What Is a Shading Coefficient?

A.M. Boyle

Generally, a shading coefficient is a measure of how well glass windows or panels are thermally insulated. This is particularly relevant in determining how well the glass is protecting the interior of a building from the heat generated by direct sunlight. The measure of the coefficient is affected by several factors, including the color, thickness, and reflectivity of the glass.

Darker colors or coatings on the glass of solar panels increase their shading coefficient.
Darker colors or coatings on the glass of solar panels increase their shading coefficient.

Solar heat can either be magnified or minimized by a glass panel, such as a window or skylight. The shading coefficient refers to the ability of a particular pane of glass to minimize solar heat as measured against a 0.13-inch (about 25-mm) thick panel of clear glass. The coefficient is usually expressed as a number between zero and one. The lower the rating, the less solar heat is transmitted through the glass, and the the greater its shading ability.

The coefficient is affected by several factors. One such factor is the color of the glass. Generally, darker colors or coatings on the glass increase the shading coefficient. This means that darker-colored or tinted windows usually allow less solar heat to be transmitted into the area being protected.

Another factor that affects the coefficient is the reflectivity of the glass. There are certain glazing materials that can be used to coat glass that give the exterior a reflective, mirrorlike quality. This means that a portion of the solar heat is reflected away from the window, thus increasing the coefficient. The thickness of the glass can also affect its ability to transmit solar heat, and often, thicker glass will have a higher shading coefficient, especially if a combination of tinting or glazing is used.

Properly measuring the shading coefficient of glass panels is particularly relevant when addressing the issue with larger buildings. Generally, a large building, such as a high-rise office tower or indoor shopping mall, has a great deal of exposed glass in its construction. There is consequently a concern regarding the affect of solar heating on interior cooling. Unless the glass panels used have a high shade coefficient, chances are that the interior of the building will become incredibly difficult to keep cool, especially during hot summer months.

There are times when a lower shading coefficient is desired. In other words, certain structures, such as greenhouses or enclosed pool areas, seek to utilize glass panels that transmit solar heat rather than block it. In such instances, glass is used that has an extremely low shading coefficient so as to accentuate the solar heat that is generated.

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