Insulation Thickness Calculator

How is insulation thickness calculated?

The quantity that sets the heat loss through a wall, roof or floor is its U-value. To reach a target U-value the total thermal resistance must be 1 / U_target. Subtract the resistance of the existing construction and the insulation has to supply the rest:

d = λ × (1 / U_target − R_existing)

λ is the thermal conductivity of the insulation and R_existing the total resistance of the uninsulated section including surface resistances. If you know the existing U-value, R_existing = 1 / U_existing.

Example

A wall with an uninsulated U-value of 1.2 W/(m²·K) is to be brought down to 0.5 W/(m²·K) using insulation with a conductivity of 0.04 W/(m·K). The extra resistance needed is 1 / 0.5 − 1 / 1.2 = 2.00 − 0.83 = 1.17 m²·K/W and the thickness is 0.04 × 1.17 = 0.047 m, or 47 mm. A 50 mm board is chosen, giving a U-value of 0.48 W/(m²·K). Heat flow through the wall drops by 60 percent.

Does doubling the thickness halve the loss?

No. The U-value is the inverse of the total resistance, so every extra centimetre returns less than the one before. In the wall above the first 5 cm cuts the loss by 60 percent and the second 5 cm adds only 15 points more. The best thickness comes from balancing investment against energy savings.

Where do I get the target U-value?

Maximum U-values for building elements are set by national building regulations and energy codes, usually by climate zone. This tool holds no table of limits: take the current value from the regulation or the project specification and enter it.

Limits

The calculation is for flat surfaces with continuous insulation. Concrete members, fixings and window reveals form thermal bridges and raise the real U-value. Condensation risk, fire class and moisture behaviour matter as much as thickness and need separate assessment.