What is cavity wall insulation?

Cavity wall insulation fills the air gap between the outer and inner leaves of a cavity wall to reduce heat loss. In new build, full-fill mineral wool batts or partial-fill PIR boards are installed as the wall is built. In existing buildings, cavity wall insulation is retrofitted by drilling small holes in the outer leaf and injecting mineral wool fibre, EPS beads, or polyurethane foam into the cavity, then re-pointing the holes. Cavity wall insulation is one of the most cost-effective home energy improvement measures available.

What types of cavity wall insulation are available?

The main types are: full-fill mineral wool batts (for new build, filling the cavity completely); partial-fill PIR or phenolic boards (new build, fixed to inner leaf with a residual 50mm clear cavity); retrofit blown mineral wool fibre (the most common retrofit, injected through drilled holes); retrofit EPS (expanded polystyrene) beads (bonded with adhesive, suitable for a wider range of cavity types); and retrofit polyurethane foam injection (less common, offers good performance but irreversible).

Is cavity wall insulation suitable for all houses?

Cavity wall insulation is not suitable for all properties. A survey is required before installation to assess: the width and condition of the cavity (minimum 50mm); the exposure zone (very exposed or coastal sites may not be suitable for blown fibre due to wind-driven rain risk); the presence of mortar droppings or debris on cavity ties; and the condition of the pointing and renders. Properties in highly exposed locations, or those with very narrow or contaminated cavities, may be better suited to external wall insulation instead.

What U-value does cavity wall insulation achieve?

A typical 1980s cavity wall (102mm brick / 75mm clear cavity / 100mm dense block / 13mm plaster) without insulation has a U-value of approximately 1.5 W/m2K. Filling the same 75mm cavity with mineral wool brings this down to approximately 0.45 W/m2K - a major improvement. For new build using a 100mm full-fill mineral wool cavity with a lightweight inner block and insulated plasterboard, U-values of 0.18-0.20 W/m2K are achievable.

Thermal & Energy noun

Cavity Wall Insulation

/ ˈkavɪti wɔːl ˌɪnsəˈleɪʃ(ə)n /

Also known as: CWI, blown fibre insulation, cavity fill

Definition

Cavity wall insulation is insulation installed within the cavity of a cavity wall to reduce heat loss through the external walls of a building. It may be installed during construction (using mineral wool batts or rigid foam boards) or retrofitted into existing cavity walls by injecting blown mineral wool fibre, EPS beads, or polyurethane foam through holes drilled in the outer leaf. Cavity wall insulation is one of the most cost-effective energy improvements for UK homes built after approximately 1920.

In practice

For new build, full-fill mineral wool batts are the most common product - semi-rigid slabs friction-fitted between the two leaves as the wall rises. They completely fill the cavity and achieve U-values of 0.20-0.25 W/m²K in a 100mm cavity. Partial-fill PIR boards are also used, fixed to the inner leaf face, leaving a residual 50mm clear cavity to manage rain penetration risk - achieving better U-values in the same cavity width.

For retrofit, a registered installer drills 22mm holes through the outer leaf in a grid pattern, injects insulation until resistance is felt (indicating the cavity is full), then re-points the holes to match the original mortar colour. The process is quick but depends on the cavity being in suitable condition. A pre-installation survey must confirm the cavity width and condition, exposure zone, and absence of major mortar bridges. Failed or poorly installed cavity insulation can cause penetrating damp by bridging the cavity.

Building Regulations

Relevant PartPart L - Conservation of fuel and power

New build wall U-value0.18 W/m²K notional (AD L 2021)

Retrofit standardPAS 2030 / PAS 2035 - energy retrofit standards

Installer certificationCIGA (Cavity Insulation Guarantee Agency)

Retrofit cavity wall insulation is covered by PAS 2030:2019 and PAS 2035:2019, the UK standards for domestic energy efficiency retrofit. Work must be carried out by registered installers who provide a CIGA guarantee. Approved Document L requires cavity wall insulation to be considered where technically feasible when carrying out major renovations to existing dwellings.

Full Building Regulations guidance

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Insulation Calculator External Wall Insulation Calculator

Cavity Wall Insulation

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