A thermal bridge occurs when the insulation envelope of a building is penetrated by a component made from a non-insulating or thermally conductive material. Heat will be transferred at a greater rate through this bridge than the heat in surrounding areas. This compromises the expected performance of the insulating material and makes for uneven surface temperatures. A thermal bridge is sometimes referred to as a cold bridge.
There are a number of locations within a building where a thermal bridge is likely to occur. The corners and junctions of windows and doors, the places where walls and roof meet, and the interfaces between walls are particularly prone to this issue. Structural elements that are assembled during construction might produce a thermal bridge because of the components used in joining the elements. Ventilation devices and balconies may also penetrate insulation layers.
The effects of a thermal bridge are, in general, inversely related to the quality of the insulation and energy performance of a building. The relative amount of thermal exchange due to bridging is comparatively low in buildings where thermal protection was not an important factor in design or was poorly implemented. In modern buildings, where a premium is placed on the efficient use of energy, thermal bridges can be the primary factor in unwanted heat transfer.
Heat exchange resulting from a thermal bridge accounts for more concerns than just the efficiency of interior climate control. Lower internal surface temperatures might be a consequence. This can lead to condensation problems in corners and at floor and wall junctions, particularly in humid climates. The accumulation of moisture can directly lead to mold growth and may be unnoticed if occurring on non-exposed surfaces.
Design and construction practices have been developed to minimize heat transfer due to thermal bridging. Care is taken that wall ties and the mechanical joiners of structural elements do not penetrate a cavity wall or insulation envelope. Extension of the floor slab beyond the envelope, exposed lentils, and the use of concrete or steel members that are combined into exterior walls can be avoided.The junction of two planes, such as walls and corners, can be made more thermally reliable by choice of material and assembly methods.
The use of infrared imaging to detect the uneven surface temperatures indicative of a thermal bridge has become common. Three-dimensional computer analysis of heat flow in building construction elements has become a powerful tool as well. This software expands on the on the one-dimensional models that have usually been used to estimate the thermal properties of buildings.