Cavity wall insulation damp can occur when insulation retrofitted into a cavity wall allows rainwater to cross from the outer leaf of the wall to the inner leaf, or where the wall was not suitable for insulation in the first place. It is most often found in walls exposed to wind-driven rain, walls with defective pointing or render, and cavities bridged by debris or other material. Cavity wall insulation remains an effective energy-efficiency measure in suitable walls, and it is not the cause of every damp problem in an insulated house. This guide explains how problems arise, the signs to look for, how the cause is investigated and the remedial options.
How a cavity wall is designed to work
A cavity wall consists of two leaves of masonry separated by a gap and held together with wall ties. Water that soaks through the outer leaf during driving rain should run down its inner face and drain away at low level, without reaching the inner leaf. Cavity trays over openings direct water back to the outside.
Retrofitted insulation is typically injected through holes drilled in the outer leaf. In a suitable wall in good condition, the insulation should remain dry. Where the outer leaf becomes saturated, however, the insulation can provide a route for moisture to cross the cavity.
How retrofitted cavity wall insulation can contribute to damp
Research carried out by UCL and BRE for the government notes that cavity wall insulation has long been established as unsuitable for brick-finished cavity walls in regions of high exposure to wind-driven rain, because the insulation can allow moisture to penetrate across the cavity from the outer leaf to the inner leaf, leading to damp within homes. From a surveying perspective, the risk usually depends on a combination of the following factors.
Exposure to wind-driven rain
The UK is mapped into exposure zones for wind-driven rain. Walls on exposed sites, and elevations facing the prevailing wind and rain, receive far more driving rain than sheltered walls, remain wet for longer and are more likely to have a saturated outer leaf.
Defective pointing, render and brickwork
Eroded or cracked mortar joints, cracked or debonded render, damaged bricks and poorly sealed window and door frames all allow more water into the outer leaf. The government research found that the brick-faced walls it inspected in highly exposed areas had high levels of cracking and other defects.
Bridging and debris in the cavity
Mortar droppings and building debris at the base of the cavity or lodged on wall ties can bridge the gap between the two leaves, even before insulation is installed. The same research found cavities blocked at the base with debris, cavity trays missing or damaged, and seals around windows in poor condition or absent. Where insulation is added to a wall with these defects, the risk of moisture crossing the cavity may increase.
Unsuitable walls and installation defects
Some walls are not suitable for retrofitted insulation because of their exposure, their condition or the construction of the cavity. Where insulation has been installed unevenly, gaps and voids can leave cold areas on the inner wall, where condensation and mould may develop. This can be mistaken for penetrating damp.
Signs that cavity wall insulation may be contributing to damp
Signs which may indicate a problem include:
- Damp patches on the inside face of external walls that appear or worsen after periods of wind-driven rain
- Damp concentrated on the most exposed elevation of the property
- Patchy damp at mid-height or around window and door openings, not only at low level
- Damp that first appeared some time after the insulation was installed
- Cold areas and mould on external walls that may correspond to gaps in the insulation
- Visible defects to the outer leaf, such as eroded pointing, cracked render or leaking rainwater goods
None of these signs is conclusive on its own.
It is not always the insulation
The Cavity Insulation Guarantee Agency (CIGA) notes that condensation or mould in a home does not necessarily mean that there is a problem with the cavity wall insulation. Damp in an insulated house may equally be caused by leaking gutters and downpipes, roof defects, plumbing leaks, a bridged damp proof course or condensation arising from high internal humidity. Our guides to the types of damp and relative humidity and condensation explain the differences.
Accurate diagnosis matters. Removing insulation that is not the cause of the problem will not resolve the damp and may increase heat loss from the home.
