Cavity Wall Insulation Calculator — Area, Cost & Savings

This calculator estimates the wall area that needs insulating by calculating the gross external wall area and subtracting windows and doors. It then provides an estimated annual energy saving based on typical UK figures from the Energy Saving Trust.

The formula

Net wall area = (Wall height × Wall perimeter) - Window area - Door area

The wall perimeter is the total length of all external walls measured around the outside of the building. Typical UK houses have a perimeter of 25-40 metres.

Energy savings estimates

The annual saving shown is the published Energy Saving Trust figure for the built form you select, because the saving depends on how much exposed external wall a property has rather than on the fill material. For a gas-heated home (Great Britain) the Energy Saving Trust gives roughly £410/year for a detached house, £240 for a semi-detached house, £180 for a detached bungalow, £140 for a mid-terrace and £110 for a mid-floor flat. These figures are based on an electricity price of 24.5p/kWh and a gas price of 6.29p/kWh.

UK Building Regulations

Under Part L of the Building Regulations (Approved Document L Volume 1, 2021 edition, England), upgrading an existing cavity wall by filling it should achieve a U-value of 0.55 W/m²K or better (Table 4.3, the renovation target where the existing element is worse than the 0.70 W/m²K threshold). For a new dwelling the limiting wall U-value is 0.26 W/m²K, with the notional (target) specification at 0.18 W/m²K (Table 4.1). Cavity wall insulation may also be installed under the ECO4 scheme (Energy Company Obligation, running to 31 December 2026), which can provide free or subsidised installation for eligible households.