Cavity Wall Insulation – A Guide

We start our series on home improvement with a look at retrospective cavity insulation, the different types and potential pitfalls.

Cavity wall insulation is an effective method of preventing heat loss through the cavity walls of a house which can, in turn, reduce the amount of money being spent on heating. An uninsulated cavity wall is said to result in around a third of all the heat generated within a house being lost, so for every £3 spent on heating a property £1 is lost through the walls alone. Other benefits of insulation include reduced condensation and fewer draughts.

Although properties built since the mid 1980’s typically use solid board insulation to partially fill a cavity during construction, many homeowners with properties built before this time consider a retrospective installation.

Although board type insulations cannot easily be insulated retrospectively, there are three main types of insulation in use in the UK that can; these include Mineral Wool, Beads or Granules and Foamed insulants.

Mineral Wool

Brands of Mineral Wool include Rockwool and Glass Wool, these are the most cost effective and widely used in the UK. They are made of spun volcanic rock or glass that is treated with a binder or water repellent during manufacture. This is then blown through the outer wall through a set of strategically placed holes. A disadvantage of using Mineral Wool is that it does not act as a water vapour barrier and will thus potentially cause damp issues if fitted retrospectively and without exceptional attention to detail.


EPS Beads (Expanded Polystyrene) are an effective alternative. Small polystyrene beads are blown into a cavity along with an adhesive. When the two bond together and settle, an insulating barrier is formed. Because of the way the beads ‘flow’ into the cavity and fill any unusual gaps, this material can be used in stone cavities. It also reduces the number of holes needed for installation making the process easier. EPS Beads are water resistant and will not transmit water across the cavity wall through capillary action.


Polyurethane Foam is injected into the cavity in liquid form through correctly placed holes drilled through the outer leaf. Once inside the foam will expand and solidify, bonding to the inner and outer walls. Not only will this prevent heat loss, it will help rectify the structural stability of the cavity walls where wall ties have been damaged and/or corroded. Relative to other methods, this is an expensive solution.

Poorly installed insulation could result in an increased risk of condensation and damp problems, which in turn can create more serious structural damage. With Mineral Wool in particular, over time the insulation will settle, leaving cold spots. As the internal temperature of the building increases, these cold spots can be subject to condensation and black mould, sometimes giving the false impression of penetrating damp. Even foam and beads can create issues if the coverage is not complete.

It should always be realised that retrospectively injecting insulation will essentially turn the cavity wall into a solid wall. Whereas it is impossible for water to track across a cavity, with the exception of along poorly designed cavity wall ties, breaching the cavity, even with a supposedly water resistant material, is increasing the likelihood of future issues – particularly when one remembers that all materials have a functional life span.

Once the insulation has been injected it is extremely difficult to remove, so you should think very carefully before choosing to have this procedure carried out.

It is very important that before insulation is installed, relevant checks are carried out by a professional to ensure that the property is suitable for retrospective insulation and that there are no undiagnosed damp or structural problems that could be compounded.

After insulation is installed, it is wise to ensure that a heat sensor is used to highlight any cold spots within the cavity.

If you have recently had your property insulated and are now experiencing problem with damp, condensation or structural issues do not hesitate to contact a surveyor on for advice.

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