Questions about detailing

What is a Thermal Bridge?

A thermal bridge (or cold bridge) is a weak point in the building's fabric insulation. They occur when a material with a relatively high thermal conductivity (a good conductor of heat) penetrates the external building envelope insulation layer. Cold from the external environment, and heat from the interior, pass through the material. They result in local heat losses and a reduction in the thermal performance of the building fabric at these points. As a result, this increases heating costs and energy use making the building less energy efficient.

Why are Thermal Bridges a Problem?

Thermal bridges are hardly an issue in very poorly insulated buildings. However, with a high-performance thermal envelope (such as the Passivhaus standard) thermal bridging is critical. Typically in an uninsulated home, about 30% of the heat loss goes through the roof, 13% through the doors and windows, 16% through the walls and 16% through the floor. When a building is designed to be exceptionally well insulated, thermal bridging, which traditionally was ignored, contributes a significant proportion of heat loss and can make a difference of 25% in a Passivhaus building. Additionally, poorly designed details (which create thermal bridges) can perform much worse than the adjacent thermal envelope. This creates considerable temperature differences in the building envelope and moisture and mould will often occur on the inside surface at these locations.

How do you detect thermal bridges?

Many thermal bridges can’t be seen with the naked eye. A quick touch temperature test with the hand can give an indication. Hold your hand over the area where you suspect a thermal bridge. If you feel a perceptible difference in temperature in a certain area, that is a strong indication of a thermal bridge. Areas of dampness or condensation can also indicate thermal bridges.

How to avoid a Thermal Bridge?

Thermal bridges are relatively easy to design out. It generally involves using software design toll calculators to calculate how building fabric components are performing. For Passivhaus all architectural details should include calculated ‘psi’ values as well as providing guidance to the contractor on how to build a junction. This will help to achieve good thermal continuity.

How do I minimising air leakage

In order to get down to PassivHaus standards the importance of the airtight layer should not be underestimated. The architects details should clearly indicate the location of the air-tight layer and this informs the contractor on where to place the membrane during the construction sequence.