Most of the solar energy gained by houses in summer enters through unshaded or poorly shaded windows, glass doors, skylights or other glazed apertures. This heat is then released, partly by convection (warming of the air) and partly by longwave radiation which cannot return through the glass. Thus, the rooms become hotter and hotter.
The most effective way to control solar heat gain is when the house is being designed. Proper attention should be given to the orientation and shading of the glazing at this stage. North facing glazing is preferable with a suitable eave overhang or pergola to provide complete shading in summer and maximum penetration of sunshine in winter.
Solar Energy Guide
The following figures indicate the percentage of solar energy entering a room (compared with an unshaded window taken as 100%). The lower the percentage the better the shading device (source of figures: CSIRO).
Inside Heavy Drapes
- Dark colour (fully drawn) 80%
- Medium colour (fully drawn) 50%
- White (fully drawn) 40%
- Silver (fully drawn) 35%
Inside Venetian Blinds or Holland Blinds
- Dark colour (fully closed) 85%
- Medium colour (fully closed) 65%
- White (fully closed) 55%
Outside Awning - (inclined to window)
- Medium to Dark colour (fully drawn) 25%
- White (fully drawn) 15%
Outside Blind or Adjustable Louvre
(installed parallel and close to window)
- Medium to Dark colour (fully drawn) 15%
- White (fully drawn) 10%
- White,( fully closed) 30%
Solar Film Attached to Inside of Glazing
- Partly Shading Window 55%
- Fully Shading Window 20%
(The figure of between 40 and 60% is determined by the shading coefficient. This is stated by the manufacturer. The lower the shading coefficient, the better the protection.)
Eaves can be designed to provide adequate protection during summer to glazing facing north, but even the widest eaves will not adequately protect glazing facing east or west. Glazing facing east or west should be shaded as much as possible.
The best way of reducing the heat flow through such windows is by using external sun shades and planting trees and shrubs. Internal sun shading devices provide less benefits because solar energy has already penetrated the glazing and the absorption of heat has already begun.
External Shading Devices
The most obvious way of controlling solar heat gain is to block the sun's rays before they strike the glass. External sun shading devices are designed to achieve this, and although their form is limited only by the ingenuity of the designer, they may be divided into the following types:
Designed to mask out the overhead sunshine; eg eaves, awnings, canopies, projecting horizontal fins, horizontal louvre systems, verandahs, pergolas.
Designed to mask out the horizontal sunshine, eg blinds, vertical louvre systems, projecting vertical fins.