🔴 The whole pamphlet, Remaking Home Heating in the UK, is published on People & Nature, and can be downloaded for free here.
The UK’s housing stock dissipates thermal energy at a rate, on average, of about 128 kilowatt hours per square metre per year (kWh/m2/yr). In consequence, that same amount of thermal energy – equivalent to roughly 60,000 hot kettles’ worth of heat each year in an average dwelling – needs to be returned to homes, to retain some semblance of thermal comfort.
In terms of improving a home’s fabric efficiency, there are many changes that can reduce the amount of supplemental heating or cooling required to maintain thermal comfort.[1] In the UK, the pressing need is for fabric improvements that retain heat inside a home—although over-heating in the summer is also increasingly a problem.
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| A construction worker insulates a house with mineral rock basalt wool mats. Photo from AdobeStock |
Some high-impact improvements to fabric efficiency can be cheap and low on inconvenience—for example, top-up loft insulation, and gap sealing to prevent draughts.
Overheating can be prevented with “cool roofs” or “cool walls”, and via the addition of brise soleil to the outside of windows. But good insulation also remains essential.[2]
A universal programme of comparatively “deep” retrofits looks like the most resource efficient and socially equitable path forward, improving homes across the board, and permanently.
Specifically, retrofit advice by the Low Energy Transformation Initiative (LETI) looks persuasive as a model for a universal programme of home retrofits. Once additional impacts of global warming and behavioural change are taken into account, LETI’s retrofit model would bring a reduction in space heating energy needs (N) down from an average of about 128 kWh/m2/year down to about 55 kWh/m2/year.[3] That is a 64% reduction overall.
So long as the leakiest homes were tackled as part of this process, and given suitable additional reforms to energy pricing, it would mean that all thermal energy needs for home space heating could be habitually met: energy needs and energy use would coincide (N=U). No one in the UK need live in thermal discomfort.
In this article I will look at LETI’s advice in detail, and put it in a wider context. I outline the LETI retrofit recommendations; and look at some practical challenges, such as physical, heritage, and aesthetic constraints, and the additional need for effective ventilation. I outline the weakness of government policies on home retrofit. Finally, LETI’s proposals concern only the renovation of existing homes. Yet the UK is forecast to require about 6.5 million additional new homes through to 2050.[4] I briefly outline the state of play with regard to pending new homes standards, and point to my own preference for a Passivhaus-aligned new homes standard.
LETI-depth retrofit
LETI’s Climate Emergency Retrofit Guide (2021) foregrounds a deep set of fabric renovations, designed to significantly improve the fabric efficiency of the UK’s existing homes and reduce space heating energy use.
The main motivator here is decarbonisation. The LETI advocate a “fabric first approach”, with fabric improvements the priority, followed by decarbonised sources of heat and on-site renewable electricity generation. Though they say they do not “seek to define Net Zero retrofit”, they provide a good set of benchmarks, and indicate how to get there.
Subsequent modelling by the UKGBC and the UK’s Construction Leadership Council (CLC) flesh out the likely costs of a LETI-depth retrofit, and the likely practical requirements in terms of skills and labour.
To their credit, LETI largely set aside economic questions such as “affordability” and focus instead on physical indices.[5] On the basis of their housing stock model, they estimate the likely physical changes in thermal performance of homes when, for instance, a given thickness of insulation is added. What do you gain, physically, by different additions, and how does that translate into reduced need for supplemental energy for maintaining thermal comfort?
The LETI baseline for thermal comfort is that internal surface temperatures should be at least 17°C when temperatures outdoors are at their minimum. With that objective, they modelled a range of retrofit options against different existing baselines of thermal performance and different minimum external temperatures—and the likely shifts in energy need that would result, in kilowatt-hours, per m2 of internal floor area, per year (kWh/m2/year).
Figure 1 shows how space heating energy needs (N) vary, between the average existing home in the UK, various retrofit standards (all very achievable), and new homes. The measure here is the amount of supplemental energy required for space heating to maintain thermal comfort.[6]
Figure 1. Comparison of space heating energy needs (“demand”)[7]
Retrofit can only very rarely reach the highest fabric efficiency standards possible in a new build—although it can get close.
