Ever since the subject of heat transmission through windows first began to be seriously investigated, it has been recognized that the externally shaded window has a great advantage in summer over the sun-exposed window in terms of comfort within a room. The heat-trap effect that everyone observes in a closed car left in the sun, even in winter, makes it obvious that clear glass is, in effect, a one-way valve for solar radiation and that the quantity of heat transmitted and imprisoned is very substantial. In the 1930s, awning tests made by ASHRAE (then ASHVE) at the University of Illinois and at Pittsburgh established rates of heat flow for various solar positions and intensities are demonstrated that shading the glass on the outside was extremely effective in reducing the total solar heat entering an enclosed space on a hot day. More elaborate studies over the last 40 years have confirmed and detailed those facts of shading that were first widely recognized in the 1930s.

These findings assumed progressively greater importance as air conditioning developed from its primitive state. However, in spite of general recognition that external shading of sunlit glass is highly effective in reducing both original and operating costs of air conditioning, the real benefits of shading have been largely ignored. It has often been easier for the architect, the engineer and the building owner simply to make use of energy in the form of electrical or absorption air comitioning to cancel out the solar heat entering through the building's fenestration. Admittedly, this was a wasteful practice in terms of energy consumption, but energy was plentiful and inexpensive.

It is not surprising that this situation has now changed. The energy conservation possibilities in the shading of fenestration have suddenly become enormously important. Properly exploited, they can often provide both old buildings and new with a major means for helping to meet the energy budget applied to the building without diminishing the occupants' comfort.

This paper explores various ways of external shading.