Thermal units
I have been looking through a brochure from a manufacturer of insulation materials and marvelling at the mix of units used to express their efficiency. My guess is that they are either using the ones that show each product in the best light or for that product’s perceived use. It made me think about writing this little summary and invite others to add unit explanations related to retrofit.
λ (or sometimes k) is the thermal conductivity of an object i.e. its ability to conduct heat. It is measured in Watts per meter Kelvin W/K. It is used to calculate heat losses from thermal bridges. It is usually ignored by DIYers and most builders.
U-value is the heat transfer coefficient that describes how well a building element conducts heat or the rate of transfer of heat (in Watts) through one square metre of a structure divided by the difference in temperature across the structure. It is expressed in Watts per meter squared kelvin W/(m²K). This means that the higher the U-value the worse the thermal performance.
R-value has the units square metre Kelvins per watt (m²K/W). An R-value can be given for a material or for an assembly of materials (e.g., a wall or a window). In the case of materials, it can be expressed in terms of R-value per metre. R-values are additive for layers of materials, and the higher the R-value the better the performance.
In some parts of the world it is normal to use R value rather than U value and doing so makes calculations of thermal efficiency of buildings simpler as the R values of the components can simply be added together for example
R(plaster) + R(brick) + R(render) = R(solid wall).
You can’t do this with U values, but there is a workaround.
If you look at the units you will see that R and U are inverses of each other, so to add U values you need to take the inverse of each U value, add them together and then take the inverse of the answer. Much more hassle but not particularly difficult with a calculator or spreadsheet. If you don’t have an inverse function on your calculator don’t worry. “Inverse” is just a way of saying “divided into 1”, so the inverse of 2 is ½ and the inverse of 3 is ⅓.
In my solid wall example above you need
1/U(plaster) + 1/U(brick) + 1/U(render) = 1/U(solid wall) then divide the answer into 1 to get U(solid wall).
Edit:
Psi-value (written as Ψ-Value) is a measurement of linear thermal heat transmittance, usually through the junctions of a building. Psi-values are measured in W/mK, showing the energy lost per metre between two thermal elements, for example, the join between a ground floor and an external wall. Psi-values are used to assess the overall thermal bridging performance of a building.
To avoid ambiguity, and as I understand it, Ψ-Value Is used for linear thermal bridges, λ value is used for point bridges, such as wall fixings.