The average (mean) supplemental space heating energy need for a UK home is presently about 128 kWh/m2/year. Many people, however, are unable to meet those needs: they live in such leaky homes, and the cost of supplemental energy is so high, that they live in the cold instead.
An “unconstrained retrofit”, on the LETI model, would bring space heating energy needs down to about 45 kWh/m2/year. A retrofit to the more stringent Passivhaus Standard would bring space heating energy needs down to about 25 kWh/m2/year. (That compares against a new build Passivhaus at about 15 kWh/m2/year.)
A “constrained retrofit” here refers to situations in which heritage or other building features constrain the scope of fabric improvements. Even under such constraints, however, it is still viable to cut supplemental space heating requirements way down.
Figure 2 sets out (in blue) the likely range of space heating energy need across the UK mainland’s housing stock. To the left of that (in orange), is where LETI think space heating energy need could and should be, if every home was reasonably retrofitted to a best practice standard—“regardless of their form, age or construction type”. This corresponds to most homes reaching an average space heating energy need of 50-60 kWh/m2/year.
Figure 2. Changes in space heating energy need (“demand”) under a LETI-depth retrofit[8]
After a nationwide program of best practice thermal retrofits, LETI reckon that almost all dwellings should only require 60 kWh/m2/year or less of supplemental heat, with most at 50 kWh/m2/year.
The target of a LETI-depth retrofit is actually 50 kWh/m2/year, but with an additional 10 kWh/m2/yr allowed to retrofit constrained homes with heritage features, like listed buildings. For simplicity, therefore, I will refer here to a target of 55 kWh/m2/yr. The present average for space heating energy use is estimated at 128 kWh/m2/year, so that implies a 57% reduction in average space heating energy use. (Once climate change and behavioural change are taken into account, further reductions would also result, on which more below.)
Figure 3 shows how LETI’s thermal retrofit standards translate into average space heating requirements per square metre, for each type of home.
Figure 3. Change in space heating need, by building form retrofit case[9]
You can see LETI’s modelling parameters at the level of individual building elements, here. In particular, LETI have sought to find the “U-value sweet spots” of different materials, to balance insulation gains against cost This is illustrated in Figure 4, which shows the diminishing returns of adding more and more insulation, in the case of one type of insulating material, with outside temperatures at 0ºC.
Figure 4. Diminishing returns for the amount of insulation added[10]
For the first 150mm thickness of insulation added to a 100m2 wall (x-axis), the flow of thermal energy escaping through the wall typically declines by 1600 watts (left-hand axis); this material accordingly reduces the “U-value” (thermal conductivity) of the wall by about 0.64 watts per square metre per kelvin (W/m2K, right-hand axis).[11] However, adding a further 150mm of insulation typically only saves an additional 220W across that whole wall, or an additional about 0.8 W/m2K.
For this material, the U-value sweet spot on a cost-per-watt basis is therefore around 100mm of insulation: shown by the “valley” in Figure 5; 100mm of this insulation has a U-value of about 0.28 W/m2K. Based on a standard U-values table for insulation materials, that could be 100mm of polyurethane foam board (PUR).
Note that the £/W sweet spot is about “bang for buck”. It is not about the constraint of absolute “affordability”.
Figure 5. Cost per watt index[12]
However, since the “baseline” thermal performance of different homes and buildings varies, LETI have also calculated recommended thicknesses of additional insulation for different baselines. As I understand it, those recommendations use a variety of different materials, with different U-values—different materials being better-suited for different tasks, and having different sweet spots in different contexts, in terms of their “bang for buck”.
You can see those recommendations in Figure 6, which shows the different insulation thickness recommendations (the y-axis) against 6 different baseline situations (x-axis), and according to four different criteria (the different markers: diamond, square, circle, triangle).
Figure 6. Thickness of additional insulation, by baseline construction, according to various criteria (-5°C external temperature)[13]
The baselines are: three existing levels of wall insulation—cavity uninsulated, cavity insulated, and solid wall uninsulated, plus baselines for when a roof, a timber floor, or a solid floor are uninsulated.
The four criteria are: a minimal limit of thermal comfort achieved; a sweet spot for best “bang for buck”; a rate of heat loss of 3.5 W/m2; and an 80% reduction in heat losses versus the baseline. Depending on the baseline, as you progressively add insulation thickness, these criteria arrive in a different order.
