Thermal transmittance & U-values

U-value and Building Regulations

While thermal conductivity (lambda) measures the ability of individual materials to resist heat, walls, roofs and even single components like windows are made of more than one material, making it more complicated.

Thermal transmittance is how a collection of different materials in combination resist heat together, so it is possible to get a single number (U-value) for any combination of materials forming a wall, roof, window, floor, etc. This number is what the building regulations need to know to ensure that each building element is adequately insulated.

U = 1/R in W/m2K or Watts per square metre per degree Kelvin

Total building heat loss calculation

The total heat loss of the building is then worked out by multiplying the area of the various elements (roof, wall, etc.) by the U value of each. For compliance, certain temperature differences between inside and outside are assumed to make all assessments comparable.

Building shape is important

It is worth noting that it’s not just the thermal transmittance of the building element that is important. Heat energy obeys the laws of physics, whether in a human body or a building. The heat inside a building will gradually make its way to the outside when it is colder outside. The heat moves out through the thermal envelope.

The thermal envelope's purpose is to reduce or slow down this heat movement as much as practical. However, the more surface area the thermal envelope has, the more surface area for heat to escape through. For this reason, it makes sense to keep buildings compact to minimise heat loss.