Window Shading

While windows are an important and attractive design feature of a home, they are also one of the main sources of summer heat gain. The same properties that allow light to pass through glass also let the sun's heat directly into the home, increasing summer air conditioning costs. Summer heat gains tend to be greatest through windows facing West or East. South-facing windows get little direct sunlight in summer, as the sun's path is almost directly overhead, shining mainly on the roof (in winter when the sun's path is lower in the sky, South-facing windows can be a good source of free heating). Any light through North-facing windows is reflected and much "cooler" - these windows are a good source of daylighting in summer.

The more windows and glass doors a home has, especially ones facing West or East, the greater the overall heat gain and the more impact on summer air conditioning expenses. Effective shading can help minimize this heat gain, saving energy and improving indoor comfort.

The Greenhouse Effect

When sunlight passes through a window or glass door, much of its heat can be trapped inside the home, just like in a greenhouse used to grow plants (or an automobile parked in the sun in summer). Rooms with large expanses of glass may have attractive views, but in summer they can become so overheated they are virtually unusable. The simplest and most effective way to prevent this is to keep the sun out with some type of window shading.

Shading Options

The most effective way to reduce summer heat gain through windows and glass doors is to block the sunlight before it hits the glass, with some type of exterior shading. Exterior shading can block up to 95% of the sun's heat. Windows facing South can be shaded from above, with roof overhangs or awnings. For windows facing West or East, louvered shutters or other types of shades that cover the entire glass area are most effective.

Solar screens and tinted window films are effective regardless of the window's orientation. Solar screens work best when installed on the exterior, covering the entire glass area. Window tint is usually installed on the inside of the glass, to protect it from damage. Both are rated for effectiveness using a number called a Shading Coefficient (or Solar Heat Gain Coefficient), which represents the percent of the sun's heat that passes through the screen or tinted film. The lower the number, the more heat will be blocked. For example, a tinted film or solar screen with a Shading Coefficient of .45 will allow 45% of the sun's heat to pass, so it will block 55% or the heat.

Landscaping is also an effective means of shading windows and glass doors. Shade trees on East and West exposures can reduce cooling costs and prevent rooms on those sides of the home from overheating.

Unfortunately, exterior shading is not always a practical option. If not, some type of interior shading would be a good idea. When purchasing interior window shades, be sure to consider energy efficiency along with aesthetics - an attractive room is nice, but not if it overheats in the summer. Look for shading devices that block a good percentage of the sun's heat, while still allowing a reasonable amount of light to enter the room.

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