You can see that, for an external temperature of -5°C, in 4 out of 6 baseline cases, best value in money terms (“bang for buck”) brings more than an 80% reduction in heat loss. For a solid uninsulated wall, the best value recommendation is to add 150mm of insulation to the existing wall. For a solid uninsulated floor the best value recommendation is about 190mm of floor insulation. For a cold uninsulated roof the best value is about 170mm of roof insulation, which again brings more than an 80% benefit.
Practicalities of retrofit
Overall, though, the basic reality of retrofit is that each home will have its own peculiarities. Implementing best practice conversions therefore requires skilled craftspeople, and high levels of oversight to ensure corners are not cut and fabric performance needlessly compromised.
LETI, RIBA and others also press a “whole building” approach to retrofit, “where the building is considered as a whole”, and evaluated by skilled thermal engineers. This is important: building physics are complicated, and the risks of ill-considered or piecemeal work are that fabric performance is compromised, or that unintended consequences arise, and could even make things worse. For instance, draught-proofing might lead to poor ventilation and damp, if controlled ventilation is not part of the plan.
A good, whole building approach can mean doing all retrofit work in one go, or in several stages—so long as everything is joined up into one holistic plan.
All of the above materials on a LETI-depth retrofit come from the Climate Emergency Retrofit Guide. It contains much more detailed information about design and performance specifications, along with case studies. It is aimed at a professional architecture and engineering readership, but is accessible for the non-specialist.
Loft insulation is an obvious requirement for anyone looking to make their home more thermally efficient. Only about 16.6 million (66%) of the about 25 million homes with a pitched roof in the UK as a whole had loft insulation in 2021. The other 8.4 million need to get it wherever possible. Buildings with uninsulated flat roofs can have insulation added on the top, outside or inside, and flats can potentially have internal floor or ceiling insulation added.
Insulating walls can be more challenging, in particular where the options for doing so are constrained. Plainly, if you want to add 150mm of insulating material to a wall, you need space to do so. If there is no cavity wall to use, then the insulation can go on the inside or outside of the wall.
As a general rule, adding insulation to the inside of a wall is more complicated than applying it outside—and more costly. There are more fiddly joints to negotiate—which risk thermal bridging—and rooms have to be approached individually. Internal insulation also takes up interior space, reducing room sizes.
Applying insulation to the outside of a building inevitably changes the way a building looks. In many cases this will be uncontroversial. However, it is unlikely to be permissible in the case of the facades of listed buildings. You can see this in Figure 7, where insulation is used alternately on the interior and the exterior of two listed buildings.
Overall, though, the basic reality of retrofit is that each home will have its own peculiarities. Implementing best practice conversions therefore requires skilled craftspeople, and high levels of oversight to ensure corners are not cut and fabric performance needlessly compromised.
LETI, RIBA and others also press a “whole building” approach to retrofit, “where the building is considered as a whole”, and evaluated by skilled thermal engineers. This is important: building physics are complicated, and the risks of ill-considered or piecemeal work are that fabric performance is compromised, or that unintended consequences arise, and could even make things worse. For instance, draught-proofing might lead to poor ventilation and damp, if controlled ventilation is not part of the plan.
A good, whole building approach can mean doing all retrofit work in one go, or in several stages—so long as everything is joined up into one holistic plan.
All of the above materials on a LETI-depth retrofit come from the Climate Emergency Retrofit Guide. It contains much more detailed information about design and performance specifications, along with case studies. It is aimed at a professional architecture and engineering readership, but is accessible for the non-specialist.
Loft insulation is an obvious requirement for anyone looking to make their home more thermally efficient. Only about 16.6 million (66%) of the about 25 million homes with a pitched roof in the UK as a whole had loft insulation in 2021. The other 8.4 million need to get it wherever possible. Buildings with uninsulated flat roofs can have insulation added on the top, outside or inside, and flats can potentially have internal floor or ceiling insulation added.
Insulating walls can be more challenging, in particular where the options for doing so are constrained. Plainly, if you want to add 150mm of insulating material to a wall, you need space to do so. If there is no cavity wall to use, then the insulation can go on the inside or outside of the wall.
