Cavity Wall Insulation (CWI) is the process of filling the gap between the inner and outer layers of external walls (the cavity) with insulating material. Most UK homes built after the 1920s have cavity walls, and filling this space can dramatically reduce heat loss.
CWI is one of the most cost-effective energy efficiency measures and is widely supported through government schemes such as ECO4 and the Great British Insulation Scheme (GBIS). Under PAS 2035:2023, CWI must be installed by certified professionals, with a full risk assessment as part of the PAS 2035 whole-house retrofit process.
The process includes:
The insulation must achieve a U-value of 0.55 W/m²K or better to meet PAS 2035 and Building Regulation standards.
Under PAS 2035, Cavity Wall Insulation must:
A borescope survey and wall tie inspection are often part of the pre-installation process.
Typically completed in less than a day with minimal disruption.
Can reduce heat loss by up to 35% in uninsulated cavity wall homes.
Leads to lower heating demand and significant long-term savings.
Eligible for 100% funding under ECO4 or GBIS for qualifying households.
Installed entirely from the outside— no changes needed to the inside of the home.
Modern CWI systems are designed to last the lifetime of the building with no maintenance.
Properties with narrow cavities, timber-framed walls, or structural issues may not qualify.
Homes in exposure zones 4 or 5 (high wind-driven rain) may require additional checks or be deemed unsuitable unless protected by render or cladding.
A full assessment of wall condition, damp, and ventilation is essential to avoid future problems.
Properties in high flood-risk areas need additional moisture management.
If CWI was installed in the past and failed (e.g. older foam systems), it may need to be extracted and reinstalled to current standards.
Cavity Wall Insulation is ideal if:
Cavity Wall Insulation remains one of the most impactful and affordable home energy upgrades available. When installed to PAS 2035 standards and supported by a PAS 2035 risk-managed approach, it provides immediate and long-lasting benefits— cutting energy bills, reducing carbon emissions, and improving year-round comfort.
With widespread funding available and minimal installation disruption, CWI is an excellent first step toward a warmer, more efficient home.
