Application Guidance: Window Retrofit Films

Window films are usually not procured directly by the end customer. They generally require installation by trained professionals. Installers are usually a great source of information on films so it is best to consult with your installer prior to selecting films for the windows in your building. The International Window Film Association is a convenient resource for finding film dealers/installers in your area.

The most appropriate window film for a particular application will depend on the characteristics of the existing windows, façade orientation, climate, and on the needed improvements in the space (e.g., reducing glare, reducing cooling and/or heating loads).

Where reducing glare is the main goal, the most relevant characteristic of the film is the visible transmittance (Tvis). A Tvis number between 0 and 1 represents the fraction of visible light that is transmitted by the film when mounted on a piece of clear glass. A lower Tvis will generally result in a more significant glare reduction. It is important to keep in mind that reducing glare in this manner may result in an increase in energy use for electric lighting.

If the main goal is to reduce cooling loads, the most important characteristic of the film is the solar heat gain coefficient (SHGC). This number, between 0 and 1, represents the fraction of heat from the sun that will end up inside the space (rather than being redirected back to the outdoor environment). A lower SHGC will result in greater reductions in the cooling load due to solar heat.

The light/heat ratio (ratio between Tvis and SHGC) is a useful metric for the impact on cooling loads of the visible light that is transmitted through the window. A higher light/heat ratio means that a film is more effective at providing daylight per each unit of solar heat that is admitted into the space.

For cases where it is desirable to reduce heating loads, the film characteristic to keep in mind is the U-value. This number provides an indication of the ability of the film to conduct heat. The lower this number is, the greater the insulation provided by the film. Such a film reduces heat loss by reducing the emissivity of the window, where lower emissivity results in less heat transfer by thermal radiation.

Window films can generally be divided into three categories:

Standard glare reducing: this includes both pigmented (“tinted”) and reflective films. These films are the least costly and are generally appropriate where glare reduction is the main goal, without particular concern for reducing cooling loads.

Spectrally selective: these films contain layers that are able to let visible light through while reflecting the invisible wavelengths of sunlight, resulting in a lower SHGC than standard films with the same Tvis. Correspondingly, spectrally selective films tend to have a higher light/heat ratio than standard films. Spectrally selective films are very popular and are generally appropriate where it is desirable to reduce cooling loads while preserving daylight. They are available with several Tvis levels and can also provide a reduction in glare.

Low-emissivity (low-e): these films contain layers that reduce heat losses from the interior to the exterior to the building, resulting in a lower U-value than standard or spectrally selective films. Low-e films tend to be more expensive than the other types of films and require more careful installation and maintenance. In early 2022, the low-e films available in the market are also spectrally-selective. These films are recommended whenever both cooling load reductions and heating load reductions are desired.

Useful links:
Efficient windows collaborative
• The National Fenestration Rating Council maintains a directory of window film performance data