As a general rule, adding insulation to the inside of a wall is more complicated than applying it outside—and more costly. There are more fiddly joints to negotiate—which risk thermal bridging—and rooms have to be approached individually. Internal insulation also takes up interior space, reducing room sizes.
Applying insulation to the outside of a building inevitably changes the way a building looks. In many cases this will be uncontroversial. However, it is unlikely to be permissible in the case of the facades of listed buildings. You can see this in Figure 7, where insulation is used alternately on the interior and the exterior of two listed buildings.
Figure 7. High level insulation strategy for listed buildings (insulation shown in red)[14]
In the UK, at least, significant aesthetic challenges are likely, outside of a strict conservation context. People are often (understandably!) attached to buildings with—for example—a merely “traditional” look, whether or not the building is technically constrained by heritage features. A mass retrofit program would fail if it is insensitive to people’s aesthetic needs, alongside the need for thermal comfort.
That said, external thermal insulation is usually the most effective and cheapest path, and least disruptive—although a full building approach will usually involve other modifications, such as draught-proofing and double- or triple-glazing windows.
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| Examples of an “Energiesprong” retrofit. Source: Green Alliance (2020) |
One way to approach externally insulating a building is to clad the entire thing in a new insulating layer, which is like throwing a tea cosy over the whole building.
A good example of that method is “Energiesprong”, a retrofitting program initiated by the government of the Netherlands in 2010. Modular prefabricated insulation is snapped onto the outside of a building, and solar PV added to the roof, with the aim of generating the amount of energy that the household consumes. You can see some before and after pictures below—the top row is from a UK pilot project in Nottingham in 2017.
The Royal Institute of British Architects’ (RIBA) 2022 “Homes for Heroes” proposal, on the other hand, specifically highlights the UK’s interwar housing as being in need of fabric retrofit. This comes from a few observations:
Apparently 50% of the houses with solid walls in England—i.e. those without cavity walls—are from the interwar period. A study of gas use intensity in London houses found that gas consumption was highest per square metre in interwar homes. Eighteen per cent of households living in fuel poverty in 2017 lived in interwar housing—although pre-1919 buildings housed the highest share (22%) of fuel poor households. Interwar housing usually has a pitched roof, so loft insulation is an easy win where it is not yet in place or is substandard. Interwar housing also comprises 3.8 million homes—so about 13% of the UK’s entire housing stock.
Interwar housing has a uniformity of style that RIBA believes is well-suited to a mass retrofit programme. Uniformity is likely to permit economies of scale in the manufacture of building elements, and a high degree of standardisation could also lift quality standards in installation.
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| Interwar housing in the UK. Source: RIBA (2022) |
RIBA suggest that “some owners will not want to alter the original façade of the building, so internal insulation will be preferable”, but that internal wall insulation “often is unappealing to residents and can restrict the volume of insulation that they are willing to install”. And so, “in most cases [the] owner-occupier’s preference will be the deciding factor in the approach taken”.
Aesthetic issues may be a significant challenge for mass retrofitting, as mentioned above. Interwar housing is as good a place as any to start to work that out. At the local level, there is the question of external insulation being “in keeping” or not with the feel of a neighbourhood. And where a certain “quaint” uniformity of style—or some other feature— is valued, then that is a design challenge for retrofit that needs to be resolved—and potentially agreed upon, simultaneously, by a row of neighbours.
Such a targeted plan only tackles some of the UK’s housing stock. Indeed, to reduce home heating demand decisively, the more urgent vintage of homes to target is the greater number of homes built through the post-war period to 2002. You can see that in Figure 8, from LETI.
Figure 8. Change in heating and hot water demand from baseline (2018) to 100% Best practice retrofit by dwelling age, form and wall type[15]
The graph shows total space heating and hot water need (“demand”) for the entire UK mainland housing stock (shown in blue), divided by dwelling age, form factor, and according to the current level of insulation. Additionally, it shows (in red) the effect of reducing those energy needs by the LETI-recommended amount, via an across the board retrofit program to best practice standards.
There is a broader lesson here—which is that any national retrofit programme needs to proceed, top to bottom, on a joined-up basis: neighbourhood by neighbourhood, street by street, home by home.
