What is an R-value? With greater emphasis being placed on the U-value of new builds as regulations tighten we take a look at what factors are involved in reaching the magical low figures designed to get building control officers’ hearts racing! We’ve already written about U-values, the measure of the rate of heat flow through a wall, floor or roof – but how exactly is a U-value calculated? We asked architect Thomas Wiedmer, who is technical manager at insulation specialist Actis, to explain. "The fabric of a building is a crucial factor in determining its overall energy efficiency. The new target Fabric Energy Efficiency rate stipulates 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. U-values are expressed as W/m2K – the rate in watts at which heat transfers through a square metre of the surface of an element when there is a temperature difference of one Kelvin between the inside and outside of the element (that is, a wall, floor or roof). A U-value doesn’t apply to any individual material used in the build up, but to each of these elements as a whole. So a brick won’t have a U-value, but a build up consisting of brick, block, mortar, insulation and so forth does have a U-value.
What each material component does have though is an 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. Think of water attempting to get out of narrow tap compared with it just cascading over the edge of a weir. T
he latter is a much easier route and the number of gallons per minute achievable is far higher in the latter scenario – the aperture of the tap gives far greater resistance than the edge of the weir. R-values are shown as m2K/W. In order to determine the R-value of each fabric we need to determine the thermal conductivity of each of the materials used in the build up – the lambda value.
Many materials have a standardised lambda value. A sort of construction equivalent of a periodic table exists defining the thermal conductivity of each material – how much heat is conducted through a cubic metre of material with a one Kelvin temperature difference between side A and side B.
In this case the volume and thickness of the material is irrelevant. The more easily a material conducts the higher its lambda value. As an 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. The thermal resistance is then calculated by dividing the thickness of a material (in metres) by its thermal conductivity (lambda value - ?).
For non-standard products verified lab tests are carried out to test the lambda or R-values enabling specifiers to declare values (?D or RD) which can be included among their calculations. The overall R-value of a multi-layered element can be calculated by adding the R-values of its component materials. U-values are often described as being the reciprocal of the sum of R-values of an element. However, the inside and outside surface thermal resistances (Rsi and Rse) needs to be added. So when determining U-values one needs to follow the following steps: 1. note lambda 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 factors 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 A simple formula to illustrate this is R = T/C C = T/R T = R x C Thermal conductivity or lambda value refers to the material only, making no allowance the thickness. Thermal resistance or R-value is the thickness of the material in metres divided by the lambda value. And thermal transmittance or U-value is one divided by the total R-value of all layers in the element build up plus the internal and external surface thermal resistances.”
By Thomas Wiedmer, Technical Manager at Actis insulation.