Cavity wall insulation has made millions of homes warmer and cheaper to heat. In the right wall it causes no problems at all. But in some houses, particularly older ones in exposed locations, retrofitted insulation has become a genuine cause of damp. Here’s when and why, and what to do if you suspect it.
How a cavity wall is meant to work
Most houses built from roughly the 1920s–30s onward have cavity walls: an outer leaf of brick or block, a gap, and an inner leaf. That gap does an important job. Rain driven into the outer leaf runs down the inside face of it and drains out at the base, without reaching the inner leaf. Metal or plastic wall ties bridge the gap to hold the two leaves together.
Filling the cavity with insulation changes that. The material is designed to resist moisture, and in most walls it does. But if water can travel across the fill, the cavity no longer acts as a barrier.
When cavity insulation can cause damp
Problems tend to arise where one or more of these are true:
- The house is in an exposed location. Walls facing prevailing wind-driven rain — common on hillsides and open ground across the Pennines and Yorkshire — take far more water through the outer leaf than sheltered walls. Retrofitting insulation into a highly exposed wall is a known risk.
- The outer leaf is in poor condition. Cracked or porous brick, failed pointing or defective render let more water in, and give the insulation more to deal with.
- The installation was poor. Gaps, voids and uneven fill, or debris and mortar snots left in the cavity, create routes for water to bridge across.
- The material has degraded or slumped. Some older fills settle over time, leaving voids at the top and compaction at the bottom.
- The wall wasn’t suitable. Rubble-filled or very narrow cavities, or walls that weren’t properly assessed, were sometimes filled anyway.
What cavity insulation damp looks like
The signs overlap with other types of damp, which is why diagnosis matters:
- Damp patches on internal walls that appeared after insulation was installed, often on the most exposed elevation.
- Patches that worsen after wind-driven rain, rather than following the seasons as condensation does.
- Damp that isn’t confined to the base of the wall (unlike rising damp), sometimes patchy or in bands.
- Mould on internal faces of external walls, where the damp inner leaf has become colder.
It’s easy to mistake this for condensation or rising damp, and just as easy to wrongly blame the insulation for damp that has another cause. Our guide to telling the types of damp apart explains the differences.
How it’s investigated
A proper investigation looks at when the damp started relative to the installation, which walls are affected and how exposed they are, the condition of the outer leaf, and the pattern of dampness inside. An inspection of the cavity itself, usually with a borescope through a small drilled hole, shows whether the fill is complete, bridged, wet or contaminated with debris. Installation paperwork and any guarantee are worth finding too.
What can be done
Depending on the cause:
- Repair the outer leaf — repointing, replacing damaged brick, repairing render. In some cases this is enough.
- Improve the weather resistance of the outer leaf in exposed locations.
- Extract the insulation where it’s wet, degraded or bridging. Specialist firms remove the fill, clean the cavity, and either leave it empty or refill with a more suitable material once the cause is dealt with.
Guarantees
Retrofit cavity insulation installed in the UK is often covered by a long-term guarantee, commonly through the Cavity Insulation Guarantee Agency (CIGA). If you believe your insulation has caused damp, find the paperwork and check the terms before commissioning remedial work, as there’s usually a claims process to follow and evidence will be needed.
Buying a house with cavity wall insulation
Retrofit insulation isn’t a reason to avoid a property. It’s a reason to look carefully at the exposed elevations, the condition of the brickwork and the internal faces of external walls, and to ask the seller for the installation paperwork and guarantee. A Home Survey Level 2 or Level 3 will flag signs of damp on those walls; where there’s a specific concern, an independent damp investigation will get to the cause. Get in touch if you’d like us to take a look.
Sources and further reading
- Cavity Insulation Guarantee Agency — CIGA — 25-year guarantees for cavity wall insulation
- Approved Document C: site preparation and resistance to contaminants and moisture — Building Regulations guidance (Planning Portal)