How to calculate thermal bridging?
There are four steps for the calculation of the negative effect of thermal bridging:
1- Determine the bridged layer
The bridge layer is a non-uniformity in a continuous material; for example, if the insulation is installed between the metal purlins, the insulation uniformity is lost because it is bridged by metal purlins. According to NZS 4214, a bridged layer is not bounded by an air space; therefore, if the wall consists of air space with a sufficient thickness so it could act as a thermal resistant layer, then the airspace must be included within the thermally bridged layer. Generally, if there is at least 20mm air adjacent to the insulation in a bridged layer, then the thermal resistance of air (R0.17) must be added to the thermal resistance of insulation.
2- Determine the portion of each parallel material
You must determine what portion of the wall consists of the high conductivity material such as metal frames. For example, a 3m2 wall has insulation with an area of 2.7m2 that is bridged by metal stud frames, the portion of insulation is 2.7/3=90% and we also have a 10% metal stud area.
3- Determine the R-value of each parallel material
To calculate the thermal resistance (R-value) of a material, divide its thickness by its thermal conductivity. For example, a 90mm thick timber batten has 0.09/0.1=0.9 m2K/W thermal resistance. Keep in mind the addition of air space resistance if available above each parallel layer.
4- Calculate the net R-value of the bridged layer
The net R-value of the bridged layer can then be calculated as below:
Where Rb is bridged layer net R-value, f1 and f2 are consecutive portions (%) of each parallel layer, R1 and R2 are consecutive R-values of each parallel layer (i.e. insulation and bridging medium)