The fabric of a building is a crucial factor in determining its overall energy efficiency. The target Fabric Enery Efficiency rate stipulate that the physical envelope of the house - the longest lasting element of every new home - plays the greatest part in ensuring this efficiency.
The target FEE stresses that greater emphasis must be placed on the U-value of a new build - the lower the figure the more efficient the envelope. A U-value doesn’t apply to any individual material used in the build up, but to each of these elements as a whole. Eat each material component does have is and R-value-or thermal resistance - which is the resistance to the transfer of heat across the material.
While U-values need to be as low as possible for maximum efficiency. R-values have to be as high as possible. In order to determine the thermal conductivity of each of the materials used in the build up -the lambda value.
Many materials have a standardized lambda value. A sort of construction equivalent of a periodic table exists to define the thermal conductivity of each material - how much heat is conducted through a cubic meter of material with a one Kelvin temperature difference between side A and side B. The more easily a material conducts, the higher its lambda value. For example, aluminium, which conducts heat very easily, has a lambda value of 160 W/mK, steel - 50 w/mK and brick 0,72 w/mK.
Thermal resistance is then calculated by dividing the thickness of a material (in metres) by its thermal conductivity (lambda value).
So when determining U-values one needs to follow the following steps:
1. Note lambas value and thickness of each individual material
2. Work out R-value of each element by dividing the thickness by the lambda value
3. Add up all the R-values and apply bridging factor
4. Include the R-values of both internal and external surface thermal resistance
5. Divide one by the total R-value to get the U-value.
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