Glass and Solar Radiation

Solar Radiation

Not to be confused with insulation, insolation refers to the incident solar radiation that reaches the Earth from the Sun. At an average distance of 150 million kilometres from the Sun, the outer atmosphere of the Earth receives approximately 1353 W/m² of insolation (NASA 1971).

This varies by around +/- 2% due to fluctuations in emissions from the Sun itself as well as by +/- 3.5% due to seasonal variations in distance and solar altitude.

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Glass and Wind Loading

The nature of wind loads

The exact effects of wind acting on a building or elements of a building are difficult to predict because wind is very rarely constant in terms of magnitude, duration or direction. For this reason it can never be precisely predicted what influence the wind will have on building elements such as glass panes at any one particular moment in time.

Research into wind speeds in many countries has produced measured data which can be used for the prediction of wind speeds which can reasonably be assumed to be the highest that can be expected over a long period of time. It can therefore be statistically predicted that over a period of time the risk that the wind speed will be greater than a specific value will be acceptably low. This is often referred to as the basic wind speed.

Albeit that the highest basic wind speed can be predicted, this should not simply be used for design purposes because several other local factors need to be taken into account.

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Glass and Solar Energy

Glass transmits solar radiation from the Sun by three mechanisms, reflection, transmission and absorption, which for solar control purposes are defined in terms of the following parameters:

Direct Solar Energy Transmittance (Te) is the proportion of solar radiation at near normal incidence that is transmitted directly through the glass.

Solar Energy Reflectance (Re) is the proportion of solar radiation at near normal incidence that is reflected by the glass back into the atmosphere.

Solar Energy Absorptance (Ae) is the proportion of solar radiation at near normal incidence that is absorbed by the glass.

Total Solar Energy Transmittance (TET) also referred to as the Solar Factor (SF) or g Value in Europe, or as the Solar Heat Gain Coefficient (SHGC) in the USA, is the proportion of solar radiation at near normal incidence that is transferred through the glazing by all means. It is composed of the direct transmittance, also known as the short wave component and the part of the absorptance that is dissipated inwards by long wave radiation and convection, known as the long wave component.

Low Emissivity Glass and Condensation

Condensation will form on any surface as soon as the surface temperature falls below the dew point of the air. External condensation will only occur on cloud free nights when there is little or no wind and usually when a ‘warm front’ follows a cold dry spell.

As gardeners know, the air temperature in their garden can vary on any day or night from one part of the garden to another. A hedge, a shrub, an open flowerbed or a projecting wall or garage can all affect the air temperature in their close proximity.

It is a combination of weather conditions and local microclimate, which can contribute to the formation of external condensation. On occasions and as a result of the local conditions it is possible to see clear and condensed windows in the same home.

However, as would be expected the formation of external condensation on glazing is also affected by the thermal insulation performance of the glass.

The thermal insulation provided by single glazing is very poor and heat from the home passes through the glass and escapes to the outside world.

As a result of this the external surface of a single glazed window is warmer than the dew point of the outside air and this prohibits the formation of condensation on the glass.

With ordinary double-glazing the level of insulation is improved. However, sufficient heat still escapes through the glass to warm the external pane sufficiently to prohibit the formation of condensation.

Low emissivity glass works differently to ordinary glass. The low emissivity coating reflects heat back into the room and as a result the amount of heat passing through the glazing is greatly reduced.

The external pane of a double glazing unit containing low emissivity glass is not warmed up by escaping heat and therefore presents a colder surface to the outside environment.

When the glass surface temperature is lower than the dew point of air and conditions are comparable to those mentioned earlier, condensation could form on the external glass surface.

Unfortunately, it is not possible to quantify the number of occasions when external condensation will occur because nobody can predict the coincidence of still air and clear night skies. However, it will be a relatively rare and transient occurrence.