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Cavity Wall Insulation Calculator

Enter wall area, cavity width and energy price to see the cost, annual saving and payback time.

How it is calculated

Inputs: wall area in m², cavity width in mm, install rate per m² of wall, annual heating degree-days (°C·days) and energy price per kWh.

volume = area × cavity ÷ 1000 (m³)
cost = area × rate
kWh saved = 1.05 × area × degree-days × 24 ÷ 1000
saving = kWh × energy price
payback = cost ÷ saving (years)

The 1.05 W/m²K is a fixed assumption: an unfilled cavity wall at U = 1.60 and a filled one at U = 0.55. Your real wall and your local degree-days may differ. Cavity width only changes the fill volume, not the cost or the saving. If the saving is zero (energy price 0), payback shows "never". Zero or negative area, cavity or degree-days, a negative rate or price, a cavity wider than 500 mm and areas above 1,000,000 m² all give no result.

Worked example

An 80 m² wall with a 50 mm cavity, installation at 25 per m², 2,500 degree-days a year and energy at 0.12 per kWh. Fill volume: 80 × 0.05 = 4.00 m³. Cost: 80 × 25 = $2,000.00. Energy saved: 1.05 × 80 × 2,500 × 24 ÷ 1000 = 5,040 kWh. Saving: 5,040 × 0.12 = $604.80 a year. Payback: 2,000 ÷ 604.80 = 3.3 years.

Try this example →

Common mistakes

  • Entering the cavity width in centimetres or metres. The field is millimetres, so 5 cm must be typed as 50; typing 5 gives a fill volume ten times too small, and a value above 500 gives no result.
  • Putting the total job price in the rate field. It is a price per m², so a 2,000 quote for 80 m² is a rate of 25.
  • Entering the energy price per MWh or in cents instead of per kWh. A price of 12 cents must be typed as 0.12.
  • Using a single winter temperature instead of annual degree-days, which is a total for the whole year. A single temperature gives a meaningless saving, and 0 gives no result.
  • Adding gross wall area including windows and doors. Only the cavity wall area that will be filled should be entered.

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 tells you how much insulation goes in, with the install cost being the wall area times your installer's rate per square metre; the annual energy saving, driven by the reduction in the wall's U-value (taken here as 1.60 down to 0.55 W/m²K), your heating degree days 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.

Wall area, cavity width, the installer's price per square metre, your heating degree days and energy price add up to the fill volume, the install cost, the energy and money saved each year and the payback time, so you can tell whether filling the cavity pays for your home.

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