “licensed”. While some of these techniques are now operational or close to maturity, they all seem to be based on closed proprietary systems.

That is fine, perhaps, for decarbonising cement and concrete production where the technology is available, and where the costs are deemed commensurate to the benefits. But it is highly doubtful that a broader rollout of such technologies internationally could work on that “cost premium” basis.

In my view, the technologies to decarbonise cement and concrete manufacture should not – and must not – be subject to intellectual property restrictions.

If it is technically available and economical, then it should be actually available to all. As with essential drugs, off-brand “generic” versions must be produced, and made available at a price competitive with, or cheaper than, dirt-cheap bog-standard materials. By subsidy or regulation, that seems to me the only real solution to decarbonising cement and concrete manufacture on a world scale.

The IEA says that these emergent technologies are most unlikely, on their own, to be competitive on price against old-school cement, with its process emissions unabated. For them to become price-competitive, carbon pricing would have to be introduced globally. However, cement is mostly produced locally to where it is used (see part 3), making it hard to police.
 
8.6. Decarbonising steel

The steel industry now relies almost entirely on thermochemical processes that use coal. Recycling is already a big thing, and producing steel from scrap only requires about one-eighth of the energy used to produce steel from iron ore. But there is not nearly enough scrap to meet present market demand for steel products. Recycled steel only comprises 30% of total global supply.

The international steel industry is focused on reducing CO2 emissions within the thermochemical paradigm, by using “green” hydrogen or biomass, instead of coal, to produce pig iron. The industry talks a good talk about decarbonisation along this route, and so do supportive states.

In Europe there’s the HYBRIT demonstration project in Sweden, intended to “demonstrate a complete industrial value chain for hydrogen-based iron and steelmaking”, and produce 1.2 Mt of crude steel a year.

China is developing Zhangjiakou as a “hydrogen energy pilot city”, with a plan to open a zero-carbon steel plant there. But as a whole, China’s steel decarbonisation plans do not go far or fast enough to meet the 1.5°C Paris target.

However, most industrial hydrogen is presently made using fossil fuels, and that is unlikely to change anytime soon. Hydrogen production itself is also incredibly energy intensive. “Green” hydrogen at any scale – or kindred alternative “flavours” of hydrogen – seems a long way off, if indeed they can ever be a practical eco-friendly reality.

Biomass combustion, meanwhile, carries its own problems: entailing significant on-site emissions, potentially colonising valuable agricultural land, and encouraging deforestation.

Innovations in steel manufacturing also face large economic barriers – from the capital intensity, long life, and sunk costs of industrial steel-making facilities; and from the low margin, fully globalised, highly competitive international market in steel products. Outside of boutique prestige projects, a universally-applied and enforced carbon price would seem to be necessary to encourage the necessary shifts in the industry at large.

Greening the international steel industry requires not just billions of dollars of investment, but trillions, according to Nathaniel Bullard, writing for Bloomberg. That’s thousands of billions of dollars, just to get to low-emissions steel, globally.

The main future promise for low-emissions steel manufacture, however, lies further from the status quo: switching from thermochemical processes to electrochemical processes – and in particular, low-temperature electrolysis. This is less capital-intensive than the alternatives, is suited to intermittent power, and produces steel in a single process, so that it does not require further refining.

Crucially, electrochemical processes use electricity, so they can be run wholesale from renewable sources of energy.

However, electrochemical steel production at scale is also still a long way away. Such a widespread shift in technology would seem to require severe economic sanctions, or universally-applied regulations, to become economically justified.

Another issue is the centrality, in proposals for decarbonising both the cement and steel industries, of carbon capture, utilisation and storage (CCUS) and/or carbon capture and storage (CCS).

Reaching “net zero” will be “virtually impossible” without the mass roll-out of CCUS, according to the IEA, especially in the case of “hard to abate” emissions, i.e. those of heavy industry and long-distance aviation.

However, CCUS at sufficient scale and energy-efficiency to course-correct existing levels of greenhouse gas emissions, let alone draw down historic emissions, in anything like the requisite timeframe, is a total pipe dream.

In the minds of corporate and state high-ups captured by fossil capital, as the IPCC’s Working Group III point out, “CCS can allow fossil fuels to be used longer”, and this reduces the degree and speed at which fossil reserves become de facto stranded assets –a necessary economic lever for limiting warming to 1.5°C or even 2°C.

The Working Group also notes that, regardless of its supposed promise, the “global rates of CCS deployment are far below those in modelled pathways limiting global warming to 1.5°C or 2°C”.

A highly critical report on CCS, published by the Institute for Energy Economics and Financial Analysis (IEEFA) in September 2022, specifically takes aim at the IEA’s advocacy for the technology.

