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How to choose internal solid wall insulation

Posted by Chris Newman on 23 September 2013 at 9:37 am

10 million properties in the UK which are solid walled. This accounts for slightly less than half of the total properties. Energy loss through the walls accounts for around one third of all wasted energy in solid walled properties and with rising energy costs and tougher building regulations on renovations, homeowners are increasingly looking for a tidy solution to their energy waste problem.  

Introducing internal wall insulation

In many solid walled properties, for example terraces, flats, listed buildings, where external wall insulation is not an option, this is fast becoming the insulation technique of choice.   Put simply, solid wall insulation is the lining of the insides of your external walls with an insulating material. The insulation is then usually covered in plasterboard to recreate a smooth internal surface.  

So what are the key things to consider?  


The thicker the insulation the greater the benefit up to a point, but there is a law of diminishing returns, taking into account increased space loss and increased cost. The optimum thickness will vary depending on your situation but it is likely to be somewhere between 10mm and 100mm. To take an average, around 50mm of phenolic insulation should bring a solid brick wall up to current building regulations but remember that the overall thickness will typically be 15mm to 30mm more than that once plasterboard and adhesive or battens are included.  


Different products have different insulation values, moisture handling properties (the way in which they deal with damp and condensation), costs and application procedures. Aerogels are the most insulative while wool (sheep or mineral) and fibreboards are at the lower end. Rigid foam boards are somewhere in the middle. Typically ‘natural’ materials are a little more expensive but aerogels buck the trend and are also costly. Insulation is either applied adhesively or with long screw fixings or both. Sometimes battens are added between the insulation and wall or on the inside face of the insulation.  Finally, plasterboard is sometimes pre-laminated to the insulation or is fixed to it afterwards.  

Moisture handling

Insulation materials are either hygroscopic (they absorb and release water vapour) or hydrophobic (water vapour will condense and form droplets on the insulation if it gets in). They are also either moisture diffuse (allowing water vapour to pass through) or closed cell. A moisture diffuse hygroscopic material such as wood fibreboard allows a building to manage moisture movement and is often a good solution in slightly damper conditions and where there is porous stone and lime mortars. Closed cell hydrophobic materials (for example a foil backed phenolic) is often suitable for masonry buildings where there isn’t much driving rain.  Moisture diffuse hydrophobic materials, such as mineral wool, will need a separate vapour barrier which brings additional risks with its vulnerability. You’ll need to decide what is appropriate for your building and then whether you adopt a ‘manage moisture’ or ‘exclude water’ strategy.  

Thermal bridging

Another important consideration is thermal bridges. These are the small areas of wall, such as window reveals, that can be overlooked or are difficult to insulate.  As they can get very cold in winter they can attract condensation leading to mould or paint damage. They can often be addressed with thinner and/or higher performing insulation.  


These are things such as wiring, plumbing and gas pipes that either come in through the wall (known as penetration services), or run along the walls. Sealing penetration services is paramount and many may need to be extended in order to do this. With wires, plumbing and gas pipes it is important to consider both how they will be accessed in future and the risks associated with leaving them on the cold side of the insulation. For these reasons it is normally best to move them onto the inside of the insulation.  

Embodied energy

Embodied energy is the term given to the total energy used to produce a product. When it comes to energy saving devices, the question is whether they save more energy than was consumed in their production. In my view, this is a bit of a red herring when discussing wall insulation. Insulation materials by their nature save energy. It’s almost a truism that if they pay for themselves in a relatively short time they will have paid for their embodied energy. Our analysis shows that even insulation with higher figures of embodied energy will see a payback in under three years. Most do it within a year. Over the lifetime of the material the embodied energy figure will largely disappear into irrelevance.  

Thermal mass

This is an oft misunderstood concept. In our climate, thermal mass is important for cooling, and not heating. With traditional buildings (for example those that need internal wall insulation), solar gains in the summer are typically low as windows areas are not excessive.  Any overheating risks can therefore usually be managed effectively with sensible shading strategies, for example closing the blinds on south facing windows during hot days.   There are quite a few things to think about when it comes to internal wall insulation and it is important to remember that what suits one building may not suit another. But it’s worth doing your research and taking expert advice and you’ll quickly narrow down your options. I can assure you, from inside my own internally insulated Victorian house, the results are very cosy and a little smug on a winter’s night.

More information

YouGen guide to insulation   From the blog

Planning Permsission not needed for (most) solid wall insulation (Jan 2013)

What is thermal bridging and should I worry about it? (Jan 2013)

About the author: Chris Newman leads on development and delivery of the Parity Projects Home Energy Masterplan.

If you have a question about anything in the above blog, please ask it in the comments section below.

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3 comments - read them below or add one


AndrewNE_ScotlandComment left on: 26 September 2013 at 7:42 pm

Hi Chris,

Thanks for your quick reply.  I will be having an in-depth insulation discussion with my Architect in the not too distant future and want to be sure we consider all realistic options and make an informed decision based on various criteria.

My description of "breathable expanding foam" was a bit vague.  The product name is Icynene - a full description can easily be found on the internet if you (or anyone else) is interested.



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Chris Newman

Chris Newman from Parity ProjectsComment left on: 26 September 2013 at 2:30 pm

Hi Andrew

So you've got a bit of a hybrid risk situation. North East and rural is lowere risk than West and coastal, but sandstone walls and 120 years old is higher risk than masonry.

I'd shy away from i) as I'm not sure there is any 'breathable expanding foam' ii) is an improvement as it won't degrade if there is any condensation.  Any problems will probably come from internal moisture as there will be no vapour barrier. iii) is an option but the risk factors might make woodfibreboad a good option.  This could also be applied direct to the walls where space is at a premium. Its important to use the correct adhesives.  There are also some new woodfibreboard products that come with a hardboard layer bound to the inner face to reduce the amount of wet trades that are required as it can then be plasterboarded and just skimmed.

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AndrewNE_ScotlandComment left on: 26 September 2013 at 10:27 am

Hi Chris,

I enjoyed your blog entry above regarding internal wall insulation.

I need to make an IWI decision in the upcoming months.  A few months ago my wife and I purchased a 120 year old solid sandstone wall house in rural NE Scotland.  There is no wall or underfloor insulation, and very little insulation in the loft.  We intend to install as much insulation as practically / economically possible.

The walls are 600mm sandstone, 50mm air gap (including vertical battens), and lath and plaster on the battens.  I understand the u value of this combined construction is about 1.4 W/m2K; not great but not bad considering.

We are considering our IWI options - our architect has a concern in terms of moisture management.  There are a number of options (including doing nothing) however we do not want to end up with a situtaion where the dew point is moved "inward" where it may cause future damage.  We are thinking about the pros / cons of various options (i) injected breathable expanding foam into the 50mm cavity (ii) injected or poured polystyrene beads into the 50mm cavity (iii) installing insulation on the inside of the existing lathe and plaster.

We would be interested to know your thoughts / experience and any specific advice you might have.  We would be reluctant to remove all the existing lath and plaster; this would be a big job and it seems to be in generally good condition including the supporting battens.



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