A thermal bridge is a part of the building envelope where heat escapes more easily than through the surrounding construction, usually because the insulation layer is interrupted or thinner. Common examples are wall-floor junctions, window reveals, lintels and balcony connections, and they cause both heat loss and condensation risk.
Thermal bridges are divided into repeating bridges, such as timber studs or wall ties that occur regularly and are included in the element U-value, and non-repeating bridges at junctions and openings, which are accounted for separately in the energy calculation.
Their effect goes beyond energy use. The internal surface at a thermal bridge is colder than the surrounding wall, so it is where surface condensation and mould appear first. Controlling thermal bridging is therefore a health and durability issue as much as an efficiency one.