One of the seemingly simplest ways to cut peak cooling load is to reduce solar gain through windows by adding window film. While it’s an option worth considering, optimizing the results requires some attention to possible interactions with mechanical, lighting, and architectural systems.

Windows 101

Depending greatly on location and design, heat gain from the sun through windows may be a significant (over 30%) portion of a building’s peak cooling load. Solar energy may be transmitted to spaces via radiation passing through glass, and via heat radiating from the glass itself when it absorbs some of that radiation. Special films that cut such heat transfer have been available for over 30 years and have become quite sophisticated. Understanding how they work involves a short physics lesson.

The light we see from the sun is less than half the energy we receive from it. Most of it is near-infrared radiation, though a small amount (3%) is also delivered as ultraviolet radiation. Window films are able to separate out much of the near-infrared from the visible, allowing the view of the outdoors to continue even as the film reflects much of the near-infrared (and most of the ultraviolet) part of sunlight. Because the film is on the inside of the glazing, even some heat that would be re-radiated from the glass into the space is blocked. In some specialized films, an extra coating (called “low-e” for low emissivity) can re-radiate heat originating in the building back into it, cutting heating bills.

Determining Savings Not Always Simple

Solar heat gain varies with the time of day and time of year, and adding the same film on all windows may reduce it more on northern exposures than elsewhere. Depending on how a building’s HVAC systems are zoned and controlled, interactions with window film may have some unexpected benefits and consequences. On the plus side:

  • As internal loads (e.g., computers) grow, reducing solar load may help avoid the need to add peak cooling capacity while cutting energy needed for cooling.
  • As building codes require higher levels of fresh air that could increase peak heating load, low-e film may help mitigate the need for additional boiler capacity while cutting energy for heating.
  • Cutting solar load may reduce the number of hours that air handler fans having VSDs must run at full, cutting their power consumption.

  On the other hand:

  • HVAC systems (especially constant volume reheat using electric coils) may require additional zoning and thermostats, and/or changes to control setpoints and schedules, to accommodate and optimize operations with lower solar load. Otherwise, reheat may simply correct for the drop in solar load, consuming more energy.
  • If visible light levels are also reduced, automated dimming systems may raise interior lighting levels or hold them at higher levels for longer periods, consuming more power.
  • Maximizing value may necessitate different film type by exposure, resulting in sides of buildings looking different, or varying how the outside appears through adjacent windows in corner spaces.
  • In very sunny areas and with certain types of glazing, greater heat absorption by glass due to addition of film may cause glass to crack. Check with the window manufacturer (especially if a warranty is still in effect) regarding the impact of film on his product.

A Few Caveats

While the films may be relatively inexpensive, installation labor cost may be significant, depending on access, window size/shape, and other factors. The in-house cost to move or re-arrange window treatments or furniture to accommodate installation should be considered part of the total job cost. Consider installation when spaces are empty during renovation or between tenancies (though this may create a temporarily spotty exterior appearance).

Set up an occupied demo area with the chosen film to assess occupant and system impacts before committing to an entire building or exposure. In northern parts of the country, be sure the demo period includes a late winter month so any reduction in visible light is fully assessed. If part of the building is shaded (e.g., by a nearby structure), set up a demo in that space as well to assess HVAC zoning impacts.

Determining the net impact of some films may require a sophisticated analysis, such as an hour-by-hour computer simulation. Low-e film, for example may both lower heating bills and increase cooling bills. Using rules of thumb could over- or underestimate total savings.

To learn more about window films, a good starting point is the International Window Film Association (