Wall U-Value Calculator (Thermal Transmittance)

What is a U-value?

The U-value (thermal transmittance) is the heat flow through 1 m² of a building element for a 1 K temperature difference between its two sides. Its unit is W/(m²·K). The lower the value, the harder it is for heat to pass and the better the insulation.

How is it calculated?

The thermal resistance of each layer is its thickness divided by its thermal conductivity: R = d / λ. The layer resistances are added, together with the resistances of the air films on both surfaces:

R_T = R_si + R₁ + R₂ + … + R_se and U = 1 / R_T

The internal and external surface resistances (R_si and R_se) are given in EN ISO 6946: R_si = 0.13 for walls, 0.10 for roofs, 0.17 for floors and R_se = 0.04 m²·K/W for the outside surface.

Example

Take a wall of 2 cm internal plaster (λ = 1.0), 19 cm brick (λ = 0.45), 5 cm insulation (λ = 0.035) and 8 mm external render (λ = 1.0). The resistances are 0.020, 0.422, 1.429 and 0.008 m²·K/W. With the surface resistances the total is 2.049 m²·K/W and the U-value is 1 / 2.049 = 0.49 W/(m²·K).

Notice that 5 cm of insulation provides 70 percent of the total resistance on its own while 19 cm of brick provides 21 percent. Without the insulation the U-value would be 1.61, so the wall would lose more than three times as much heat. The λ values here are only examples.

Where do I get thermal conductivity?

For insulation products the declared conductivity (λ_D) is on the label and in the declaration of performance. Design values for materials such as brick, concrete and plaster are tabulated in EN ISO 10456 and in national standards. This tool gives no material table from memory and asks you to enter the value.

Limits

The calculation is for a section of uniform layers without thermal bridges. Concrete columns and beams, fixings and metal profiles raise the real U-value. Whole building compliance and condensation checks (EN ISO 13788) are separate calculations.