HTHOMETALLYTHE PRACTICAL HOME GUIDE

Insulation/ CALCULATOR

Assembly heat loss

Compare heat flow through a wall, roof or window area.

Use net area; exclude openings calculated separately.
Use U in W/(m²·K) from a calculation of the complete construction or a product test report, including repeating thermal bridges. For windows, use the whole-window value. Insulation conductivity λ, R-value and US-unit U-factors are not interchangeable with this input; 0.30 is only an example.
Use indoor minus outdoor temperature for the stated heating condition. A difference of 25 °C equals 25 K; do not enter the indoor temperature alone.
Transmission heat loss

600 W

Equivalent heat rate

0.60 kW

Steady transmission through one assembly at the entered temperature difference. Excludes ventilation, air leakage, other surfaces and junction bridges not already included in U. Results in W or kW are a heat rate, not energy in kWh. Zero at ΔT = 0 does not establish zero total heating demand. This is not whole-building heat loss, boiler sizing or annual savings; heating-system efficiency is not included.

FROM ESTIMATE TO ACTION

Use the result, not just the number.

01

Separate walls and openings

Use the net area of the construction in m². Subtract windows and doors counted separately; for a window, use the area matching its whole-window rating, including the frame. Calculate sections with different U-values separately, then add their heat rates in watts without counting any area twice.

02

Verify the complete construction U-value

Use U in W/(m²·K) from a documented construction calculation or product test report, not insulation conductivity λ or its R-value. For a window, use the whole-window value, not centre-of-glass alone. Check the units: a US U-factor cannot be entered unchanged. The example 0.30 is not a value for every wall.

03

Compare the same heating condition

Set ΔT to indoor minus outdoor temperature for the same stated condition, not indoor temperature alone. Halving U halves this heat rate only if area and ΔT stay fixed; it does not halve the energy bill. W and kW are power, not kWh. Ventilation, air leakage and junction losses not included in U remain outside this estimate; do not size a boiler from it.

Inputs to verify

Assembly area (m²)
Use net area; exclude openings calculated separately.
Assembly U-value (W/m²K)
Use U in W/(m²·K) from a calculation of the complete construction or a product test report, including repeating thermal bridges. For windows, use the whole-window value. Insulation conductivity λ, R-value and US-unit U-factors are not interchangeable with this input; 0.30 is only an example.
Temperature difference (K)
Use indoor minus outdoor temperature for the stated heating condition. A difference of 25 °C equals 25 K; do not enter the indoor temperature alone.

Continue your calculation

Insulation

Extra insulation thickness

Estimate an ideal continuous layer needed to reach a target U-value.