Insulation Overview

Of the 27 million homes in the UK, an estimated 7.8 million have solid walls, while 18.9 million use cavity wall construction. The remainder use a mix of timber frame and other wall constructions. The vast majority of the solid masonry walls in the UK are uninsulated, leading to increased heating costs and contributing to climate change.

Insulating these properties, particularly historic or listed buildings, from the outside is not always possible. External wall insulation or EWI systems can change the appearance of facades or building lines, running into planning challenges on grounds of aesthetics, historical conservation or even neighbouring land usage and boundaries.

Therefore, internal wall insulation or IWI can offer the only valid strategy for a number of homes. These systems are not without their own challenges however. Installing a solution that is not suited to the wall may lead to condensation, mould growth or structural decay, which may not always be easily detectable. 

Internal wall insulation also comes at the cost of space in the room, and may impact on or conceal internal historic finishing. The thinnest possible systems are therefore highly sought after, and these systems, collectively referred to as thin internal wall insulation or TIWI are the subject of the Leeds Beckett/BEIS study we’ll be discussing today. 


This study, released in March 2021, took place between 2017 and 2019 and combined six complimentary research phases, with the goal of producing a well rounded assessment of the risks and benefits of TIWI systems across various types of solid wall application. 

Part of the report surveyed 100 homes of solid wall construction. The overwhelming majority of these required some remediation. There are often internal obstacles requiring removal, or preservation in listed buildings.

In most properties, services such as telephone and network sockets or electrical outlets need to be repositioned. Radiators, heating pies and even boilers may also require relocation.  All of which usually requires specialist contractors and extends the timescale and budget required for works to be completed. 

In some cases, particularly with older buildings, internal features such as decorative plasterwork may be protected or listed, sometimes requiring expensive and time consuming conservation or reinstatement work to be undertaken. 

Taking these cases together, the survey determined that up to 95% of homes had some form of obstacle that would require repositioning if IWI were applied to the external walls.

Irregularities in walls, or walls that were not built level or straight, add further complications, as uneven plaster often has to be removed, or levelling layers or battens applied. Such remediation may add additional depth to the finished wall system over and above the thickness of the insulation. 

The surveys also identified that in almost all cases remedial works will be needed prior to any wall insulation being installed. For example; 

- Damp was observed in 9 out of 10 homes

- A quarter of walls were already damaged 

- 13% of walls were not flat, posing issues for rigid board solutions. 

- and One in 10 homes had no ventilation

Additionally, half of all walls already had plasterboard installed which would need to be removed before any retrofit could take place. 

Social Factors

Also included in this section was a social survey, asking 180 members of the public a questionnaire to gauge their perception of retrofit work and explore social barriers to IWI. This survey suggested that the most important motivators for having retrofits were to reduce energy bills, provide good value for money and making homes warmer. Improvements to house prices and appearance were not considered as important. 

The better-known measures were the most popular (i.e. Photovoltaics, double glazing in windows and new boilers) and since IWI was the least-well-known it was also the least popular retrofit option identified in the survey results.

The literature review identified that adding internal wall insulation can typically reduce greenhouse gas emissions by 5% to 10% in residential properties. Although this sounds small, it is only one component of an emissions reduction strategy, with double glazing, roof insulation, and airtightness, particularly around doors and windows, being other excellent passive solutions. Active solutions such as more energy efficient boilers can also make a significant impact on the emissions associated with a dwelling.

This drive to reduce the carbon emissions associated with dwellings has been a major driver behind government policy for several years, with a range of subsidised insulation schemes operating to facilitate upgrade works. 

Most recently, in the five years leading up to 2020, the Energy Company Obligation (ECO) aimed to improve the energy efficiency of over one million homes via funding from utility companies.  The Government mandated that a proportion of these (21,000) must be solid wall insulation retrofits, including internal wall insulation system. 

Solid Wall Insulation retrofits however remain relatively uncommon due to their complexity. Cavity wall insulation retrofits on the other hand, are simpler to install and more cost effective, leading to there now being significantly more uninsulated solid walls (around 6 to 8 million homes) than uninsulated cavity walls. That said, the ECO targets were met, with 1,003,500 homes insulated by the end of June 2020. 

