Buildings loose heat to the surroundings by a combination of air leakage and thermal conduction through the building envelope, including through the ground floor. Air leakage or infiltration has been addressed over the past decade by the introduction of pressure testing which ensures that ever more stringent air tightness standards are met by paying attention to sealing details at junctions and across porous wall constructions. Conductive heat losses have been subject to the same tightening of standards, primarily by the progressive lowering of U-values.
Conductive losses through the building fabric can be split into two categories:
a) Plane heat losses: through the main elements of the building fabric (roof, walls, windows and floor). The U-value (W/m2K) of a construction multiplied by the area of that construction gives the heat loss in (W/K).
b) Thermal bridge heat losses: through corners, junctions, and structural elements penetrating the insulation layer.
The relative importance of each mode of heat loss for current new build properties in the UK very much depends upon the type of building under consideration and the level of performance being aimed at. In all cases, thermal bridge heat losses are responsible for an increasing percentage of the overall building heat loss as airtightness and fabric U-values have been improved in UK Building Regulations. For detached housing, it is common for thermal bridges to account for 30-50% of conductive losses, as calculated by thermal modelling. For multi-residential projects (apartments) this figure could be 20-30%, and balcony connections can be a major contributor to the total thermal bridge heat loss if effective thermal isolation is not included in the design.