Cavity wall insulation: damp problems

Cavity wall insulation can cause damp. Find out if your home's likely to be affected.

When we went undercover to investigate the quality of insulation suitability inspections in 2011, our findings suggested that you can’t always rely on assessors to carry out thorough inspections or warn of potential damp problems – even those from big brands such as Marks & Spencer, Npower and Tesco.

Posing as a consumer considering cavity-wall insulation, we invited eight insulation companies to assess a house for suitability. 

Cavity walls and wind-driven rain

We wanted to see whether the installers would correctly recognise and report that it was unsuitable for cavity wall insulation, as it had walls that were regularly exposed to wind-driven rain.

We secretly filmed the sales visits and showed our findings to an expert chartered surveyor. Our video footage below shows the poor advice we received, and reveals some staggering price differences. 

Cavity-wall insulation secret filming video

Checklist: is my home at risk of damp problems?

Cavity-wall insulation causing damp is very rare, but it’s worth checking whether your home’s at risk. You can use the checklist below to assess your home’s damp risk. Ask any potential installer about these factors, too.

Damp could occur in properties as a result of cavity-wall insulation if there is a combination of these factors:

• Your home is exposed to severe levels of wind-driven rain (zones three or four in our map below)
• Your home is located in an unsheltered position, eg not protected by trees or other buildings
• The external walls are poorly built or maintained with, for example, cracks in the brickwork or rendering.

Published guidance by the Building Research Establishment says that in these cases there is ‘an increased risk of rain penetration if a cavity is fully filled with insulation’. Rain could penetrate the outer wall, bridge the cavity via the insulation material and transfer moisture to internal walls, causing damp. 

If you are in any doubt as to what kind of damp you have, see our advice on what kind of damp is affecting your home.