Of all the notified ECO-funded measures installed to the end of June 2020, 65% were insulation measures, including cavity wall insulation (33%), loft insulation (22%), solid wall insulation (7%) and ‘other insulation’ (3%). The remainder are mostly heating measures, with 23% boiler measures and a further 13% being assigned as ‘other heating’ measures.

The current environmental policy that the UK government is working towards is being net-zero by 2050. This means a reduction in carbon emissions by 100% compared to 1990 levels. According to the Committee on Climate Change (CCC), energy use in homes accounts for approximately 14% of the UK carbon emissions, of which 63% is for space heating. 

Thermal insulation reduces the amount of heat lost to the external climate through any given element, and is therefore a good way to reduce these emissions. The CCC argues that “near-complete” decarbonisation of how we heat our homes would be required for the UK to meet its emission reduction targets. This can be achieved through a combination of changing to cleaner fuels and reducing the overall amount of energy required.

The Committee on Climate Change’s scenarios for meeting these “fifth carbon budget” targets require insulation of about 7.5 million more walls and lofts from the number recorded in 2015.  

The CCC argues that in many areas current policy is failing to drive uptake of energy efficiency measures in homes, including for highly cost-effective measures such as loft insulation. The committee further recommends that “policy needs to incentivise efficient long-term investments, rather than piecemeal incremental change” in order to improve the overall picture around insulation retrofit. 

Hygrothermal Factors

The review also showed that internal wall insulation may introduce some moisture accumulation and condensation risks, mainly as a result of the masonry wall becoming colder. 

With an uninsulated wall, the masonry is heated by the internal climate of the building. This serves to drive water vapour further into the wall from inside to out, and also makes it possible for the wall to lose moisture by drying. 

Insulating the wall internally means that this heat is trapped inside. While this is great for the thermal comfort of the building, and the heating bills of the occupiers, it does mean that the masonry is now at a lower temperature than it was before. 

This drop in temperature means that the moisture in the wall, previously at a temperature which would keep it as water vapour, can now condense to form liquid water. The colder masonry also means that any driving precipitation can be driven further into the wall by solar radiation, and this increased quantity of moisture can accumulate behind the insulation. 

If a non-permeable render or other coating is applied to the external surface of the wall, then any moisture from interstitial condensation, or any that already existing in the masonry can be trapped there. These issues are greatly reduced if the insulation is permeable, but can still pose a problem worthy of careful consideration.

As more solid walls are retrofitted with insulation, more data around its effects, both economic and hygrothermal becomes available, and various government backed schemes have boosted the numbers of such projects being undertaken.

A number of homes with various masonry walls were surveyed more thoroughly in the Thin Internal Wall Insulation Study, and the results are shown here. (Table 5.1, annex A)

With a complete renovation, the resulting wall showed little to no risk of condensation, confirming that a holistic approach to internal wall insulation can be done without introducing significant issues relating to moisture management. 

The main exceptions here were at services, such as in gas meter cupboards. 

As these were left without insulation, they represent a cold bridge. Such cold bridges suffer from increased condensation risks as the upgraded insulation in other areas of the dwelling serves to make the temperature drop in uninsulated areas more severe. 

This lower localised temperature makes the formation of condensation, damp and mould more likely than would be the case were the surface temperatures more uniform. 

Services and openings can also give rise to cold spots in the structure associated with the air leakage if they are not properly sealed. Such air leakage can introduce colder external air into warmer and more moisture laden internal environments, which can have a similar effect to cold bridges caused by non-uniform insulation. 

A holistic approach, providing a range of measures ranging from internal wall insulation, as in this study, through loft insulation, airtightness measures, double glazing and improved ventilation therefore contributes to the reduction in the risk of condensation. 

This type of retrofit, based on the interlinked principles of Heat, Air and Moisture movement, is the key to designing a successful hygrothermal upgrade package. We’ve covered these principles and the effects each has on the others previously in our webinar series, and this is available for on-demand viewing on our YouTube channel if it’s of interest. 

