Cavity wall insulation is used to reduce heat loss through a cavity wall by filling the air space with a porous material. This immobilises the air within the cavity (air is still the actual insulator), preventing convection, and can substantially reduce space heating costs.

During new build, cavities are often filled with glass fibre wool placed between the two leaves of the wall, but many other building insulation materials offer various advantages and many others are also widely used. For existing buildings that were not built with insulated cavities, a fibrous material such as cellulose insulation is blown into the cavity through suitably drilled holes until it fills the entire wall space, or foam is injected. Although some foams used in the past, such as urea-formaldehyde, are no longer used (some people are allergic to this material, which is very difficult to remove once inside the wall), others such as polyurethane, have taken their place.

In Ireland, government grants are widely available to help with the cost of cavity wall insulation.