Additional measures include airtightness, which is an important way to reduce heat loss, and an essential component of retrofit. For instance, draught-sealing at a flat rate of £400 can typically bring the number of “air changes per hour” down to 0.5, according to a recent report commissioned by the government. This increase in air-tightness typically means that a smaller heat pump is required.
However, air-tightness comes with risks of poor ventilation, humidity, condensation and damp. Therefore, effective ventilation is also important. You need to “build tight, ventilate right”. You can use cheaper “mechanical extract ventilation” (MEV) systems, which simply ventilate air—and with it, heat from inside. Or you can get a more energy-efficient, but more costly, “mechanical extract ventilation with heat recovery” (MVHR) system. Research compiled for the Climate Change Committee’s 6CB (2020) estimates that a standard MEV system cost around £550, whereas an MVHR system cost £1,700-£4,100 per home. Wherever possible, MVHR seems preferable, as the norm.[16]
Responsibility for a national programme must ultimately reside with the government. How it happens, however, needs to be managed democratically on a local basis, through a devolution of expert responsibilities and decision-making.
Government policy on retrofit
2012 was a comparative high point for home fabric efficiency improvements in the UK. In that year, there were more than 1.5 million loft insulations, and around 600,000 cavity wall insulations —the product and culmination of two schemes introduced by the New Labour government.[17]
Those supportive policies were cancelled by the incoming coalition government under David Cameron, part of his campaign against “green crap”.[18]
In 2014, a statutory instrument was put in place, “to ensure that as many as is reasonably practicable [!] of the homes in which such persons live [those living in fuel poverty] have a minimum energy efficiency rating of Band C”, according to the EPC certification standard, in England.
Figure 9. Installation of home fabric efficiency measures (2010-2024)[19]
The number of fabric efficiency measures installed annually in the UK housing stock has remained very low, barely rising above 400,000 in 2024. Note that the Energy Company Obligation (ECO), which pays for most fabric efficiency improvements, is paid for out of the pockets of households themselves, via the “social obligation” levy on energy bills.
Labour’s 2019 manifesto, under Jeremy Corbyn, committed to a “Warm Homes For All” plan, with the retrofit of “almost all” of the “27 million” homes in the UK, “to the highest energy efficiency standards feasible for each building by 2030”. (In 2020, there were actually about 28 million households, and about 29.5 million dwellings in the UK).[20]
By April 2023, Labour’s plans under Keir Starmer had morphed into the more moderate ambition to “upgrade the 19 million homes that need it”; nevertheless, this meant retrofitting “up to 2 million homes a year”. In 2024, that ambition was itself “stood down” as part of the party’s U-turn over commitments to spend £28 billion a year on greening the economy. The 19 million homes would possibly now take 14 years to be realised, according to Starmer; it was replaced instead with a commitment to upgrade 5 million homes by 2030.
Present policy now commits to upgrading “up to 5 million homes” across the UK, over the present five-year parliament. Scheduled funding for that task is set at £6.6 billion over those five years (2024-29), with the same level of support following during the parliament after that (2029-), if Labour win the next election.
New homes standards
We require high standards of fabric efficiency in all new homes. An ambitious new standard for all new homes was first planned under New Labour in 2009, and was due to come into force at the start of 2016. That was ditched by Cameron in 2012.
An alternative “Future Homes Standard” (FHS) was then proposed by the Conservative government in 2019—intended to come into force in 2025, and impose improved fabric efficiency and ventilation standards for all new homes (alongside obligations for low-carbon heating and electric vehicle charging infrastructure).
The details of the FHS standard have been slow to materialise, and the FHS looks likely to be lacklustre in content.
Initial consultations by the government with the various industry “stakeholders” in 2019-2020 were followed by further consultations in 2023-24 on the possible technical content of the FHS. An industry body called the Future Homes Hub (FHH) published its own Ready for Zero report in 2023, in which it outlined five possible specification standards. However, the government’s second technical phase of consultations offered up for discussion only the “two weakest options” for the FHS outlined by the FHH.
It remains to be seen if the Labour government will do anything to intervene and upgrade the final fabric efficiency specifications for the FHS. These are due to be announced in 2025, along with an implementation date. However, recent actions do not inspire confidence: notably, the government cancelled the previous government’s 2035 phase-out plans for gas boilers, and has explicitly stated that gas boilers will now be allowed in new homes under the FHS.
