A thermal bridge occurs when heat or cold transfers across an external surface of a building. This can cause heat to escape from the building or cold to enter. Thermal bridging occurs when a thermally conductive material (ie a material with low resistance to heat flow) can penetrate or bypass the insulation layer. For example, insulation is often placed between timber joists in roofs, however the joists themselves may conduct heat and reduce the effectiveness of the insulation by acting as a thermal bridge along which heat can be transferred out and lost. Thermal bridging can greatly reduce the effectiveness of insulation, so it's crucial to minimise thermal bridges during the design of a new build or refurbishment.