Should we insulate solid walls?

There are a wide range of issues associated with insulating solid walls and this has been illustrated by some of the results seen with the ARBED 1 programme in Wales. With our current obsession with energy and carbon we have tended to ignore the advice of conservation builders and eco-builders and taken construction detailing decisions from the mainstream industry. This has meant specifying high efficiency insulations like polystyrene and other non-breathable insulations for terraced housing.

On paper the efficiencies that this type of material brings are impressive, unfortunately, it is just incompatible with the majority of terraced housing. So, we are now seeing issues like:

  • damp internally (associated with non-breathing external insulation)
  • mould growth (associated with poor ventilation and non-breathing external insulation)
  • cold bridging (associated with poor installation of insulation)
  • external finishes failing due to frost damage (associated with internal insulation)

Even the folk at BRE are now aware of these issues, but their focus is on more research rather than stopping this madness that we are inflicting en masse to our terraced housing stock.

CADW are thinking about trying to change the default situation with regards to insulating solid walled buildings, i.e. having to get permission to install wall insulation, rather than having to apply for not insulating (as per current situation). In the age of carbon reduction this seems like madness – not insulating walls?? However the rationale behind this stance is set to become better known and appreciated. We have already found that solid walls insulate a lot better than computer programmes tell us (see earlier blog about the SPAB report), we also know that dry walls are up to 38% more efficient than wet ones. So why not just let the walls breathe and remain dry?

By using insulation we can ruin this natural process and hence cause problems. If we insulate the outside we alter appearance etc and we can also radically change the breathability of the structure. If we insulate the inside we reduce the amount of warmth flowing through the walls, but then this can lead to problems with the outer surface of the walls as the wall itself is colder and hence more prone to damage through frosts etc.

If we did nothing (apart from ensuring that the wall could breathe properly) then the walls would do what they were designed to do and this is much healthier both for the walls and the occupants. A few coats of limewash or silicate paint to provide a wearing layer for the structure would be enough to help keep the walls dry, as efficient as they can be and also cost us all a lot less in the short and long term.

Personally I think that these decisions need to be made on an individual basis as each house if different. However, overall guidance would be that you always need to:

  • use breathable insulations on older solid walled houses;
  • maintain a breathability gradient from the inside to the outside (inside less breathable than the outside);
  • ensure that internal insulation is designed to allow some heat through;
  • ensure good detailing work especially around window reveals etc;
  • keep walls dry using breathable paints and finishes.