Thermal bridge (Wärmebrücke)

Definition

Thermal bridges are weak points in the building envelope where heat is locally conducted to the outside more strongly than through the surrounding regular surfaces. A distinction is made between geometric thermal bridges (such as external corners, cantilevers and bay windows), where the outer surface is larger than the inner surface, and constructional thermal bridges, which result from different material conductivities — for example an uninsulated reinforced-concrete lintel within an otherwise masonry external wall. Linear thermal bridges are evaluated via the Ψ value (Psi value) in watts per metre and kelvin (W/(m·K)). Point thermal bridges use the χ value (Chi value). In KfW funding a flat-rate thermal bridge surcharge of 0.10 W/(m²·K) can be applied — or, with detailed proof, better values. The exact calculation follows DIN 4108 supplement 2 or uses thermal simulation.

When is the term used?

In renovation practice, thermal bridges are doubly relevant: from an energy and a building physics perspective. Energetically they raise the heating demand and can tip the calculation for higher Efficiency House standards (EH 55/40). From a building physics perspective the internal surface temperature drops so far that relative humidity condenses — mould growth becomes likely. In higher-tier KfW renovations a detailed thermal bridge calculation is often required. In Munich buildings from the 1960s and 70s, reinforced-concrete floor slabs and cantilevered balcony slabs are frequently critical thermal bridges. Solutions include thermal break elements (insulation baskets) or interior insulation with flank insulation.

Example