Cavity wall insulation has been the subject of one of the more complicated chapters in UK home energy efficiency. Millions of properties had it installed between the 1990s and 2010s under government-funded schemes, often without adequate surveys. A significant proportion of those installations subsequently caused damp problems, and there's been an ongoing remediation effort to remove poorly installed or unsuitable cavity fill from affected properties.
That doesn't mean cavity wall insulation is bad. In suitable properties, correctly surveyed and installed, it's one of the most cost-effective energy improvements available. The key is understanding what makes a property suitable, and insisting on proper survey work before proceeding.
How Cavity Wall Insulation Works
A cavity wall consists of two leaves of masonry (typically brick outer and block inner) separated by an air gap of 50-100mm. The gap was originally designed to prevent rain penetration: water hitting the outer leaf can't bridge to the inner leaf if there's an air gap between them.
Cavity wall insulation fills this gap, usually with either blown mineral wool fibre, expanded polystyrene beads (EPS), or injected polyurethane foam. The insulation reduces the rate at which heat moves through the wall, improving the thermal performance significantly: a standard uninsulated 50mm cavity wall has a U-value of around 1.6 W/m2K; filled with insulation, this drops to approximately 0.5 W/m2K.
The problem is that insulation in the cavity can also facilitate moisture transfer if the outer leaf is porous, cracked, or exposed to high rainfall and wind, or if the insulation is poorly installed, leaving gaps or bridging the DPC.
What Makes a Property Suitable
A pre-installation survey is not optional. A competent survey includes:
Exposure assessment. The cavity wall insulation industry grades properties by wind-driven rain exposure. Properties in sheltered locations in low rainfall areas (much of England) are generally lower risk. Properties on exposed hillsides, facing prevailing wet winds, or in high rainfall areas are higher risk and require more careful assessment.
Wall inspection. The outer leaf must be in good condition: no cracked or spalling bricks, no failed pointing, no cracks in the masonry, no evidence of penetrating damp. Installing insulation in a wall with defects traps moisture and causes internal damp.
Cavity inspection. An endoscope survey (a camera inserted through a drilled hole in the mortar) checks the cavity for debris, mortar snots (lumps of mortar that bridge the cavity), and whether the cavity is clear and of adequate width. A cavity full of mortar snots will prevent insulation from distributing evenly.
Existing insulation. Some properties already have partial cavity insulation. Topping up with a different type of insulation requires careful assessment.
Grant Funding
The ECO4 scheme (Energy Company Obligation) and the Great British Insulation Scheme both fund cavity wall insulation for eligible households. Eligibility is based on income, property EPC rating, and whether the property is a rental (landlord obligations) or owner-occupied. TrustMark-registered installers administer the grants and the work is free or heavily subsidised for qualifying properties.
Check eligibility through the Simple Energy Advice service (simpleenergyadvice.org.uk) or by contacting a TrustMark-registered insulation installer. The fact that installation may be free under a grant doesn't mean the survey requirements can be skipped: a poor installation in an unsuitable property causes expensive damp problems regardless of who paid for the insulation.
If you've been told you're getting free cavity wall insulation and no survey has been offered, insist on one. A five-minute visual inspection from the street is not a survey. An endoscope inspection and exposure assessment is the minimum. If the company won't do it, don't proceed with them.
If Your Cavity Insulation Has Caused Damp
Thousands of UK homeowners have experienced damp and mould following cavity wall insulation installation. If you suspect your insulation is causing damp, the options include:
Extraction. The insulation can be removed by drilling a grid of holes and using a high-powered vacuum to extract the fill. This reverses the installation. It's expensive (£1,500-£4,000 depending on property size) and doesn't always remove all material.
Complaint through CIGA. The Cavity Insulation Guarantee Agency (CIGA) provides 25-year guarantees for cavity wall insulation installed by registered installers. If your installation is causing damp and was done by a CIGA-registered installer, you can make a claim under the guarantee. CIGA will investigate and, if the installation is found to be defective or unsuitable, will fund remediation.
Trading Standards and legal action. If the installation company is no longer trading and the CIGA guarantee doesn't apply, Trading Standards can investigate mis-selling claims. Legal action through the courts is possible but requires evidence of negligent installation or inadequate survey.