high-reflectivity window film
A high-reflectivity window film is a type of window film that helps block summer heat gain. High-reflectivity films are best used in climates with long cooling seasons because they also block the sun's heat in the winter.
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The effectiveness of these reflective films depends on many factors:
Silver, mirror-like films typically are more effective than the colored, more transparent ones. East- and west-facing windows, because of their greater potential for heat gain, can benefit more from these films. North-facing windows won't really benefit from them at all, and south-facing windows may benefit somewhat, but the benefit could be offset by the reduction of heat from the winter sun.
These films have some overall disadvantages:
These reflective films are available to apply yourself over existing windows. Some window manufacturers also make reflective glazing or glass.