Thermal bridges occur in a building at junctions or changes in materials which results in a higher heat loss when compared to the uniform areas. Examples are at the heads of windows and doors, corners, floor junctions etc.
Thermal bridging can have a number of negative impacts. It can have a significant impact on heat loss and energy efficiency as well as facilitating condensation and mould growth.
To account for linear thermal bridging, designers must use a “Y” factor which is a fraction of additional heat loss added to the overall calculations. Designers have the option of using a default values or calculating the actual “Y” factor for the thermal bridges. The default values can be a conservative estimate which can lead to additional cost elsewhere in the specification.
The default “Y” factor of 0.15 (15% additional heat loss) is used where no calculations are performed and where the Acceptable Construction Details (ACD’s) were not used and signed off during construction.
A default “Y” factor of 0.08 (8% additional heat loss) can be used where the Acceptable Construction Details were used during design and construction and have been signed off as appropriate.
The designers can calculate the actual “Y” factor by measuring the length of the thermal bridges and multiplying this by the additional heat loss associated with the thermal bridge. This may involve thermally modelling some junctions if they are not in accordance with the ACD’s. In the vast majority of cases this will result in a lower “Y” factor than the defaults.
Understaning thermal bridging
The video from Xtratherm gives a visual representation which helps gain a better understanding. In the last part of the video you will see a visual representation of the default and calculated “Y” values. The hole in the gable wall of the house represents the heat loss associated with each “Y” value. You can also see the selection of the Acceptable Construction Details for each of the locations where a thermal bridge exists.