The importance of thermal bridges

Thermal bridges are areas of increased heat flow across otherwise thermally insulating materials or constructions. They are a particular problem in building construction because it is rarely possible to provide a perfectly continuous and unbroken layer of insulation around the whole of the space which is to be insulated. In addition to the increased heat flow, a thermal bridge also has an effect on surface temperatures, creating in winter a drop in surface temperature on the inner warm side of the construction, and a rise in surface temperature on the external cold side. The lowering of the temperature on the inside brings with it the risk of surface condensation and mould growth, whereas the rise in temperature on the outside provides an opportunity for detecting the bridge by means of infrared thermography. The importance of a thermal bridge depends on a number of factors, including:

• The level of insulation which is required

• The area of the thermal bridge relative to that of the insulation material

• The thermal conductivity of the material of the bridge relative to that of the insulation material in which it occurs

• The geometry of the thermal bridge.

The three most common materials that create thermal bridges in building construction are timber, concrete and steel. When one of these materials forms a thermal bridge in an insulation layer, then, as the specified U-value is reduced, the thickness of insulation required to meet that U-value increases more and more rapidly. This effect is illustrated in Fig. 15.1, where insulation thickness is plotted against U-value for a typical construction element. In the lower curve, it is assumed that there is no thermal bridge in the construction, and that the insulation layer is perfectly continuous. In the upper curve, it is assumed that the insulation is bridged by 100 mm x 50 mm timber joists or battens. Note that the presence of the thermal bridge causes the necessary thickness of insulation to increase very rapidly as the U-value is reduced. Furthermore, the upper curve asymptotes to a minimum U-value, determined principally by the dimensions and properties of the thermal bridge, below which it is impossible to go.

This example demonstrates three things:

• At the U-values currently required for building elements, thermal bridges are highly significant

• Because of their significance, thermal bridges must either be avoided or be protected by thermal insulation

• Because a thermal bridge has such a large influence on the magnitude of the U-value of a construction element, the calculation procedure must be sufficiently sophisticated to take proper account of the bridge - otherwise the calculated U-value will be wrong.

The importance of adopting an appropriate calculation procedure for a construction element arises because the majority of simple methods tend to underestimate the effect of the bridge. A simple method will often give a result that is less than the correct U-value, giving the impression that the element is better than it really is. It also follows that when the U-value of a traditional construction element is re-calculated using currently recommended procedures, the new U-value is often found to be higher than the previously accepted value.

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