How much insulation is it best to use on solid walls?

Wood fibre board insulation being used externally

Many of you will be aware that I am slightly reticent in recommending certain insulation types on older buildings. This is because of the risks associated with damp, thermal bridging and structural failure. So assuming that you are planning on using a breathable insulation (like wood fibre), how much should you apply?

The Part L of the Building Regulations wishes any refurbishment to get to around 0.3 – 0.35 U value for a solid wall. It thinks that all solid walls have a starting value of around 2.1 which is actually normally much worse that they are. This has led to companies putting on around 120mm thick insulation. Knowing the starting point of your wall is really important as thick walls can be as low as 0.8, so around three times better than predicted. In Wales most solid walls are around 300mm thick and these have an average of 1.51 (from Caroline Rye). So around 30% better than predicted.

In order to get Building Control approval for anything more than 0.3 then you will need to cite that this is a breathable wall and hence special considerations are available for it.

The recommendations I would give change depending on whether you are applying External or Internal Wall Insulation. The base reason also changes.

External Wall Insulation (EWI)

The situation with EWI is that whichever material you choose the real key is to get thermal bridging and rain protection sorted. Reveals, roofs, rainwater systems, etc are all really important to get right. It can be more effective to put 20mm over all the surfaces (inc reveals) than it is to put 100mm just over the walls (not in the reveals). So it is key to get the specification and detailing right.

The more insulation you put on, the less effective the insulation becomes in relation to its thickness. This means that the first cm of insulation reduces heat loss the most, the second will provide less heat loss, the third less still. So there is a rule of diminishing returns at play at here. So there is no point putting on loads of insulation when you will get a similar return from less. As a rule of thumb here to get down to 0.3 you will need 100mm of insulation, However, this assumes that you are starting with a U value of around 2.1. So if your wall is 1.51 then you should be able to use a 80mm board, but remember to put on the 20mm reveal boards!

Even with a breathable system you will need to ensure that it is weathertight. So make sure that the required work is done to rooflines, eaves, guttering etc. 

Any EWI needs to have a protective covering and again this needs to be breathable and also long lasting and so we would recommend a lime render combined with a lime proof mesh and a breathable paint finish (silicate paints are ideal for this).

Internal Wall Insulation (IWI)

Wood fibre board installed internally (from Inglehome)

IWI has the consequence of making the outer side of the wall colder and this can lead to a number of problems including damage from freeze / thaw. Using a vapour membrane also draws in moisture into the wall, so it is really important to use the right insulation system here. Breathable systems can be used without a vapour membrane (unlike the conventional insulation systems that either have built in vapour control, or require a separate membrane). This lack of barrier makes the system a lower risk in the long run.

Research is showing that a safer U value to aim for, is around 0.7 to 0.8. This means that many thick walls may already have this type of value and so not need any insulation. However, most walls are around 1.51, so in order to get down to 0.7 you only need 40mm of wood fibre board. Having a wall that allows a little more heat through it than recommended by Part L is important as this helps to keep the wall dry and safe. 

If installing IWI it is also really important to remember thermal bridges. So reveals, between floors, fold backs along party walls etc need to be assessed and treated appropriately.

If using a wood fibre system you would need to use a breathable finish on the walls. So a lime plaster skim and a claypaint would be ideal.