Internal wall insulation

Internal wall insulation (IWI) can be used to insulate solid walls where external wall insulation (EWI) may not be suitable due to issues like access or planning constraints. 

It does have its own limitations including the disruption involved from installing it. Occupants may need to re-jig their living arrangements while rooms are being worked on or move out entirely if the whole house is upgraded. Redecoration will be definite at least!
If the property has a lot of internal architectural features, these may be lost or hard to replicate and reinstate if IWI is fitted.  Handmade plaster cornices, ornate mouldings or features such as wood panelling or original picture and dado rails are prime examples. Generally speaking it is going to be much more suitable if kitchens and bathrooms are being refurbished….and don’t forget  fitted furniture like cupboards may have to be removed and reinstated depending on location.. 

Keep in mind rerouting and reinstating services and terminations such as sockets and switches, radiators, and pipework–these need to be removed and then reinstated. With electrics you may have to replace cables altogether –an increased heat load will require larger diameter cables run in trunking and there also may be possible reactions with cable sheathing and polystyrene insulation.  

Loss of dimensions is another major factor.  If rooms are small to start with and require insulation on two or three elevations, this will reduce floor area, in extreme cases perhaps to the point that they are no longer functional.

If IWI passes these checks, next we need to ensure the type of system used will be suitable for the existing fabric of the building.  

There are several system designs, but all  fall into two main categories of vapour permeable and vapour sealed systems. A vapour permeable system such wood fibre boards are ideal for traditional, “breathing”  buildings. There is no need for a vapour barrier, so no risk of a vapour barrier failing in the future.  Wood fibre board is not only  vapour permeable but also hygroscopic which helps with moisture balance in the building. The main drawback to vapour permeable systems is that thermal performance may not be as effective compared to vapour tight materials and thickness is limited in this context since  the thicker the material the less vapour permeable it will be.  Along with the insulation itself, the  internal wall finish would need to be vapour permeable so a lime plaster is ideal –if it is not, the construction is no longer fully breathable and must be considered very carefully indeed.

Vapour sealed systems are used on vapour-sealed constructions.  Many modern building techniques, including pre fabricated, modular and some timber frame builds will use vapour barriers and vapour retarders to mitigate moisture vapour traveling through their fabric and causing rot and damage,  With these there must be a continuous vapour barrier otherwise using vapour sealed materials like PIR board can cause issues if moisture gets caught in the insulated structure (this is called interstitial condensation). Vapour sealed systems can vary in cost –often materials are relatively cheap but installs can be tricky to ensure they are vapour sealed. When installed correctly, they usually have better thermal performance than permeable systems.  

With IWI we need to address something called  thermal bridges which may be created through installing insulation. Thermal bridging is one of the main reasons for the performance gap, where a property fails to perform as expected after insulation is installed. Briefly, a thermal bridge is usually where a gap occurs between insulating materials. If using a stud work system, for example, installing insulation in between studs can create thermal bridging, so over-boarding another insulation layer across the studwork can mitigate the issue.  “Point” thermal bridges can be created  via bolts or other mechanical fixings. 

Window and door reveals will be an issue as unless you're moving the windows back into the insulation, you will be creating a thermal bridge at the window reveal. To mitigate this, you would need to insulate the face of the reveal too.  Bear in mind that it’s unlikely you'll be able to fit a substantial depth of insulation layer as you would need to return back to the window frame, so changing material for these areas can work. Aerogel is very useful for this–it is very expensive so we prefer to use it in small areas, but it  has a high thermal resistance to very low profile ratio.  Also, care would need to be taken when installing IWI at junctions and interfaces, like floors and ceilings, so there is a continuation of the thermal envelope that  avoids thermal bridges at these points.

As mentioned before, vapour barriers can be difficult to maintain across aspects and interfaces. If the barrier fails this can cause interstitial condensation and hidden damp inside the structure. Fitting vapour sealed systems or structures that are meant to be permeable can cause internal damp and mould and affect the external walls as well. Water may be held for longer in the wall as the newly insulated wall wouldn’t be able to dry out from the inside and will be susceptible to frost damage and spalling.  Walls can be treated externally to reduce the water absorption by using silicone external coatings. 

It is absolutely crucial that existing Issues like penetrating damp or structural cracks are addressed prior to installing internal insulation.  And as  with any form of insulation installation, you need to ensure that ventilation is going to be sufficient (especially if along with the insulation you are upgrading windows and sealing draughts). .  

So, to summarise, Internal wall insulation is a very effective way of insulating the walls in your home but is one of the riskiest methods if not installed correctly.  As with any retrofit works, we recommend your property be properly assessed using PAS2035 standard and installers who work to this too.