U-value calculator for walls
The U-value tells you how much heat passes through one square metre of wall for each degree of temperature difference between inside and outside. The lower it is, the better the wall insulates. It's the figure energy regulations, energy certificates and insulation manufacturers all use.
Choose the existing wall and the insulation material, or enter the thermal conductivity (λ) from the product sheet. The calculator shows how much an insulation layer improves the wall and, if you enter a target, how thick the insulation must be to reach it.
Your measurements
Typical values — use the manufacturer's figure if you know it.
Result
- U-value without insulation
- 2.09 W/(m²·K)
- U-value with insulation
- 0.30 W/(m²·K)
- Total thermal resistance (with insulation)
- 3.34 m²·K/W
- Heat loss through the wall reduced by
- 86%
- Heat flow before, at the temperature difference
- 41.8 W/m²
- Heat flow after
- 6.0 W/m²
Simplified calculation for a homogeneous wall: plaster layers, thermal bridges and fixings are ignored, so the real U-value is slightly higher.
How the calculation works
- Thermal resistance of each layer: R = thickness (m) ÷ λ.
- Total resistance: R_total = Rsi + R_wall + R_insulation + Rse, with surface resistances Rsi = 0.13 and Rse = 0.04 m²K/W for walls (EN ISO 6946).
- U-value = 1 ÷ R_total, in W/(m²·K).
- Heat flow = U × temperature difference, in W per m² of wall.
- Thickness for a target U = λ_insulation × (1 ÷ U_target − Rsi − Rse − R_wall).
Worked example
A 25 cm solid brick wall (λ = 0.81) has R = 0.25 ÷ 0.81 = 0.31 m²K/W and a U-value of about 2.09 W/(m²·K). Adding 10 cm of EPS (λ = 0.035) adds R = 2.86, and the U-value drops to about 0.30 — 86% less heat loss through the wall. At a 20 K temperature difference, the heat flow falls from 41.8 to 6.0 W per m². To reach U = 0.20, the same wall needs about 15.8 cm of that EPS.
What the calculator assumes
- The λ values in the list are typical figures; real products differ — use the declared value (λD) from the product sheet when you have it.
- Surface resistances follow EN ISO 6946 for walls with horizontal heat flow.
- In imperial mode, U is shown as a U-factor in Btu/(h·ft²·°F) and R as a US R-value; λ stays in W/(m·K).
When to adjust the result
- This is a simplified calculation for a homogeneous wall: plaster layers, air gaps, thermal bridges and fixings are ignored, so the real U-value is slightly higher.
- It doesn't assess moisture: internal insulation in particular can cause condensation in the wall and needs a hygrothermal check.
- Legal minimum values differ by country and by type of work. For an official figure (for example for a grant or a regulation), have it calculated by a qualified energy adviser.
Frequently asked questions
What is a good U-value for a wall?
Uninsulated solid walls are often between 1.5 and 2.5 W/(m²·K). Insulated walls in modern energy-efficient buildings are typically around 0.15–0.30. The requirement that applies to you depends on your country and on whether it's a new building or a renovation.
What's the difference between U-value and R-value?
They're inverses: R is the thermal resistance (higher is better), U = 1 ÷ R is the heat transmittance (lower is better). In the US, R-values are quoted in imperial units; 1 m²K/W equals R-5.68.
Does doubling the insulation halve the heat loss?
Not quite — and the benefit shrinks with each extra centimetre. Going from 0 to 10 cm makes a huge difference; going from 20 to 30 cm improves the U-value much less. That's why there is an economic optimum.