This section of the study verifies this, as walls that are simply insulated without thought to the amount of ventilation or the condensation risk, tended to perform worse than cases where holistic approaches were taken.

Whole house approaches also need to consider thermal bridging through insulation fixings, and air infiltration around doors and window reveals. This means that how the insulation is detailed has a significant impact on the performance. 

Most of these tested dwellings used butt jointed IWI boards. These butt joints were visible through a thermal camera, indicating some heat loss occurring due to the non-overlapping joints at the board edges. While this was not found to be significant enough to introduce a localised condensation risk, it could allow unmitigated airflow through the insulation, worsening the performance through a process known as thermal bypass.


The study looked at a number of Thin Internal Wall Insulation options, which were: 

- Phenolic foam, 

- Rigid Polyisocynaurate or PIR foam, 

- Aerogel, 

- Expanded Polystrene or EPS, 

- Cork with Lime Render

- Latex rolls, 

- and thermal paint. 

Each has different advantages, but one of the main ways to compare them is through their thermal conductivities. The thermal conductivity, or lambda value of an insulation, measures how much heat can be conducted through a material, and does not account for variations in thickness. This makes it a useful indicator or which materials will be most effective given a fixed thickness.

Phenolic insulation is a closed cell rigid insulation, composed of a plastic foam core sandwiched between facing materials. The foam core consists of high solids, mixed with a phenolic resin. Typically, it has a thermal conductivity of 0.018 to 0.02 Watts per metre Kelvin. This is less than half that of most mineral wool products, and for thermal conductivity, the lower the value, the better. 

Phenolic boards have a high closed cell content and a fine structure, making them quite dense. These rigid boards are typically laminated onto pure aluminium foil facings, glass tissue facings or plasterboard. Because of this dense structure and impermeable facing materials, Phenolic boards are usually highly resistant to the passage of moisture vapour. 

PIR, or polyisocyanurate insulation, is similar to phenolic in that it is formed using a closed cell foam between facings to make a rigid board, but has a slightly worse thermal conductivity, at 0.022 to 0.025 W/mK. PIR is similar to PUR, or polyurethane insulation and like phenolic foam is typically resistant to the passage of moisture vapour. It is typically used for insulating between or over framed walls, or as insulated plasterboard.

Aerogel insulation is a topic that we have covered in a number of webinars now. It consists of a synthetic ultralight material in a gel form, which is embedded into a blanket for ease of use as insulation. Our Spacetherm Aerogel has a thermal conductivity of 0.015W/mK, and is also vapour permeable, enabling moisture movement through the insulation layers in either direction.

EPS stands for Expanded Polystyrene, which is the bubbled foam insulation often used in packaging. Its thermal conductivity varies from around 0.032 to 0.038, and like other forms of rigid foam it limits the transfer of moisture vapour.

Cork, taken from the bark of cork trees has a thermal conductivity of 0.036 to 0.038 and is permeable to moisture vapour. External render or internal plaster are typically applied directly to the cork board, negating the requirement for an additional board or lath system. The cork system in this study had a factory applied lime render face, but did not specify the depth of the render.

Latex rolls consist of a latex foam with a fibreglass layer. It is applied much like a thick wallpaper. The thermal conductivity varies between such products, but they typically only offer a minimal level of thermal insulation upgrade. 

Thermo paint is a specialist surface coating that restricts the transfer of heat. It has the advantage of being the thinnest system, but offers minimal thermal value and is best used in conjunction with other systems. In this study, it was combined with the Latex rolls.

Case Study Houses

(Table 0-1, Annex B)

The main part of the study looked at three houses with similar constructions for qualitative tests on different Thin Internal Wall Insulation systems and how they perform under real world conditions. Once each insulation system was installed, the u value; airtightness; whole house heat loss; thermal comfort; and heat up and cool down times was assessed. The properties tested were all mid terraced houses with solid masonry walls of 230mm (or 9 inches) brick. 

Several features of the buildings were considered notable: no carpets were fitted, which may have reduced the airtightness of the dwellings and each had a cellar and room in the roof of some description. 