I have commented in more detail on the IEA’s approach in Appendix 5, in the PDF version.

8.7 Actions by architects

I began this series with reference to some worker-led groups and professional bodies across architecture, design and engineering that have become prominent advocates for decarbonising the built environment. In the UK, these include the Architects Climate Action Network (ACAN), Low Energy Transformation Initiative (LETI), Architects Declare (AD) and the Royal Institute of British Architects (RIBA).

Each of those has published a stream of pamphlets, petitions, guidance documents, case studies, regulatory and policy advice, and benchmarking objectives, all aiming to shift the built environment professions internally, and shift external norms and expectations.

They consider not just emissions, but also the built environment’s direct impacts on other aspects of the natural environment, such as wildlife habitats, biodiversity, water systems, soil health, and people’s health and wellbeing.

15 Clerkenwell Close, London. Photo by Chris Wood / Wikimedia Commons

The intellectual momentum behind these proposals is significant. What they amount to practically and at scale remains to be seen.

In architecture, there are political tensions that need to be worked out. One of these is that large architecture firms usually get paid to design new buildings and shepherd them through construction. But what if one of the climate-aware architect’s main tasks is to halt most new construction? What if the only way to be a climate-responsible architect is to discourage, not encourage, large projects with high rates of material use and large carbon footprints?

What if the world needs architects and engineers to divest from surplus projects in rich countries, and be redeployed delivering services to the global poor?

In 2020, the famous architect Norman Foster, an early signatory to Architects Declare, withdrew his firm’s support, because of his enthusiasm for airports.

The prestige buildings mentioned above for their novel use of low-carbon materials were all new builds, with stated tonnages of embodied carbon. Few people actively want to see an end to construction – and indeed new construction is necessary to meet urgent social needs.

But the onus needs to be on those new buildings causing emissions in the built environment proving their social worth, both in terms of redressing inequalities, and in an international context of limited and shrinking carbon budgets. Certainly this is the case in rich countries, but also wherever capital pours into construction.

In this respect, it was nice to see Anne Lacaton and Jean-Philippe Vassal win the prestigious Pritzker architecture prize in 2021, for their spatially generous, and beautiful, retrofitted expansions of existing social housing blocks in France. Retrofits like that, which improve buildings’ operational performance, at minimal cost in terms of embodied emissions – should be the model.

Most buildings construction worldwide is designed with little regard for the climate, and with more of an interest in catering to architects’ clients – whose agendas will not often be led by environmental concerns. For that reason, compulsory regulations on embodied and operational carbon are essential, internationally.

Voluntary benchmarks set by RIBA and LETI give a sense of scale, when thinking about what reasonable legal limits on embodied carbon might look like, on a per-project basis.

However, legally-binding hard limits on the carbon footprints of new construction as a whole are needed, on a per-country, or regional basis. Plus tough benchmarks for the carbon intensity of the delivery of services, with legally-binding obligations prioritising the collective provision of essential and universal social goods, such as housing.

In its 2030 Climate Challenge (version 2, 2021), RIBA proposes to almost halve embodied carbon for new buildings in the UK by 2030. The target is for an embodied carbon performance of less than 625 kgCO2e/m2 for domestic buildings, and less than 750 kgCO2e/m2 for non-domestic office buildings. There are interim targets for 2025, and additional targets for school buildings (see below).

LETI has published more stringent targets for 2030: less than 300 kgCO2e/m2 for domestic buildings (over 6 storeys), schools, and retail; and less than 350 kgCO2e/m2 for offices.

For comparison, 15 Clerkenwell Close, the (deluxe) residential and office building mentioned above, had declared embodied emissions of 335 kgCO2e/m2.

Source: adapted from RIBA (2021), 2030 Climate Challenge

For a new home of 70 m2, according to the RIBA 2030 benchmark, the embodied carbon would have to be under 43.75 tonnes CO2e. For the LETI 2030 benchmark, it would be under 21 tonnes CO2e.

But in the UK, “business as usual” residential construction, under current regulations, produces an embodied carbon footprint of 84 tonnes CO2e for the same floor area.

The architects’ associations are saying this footprint needs to be slashed by nearly half (RIBA), or more than three quarters (LETI).

🔴 Go to part 9

🔴 Go to Contents and Introduction

Download the whole series as a PDF here

[1] See Feilden Clegg Bradley Studios’ whole life carbon review tool

[2] For a 1m2 area of wall made of hempcrete, offering a typical level of insulation (R-20 grade, with a heat transfer coefficient of 0.27 Watts per meter-square Kelvin), process emissions come to about 36 kg CO2e. Typically, this is roughly equal to the carbon sequestered during the hemp’s growth phase.

