A thermal bridge, also known as a cold bridge is an area of a building construction which has a significantly higher heat transfer than the surrounding materials. This is typically where there is either a break in insulation, less insulation or the insulation is penetrated by an element with higher thermal conductivity. To succeed in the latest versions of SAP, it is necessary to address thermal bridging.
Where a building is situated in a cold climate such as the UK can result in additional heat loss at these points causing significant mould growth. Moulds produce allergens (substances that can cause an allergic reaction), irritants and even toxic substances. Inhaling or touching mould spores may cause an allergic reaction, such as sneezing, runny nose, red eyes and skin rash. Mould can also be the cause of some asthma attacks and other respiratory illnesses.
As dwellings become better insulated, the importance of thermal bridging has increased. In very well insulated dwellings, the effect that thermal bridging can have on the overall thermal performance of a dwelling can be significant. Recent research undertaken has shown that thermal bridging can be responsible for up to 30% of a dwelling’s heat loss.
The heat loss associated with these thermal bridges is expressed as a linear thermal transmittance (Ψ-value) – pronounced as ‘psi-value’.
Thermal bridges occur within the building fabric where, because of the geometry or the presence of high conductivity materials, heat flows are two or three dimensional. For many situations, simple calculations are no longer sufficient to determine thermal performance correctly and it is necessary to analyse the construction using numerical modelling. A number of numerical modelling software packages are available where these specify the geometry, the materials and the boundary conditions of the model in two- or three-dimensions as appropriate.