Insulation Thickness Calculator (for a Target U-Value)
How thick a layer of insulation must be to bring an existing wall, roof or floor down to a target U-value.
Results
Insulation thickness needed
78.1
mm
Thermal resistance to add
2.232
m²·K/W
Reduction in heat loss
78.1
%
The dashed line marks the value you entered.
What this tool does
Thermal resistances add up, so the insulation has to supply the difference between the resistance you want (1 ÷ U target) and the one you already have (1 ÷ U current); its thickness is that difference times the material's conductivity λ. The first centimetres do most of the work — going from 1.6 to 0.35 W/m²K cuts losses by almost 80 % — while each further step costs more thickness for less gain, which the chart makes plain. Take λ from the product's datasheet, and check the U-value your local building regulations require.
Formula
d = λ × (1 ÷ U_target − 1 ÷ U_current)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ut | Target U-value | W/(m²·K) |
u0 | Current U-value of the wall or roof | W/(m²·K) |
la | Insulation thermal conductivity λ | W/(m·K) |
D | Insulation thickness needed | mm |
RA | Thermal resistance to add | m²·K/W |
RD | Reduction in heat loss | % |
Worked example
- Target U-value0.35 W/(m²·K)
- Current U-value of the wall or roof1.6 W/(m²·K)
- Insulation thermal conductivity λ0.035 W/(m·K)
- Insulation thickness needed78.1 mm
- Thermal resistance to add2.232 m²·K/W
- Reduction in heat loss78.1 %
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.