Building Heat Loss Calculator

Heat loss is the number behind almost every energy decision in a building. It sizes the heating system, it determines the annual fuel bill, and the change in it is what any insulation upgrade is actually buying.

The calculation is a sum of areas multiplied by U-values, plus ventilation. A U-value is the heat lost per square metre per degree of temperature difference, so a lower number is better — modern walls are around 0.2 and uninsulated solid walls above 1.5.

The calculation

Heat loss coefficient = the sum of each element area × its U-value, plus ventilation losses, giving watts lost per degree of temperature difference. Peak load = coefficient × design temperature difference. Annual demand = coefficient × degree days × 24 ÷ 1000, which converts to kilowatt hours.

Worked example

Walls of 120 at 1.5 lose 180 W/K, a 60 roof at 0.4 loses 24, a 60 floor at 0.7 loses 42 and 20 of glazing at 2.8 loses 56 — 302 W/K of fabric loss. Adding 50 W/K of ventilation gives 352 W/K. At a 23 K design difference the peak load is 8.1 kW, and across 2,500 degree days the annual heat demand is about 21,120 kWh.

Where to spend on insulation

The elements losing the most are where money works hardest. Here the walls lose 180 W/K on their own — more than everything else combined — so wall insulation dominates any retrofit, while replacing the already-decent roof insulation would barely register.

Frequently asked questions