How to calculate the U-value of a construction
Thermal Conductivity (Lambda Value)
Thermal conductivity, or Lambda value, 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 (W/mK). The lower the value, the better the thermal efficiency of the material.
|Rock or Glass Fibre Insulation||0.040W/mK|
Thermal Resistance (R Value)
Thermal resistance, or R value, is measured in W/m2K and is equal to the thickness divided by the conductivity. Resistances of each material are added together to determine the overall resistance of the construction. The higher the R value, the more efficient the insulation, i.e. 100mm PIR Insulation = 0.1 ÷ 0.020 = 5.0W/m2K 100mm Rock or Glass Fibre Insulation = 0.1 metres ÷ 0.040 = 2.5W/m2K
Note – Surfaces also provide thermal resistance and there are standard figures for these resistances that must be taken into account when calculating U-values:-
Internal Surface Resistance = 0.13W/m2K
External Surface Resistance = 0.04W/m2K
Thermal Transmittance (U-value)
Thermal transmittance, or U-value, is a measure of the rate of heat loss of a roof or wall construction. 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. Effects of support bars and brackets causing thermal bridges, and fixing screws should also be taken into account.
Calculating U- values
U-value = (1 ÷ R)
Where R = Material Thickness (m) ÷ Thermal Conductivity (W/mK)
* Example:- To find the U-value of a roof construction with 210mm thick insulation with a thermal coefficient of 0.040W/mK:
R = [(0.210 ÷ 0.040) + (0.13 + 0.04)] = 5.42W/mK
U-value = (1 ÷ 5.42) = 0.18W/m2K
* Example:- To find the U value of a roof construction with 100mm thick PIR insulation with a thermal coefficient of 0.020W/mK, and 50mm Rock Fibre insulation with a thermal coefficient of 0.040W/mK:
R = [(0.1 ÷ 0.020) + (0.05 ÷ 0.040) + (0.13 + 0.04)] = 6.42W/mK
U-value = (1 ÷ 6.42) = 0.16W/m2K
|50mm Rockwool / 80mm PIR||0.18W/m2K|
|50mm Rockwool / 100mm PIR||0.16W/m2K|
|50mm Rockwool / 120mm PIR||0.13W/m2K|
*Note: The U-values given in the above examples and table are simplified as they do not take into account any metal spacer system, screws or air gaps.
Values of internal and external resistance are taken from BS 5250 Code Of Practice for Control of Condensation in Buildings.
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