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Calculating U Values

In order to calculate U-Values it is important to first understand, Thermal Conductivity, Thermal Resistance and Thermal Transmittance.

Lambda (Thermal Conductivity)

Thermal conductivity is the rate at which heat is transmitted through a material, measured in watts per square metre of surface area for a temperature gradient of one Kelvin per metre thickness - simplified to W/mK.

The Lower the value, the better the thermal efficiency of the material.


PIR Board 0.022 W/mk
Glass Fibre Roll 0.044 W/mk

R-Value (Thermal Resistance)

Thermal resistance is measured in m2K/W and is equal to the thickness of the material (in metres) divided by the conductivity of that material. The resistances of each material within an element are added together to determine the overall resistance of the element. The higher the R-value, the more efficient the insulation.


PIR Board 0.022 W/mk 100mm = 0.1 metres ÷ 0.022 = R-Value 4.54 m2K/W
Glass Fibre Roll 0.044 W/mk 100mm = 0.1 metres ÷ 0.044 = R-Value 2.27 m2K/W

N.B. Surfaces and cavities also provide thermal resistance and there are standard figures for these resistances that must be taken into account when calculating U-values. 

U-Value (Thermal Transmittance)

Thermal transmittance, commonly known as the U-value, is a measure of the rate of heat loss of a building component. It is expressed as watts per square metre, per degree Kelvin, W/m2K. The U-value is calculated from the reciprocal of the combined thermal resistances of the materials in the element, air spaces and surfaces, also taken into account is the effect of thermal bridges, air gaps and fixings.

Calculating U-Values


U-Values can be calculated using the following formulas:

Firstly calculate your total Thermal Resistance (R-Value)

Thickness of product in meters ÷ Lambda (Thermal Conductivity)


PIR Board 100mm = 0.1 ÷ 0.022 = 4.54 R-Value
Glass Fibre Roll 100mm = 0.1 ÷ 0.044 = 2.27 R-Value

Then, simply add together the R-Values of each product used in the application (including any air gaps) and apply the following formula:

1 ÷ Total of all R-Values


PIR Board 100mm + Glass Fibre Roll 100mm =
Total Combined R-Value = 6.81
U-Value = 1 ÷ 6.81 = 0.147 W/m2K

N.B. The above examples do not allow for any thermal bridges, air gaps and fixings etc.

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