House A is oriented North South and the building has relatively non-airtight wooden front and rear external doors, although functional double-glazing PVC units were fitted in each room, The mid height basement and room in roof were not insulated during these tests. In addition, the cellar door was a simple internal wooden door and the cellar itself had an exposed South wall as the terrace was built on a slope. 

House B had the same depth of 230mm brick, but was oriented East West. It had a full cellar and insulated loftspace, again which were left as is for the purposes of this test. 

House C, facing North/South had a particularly draughty cellar door, though the front door and all windows were double glazed.

As mentioned, the dwellings were solid wall terraced homes with 2 external walls which, according to the English Housing Survey, represents the most common form of solid wall dwellings with approximately 2.1 million being found in England. 

It is worth pointing out that with only 2 external walls, when the external doors and windows are taken into consideration, the available area for insulation as a proportion of the entire thermal envelope of the dwelling can be relatively small. As the English Housing Survey states that over 50% of homes are either mid terraces or flats, this may be important when considering the national impact of IWI and TIWI. This also suggests that homes with larger wall areas may achieve higher savings than those measured in this project.

Thermal Results

House A was tested with a typical IWI solution of 70mm of phenolic, as well as with 27mm PIR, and finally with our own Spacetherm Wall Liner at 13mm. Each system insulated 27% of the external envelope. This accounted for 30.83m², of which 6.18m² were applied as returns to the party wall. 

House B, a mid terraced house, was tested with 22mm of EPS, and with 20mm of cork render, covering 19% of the external envelope. 

The final house - house C, used 10mm latex rolls, and the same but with a layer of thermal paint. This covered 32 and 38% of the external envelope respectively.

In-situ u-value measurements were undertaken at multiple locations to achieve an average value, using an array of Heat Flux Plates (HFPs) positioned in a grid across the plane element area. The location of each HFP grid array was selected using thermal imaging to avoid regions which were deemed to be affected by thermal bridging at nearby junctions. These were accounted for in thermal bridging calculations.

The HFPs were kept to consistent locations across the tests to facilitate comparisons, and these were North facing walls (or for house B, West facing with external plywood shielding to remove solar radiation). 

In house A, the uninsulated u-value of the walls was 2.11W/m²K. A typical insulation strategy (such as the Phenolic system at 70mm) would bring this down to around 0.3W/m²K. The two Thin Internal Wall Insulation systems achieved 0.78 with the 27mm PIR system, and 0.76 with the 14mm Aerogel system. 

Respectively, this equates to a reduction in the u value of 86%, 63% and 64%, which is a significant amount in such little space. 

House B started with a u-value of 2.01W/m²K, which was reduced by 49% using the EPS system to 0.98W/m²K, and 32% to 1.36W/m²K using the 20mm cork render. 

The latex rolls in house C reduced the u value from 2.10 by 38% to 1.3W/m²K. The thermo paint reduced this by a further 4%, bringing the u value to 1.25W/m²K.

Most of the products succeeded in achieving their retrofit target U-value. The only notable performance gap was for TIWI 4 (cork lime render) for which the insulation thickness was uncertain. Further, the cause of the underperformance is possibly due to the thickness of the primary insulation layer being less than specified.

Airtightness Results

As we have covered in other webinars in this series, the airtightness of a building, or infiltration rate, is a measure of the uncontrolled ventilation. This air exchange, together with purpose-provided ventilation, establishes the total ventilation rate for the building fabric, determining how much heat is lost due to air exchange with the external environment. 

Heat, air movement and moisture management are closely linked, so having poor airtightness in a dwelling can lead to significant thermal losses, and in severe cases, even condensation. Therefore, it is vital that airtightness is considered as part of the retrofit system.

While approved document L1B does not specify a methodology for ensuring airtightness in refurbishment projects, this study took the approach that is suited for new builds, by performing a blower door test.

