The control or otherwise of solar radiation is an important part of building design. In a relatively hot climate it represents one of the most significant sources of potential summer heat gains. Even in a relatively cold climate, direct solar radiation can be a source of extreme local discomfort, equivalent to a 1000W electric bar radiator for every square metre of exposed window.

At the same time it represents one of the most important sources of sustainable heating energy that we have at our disposal as designers. Also, the luminous efficacy of the diffuse sky (the number of lumens of light for each Watt of heat energy introduced) makes it still the most efficient form of interior lighting, more-so then even fluorescent lights. Thus, the selection and design of a shading device is usually a compromise between heat rejection in summer and heat capture in winter, all whilst maximising the view out and the percentage of clear sky visible from it.

Figure 1 - The four fundamental shading strategies.

The use of appropriate solar controls is very important, especially within air-conditioned buildings. Proper shading greatly reduces what is essentially a needless waste of energy trying to cool a space with large areas of unprotected glazing. The diagram above indicates the four fundamental shading strategies available to you. It is essential that the designer understand the advantages and disadvantages of each in order to select and apply them correctly.

External v Internal Shades

Both external and internal shades control heat gain. External shades are more effective than internal shades because they block the solar energy before it enters the window. When using an internal shade, such as blinds or a curtain, the short-wave radiation passes through the glass and hits the shade. Depending on the colour of the shade, some percentage will be reflected straight back out the window, but the rest will be absorbed by the shade itself, effectively heating it up.

The energy from the hot blind is given off as long-wave radiation, half into the space and half from the other side back towards the window. As discussed in the greenhouse effect topic, window glass is opaque to long-wave radiation so it gets trapped between the window and the blind and ends up heating the air within this space. This heated air will tend to rise, exiting out the top and drawing in cooler air from below. This forms quite an effective thermosyphon that continually draws cool air from the bottom of the space, heats it up and pushes it out the top underneath the ceiling. Over a whole day this can significantly increase internal room temperatures. Also, as the return-air ducts of most air conditioning systems are in the ceiling, this hot air can add significantly to air- conditioning loads.

Figure 2 - The effects on internal heat flow of external versus internal shades. If user control is necessary, use blinds encased in a double-glazed unit where excess heat gains are vented to the outside.

Thus, even though internal and external shades seem to be doing the same job (protecting the occupants from solar radiation), their effect on the performance of the building is quite different. Where possible you should always use external shading devices. If you must use internal shading devices, then:

• The best option is to house them inside a double glazed unit with vents at the top and bottom of the external leaf. This isolates the heat from the blind and makes the long-wave opacity of glass work for you not against you. There are many such products commercially available.
• The next best option is to use a sealed unit that, when closed, does not allow the vertical circulation of air. This can be as simple as long curtains that extend down to and rest on the floor (retarding the entry of cool air) with a sealed pelmet at the top (retarding the exit of heated air from the top).

Comparative Shading Coefficients

The table below compares the Shading Coefficients (SC) of various shading devices. The SC is simply the fraction of solar radiation transmitted by the specified device, compared to that transmitted by an unprotected sheet of double-strength clear float. The lower the SC the less solar heat passes through, thus the more effective the device.

When deciding which devices to use and where to use them, the designer must consider a whole range of issues, whether they will be opened and closed daily as needed or just put up for the hottest season, whether they will adversely affect natural lighting level or even produce more glare, and how they might affect any natural ventilation strategy.