Cavity walls are constructed in two layers; an inner wall and an outer wall. The gap in-between is referred to as the cavity. New homes built in the last decade have been constructed with the cavity pre-filled with insulation, but if your home is older it may not have insulated walls.
Cavity walls can be filled with insulation materials such as mineral wool or polystyrene beads, which will increase the thermal efficiency of your home and could save over £100 per year on heating bills.
The installation will cost in the region of £100 – £350 and if you are over seventy or are receiving qualifying benefits, you may get help with the costs through government grants.
The very first cavity walls were introduced in the UK in coastal areas during the 1920’s; the reason they were used was to reduce dampness and water ingress from driving rain, rather than for insulation purposes.
It was only later, when it became apparent that cavity walls had excellent thermal properties, that they were used in houses across the country to provide insulation.
Filling these cavities with insulation retrospectively can increase the energy efficiency of your house, but can cause problems if it is not done correctly.
Damp and Condensation
Wind-driven rain can penetrate single-leaf walls, and cavity walls are intended to allow any water penetrating the outer leaf to run down the inside of the cavity to foundation level, preventing dampness on the inner leaf.
When the cavity is filled with insulating materials there is a danger that the insulation will bridge the gap between the walls and allow the transfer of water to the inner wall.
New homes are generally built with waterproof rigid insulation boards attached to the inner leaf, and a narrow gap between the insulation and the outer leaf to prevent water ingress.
If you are going to install cavity wall insulation it is worth considering how much driving rain the walls will be exposed to, and what type of material is used for the insulation.
The building regulations approved document C includes a map produced by the BRE (Building Research Establishment) which details the exposure zones in the UK for driving rain.
The zones are from 1 to 4, with zone 1 representing the east of the country where there is less rainfall, progressing through to zone 4 in the most westerly areas of the UK. The documents also give guidance on cavity width and the types of materials to use in each of the zones.
Another possible problem with cavity wall insulation, particularly if it is not installed correctly, is increased condensation. If the cavity is not completely filled there is a chance that the insulation can settle in the cavity over time and leave empty space at high level in the wall.
This could result in condensation and mould growth just below the ceiling on inside walls.
A professional installer should complete a thorough survey of the walls after installing the insulation to check that the entire cavity is filled; make sure your installer carries out this important task.
If any air gaps are left in the cavity between insulation materials, they can cause ‘cold spots’ on the inside wall which will invite condensation. The common areas to experience cold spots are between the windows, close to ground level, and high on the walls in upstairs rooms.
Wall Tie Corrosion
Wall ties are made of iron or steel and support the structure of the property by pinning the inner and outer walls together. In a dry cavity there is no risk of corrosion, but if exposed to water, wall ties can quickly begin to rust.
Wall ties can be inspected prior to the installation of insulation to make sure no corrosion is present. If wall ties have to be replaced it can be a very expensive process, as the brickwork and insulation needs to be removed and re-fitted to grant access to the wall ties.
The majority of walls are filled by blowing mineral-wool fibre into the cavity, which is not waterproof. Other materials that can be used are foam or polystyrene beads. These tend to be more expensive than mineral wool, but are waterproof if installed correctly.
They will take 2-3 days to inject into the walls of a house, which accounts for the extra cost, but are not without their own issues.
Polystyrene beads are individually waterproof, but must be bonded together with adhesive on-site before injection. If they are not bonded correctly it can lead to loose beads and air gaps within the cavity. Foam insulation is also mixed and prepared on-site, so must be done correctly.
If you are having your cavity walls insulated, make sure you use a professional installer who is registered to the CIGA (Cavity Insulation Guarantee Agency).
As well as ensuring that all their work meets current building regulations and standards, the installer should also register the work with the CIGA who will provide you with a 25 year guarantee on the insulation.
This will give you a safeguard against any problems arising due to the work carried out during the installation and any problems related to the insulation itself.
Cavity wall insulation is a very effective way to make a home more energy efficient and to reduce heating bills. When installed correctly it could save over £100 per year, and at a cost of roughly £100-£350, it will quickly pay for itself.
Cavity wall insulation problems are rare, but if you live in an area with a high level of wind-driven rain it may be worth investigating the different types of materials available to protect the property from any potential future problems.
Government subsidised grants will only run to the expense of standard mineral-wool fibre insulation, so if you are reliant upon financial help to have insulation installed, you may not have the option of selecting more expensive, waterproof materials.
Regardless of the type of insulation you have fitted, it is essential to have it done by a CIGA registered installer so you qualify for the 25 year guarantee.