These blower door tests showed little significant variation in the infiltration rates before and after the TIWI was installed. This contrasts previous observations for IWI retrofits where infiltration was reduced. Most other field trials, however, have the installation of IWI only as part of the retrofit and it is possible that savings identified on other case studies may have been linked to these ancillary activities such as sealing around pipes, vents, loft hatches, windows and doors. The major infiltration routes identified were via the suspended timber ground floors and poorly sealed cellar and external doors, as well as through service penetrations, boxed in pipe routes and plug sockets.

Overall Results

The heat transfer coefficient (HTC) is a metric of a building’s thermal performance that quantifies the total rate of heat loss from the entire thermal envelope of a building in Watts per Kelvin of temperature differential (W/K) between the internal and external environments (ΔT). The HTC is an combines the heat loss rates from plane elements, thermal bridges and air exchanges. 

The HTC of a house does not include heat exchange with adjoining dwellings, only heat loss to the external environment. Each Test House had two adjacent dwellings, therefore consideration had to be made to either minimise heat transfer between neighbours as this has been shown to reduce the accuracy of the coheating test. The temperature was modulated to allow for this, using a lower set temperature than the standard. The party walls were also monitored using Heat Flux Plates.

As the area of walls insulated was much less than the total area of the insulation envelope in all cases, the overall heat transfer coefficient was much lower than the improvement to the insulated wall u values. 

In house A, while the IWI system reduced the u value by 86%, the whole house thermal performance only increased by 18%. Similarly, the PIR system (providing a 63% improvement in the wall u value) only provided 15%. The system using our Spacetherm Aerogel insulation, which achieved a u value 64% better than the uninsulated wall, achieved a 13% improvement over the whole building. This shows that while insulating the walls does make a significant improvement, other factors should also be considered, including airtightness, double glazing and loft insulation.

House B, which had u values 49 and 32% better than the uninsulated wall, saw HTCs of 15 and 17%, while house C (which achieved a 38% reduction, followed by a further 4% with the thermal paint) had heat transfer coefficients across the house of 10%, and a further 7%).

Heating Response

Another possible advantage for IWI and TIWI products is that post retrofit, homes may heat up more quickly and cool down more slowly, thus improving thermal  comfort for the occupants. 

To evaluate this, data from the simulated occupancy trials conducted in the test houses were analysed. The results indicated that neither IWI nor TIWI significantly improves heat up rates. TIWI may influence cool down rates, however, the effect is not substantial and appears to not be linked to the insulation’s ability to reduce heat loss. 

Thermo-reflective paint appears to have the greatest impact, reducing temperature drops by 0.9°C over a single evening cool down. However, uncertainty is very high and some TIWI were found to accelerate cool down rates, perhaps because thermal mass was behind the insulation, or have no effect at all. 

Verification Of Results

The results of the study were also verified through dynamic hygrothermal simulation and lab based materiality testing. The simulations showed that the results of internal wall insulation vary wildly with other factors, such as air infiltration. Different simulation values for airtightness resulted in variations in the final output between 3 and 59%.

To investigate how thermal bridging is affected by TIWI, thermal models were created using Test House A as the base case in the TRISCO thermal modelling software. Each internal insulation configuration was modelled in turn with and without reveals and party walls insulated. Prior to the retrofits, intermediate floors and windowsills and jambs were identified as having a condensation risk. 

Applying IWI and TIWI to the external walls increased this risk where reveals were not also insulated. Insulating reveals removed the risk. Thermal bridging and condensation risk were generally less extreme when TIWI, which has lower thermal resistance, was used. As we discussed earlier, these risks become more pronounced the greater the variation in performance between insulated and uninsulated areas grows.

Additionally, decorative coving below intermediate floors, which was not an area of condensation risk prior to retrofits, presented a condensation risk following all TIWI and IWI retrofits and so may in all cases need to be removed before retrofit. The risk was most pronounced in IWI though became less extreme in TIWI with lower levels of thermal resistance. This has implications for the costs of IWI and TIWI retrofits since it is not known how many solid wall homes may have decorative coving.

Internal insulation also caused water content of the solid wall to fall more slowly than the base case uninsulated wall, potentially increasing the risk of mould and rot. This means that masonry walls do tend to have more water content when internally insulated. The hygrothermal study found that this also applies to the internal leaf of cavity wall construction, with some internally insulated cavity wall constructions seeing an increase in moisture content over the simulation period. 

