Cavity Wall Insulation Calculator

About this calculator

Cavity wall insulation fills the gap between a building's inner and outer wall leaves with insulating material, and its economics come down to three linked numbers: the fill volume, wall area × cavity width, which sets the material and install cost; the annual energy saving, driven by the reduction in the wall's U-value and the local energy price; and the payback period, install cost ÷ annual saving, which tells you how many years until the insulation pays for itself.

Homeowners and energy assessors run this calculation before committing to insulation work because cavity fill is one of the higher-return home efficiency upgrades in cold climates, typically paying back within a few years — but the payback period swings a lot with local energy prices and wall area, so a number specific to your home is far more useful than a generic industry average.

This calculator takes your wall area, cavity width and energy price and returns the install cost, the annual saving and the payback time, so you can judge whether cavity insulation is worth doing for your specific home.