When the ambient temperature within a building is within or above the comfort zone, any additional heating of the interior due to solar gain will result in discomfort. Architects therefor design solar shading devices to prevent this. However, at cool times of the year, it is generally desirable to allow solar radiation to pass directly into the room to provide a useful heating effect. This response – between blocking excess gain in summer vs optimizing gain in winter- can be provided either by the shading device being moveable, or by it being geometrically selective.
Solar Radiation Entering A Room Can Have Two Closely Related Thermal Effects
Radiation absorbed onto room surfaces will lead to an increase in air temperature, and mean radiant temperature, and solar radiation falling directly on to an occupant will lead to an increase in the mean radiant temperature experienced. A third important effect which is not strictly thermal is that high intensities of radiation from direct sun or even the diffuse sky can cause discomfort glare, or disability glare where an occupant’s visual performance will actually be impaired.
The function of shading is to eliminate these three effects but it is itself subject to certain other constraints. Maintenance of air flow through the building is of great importance during the cooler hours of the day in non air-conditioned buildings, there will be a need for admitting controlled levels of diffused daylight, and in most cases there will be a requirement for views out of the window.
To carry out the shading function one or more of a number of shading devices can be employed. The devices can be classified into three types. Moveable opaque can be highly effective in reducing solar gains but eliminates view and impedes air movement, e.g., roller blinds, curtain, etc. The second type, louvers, may be removable, adjustable, or fixed-affect view and air movement to some degree. Finally, fixed overhangs are easy to provide with an overhanging roof or balcony. They also give protection to walls and openings from rain with little to no effect on view and air movement.
In practice, shading devices are very varied in form and sometimes consists of combinations of the above types.
Movable Blinds or Curtains
These have limited application in non air-conditioned building since they seriously impede air flow. In air-conditioned buildings, where the out- side air temperature will be above room temperature, windows will be glazed and the question of airflow does not arise. In this case, an opaque (or translucent) blind could be considered as a means of reducing cooling load, effects of direct radiation on occupants, and glare.
The more reflective the fabric, i.e., lighter the color, the more radiant energy will be reflected back out of the building. However, many light colored fabrics also transmit quite a high proportion of radiation (typically about 15%). Because of this the blinds will look quite bright when insolated, becoming a secondary daylight source. However, this transparency has prompted some designers to specify blinds of dark colors, which is a mistake because this will lead to absorption rather than reflection and will generate heat inside the building.
Fabrics are available which have aluminized finished on one side and are preferable to ordinary fabrics. These are highly opaque but still look light colored from the inside of the building when illuminated from inside.
Geometric Shading – Overhangs and Louvers
In both these cases, obstructions are used to block only that part of the sky through which the sun passes (or that part of the sky which is sufficiently bright to cause problems). In principle this leaves the low angle view and the ingress of air, unimpeded. The traditional method of shading in the tropics is by the use of fixed overhangs together with careful consideration of orientation.
Effect of Shading on Daylighting
It is clear that the presence of a shading device must affect both the unwanted solar gains and the useful daylight. How can we evaluate this and optimize the shading design? The first step is to recognize that the need for shading does not coincide in time with the limiting day- light conditions. By this we mean that when shading is required there is a surplus of radiant energy, perhaps 10 times more than required for illumination, whereas in the limiting case of just sufficient sky luminance to provide daylight within the room, no shading is required for thermal or glare control. This suggests that if the shading is moveable, the problem is solved, provided when the shading is deployed, it still transmits sufficient light and in a useable direction, into the room.