This implies that there could be an increased risk of moisture build up on the inner brick leaf in IWI retrofits, something that may not occur for TIWI retrofits. It is worth noting however that the rate of drying was slowed (therefore risk of moisture problems increased) in all the TIWI retrofits, and the extent to which the drying was slowed down was relative to insulating properties of the materials.

Risk Of Timber Rot

BS 5250 explains that a relative humidity over 80% allows mould growth to begin to occur in unventilated air spaces. In hygrothermal simulations a temperature of 20°C is used as a threshold of risk. The simulation looked at each of the insulation options in reference to these figures, looking at the point in the walls where timber joists would be inserted to. This allowed the survey to assess the risk of rot within  timber joists in the wall - which have previously been identified as potential issues with solid walls and internal insulation strategies. 

(Table 6-2)

This analysis found that the uninsulated wall had 4523 hours above 80%, of which only 40 were above 20°C. This equates to 0.2% of the time being at risk. Adding any insulation strategy increased this risk. The full IWI system using phenolic insulation represents the greatest risk, at 11.3% of time above the risk threshold levels (2965 hours above both conditions), while the thin internal wall insulation systems have considerably less risk. The thin PIR system was above 80% for 17869 hours, with 1398 being over 20°C. This equated to 5.3% of the time at risk. 

The system using our Spacetherm insulation was about half of this, with far fewer hours above 20°C, due to the improved thermal qualities of the insulation. With 15,509 hours above 80% relative humidity, of which only 742 of them were above 20°C, the system using aerogel insulation was at risk 2.8% of the time. 

The EPS caused a risk 4.3% of the time, while the joists in the cork system were at risk only 0.7% of the time. The Latex insulation system caused a risk 0.6% of the time, with an additional 0.2% when the thermo-paint was applied. 

It is important to take these figures into account alongside the thermal performance figures from earlier. Like everything else in construction, it is about balancing competing factors to achieve the correct outcome for a specific project.

In general however, moisture vapour permeable systems (such as the cork, or our own aerogel insulation) performed better on average in this metric than impermeable ones and the moisture contained within the fabric is permitted to dry out in either direction according to prevailing temperature gradients. .

 Lab testing was also carried out on solid walls as part of the research. These lab based tests showed that the risk of surface condensation was reduced, by similar degrees, regardless of whether a standard IWI or Thin IWI was installed. It also showed that the risk of interstitial condensation in solid walls was ‘only somewhat introduced’ when a thin internal wall insulation was applied, but ‘substantially introduced’ when a thicker, standard internal wall insulation was installed.

Despite being significantly thinner, TIWI was almost as effective at reducing whole house heat loss as full IWI systems. IWI systems were shown to introduce increased moisture to the wall, particularly in and timbers such as joists bedded into them, as were solutions that had higher inherent vapour resistances. Permeable TIWI’s, ie the aerogel/MgO laminate, showed a reduction in moisture in the wall and therefore will help limit any risks of interstitial condensation causing rot etc to timber joists ends etc.


So in these test houses, and in the survey results, we’ve seen that thermally upgrading properties with solid masonry walls poses several challenges. Balancing the delivery of a meaningful and cost effective thermal performance upgrade against the effects of changes to moisture movement and the loss of internal space means there are no simple solutions. 

What is important in these projects is developing a good understanding of the impacts of each retrofit measure when considered holistically, and how these measure may impact on one another. It’s also helpful to manage expectations of what is achievable in performance terms as its often the case that a more balanced approach can deliver a better “whole house” result than simply fitting more insulation into a wall or spend most of the budget on photovoltaic panels.

This Webinar Includes
  • Discussion of Leeds Sustainability Institute Thin Insulation Study
  • Characteristics of Thin Internal Insulation Systems
  • Performance of Aerogel Wall Liner
  • Review of On-site Survey Data
  • Whole house energy performance
  • Hygrothermal Performance
  • Modelled vs Measured