Insulating window film is thin, plastic film which you stick onto the glass. Some types are glued on, but most stick on by “static cling”. It is often confused with “window kit shrink film”, which is clear plastic film insulation that is mounted to the window frame and heated with a hair dryer to make it clear. There are different types with different advantages and disadvantages.
Heat energy flows into and out of a house through the window panes as two types of radiation: ultraviolet (UV), which is the higher frequency radiation, and infrared, which is a lower frequency radiation. The heat from the sun enters as UV radiation. It enters as direct sunlight and also as indirect sunlight. On a bright day much enters as indirect sunlight.
Heat flows through the panes in either direction as infrared radiation when the temperature is higher on one side of the glass than on the other side. It leaves the house as infrared radiation on cold days and enters on hot days.
Reflective Window Film
Reflective window film is designed to block UV radiation in the hot months to keep out the heat. There are two kinds of reflective window film: “mirror reflective” and “tinted reflective”. Mirror reflective film looks like a mirror from outdoors, but is practically transparent from inside the house. It is the best way to block UV radiation and keep the home cooler in the summer. Mount it only on the windows that the sun shines directly through. It works better on east- and west-facing walls, because the sun’s rays enter at lower angles.
Tinted reflective window film darkens the windows to increase the light that enters the home. Tinted vehicle windows have reflective windows film. Companies offer a wide range, from film that reflects very little light and is virtually invisible from inside, to film that makes the home significantly darker. Some hardware stores offer a range of these films, but only one may be in the store, the others must be ordered.
Heat Control Window Film
The label “Heat Control Window Film” is given to window film that is reflective to block UV radiation and in addition has a “low-E coating” to reduce infrared radiation. UV radiation mainly enters the house, increasing the home’s need for cooling in the summer, whereas, infrared radiation mainly transfers heat out of the house in the winter, increasing the need for heating.
Heat control window film is labeled in different ways. It may be labeled, “blocks UV rays” or “low-E coating to retain winter heat”.
Low-E coating is a thin layer of very tiny metal particles which blocks infrared radiation. Infrared radiation, as explained above, transfers energy from the warmer to the cooler side of a window. Low-E stands for “low thermal emissivity”. Emissivity is the measure of how much infrared radiation will pass through it, averaging about 30% for window films.
Heat control window films are classified by their “clarity”. Those with the minimum clarity provide the most superior insulation, but allow the least light to enter the house. They are mounted by static cling, so one can easily replace them if they make any rooms too dark. The websites of some window film manufacturers show how much a room is darkened by each of their films and give their emissivities, but they do not take into account how much sunlight enters the rooms. Some companies require that their installers put on their films.
Which Type Should You Install On Double-Glazed Windows?
Reflective window film, since it reflects sunlight, is quite effective at lowering air conditioning costs in homes with double-glazed windows. However, heat control window film is much less effective with double-glazed windows because these windows have been designed to block infrared radiation. The inner and outer surfacess of each piece of glass reflect it, and more importantly, the gas trapped between the panes.
For many other ways to insulate your windows, read How to Insulate Windows.
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