How the Cavity Wall Insulation Calculator Works

Enter your property type, its approximate wall area or number of bedrooms and your heating type. The calculator estimates the installed cost for your home, expected annual saving on heating bills at current energy prices and how long investment takes to pay for itself.

The underlying numbers, installers typically charge £15 to £30 per square metre of wall, which puts a mid terrace at roughly £650 to £1,800, a three bed semi at £650 to £2,500 and a detached home anywhere from £1,500 to £5,000 depending on size and access. Quotes vary widely between firms for identical jobs, so always compare at least three.

Does Your Home Have Cavity Walls

Most UK homes built between the 1920s and 1990s have cavity walls, two leaves of masonry with a gap of 50mm or more between them. Homes built after the mid 1990s were generally insulated during construction and pre 1920s homes usually have solid walls needing a different approach entirely.

Two quick checks tell you what you have. Look at the brickwork, if every brick shows its long face in a stretcher bond pattern, wall is almost certainly a cavity. Bricks alternating long faces and short ends suggest a solid wall. Then measure wall thickness at a window or door reveal, cavity walls run around 250 to 270mm while solid walls come in nearer 220mm. An installer confirms with a borescope through a small drilled hole before quoting.

What the Installation Involves

The job is quicker than most people expect. The installer drills 22mm holes through outer leaf at roughly one metre intervals, injects insulation under pressure, then plugs the holes with matching mortar. A typical semi takes two to four hours, all from outside, with no internal disruption beyond drilling noise.

Three materials dominate, blown mineral wool at cheapest end, bonded polystyrene beads which resist moisture and suit wetter locations and polyurethane foam at premium end. One caution on foam, some mortgage lenders have grown wary of polyurethane installations, so if you might sell or remortgage, check with your lender first or stick to mineral wool or beads. Whatever material, use an installer registered with CIGA, whose independent 25 year guarantee transfers with the property and gets requested by solicitors at sale.

The Savings and the Damp Question

Filling a cavity roughly triples wall’s thermal performance, taking a typical U value from 1.6 down to around 0.5 W/m²K. In money terms that means annual heating savings of roughly £110 to £180 for a mid terrace, £180 to £285 for a semi and £300 to £480 for a detached home at current gas prices, with payback typically inside three to six years. It also commonly lifts an EPC by a band, which matters increasingly for landlords and sellers and if a heat pump is in your future, insulating first means a smaller and cheaper unit.

The damp reputation deserves a straight answer. Properly installed insulation in a suitable property rarely causes problems. The failures cluster in specific situations, homes in severe wind driven rain zones along western coasts, cavities narrower than 50mm and walls with defects like cracked render or failed pointing. A proper pre installation survey screens for all of these, which is why any installer who quotes without inspecting should be shown the door.

Grants and VAT

Two funding routes can cut cost dramatically. The ECO4 scheme, running until December 2026, can fund cavity wall insulation in full for households on qualifying benefits in less efficient homes. The Great British Insulation Scheme extends free or subsidised installation to homes in lower council tax bands regardless of benefits. Even paying privately, cavity wall insulation currently carries 0 percent VAT until 31 March 2027, which quietly trims the bill against older price guides.