Cold bridges are sections through the fabric of significantly lower thermal resistance than the rest of the construction. These happen particularly around openings and at junction of walls/floors and walls/roofs. Concrete and steel framed buildings are particularly prone to cold-bridging unless these elements are individually insulated. Cold-bridging is the result of localised areas of low thermal resistance caused by the presence of elements with a high thermal conductivity.
Typical example are steel frames, window frames and openings, concrete frames etc.
The result of cold-bridging is localised areas of increased heat loss/gain which can be readily identified with thermal imaging equipment and can considerably increase overall heat loss.
Cold bridging usually shows itself through so called pattern staining whereby the cold-bridge through an external frame attracts moisture as surface condensation which attracts dirt and dust. Surface condensation that gives rise to mould growth is the result of massive cold-bridging where the whole structure lacks sufficient insulation.
As thermal standards have risen so cold-bridging has become more significant, and even small elements such as roof fixings and metal cavity ties can significantly affect performance.
The effect of Cold-Bridging is to reduce the effectiveness of any insulation. Many countries allow for this in the calculation of the maximum allowable rate of heat loss (U-Value) or minimum Thermal Resistance (R Value).
This is done by averaging out the effects of the cold-bridge over the whole structure.
Losses through mortar joints in lightweight blockwork is particularly significant, as are losses through steel framed structures.
The Average U-value / R-Value takes account of all these losses and this value should be used in calculating envelope heat losses/gains during the year.
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