How the problem is investigated
An investigation into suspected cavity wall insulation damp should explain the cause before any cure is recommended. Depending on the property, it may include:
- External inspection – the exposure of the site and of each elevation, pointing, render, brickwork, window and door seals, sills, rainwater goods and ground levels
- Moisture and environmental readings – moisture meter mapping of the internal walls, with temperature, relative humidity and dew point readings to assess the risk of condensation
- Thermal imaging – to identify cold areas that may indicate missing, slumped or wet insulation, as well as thermal bridges
- Borescope inspection – a small camera inserted through a drilled hole to view the cavity, the condition of the insulation, any debris and the wall ties
- Core samples – removal of small samples of insulation, and in some cases masonry, to establish whether the insulation is wet and how far the moisture extends
- Calcium carbide testing – where needed, to measure the moisture content of masonry or plaster samples
Where a borescope inspection or core sampling is needed, we arrange it through a specialist and will explain beforehand what it involves. Because a CIGA guarantee is not valid if the insulation has been altered or disturbed, the guarantee position should be checked before any intrusive investigation or removal takes place.
Remedial options
Remedial works should be proportionate to the cause and extent of the defect. Depending on the findings, options may include:
- Repairing the outer leaf – repointing, repairing render, replacing damaged bricks and resealing windows and doors
- Rainwater goods – repairing gutters, downpipes and overflows that are saturating parts of the wall
- Clearing bridging – removing debris from the base of the cavity and addressing missing or damaged cavity trays
- Partial or full extraction – removing the insulation from the affected areas or the whole wall, usually by a specialist contractor. The method depends on the type of insulation, and some materials are more difficult to remove than others
- Drying and reassessment – allowing the wall to dry, then considering with specialist advice whether a different form of insulation is appropriate
- Internal repairs – replacing damaged plaster and decorations once the wall has dried
Water-repellent treatments to the outer leaf are sometimes suggested. The government research referred to above concluded that brick-faced cavity walls were unlikely to be in good enough condition for such treatments to reduce moisture penetration without defects first being repaired.
CIGA guarantees
Where cavity wall insulation was fitted by a registered installer, it may be covered by a guarantee from the Cavity Insulation Guarantee Agency (CIGA), which provides independent 25-year guarantees for cavity wall insulation fitted by registered installers in the UK and the Channel Islands. In general terms:
- The guarantee covers defects in materials or workmanship
- It remains valid for subsequent owners or occupiers of the property
- It is not valid if the insulation has been altered or disturbed
- Problems should be reported to the installer first, and to CIGA if the matter is not resolved
- CIGA can confirm whether a property is covered if the original paperwork cannot be found
Where CIGA concludes that there is a defect in the insulation or its installation, remedial work is carried out under the terms of the guarantee, which in more complex cases can involve clearing the insulation from the cavity. CIGA also states that it is not necessary to obtain legal advice or engage a claims handling company simply to start the process of investigating a concern. Homeowners should read the terms of their guarantee and contact CIGA directly for current details.
How an RICS Chartered Surveyor can help
An RICS Chartered Surveyor can provide an independent assessment of whether cavity wall insulation is contributing to damp, or whether the problem has another cause. Our damp and mould survey considers the building fabric, exposure, moisture levels, ventilation, heating and thermal performance, and the written report includes photographs, a risk rating and prioritised recommendations. Homeowners and buyers wanting a second opinion on a contractor’s findings can book an independent damp survey. As Lansdown Associates does not install or extract insulation, our recommendations are based only on the evidence.
Frequently asked questions
Does cavity wall insulation cause damp?
Not in every case. In a suitable wall that is in good condition and not severely exposed, insulation should remain dry. Problems are more likely where the wall is exposed to wind-driven rain, the outer leaf is defective or the cavity contains debris.
How can I tell whether my house has cavity wall insulation?
A regular pattern of filled drill holes in the mortar joints of the outer leaf often indicates retrofitted insulation. A borescope inspection can confirm the position, and CIGA can confirm whether a guarantee is registered for the property.
Should the insulation be extracted?
Only where the investigation shows that the insulation is contributing to the damp and that other repairs are unlikely to resolve the problem on their own. If the property is covered by a CIGA guarantee, report the concern before any insulation is removed.
Speak to an RICS Chartered Surveyor
If you suspect that cavity wall insulation is causing damp in your property, our RICS Chartered Surveyors can investigate the cause and advise on proportionate remedial options. For a free quotation, call 0151 374 0905 or email office@lansdownassociates.co.uk (Monday to Friday, 09:00 to 17:00). We usually respond within one working day.
