A typical dwelling built to 2006 standards loses more than 30% of its heat through junctions and around window openings by a process called ‘thermal bridging’.
For the first time in 2013, UK Building Regulations began to measure heat loss through junctions, this is measured by Y-values (also known as ‘psi’ values). The Y-value in each SAP assessment has a big effect on build materials and cost. If you don’t understand Y-values in your build, you may be paying unnecessary costs.
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| Thermographic Image Showing Heat Losses Through The External Walls |
It is important to familiarise ourselves with terms such as:
• 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.
• 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.
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| Thermographic Image Showing Heat Loss From Balconies |
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.
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| Thermal Bridging Calculations For a Steel Structure |
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.
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).
I n 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
• 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
A lthough 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.
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