Improving our insulation is a great way to save on energy for heating the home, but there are factors that can undermine attempts to insulate our houses – a key one is thermal bridging.
The objective of insulation is simple; to reduce the flow of heat out of a building. However, you can insulate large swathes of a house, but still leave channels of highly conductive material, called thermal bridges, which will leak heat to the exterior. Some materials, such as metal, are much better at conducting heat than others. If these materials penetrate the insulation envelope they can cause problems.
Graphic: The balcony causes a thermal bridge. The concrete slab extends from the inside of the building to the exterior. It will also be associated with a window or door that breaks the insulation layer further. Steel ties are used to support the wall cavity. These bridge the insulation layer, and although they are very small they have a high linear heat transfer rate.
It has been reported that more than 60% of domestic energy use goes towards space heating . Having good insulation can dramatically reduce the amount you spend on heating, and your impact on the environment. However it is important to ensure that adding extra insulation to your house is having the desired effect.
Thermal bridges provide an easy path for heat to escape. Heat loss can be worked out for any “detail” of a building’s construction. This could be a balcony, where a concrete slab penetrates the insulation layer, or an architectural element like the corner of a building or the insulation gap around a window [2, 3].
Each such detail has a property called a linear thermal transmittance associated with it. This is the rate of heat that can be transmitted from one side of the detail to the other. Linear thermal transmittance is represented by the Greek letter Psi (Y), so is sometimes referred to as a psi value.
In order to work out the heat loss due to thermal bridging (HTB) you need to multiply the psi value by the length of the detail. This is the length between the warm interior end and the cold external end. Once this has been worked out for each detail that could be the thermal bridge you add up the results for each detail in the building to get a total heat loss. In other words, for those of us who like formulae,
HTB = ∑(L x Y)
HTB = Heat loss from thermal bridges W/m2K
Y = Linear thermal transmittance (W/mK)
L = Length of detail (m).
∑ = The sum of *
The Greek letter sigma (∑) is not a variable, but rather an instruction to add up a series of variables. This means that L x Y should be calculated for every possible thermal bridge, and then all of them should be added up to determine the total effect of all the thermal bridges.
The most complicated part of this calculation is working out what the psi values are for each possible thermal bridge. A qualified assessor can be commissioned to conduct thermal modelling for the house and determine the precise Psi values, but for most purposes it is better to use an appropriate published value. An appendix to the Government’s Standard Assessment Procedure for Energy Rating of Dwellings (SAP 2012)  lists the psi values of various architectural details (Appendix K, page 80). If the building has been independently assessed and is known to comply with these standards then the “approved” values can be used. Otherwise the values in the default column are more appropriate.
It is easy enough to look up the numbers that apply for each part of your house, and do a bit of measuring to work out the lengths of these features. Then with a bit of basic maths you will soon know which parts of the building are losing the most heat, and what the overall heat loss is.
A lot of thermal bridges are inherent to the building’s design. If you are building your house from scratch then you have a lot of options to reduce heat loss, for instance by including thermal breaks in large concrete elements, and not using highly conductive metal wall ties from cavity walls [4, 5]. Unfortunately thermal bridges are a lot harder to remove when retrofitting a building, but knowing where the heat loss is occurring can help you to plan where insulation needs to be added (even a thin amount is better than none) and where it is doing all that it can.