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U-value calculator

This U-value calculator takes up to five layers with thickness and thermal conductivity, the type of element and gives you the U-value, thermal resistance and the heat loss straight away.

Source: Wikipedia – Wärmedurchgangskoeffizient (U = 1 / (R_si + Σ d/λ + R_se), EN ISO 6946). Updated: .

cm
from inside to outside; thickness 0 = layer not used
W/(m·K)
from the material data sheet; example values below
cm
W/(m·K)
cm
W/(m·K)
cm
W/(m·K)
cm
W/(m·K)
m²
for the heat loss; 0 = do not calculate
K
e.g. 20 °C inside and −5 °C outside = 25 K
Kd/a
guide value for central Europe about 3500 Kd/a; use your location's figure
/kWh
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Result

U-value
U-value 0.16 W/(m²·K), thermal resistance 6.24 m²·K/W
Layers and thermal resistances
LayerThicknessλ in W/(m·K)R in m²·K/W
Inside surface (R_si)––0.13
Layer 11.5 cm0.70.021
Layer 236.5 cm0.142.607
Layer 312 cm0.0353.429
Layer 41.5 cm0.870.017
Outside surface (R_se)––0.04
Total (R_T)––6.244
Heat loss
  • At 100 m² and 25 K temperature difference about 400 watts are lost.
  • Over a year that is about 1,345 kWh (degree days 3,500 Kd/a), roughly 134.52 at your energy price.
  • U = 1 ÷ R_T = 1 ÷ 6.244 = 0.16 W/(m²·K).
Simplified calculation after EN ISO 6946 for uniform layers. Thermal bridges, fasteners, cavities, moisture and ventilated layers are not included; for compliance, use manufacturer and standard design values.

How it is calculated

An external wall of 1.5 cm interior plaster, 36.5 cm perforated brick, 12 cm insulation and 1.5 cm render has a U-value of about 0.16 W/(m²·K). The calculator works it out from your layers.

How it is calculated

Each layer has a thermal resistance R = thickness ÷ conductivity. Add the surface resistances: for a wall 0.13 inside and 0.04 outside m²·K/W, for a roof with upward flow 0.10 and for a floor with downward flow 0.17 (EN ISO 6946). The U-value is the reciprocal of the sum: U = 1 ÷ (R_si + Σ d/λ + R_se). The lower the U-value, the better the element insulates.

Example conductivities

Typical orders of magnitude in W/(m·K): reinforced concrete 2.3, sand-lime brick 0.7 to 1.1, perforated clay brick 0.08 to 0.16, timber about 0.13, interior plaster 0.70, render 0.87, mineral wool and EPS insulation 0.032 to 0.040. The manufacturer's figures or design values always take precedence.

Heat loss

With area and temperature difference the calculator shows the loss in watts, and with degree days the annual use in kilowatt-hours.

Related: the heating and cooling power for rooms, the roof calculator and the heating cost comparison.

Frequently asked questions

How do you calculate the U-value?

Divide each layer's thickness in metres by its conductivity, add them up, add the inside and outside surface resistances and take the reciprocal.

What is a good U-value?

The lower the better. An uninsulated brick wall is often around 1 or more, a well insulated external wall is below 0.2 W/(m²·K).

What is the difference between U-value and lambda?

Lambda is a property of the material per metre of thickness, the U-value describes the whole element with all layers and surfaces.

Why do the surface resistances differ?

Warm air rises. With upward heat flow the transfer is better (0.10), downward worse (0.17), sideways in between (0.13).

Does it work for windows?

No. Windows have their own figures (U_w, U_g, U_f) and thermal bridges at the frame. The calculator is for flat, multi-layer elements.

Sources and legal basis

As of: · Editorial policy

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