THERMAL BRIDGING   

INTRODUCTION

Material contained in this section will include:

• The definition of a thermal bridge
• Types of thermal bridges (repeating, non-repeating and geometrical)
• Psi (ψ) values
• Methods of calculation (1-D, 2-D and 3-D)
• Sequencing of construction processes and examples of common occurrences of thermal bridging

Thermal bridging can have a significant impact on the thermal and energy performance of the building envelope. In the past, when dwellings were relatively poorly insulated, thermal bridging had little influence on the overall thermal performance of the building. However, as dwellings have become better insulated, the relative importance of thermal bridging has increased. In very well insulated dwellings, the proportional effect that thermal bridging can have on the thermal performance of a dwelling can be significant.

For example, in a notional semi-detached dwelling (89m² floor area) with a total fabric heat loss of just over 90W/K and a y value of 0.08 (roughly equivalent to a 2006 Part L1A compliant dwelling), then thermal bridging is likely to account for 16% of the dwellings total fabric conduction heat loss (see figure below). If no additional measures are taken to improve thermal bridging beyond this level (i.e. y value remains at 0.08) but the total fabric heat loss reduces by 25% by 2010 and 44% by 2013, then thermal bridging could account for almost 30% of the dwellings total fabric conduction heat loss by 2013.

Percentage of heat loss attributable to thermal bridging

Even when measures are taken to reduce thermal bridging, in very well insulated dwellings, thermal bridging can still account for a significant proportion of the overall fabric conduction heat loss (see figure below).

Percentage of heat loss attributable to thermal bridging

In addition to an increase in fabric conduction heat loss, thermal bridging can also result in:
• An increase in solar heat gains during the summer
• Reduction in internal surface temperatures
• Cold spots occurring within the building
• An increased risk of both surface and interstitial condensation, which may result in mould growth and pattern staining
• Reduction in indoor air quality (due to condensation and mould growth)
• Damage to building components

Although the aim, particularly in low and zero carbon dwellings, should be to try and design and construct dwellings that are thermal bridge free, it is important to realise that it is not generally possible to eliminate thermal bridging altogether. Instead, efforts should be made to minimise thermal bridging as much as is possible.