Over 60-100 years of use, hempcrete re-carbonation can re-absorb up to around 40% of the original process emissions, according to the study just cited. So hempcrete’s “cradle-to-grave” carbon footprint ranges up to, potentially, the sequestration of about 15 kg CO2e per m2 for the same R-20 piece of wall.

Different lifecycle emissions of hempcrete are associated with different densities of “mix” and different “model” estimates for process emissions. See here for a useful comparison.

[3] See pages 34-35 in the 2023 Global ABC report.

[4] I said that the only extra net gain in carbon sequestration occurs when a new tree grows in the place of the felled one. This is seeing things according to a “debt-then-dividend” (or “carbon repayment”) model. However, the other way to look at it is that you start with the dividend; then once you fell a tree you are simply back to zero.

These are two different carbon accounting conventions – and they are most salient when applied burning biomass, and releasing biogenic carbon into the atmosphere. However, they are also relevant to the stock and flow of timber-based products. Sustainable forestry – amongst other things – is the practice of managing the “flux” in the stock of carbon in a forest over time. In carbon accounting terms, the gains versus losses are recorded under the category “Land Use, Land Use-Change and Forestry” (LULUCF).

But whichever way you look at it, the “flux” of biomass growth is hardly instantaneous. Bamboo produces wood much quicker than most other plants: for most timber products, you might expect forest regrowth to return to pre-harvest levels (“carbon sequestration parity”) after around 40 years.

Crucially, the forest by-products of logging also need to be removed, in such a way as to avoid decay on land, which itself releases methane – and to reduce the risk of forest fires. (Note that both living and dead trees transport methane into and out of the atmosphere.)

[5] A recent literature review compiled the reported cradle-to-gate emissions of some CLT manufacturers, based in Europe, North America, and Australia (that is, the emissions flux from growth through manufacture, to point of sale). The average declared sequestration was -643.6 kgCO2e/m3 of CLT. That is, the CLT has a negative carbon footprint – when also taking account of the other, energy-based emissions that arise during manufacture.

However, a better way to assess the sequestration potential of bio-based products like CLT, is to look at manufacture in the context of land-use; and to consider the whole life-cycle, cradle-to-grave. In particular, it is important to consider what happens to forest residues after logging, and what happens to timber products at the end of their life – forest residue decay and land-fill decay are both a significant source of greenhouse gases, including methane.

One recent study cited in the Global ABC report does both of these things. It modelled a 100-year lifecyle for CLT produced in the Southeastern United States, from high-productivity pine plantations, and found that net greenhouse gas emissions could be as low as -1,445 tonnes CO2e per hectare (100m x 100m). This modelling is based on current modes of CLT production: it assumes in this instance that the drying kiln is powered by gas, that all vehicles are powered by diesel, that forests are “clear cut”, and that all harvest residues are left on the forest floor to rot. CLT recycling rates are presently low, and the study models 50% of manufactured CLT going to land-fill at the end of a 60-year lifetime of use.

(It is also common – because economical – for timber mills and processing plants to generate heat by burning wood residues and cut-offs. This releases more greenhouse gases than burning fossil fuels – combustion of dry wood produces more CO2 combustion emissions per unit energy out than coal does. Use of wood residues increases emissions compared to the use of gas.)

All of which indicates that the sequestration potential of CLT could be improved yet further by reducing emissions at all those stages in the lifecyle – and that timber plantations for CLT should be cultivated in more ecologically beneficial ways.

In the case of bamboo CLT, the high rate of carbon sequestration tends to be offset, for the time being, by a very emissions-intensive process of fabrication, when compared to other forms of engineered timber. The same Global ABC report outlines some of what is required in the case of bamboo CLT – carbonisation, high-temperature air drying, synthetic glues and anti-mould treatments. According to recent modelling, that can mean that the CO2 emissions during the manufacturing stage for bamboo CLT may end up comparable to those of steel on a per kilogram basis. Those emissions produced during the manufacturing stage are only offset ~35% by the biogenic carbon sequestered in the wood.

However, bamboo is also lightweight. I cannot find any data comparing the lifetime emissions of bamboo CLT and steel on an end-use basis. For example, if a lower mass of bamboo than steel imparts similar tensile strength, then that would reduce its real-world embodied emissions.

Another recent study calculated that, on average, CLT manufacture requires felling about 12m2 of forest for every m2 of a finished CLT building. However, in that study, the length of the trees’ growth phase and the time to harvest is unspecified. The same study reckoned that, if all new buildings worldwide to 2060 were built of CLT, supplying the timber would require 310,000 km2 of forest – about 0.8% of the world’s “available” forested area of 40.6 million km2. (The volume of construction required is based on international agencies’ forecasts, discussed below.)

[6] Based on the reported 2011 emissions, the authors estimate ~2 Gt CO2 sequestered annually.

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