Cavity wall insulation

A question about cavity wall insulation may not be one that you’d expect to be asked when talking about old buildings. Generally, cavity walls are regarded as a modern form of construction but the subject has cropped up a couple of times in the Q&A sessions at the SPAB Old House Eco Courses that I lead with Marianne Suhr. Since I wrote the section on insulating cavities in Old House Eco Handbook, it usually falls to me to respond.

Cavity walls do occur in older buildings and they appear to date back to at least the mid-nineteenth century, although we may discover they predate this: National Energy Services is currently running a competition to find Britain’s oldest cavity wall dwelling.

The idea of introducing a cavity generally seems to be to do with preventing penetrating damp. With this in mind, I would always recommend seeking impartial expert advice before filling cavities, certainly early ones and especially in locations where wind driven rain is an issue. It goes without saying that any existing damp problems must be resolved prior to the installation of any type of insulation.

Before filling a cavity, it’s essential to ensure that it’s at least 50mm wide. In an older building this is often not the case. Indeed, the cavity may be inconsistent and may not even exist in some parts of the wall so a thorough investigation should be carried out to check what’s going on, possibly using CCTV. Whenever cavity wall insulation is planned, thermal imaging both before and after installation is advisable. This will help identify any areas that have not been fully filled as well as potential thermal bridges.

Installing the insulation involves drilling a series of holes, usually about 22mm diameter, at regular intervals across the external wall horizontally and vertically. If the wall is of brick, this should be done through the mortar joints, although inevitably the corners and edges of the bricks can be damaged. Where there’s a rendered finish, unsightly blemishes will inevitably be caused across the face of the wall. Through the holes the installer pumps in either loose-fill polystyrene bead, mineral wool or a foam insulant. In older buildings I wouldn’t advise foam because it lacks breathability and is irreversible as it adheres permanently to the structure. This also means that materials cannot be recycled in the future.

Once the installation is complete, all the holes must be filled to avoid moisture and wind penetration. A matching mortar should be specified with the contractor at the outset – in the case of old buildings this should generally be of lime.

Image: ©Roger Hunt