A thermal bridge in the context of construction is an area of the building envelope which has significantly higher heat transfer that the surrounding materials.

In broad terms there are two types of thermal bridges to watch out for. There are material thermal bridges and there are geometrical thermal bridges.

Material thermal bridges are created when materials with a high thermal conductivity interrupt the thermal envelope of a building. These can be relatively severe, like a steel beam passing from the inside to the outside of a building, or relatively minor like a timber stud within a cavity wall. There are two factors which determine the severity of a material thermal bridge. The thermal conductivity of the material and the size of the interruption in the thermal envelope.

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Probably the most familiar thermal bridge to anyone living in temperate or cool climates is the single glazed window. We have all noticed that as the temperature drops outside so to does the inside surface temperature of glazing. This typically results in condensation on the inside of the glazing (or in many cases the aluminium framing). The same thing can also happen with external walls in areas where the heat transfer is greater. When the external temperature drops the internal surface temperature follows allowing condensation to form. This moisture is a key ingredient necessary for growing mould.

Geometrical thermal bridges are the other reason for increased heat transfer. These are caused by the shape of a particular part of the thermal envelope rather than the materials. A good example of these are the external corners of buildings. If you look at the corner of a house in plan you will see that it has a large amount of external surface through which the wall can lose heat and a relatively small internal surface from which it can draw heat. The result is that the internal surface temperature of external corners will always be lower than the rest of the inside of the wall during cold weather. In the image you can see how this effect has al2015-06-07 14.12.01lowed condensation to form on the inside of the external corner. You can quite clearly make out where the lowest temperatures have occurred due to the distribution of the black mould.

Frequently in architecture these two types of thermal bridges overlap. A steel post embedded within an external corner of a wall. A steel beam at the apex of a cathedral roof. In these instances you are asking for trouble with condensation, corrosion and mould.

The answer is generally to avoid these kinds of situations. Where unavoidable you need to create a ‘thermal break’ in the assembly or use materials with a lower thermal conductivity. In our office if we are concerned about a potential thermal bridge we use software to model the detail and check the internal surface temperatures of the assembly to ensure that condensation will not cause problems.

For very highly insulatedWall Foundation Detail with Nib buildings thermal bridges have other implications. Due to the very low levels of heat lost through the walls, windows, floor and ceiling, the relative amount of heat lost through thermal bridges can become quite high. For this reason, when designing a Passivhaus, thermal bridges need to be avoided or minimised wherever possible. Those thermal bridges which remain need to be quantified and added into the thermal modelling of the project in order to produce an accurate model of the thermal performance of the building.