Thermal bridge
Also known as: cold bridge, heat bridge
Short definitionA spot that pierces or interrupts the insulation and passes heat far more easily than the panel around it. Steel columns, bolts, hanger rods, door frames and the floor–wall junction are the best known.
Steel conducts heat at about 50 W/mK, polyurethane foam at 0.022 W/mK; the gap is more than two thousand times. A single bolt running right through the panel therefore carries as much heat as a wide area around it. The result is more of a nuisance than the loss itself: sweating and mould at the outer end of the bridge, frost and icicles at the inner end.
The remedy is applied at the drawing stage. Structural steel is kept outside the room, the ceiling is hung from outside and thermal breaks are fitted to the hanger rods. In a freezer room the inner sheet is cut at the wall–floor junction and at the threshold to break the metal continuity. Pipe and cable penetrations are filled with foam and sealed on the warm side. Every hole opened later for racking or pipes needs the same care; a thermal camera shows the spots that were missed.