Cavity Wall Insulation


damp causes

What is the purpose of a cavity wall?

The purpose of the cavity wall is to help prevent rain water from penetrating through the outer wall to the inside of the property.

Cavity masonry walls were introduced on the exposed western coasts in the early nineteenth century, to stop penetrating dampness to the inside surfaces due to wind-driven rain. Due to the success at preventing penetrating dampness the construction of cavity walls spread to other, dryer, parts of the country, with the added benefit of the natural insulation created by air layer trapped within the cavity.

Since the late 1940’s this insulation quality has been enhanced by using lightweight blocks, rather than standard brick, to build the inner leaf of the wall. But the main reason for building a cavity wall hasn’t changed, it’s to keep the rain out.

What is cavity wall insulation?

Materials commonly-used include mineral wool, polystyrene beads or foam, the insulation is blown into
the wall cavity through large holes drilled into the outside elevations of your property, the holes are then
filled in with mortar, but leave evidence of the installation.

The government is keen for home-owners to insulate their homes, but is this a good idea and what are the
down sides of installing retro fit cavity wall insulation into your property and why are there a large
number of companies now being set up to remove cavity wall insulation if there are no side effects or

If your home was built from 1920 onwards, it’s likely to have cavity walls, for cavity wall insulation to be able to be installed, the wall needs to have a cavity of at least 50mm wide and the walls and pointing need to be in good condition, to be suitable for cavity wall insulation.

Since the early 1980s, Building Regulations have required new houses to be built with insulation material in the cavity. As long as they are built properly, this insulation should not compromise the walls resistance to prevent rain penetration.

The insulation in this new housing stock, is fixed to the inner leaf, leaving a cavity between the installed insulation and the inner face of the outer brickwork to intercept any rainwater that penetrates the outer brick leaf. The insulation material is usually in the form of rigid foam boards, which are intrinsically waterproof, or semi-rigid mineral-wool or ‘glassfibre batts’, where the fibers are aligned vertically so any penetrating rainwater drains downwards in the cavity and not have the chance to penetrate across to the inner leaf.

damp in brickwork

Cavity wall insulation found to be saturated note damp spot on mortar bed were brick has been removed.

Why is cavity wall insulation installed this way into new property?

This is because the Building Research Establishment has found that single-leaf brick walls ALWAYS leak when exposed to wind-driven rain. The leakage occurs at the vertical or ‘perp’ joints between adjacent bricks, because of cracking which occurs due to shrinkage in the mortar on drying.
So we have to ask the question why does building regulation and BRE recommend the cavity is maintained to prevent penetrating dampness in new build properties, but when retro fit cavity wall
insulation is installed, the cavity is completely filled with no cavity being left?

The government has announced plans to accelerate the installation of free cavity wall insulation, as part of its response to the issues of rising energy costs. Specifically, cavity wall insulation is now to be offered free to all homeowners which are elderly or on benefits, the question has to be asked, who is going to pay for the repairs to the defects caused, when the cavity wall insulation system go’s wrong ?

In the past, the only times rainwater penetration was a problem within a property, was when the wall ties were dirty with mortar droppings or installed sloping downwards from outer leaf to inner leaf, or the cavity itself being blocked at low level with mortar droppings or other debris.

Mortar found in the base of a cavity resulting in rising dampness

Mortar found in the base of a cavity resulting in rising dampness

In these cases penetrating rainwater could track across the cavity and show up as damp patches on internal decorative surfaces, but the solution for the repair works was relatively simple, cut out bricks as required and clear the rubbish out of the cavity, or replace the offending wall ties and the problem was solved.

The problem being these defects have not gone away but they regularly only become an issue once cavity wall insulation has been installed, they then become compounded when the cavity wall insulation becomes wet, as this leads to more extensive dampness issues, loss of any thermal value gained by the insulation and infact the wall becoming colder costing more in fuel costs.


Many owners of the properties we inspect, complain that following cavity wall insulation being installed, they noticed an increase in mould and condensation. We often find sealing up of airbricks with silicon by the cavity wall installers, they should infact remove the installed airbricks and install high flow ventilation and cavity liners to ensure the ventilation path to the sub-floor is maintained, to prevent fungal decay and insect infestation affecting the installed timbers and allow a natural ventilation path for the moisture vapour to move out of the property, instead of becoming trapped.
Air bricks sealed with silicon by cavity wall installer

Air bricks sealed with silicon by cavity wall installer

Damp problems caused by cavity wall insulation in new housing stock

Dampness problems caused by cavity wall insulation have not generally occurred in houses where the insulation was built-in from new, although this is now changing and accounts of dampness caused by built-in cavity batts are starting to be located.
Dampness problems within a ten year old property, due to wet cavity wall insulation and dry lining system

Dampness problems within a ten year old property, due to wet cavity wall insulation and dry lining system.

