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The calculation method used to determine the thermal transmittance and heat loss attributable to a thermal bridge is dependent upon the type of bridge encountered.

For repeating thermal bridges, the additional heat flow attributable to the thermal bridge is taken into account in the calculation of the U-values of the various elements containing the thermal bridges. The thermal transmittance of the bridged construction can be calculated using a simplified calculation method described within BS EN ISO 6946: 2007 (British Standards Institution, 2007) and BRE Report BR443 (Anderson, 2006). This method, commonly known as the combined areas method, involves calculating an upper and a lower limit of thermal resistance for the bridged construction. The arithmetic mean of these two values is then used to determine the U-value of the construction. Various corrections can be applied to take into account factors such as:

• Air gaps in insulation.
• Mechanical fasteners penetrating an insulation layer.
• Precipitation on inverted roofs.

Further details on the calculation procedure and its applicability are contained within BSI EN ISO 6946: 2007 (BSI, 2007).

For non-repeating and geometrical thermal bridges, the heat flow attributable to the thermal bridge is calculated and added separately. A variety of numerical modelling software packages are available that are capable of determining ψ values from 2 or 3 dimensional heat flow calculations, some of which use CAD drawings of the detail to input the geometry. An example of a freely available software package is the LBL’s THERM which can be downloaded from the following website:

The following diagrams illustrate some of the outputs from THERM.

Modelling outputs from THERM

Modelling outputs from THERM

The criteria for undertaking such calculations is contained within BS EN ISO 10211 (BSI, 2007 and BSI, 2003). Guidance on numerical modelling techniques can also be found within BRE IP 1/06 (Ward, 2006) and BR 497 (Ward and Sanders, 2007). Alternatively, ψ values can be obtained from measurement or from an encyclopaedia (see Lowe & Bell, 2000).

If the ψ value is known, the additional heat loss through each thermal bridge can be calculated using the following formula:


where L is the length of the thermal bridge in m over which the ψ value applies.

The resultant values for  formula2  can then be added to the heat loss through each of the elements of the fabric, using the formula below, to obtain the total fabric conduction heat loss.


where U is the U-value of each element in W/m2K and A is the area of each element in m2


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