Modern, High-Performance Window Films

To avoid the unfortunate consequences of excessive solar heat gain, window film manufacturers have been working to develop modern products which block more heat than light, doing it by reflection of the unwanted infrared radiation contained in the solar rays,while still admitting adequate amounts of visible light, necessary to see the outside, and to provide good daylight illumination of the interior.

In order to get the right kind of window film for your window, look for the Visible Transmittance(VT) value for the film when applied to a single pane of clear glass. Then find the Solar Heat Gain Coefficient (SHGC) for the combination. If the VT value is greater than the SHGC value,you have the right kind of film for a hot climate window, but be sure to check the window manufacturer’s warranty before applying the film.The Light-to-Solar-Gain (LSG) ratio is the VT value divided by the SHGC value. The higher this ratio, the more spectrally selective the film and the better it is for hot climates. It is difficult, however, to achieve LSG values greater than about 1.2 with window films.

Soft multi layer coatings inside double pane windows, however, can achieve values as great as 1.6, providing good transmission of visible light (no dark appearance) as well as good solar heat gain rejection. In order to achieve good solar heat rejection with either a window coating or an applied window film, the SHGC value needs to be low (below approximately 0.3 to 0.5) to be effective—the lower the better. With an LSG ratio of only 1.1, for example, this means that the visible transmittance (VT) value will be about0.33 to 0.55. Low values in this range can impart a dark appearance to the glass and may be objectionable. For sun rooms with vast expanses of glass exposed to the direct sun for much of the day, however, values as low as 0.3 to 0.4 for the VT should not be objectionable, especially since these values can produce SHGC values below 0.3 to 0.4 with window films having LSG values greater than 1.0.