Problems with gaps in the insulation, or only some walls being insulated

Even where cavity wall insulation does not transmit rainwater across the cavity, it can still create dampness problems through increased condensation. Most houses were cavity walls have been filled with blown mineral fibre, suffer from gaps in the insulation and it is these voids which cause condensation and black mould on the walls inside, due to cold spots where the wall surface has fallen below dew point temperature. This is because, in a house which has been only partially insulated, the temperature still rises and with it so does the humidity ‘the amount of water vapour held in the air’. Any uninsulated wall areas results in cold spots and a higher risk of condensation. Common areas for condensation and black mould growth are near ground level, between windows and at ceiling level in upstairs bedrooms.
Void in cavity wall insulation resulting in cold spot and dampness affecting the internal wall.

Void in cavity wall insulation resulting in cold spot and dampness affecting the internal wall.

Before the insulation the installers are supposed to do a thorough survey of the cavities, and proceed only if the wall meets strict British Standards. In practice this is never undertaken as it is completely impractical and hardly any cavity walls meet these standards, as they all have unfilled mortar joints, debris dropped down the cavity and wall-ties covered with mortar droppings, but the installers press on regardless, as time is money, these imperfections and obstructions catch the insulation and stop it from filling the cavity evenly or are knocked down to the base of the wall due to the installation process, resulting in a bridging of the installed damp proof course, resulting in the additional problem of rising dampness and fungal decay of the installed floor joists.

Another problem is insulation sinking to the bottom of the cavity, leaving cold areas at the top of the house or below windows, this being referred to as slump, this becomes more extensive if the short cuts are made by the contractors to save money by installing insufficient fiber.

Wall tie corrosion

Wall ties are vital for the structural integrity of a cavity wall, they provide the lateral support for the external leaf of brickwork. Original wall ties were made of iron or steel, they will inevitably eventually rust, but in dry conditions they should last for many years. In persistently damp conditions, however, they can corrode much quicker and replacing them is a costly and time-consuming process and may involve cutting out individual bricks from the outer leaf were isolation is required. Replacing corroded wall ties becomes much more difficult in a building with cavity wall insulation, as the insulation itself has to be removed around each tie and then replaced afterwards.
Anyone thinking of having cavity wall insulation installed should first have the condition of their home’s wall ties assessed using the method described in BRE Digest 401.


Corroding cavity wall tie in a wall with cavity wall insulation installed.

Who regulates the Cavity Wall Insulation industry?

I have been asked this question many times, when the walls of a client’s house have become damp and affected by mould, or they have dry rot in the floor voids, in every case the installers and their “guarantee” provider CIGA (the Cavity Insulation Guarantee Agency) has refused to acknowledge that the cavity wall insulation was the cause of their problems, it is then up to the home owner to instruct a qualified damp and timber surveyor who is a member of the PCA, to undertake a survey and provide a report, to prove otherwise, which we have successfully done may times.

In every case the installers and CIGA have insisted the dampness/ fungal related problems were due to construction faults in the building, even though these defects were supposed to have been identified by the “surveyor” prior to installation, even when their installers had sealed up the sub-floor ventilation with silicon instead of installing cavity liners and high flow sub-floor ventilation units.


There are many home owners who have had cavity wall insulation installed, how many have experienced problems, nobody knows, as there has never been a conclusive study undertaken, this is likely due to the fact the industry and the government do not want the home owners to know the full existent of the issues.
Unfortunately there are a very large and increasing number of people who have suffered serious problems, and who have found it very difficult or impossible to get these problems recognised and rectified without going to the expense of instructing a qualified surveyor.

The cavity insulation industry and government agencies, do themselves no favours by pretending that these problems do not exist.
My own opinion is the installation of cavity wall insulation into an existing cavity wall is a bad idea, with the potential to create dampness and fungal decay problems, were rectification costs far outweigh any savings in fuel costs, or corresponding environmental benefits.

The most effective way to add thermal insulation to the walls of an existing home is on the inside, where the installation can be carried out under controlled site conditions, and any subsequent defects easily spotted and rectified.

The installation of insulation shouldn’t be undertaken without a full survey being undertaken, this must take into account the ventilation needs of the property, to maintain a healthy living environment as well as save energy.

Installing insulation in an existing cavity wall by drilling holes in the outer leaf and pumping material into a cavity in the hope that everything will be alright, even though you can’t see it, creates possibilities for problems.

If you have dampness related problems affecting your property give us a call you deserve to live in damp free house and healthy living environment