In my view, all new homes should be built to the Passivhaus standard. This may well bring a cost premium, but that should be subsidised by government wherever it impacts on the upfront costs of social housing. As we saw above (Figure 1), a Passivhaus new build home should be able to maintain year-round thermal comfort with the application of just 15 kWh/m2/year of supplemental heating or cooling.[21]
□ This is the fourth and last excerpt from Remaking Home Heating in the UK, a People & Nature pamphlet by Tom Ackers, which can be downloaded for free here.
🔴 The first two excerpts, (1) on policy proposals and (2) on fuel poverty, were published by People & Nature yesterday. The third excerpt, on electrification and heat pumps, is here.
🔴 More People & Nature commentary and analysis of the decarbonisation of home heating is here.
References
[1] For an outline, see Decarbonising the Built Environment: a Global Overview, part 9
[2] “Cool roofs” and “cool walls” alike make use of reflective materials (eg, metal, light-coloured renders or light-coloured paints) that reflect away incident sunlight. However, in the absence of good insulation, those same materials can radiate heat away from the building when it is cold outside
[3] The actual target is 50 kWh/m2/year, but with an additional 10 kWh/m2/year allowed for retrofit constrained homes with heritage features—like listed buildings
[4] Estimate from the Climate Change Committee (CCC), Sixth Carbon Budget (6CB, 2020). See 6CB Dataset (v2, Dec 2021)
[5] The exception here is the calculation of “U-value sweet spots” for the recommended materials to be used: the aim being to balance insulation gains against cost
[6] Whereas I use the term energy need (N), LETI refer instead to space heating energy “demand”. See the Introduction to Remaking Home Heating in the UK
[7] Source: LETI (2021), Climate Emergency Retrofit Guide, page 60. (AECB is the Association for Environment Conscious Building.)
[8] Source: LETI (2021), p.61
[9] Source: LETI (2021), p.57
[10] Source: LETI (2021), p.185
[11] Kelvin is the standard (SI) unit for measuring temperature. A temperature difference of one kelvin is equal to a temperature difference of one degree Celsius; however, 0K is equivalent to -273.15°C. U-values measure the rate of thermal energy. In the case of a wall, a U-value of 1.0 W/m2K means 1.0 watts of heat flow, per metre square of wall, per 1°C of temperature difference between the inside and the outside of the wall
[12] Source: LETI (2021), p.185
[13] Source: LETI (2021), p.188
[14] Source: LETI (2021), p.163
[15] Source: LETI (2021), page 59
[16] A recent government report (Cost-Optimal Domestic Electrification (CODE) Final Report, 2021) suggests that, alternatively, households might adopt “better ventilation practices (e.g. avoiding drying clothes indoors, putting lids on pots while cooking, opening windows when more ventilation is needed, etc.) which are not necessarily costly or mechanical” (page 70). However, I think that these “behavioural” adjustments may be impractical for many households—especially where there are indoor space constraints and a lack of outdoor space for drying laundry. Improved mechanical ventilation is a surer way to secure thermal comfort
[17] These were the Carbon Emissions Reduction Target (CERT) (2008-2012) and the Community Energy Saving Programme (CESP) (2009-2012). See CCC (2013), 2013 Progress Report to Parliament, page 116; CCC (2022), Independent Assessment: The UK’s Heat and Buildings Strategy, page 12
[18] By January 2022, Carbon Brief was able to calculate that the end of the CERT and CESP, and all the other “green crap” changes, had added £2.5 billion to UK energy bills—costing the average household £40 per year. Most of that (90%) was from energy price rises to that point, with the remainder coming from the cost of energy providers going out of business
[19] Left: CCC (2022), Independent Assessment: The UK’s Heat and Buildings Strategy, p.12; right: DESNZ (2025), Household Energy Efficiency Statistical Release. Note that wholly privately funded installations are not included. The figures in the first graph are for the UK; the figures in the second graph are for Great Britain only and exclude Northern Ireland
[20] The 2019 Corbyn Labour policy foresaw 250,000 new skilled jobs in the construction, heating and retrofit industries, and 200,000 new indirect jobs
[21] See LETI (2021), Climate Emergency Retrofit Guide, page 53 and page 60
