Emissivity is an indicator of the amount of long-wave infra-red radiation which a surface (such as the façade of a building) will emit to its surroundings. According to Kirchoff's law the emissivity of a surface is equal to its radiant absorptivity at a given temperature and wavelength. It is expressed as a proportion of 1, so if a surface absorbs 40% of the long-wave infra-red radiation incident on its surface, it has an emissivity of 0.4.
All bodies which are hotter than 0°K emit thermal radiation and absorb thermal radiation. In general, the higher the temperature of a body, the lower the average wavelength of the radiation it emits. The range of terrestrial temperatures experienced within the built environment is comparatively ‘cold’ compared to the sun and so they are radiating at a much longer wavelength. This anomaly allows us to distinguish between short-wave solar radiation and long-wave infra-red radiation (terrestrial radiation).
Typically, glass is relatively transparent to short-wave solar radiation, but opaque to long-wave infra-red radiation. This is said to produce a ‘greenhouse effect’, where solar radiation enters a space, and heats it up, but the resulting long-wave infra-red radiation emitted by the hot internal surfaces is unable to escape.
However, glass has a relatively high emissivity. This means that although it does not transmit long-wave infra-red radiation incident on its surface, it does absorb it. This absorbed ‘heat’ is then re-radiated.
In cooler climates this means that long-wave infra-red radiation that builds up inside a building is reflected by the glass back into the space, rather than being absorbed by the glass and then partially re-radiated to the outside. This reduces heat loss and so the need for artificial heating.
In hotter climates, a low-e coating means that long-wave infra-red radiation outside the building is reflected back out of the building, rather than being absorbed by the glass and then partially re-radiated to the inside. This reduces the heat build-up inside the building and so the need for cooling. In hotter climates, a low-e coating might be used in conjunction with solar-control glass to reduce the amount of short-wave solar radiation entering the building.
Low-e coatings can reduce the emissivity of glazing from more than 0.8 to 0.2 or even as low as 0.04. Coatings tend to be either hard ‘pyrolytic’ coatings which are applied to molten glass, or soft ‘sputtered’ coatings, which generally need to be protected within the sealed unit.
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