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Window Films
For existing buildings, many of the solar control benefits of tinted, reflective and low-e glass are available by applying window films to existing windows. Window films are typically thin layers of polyester film that can either be clear, tinted or reflective. Some films use thin coatings similar to those used in low-e windows. Films can reduce solar heat gain by as much as two thirds, and some films will provide good solar control while allowing good visible light transmission. Quality films also reduce fading of fabrics and provide some safety from broken glass. Most manufacturers offer guarantees that the film will last for at least five years. The following table presents several examples of the performance characteristics of window films applied to clear glass. Films are available from manufacturers that can meet a wide range of solar control and visible light design criteria.
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Example Performance of Clear Glass With and Without Window Films |
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|
Glazing Type |
Shading Coefficient |
Solar Heat Gain Coefficient |
Visible Light Transmittance |
|
Single-pane ¼" glazing |
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|
Clear glass |
1.00 |
0.86 |
90% |
|
Clear with tinted film |
0.50 |
0.43 |
48% |
|
Clear with reflective film |
0.29 |
0.25 |
15% |
|
Clear with spectrally selective film |
0.51 |
0.44 |
69% |
|
Double-pane ¼" glazing |
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|
Clear glass |
0.87 |
0.75 |
81% |
|
Clear with tinted film |
0.56 |
0.48 |
43% |
|
Clear with reflective film |
0.42 |
0.36 |
16% |
|
Clear with spectrally selective film |
0.58 |
0.50 |
62% |
The amount of radiant solar energy that enters a building depends on the type of glazing, any surface treatments of the glass, the window’s orientation and exterior or interior shading. For all of these reasons, cooling load reduction and energy savings will vary significantly depending on the specific application. Manufacturers of window films often have computer programs available to help estimate savings for specific cases. In general, clear east and west windows will gain the most heat in the summer. In the winter, the south windows will gain the most. If your building has unshaded clear glazing on the east, west or south sides, you may want to consider adding window film. In order to take advantage of natural daylight, consider using film that allows plenty of natural light to enter (high VLT). For reducing cooling costs, priority should be given to west or east facing windows. South facing windows are the second priority. North windows are the lowest priority from an energy and comfort point of view, although it may be necessary to include north windows in a window film retrofit for aesthetic and appearance reasons.