Thursday, 18 March 2021

Field tests on thin insulation for internal wall insulation

There is a new report out: Measuring Energy Performance Improvements in Dwellings Using Thin Internal Wall Insulation [1]. Researchers at Leeds University performed field tests on six kinds of thin internal wall insulation: PIR, aerogel, and EPS (all laminated onto a board), cork render, latex rolls and thermo-reflective aerogel paint. They were all compared to a typical thickness of phenolic foam insulation - the conventional option. The products were installed on three different solid-wall houses (two systems tested in each). The team measured U-values (thermal conduction through the wall) directly and also the impact on air tightness. They used simulations to study the effects on damp and risk of frost damage. They recorded costs of installation, separating out the costs of materials and labour and decorative finishes. They interviewed the installers to find their opinions - you very rarely find these reported! Here I describe the main results. 

A tradeoff between cost thickness and performance.

Reduction in heat loss (percent reduction in measured U-value) obtained from Phenolic foam board and six types of thin insulation. Paint: thermo reflective aerogel paint, Latex: latex roll, Cork: cork render. The thickness is the total thickness including air gap and plaster skim, where appropriate. Data from [1]

The products were:

Phenolic foam board The conventional option
PIR plasterboard laminate Also common in retrofits that are not subsidised
EPS plasterboard laminate The cheapest laminate available, common in DIY retrofits
Aerogel blankets The thinnest laminate available
Latext roll Another novel product, that can be applied like wallpaper
Thermo-reflective aerogel paint The thinnest product available


As always, there is a tradeoff between cost, thickness and performance as you can see in this chart. Aerogel is a bit of an outlier: only two thirds the thickness of PIR of similar performance - but costs 50% more. The paint also contains aerogel but is so thin that very little is achieved - I have suffered salesmen bending my ear trying to convince me their insulating paint was fantastic but I never believed them and I am not surprised here.

Thin insulation is worth considering for tricky places.

You may be tempted to consider thin insulation everywhere, and reducing heat loss by 65% is definitely worth having. But this is still twice the heat loss you would get with a full thickness. I would consider it mainly for particular places where normal thickness would pose problems, for example:

  • On a wall adjacent to a staircase, where thick insulation would leave the stairs very narrow.
  • Where thicker insulation would block doors or windows opening
  • For window reveals, where thick insulation would block too much light


Labour costs vary as well as materials.

Thin insulation is somewhat cheaper bit cheaper to install, though the labour costs are similar. This chart (from the report) shows the costs broken down. The cork render costs more to install as it takes two visits, but if you like the appearance you do not have to decorate afterwards which offsets some of the extra £s. Latex roll is very easy to install as you apply it like wallpaper; the paint is the cheapest - but not such good performance.

Installation costs per m2, broken down into components - from [1].


The costs in the chart are averages. Complicated walls with multiple windows or bay windows are more expensive to insulate both because of extra time and wasted material. The more cutting you have to do, generally speaking the more waste.

Aerogel, being breathable, reduces risk of rotting timbers.

The tests did not run long enough to evaluate risks of moisture over time; this part of the research used simulations. It found that the aerogel, which is 'breathable' had lower risk of damp and mould than PIR with similar insulation performance. For joist ends, the length of time when conditions could allow mould to grow mould and timber to rot was halved in the aerogel simulation. Insulation made the walls colder and wetter on the outside, but not enough to risk frost damage.

You may be tempted to leave details such as coving or cornicing in place and insulate around them but the simulations found this would be a bad idea; you get thermal bridges and possible condensation on those surfaces. Fortunately, it is possible to find craftsmen who can recreate them on top of the new insulation. We did in our house and the only difference is the new stuff is cleaner.

No improvement in air tightness

The team tested effects on air tightness as well and found no difference before and after. Insulation is sometimes thought to improve air tightness but seemingly not here - although there was considerable scope for improvement in the houses they used.

Installers do not regard installing insulation as particularly skilled - no training needed.

It is rare to see the opinions of installers reported, so this was particularly interesting. The installers said they do not like doing this sort of job as it is so time consuming. Skirting boards, electric sockets, shelves, thermostats, - all have to be removed and replaced afterwards while fireplaces and windows have to be worked around. This is all very finicky to do accurately. Nonetheless, they do not regard this as a specially skilled job that needs training and are generally unaware of the PAS 2030 standard that is required when installing under grant schemes. They would only be interested in training if this gave them a strong competitive advantage or taught them methods that would save a lot of time.

EWI is far more popular than IWI - but mixtures are also common.

EWI is much more popular than IWI even though it is significantly (though not overwhelmingly) more expensive. According to BEIS's costing report from 2017, internal wall insulation typically costs £95/m2 while external wall insulation costs £116/m2 [2]. However, under the ECO grant schemes, there are 17 times as many installation of external wall insulation as internal [3]. EWI has a number of technical advantages, is less disruptive for the residents, and is generally preferable where it is an option, but having to get planning permission can be a showstopper.

In practice, many houses (like mine) end up with a mix of internal insulation at the front (where external would cover up nice architectural features) and external insulation in other places where aesthetics are not so important. EWI can be beautiful too, but it is always different. So, knowing the pros and cons of different products for internal insulation is useful.


[1] Measuring Energy Performance Improvements in Dwellings Using Thin Internal Wall Insulation (Leeds Sustainability Institute for BEIS) March 2021

[2] What does it cost to retrofit homes (Cambridge Architectural Research for BEIS) 2017

[3] Household energy efficiency statistics (BEIS) 2021


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