The basic measure of how much heat energy is conducted by any building material, including thermal insulation, is thermal conductivity. It is also called the lambda (λ) value, or k value.
A material’s thermal conductivity is the number of Watts conducted per metre thickness of the material, per degree of temperature difference between one side and the other (W/mK).
As a rule of thumb, the lower the thermal conductivity the better, because the material conducts less heat energy.
Thermal conductivity is a property of the material and does not take into account thickness. Two different thicknesses of the same material still have the same λ-value.
To compare the relative performance of different thicknesses of materials means working out their thermal resistance (units: m2K/W).
Thermal resistance is calculated by dividing the thickness of the material by its thermal conductivity, giving an R-value specific to that thickness.
As a rule of thumb, the higher the thermal resistance the better, because there is a greater resistance to heat transfer.
A U-value is a measure of thermal transmittance, or the amount of heat energy that moves through a floor, wall or roof, from the warm (heated) side to the cold side. It is the number of Watts per square metre of the construction, per degree of temperature difference between one side and the other (W/m2K).
As a rule of thumb, the lower the U-value the better, because the complete construction transmits less heat energy.
We work closely with all our suppliers to provide specialist advice and support. Take a look at our extensive range of building insulation, and contact your local branch where the team will be happy to discuss your needs and assist with product specifications and U-value calculations to ensure you make the most suitable product choice for your project.
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