Windows and glazing

In recent years there has been rapid development in the technology of the building envelope, especially in the sphere of glass. Glazing systems are now possible which react to environmental conditions such as light and heat, yet these are merely a foretaste of things to come. Also there have been considerable advances in the thermal efficiency of glazing, with U-values now commercially better than 1.0 W/m2K. Table 5.1 shows the heat transfer characteristics of seven glazing systems.

Table 5.2 illustrates the impact of solar gain according to orientation by giving the net U-values.

Windows have many benefits, aside from the obvious. Nevertheless, they are the main weak thermal link when incorrectly specified. Discomfort arises in summer, not just from the rise in air temperature due to heat gains, but also due to the rise in radiant temperature from the glass surface itself. Radiant effects are further increased if the


Single glazing

Double glazing

Triple glazing

Double with Low E

Double with Low E and Argon

Triple with 2 Low E and 2 Argon

Double with Aerogel

Table 5.1

Comparison of typical heat transfer through different glazing options


U-value ( W/m2K) with solar gain



Single glazing Double glazing Triple glazing Double with Low E Triple with Low E


Table 5.2

Effective net U-value taking account of solar heat gain occupant experiences unshaded sunlight. In winter, cold window surfaces cool the adjacent internal air, which then falls under the buoyancy effect leading to a cold downdraught. This would also be accompanied by a cool radiant temperature. Along with the change in temperature, there may well be an asymmetric temperature field leading to greater discomfort.

As pressure has increased to improve the thermal efficiency of buildings this has forced the pace of developments in glass technology. The following are some examples.

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