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.
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.