What is thermal bridging in a new house?

How it works in Ireland

Insulation only works where it is continuous. A thermal bridge is a place where something conductive, such as a concrete lintel, a steel beam, a block wall carrying through a floor or a poorly insulated window reveal, lets heat bypass it. The inner surface at that spot is colder than the wall around it, which is where condensation and mould tend to appear.

Technical Guidance Document L expects thermal bridging to be limited, and it is allowed for in the energy calculation. The calculation can use default values, which are conservative, or the designer can use values from calculated junction details, which usually improve the result. Good detailing therefore helps both comfort and the BER.

Ask your designer to show on the drawings how each key junction is insulated, and to name the three most important ones. Then check those on site before they are covered up: the lintels over openings, where the wall meets the floor slab, and where the wall meets the roof. A photograph before closing up is cheap evidence for your records.

What to ask

Before the build, ask which junctions the designer considers critical and whether the energy calculation uses default or calculated values for them. During the build, walk the site with the drawings when the lintels, floor edges and eaves are exposed, and ask your builder or certifier to confirm that insulation carries through as drawn. If you find a gap, raise it straight away, because correcting it after plasterboard and finishes are fitted is far more expensive.

A common mistake

Looking only at the thickness of insulation in the walls. The junctions are where a good wall loses its performance, and they are hard and costly to fix after plastering.