NOTE: This edited article is out of Buildings Industry News from several years ago.
According to the U.S. Department of Energy, 25 to 35% of energy wasted in buildings is due to inefficient windows. The California Energy Commission estimates that around 40% of a typical building’s cooling requirements are due to solar heat gain through windows. Given these facts, it’s likely that you could make big strides in energy savings by improving this exterior building element.
When it comes to making sure your windows are as energy-efficient as possible, you’ve got a few options. So that you can make the best choice for your facility, we’re comparing standard solar-control window film, Low-E window film, and Low-E window characteristics. We’re evaluating these choices based on:
- Insulating performance. During cold weather, the insulating effect of windows has a direct impact on how the interior space feels. The better the window’s insulating performance, the warmer the room feels in cold weather. According to the Efficient Windows Collaborative, when glass surface temperatures fall below 52 degrees F. is when building occupants start to notice thermal discomfort.
- Payback and ROI. When installing new windows or window film, the amount of time it will take to see a payback is important; if payback is quick, it’s more likely to be a realistic option for your facility.
- Initial cost. Upfront costs are top-of-mind for any commercial building purchase.
- Installation process. The installation method used, and the amount of time required to install new windows or window film, is a factor to take into consideration. Building occupants may need to be temporarily relocated, and parts of the building may not be accessible during certain periods of time.
- Warranty. Knowing how long you have to report problems or correct installation issues can save headaches down the road if you discover a problem with your windows or window film.
- Effect on outdoor views. Saving energy is important, but you don’t want those efforts to prevent building occupants from being able to see outside and enjoying views while they’re inside.
Standard window films (Reflective and lower-reflectance films)
Insulating Performance
U-value and emissivity values vary for standard window films, depending on the type of film, with emissivity values ranging from 0.70 to 0.81, which means that 19 to 30% of the room’s heat is reflected back into the room in the winter to help reduce heating costs.
While providing excellent savings during cooling season, these window films reduce solar gain through windows year-round, even when heat gain might be helpful (during the winter). In climates with prolonged heating seasons, these window films can increase the amount of heat needed from the building’s HVAC system. Overall, these films provide excellent net savings and an attractive ROI.
Typical ROI depends on many factors for any window film or window replacement project, such as type of film, window-to-wall ratio, type of existing glass, energy rates, type of heating and cooling system, overhangs, adjacent buildings, etc. For this article, a modern 10-story, 150,000-square-foot building was used as the model. It was evaluated for energy savings across a variety of climates based on national average energy rates using the U.S. Department of Energy’s EnergyPlus program.
Low-E window films
Insulating Performance
Low-e window film U-value and emissivity values can range, depending on the type of film. Conventional Low-E window film has an emissivity rating of 0.33, which means that 67% of the room’s heat is reflected back into the room in the winter. There are newer Low-E window films available with emissivity ratings as low as 0.07, which means that 93 percent of the room’s heat is reflected back into the space. Traditional Low-E window film improves insulating performance of existing windows by 44%; higher-performance, newer Low-E window film can improve insulating performance by as much as 92%.
Low-e films offset the loss of free solar heat during the heating season (unlike standard window films) and normally provide both cooling and heating savings. Low-e films improve window insulating performance 24/7, while only limiting solar heat gain in winter for 2-3 hours per day on sunny days (which are less frequent than in warmer months). The Low-E coating reduces heat loss in the winter; during the summer, it reduces non-solar heat gain.
ROI and Initial Cost
Payback for Low-E films ranges from 2-5 years. These films usually offer a faster ROI due to all-season performance and typically high heat rejection properties in summer. Low-e window film can be installed in most commercial spaces for between $5 and $8 per square foot. Some recently introduced high visible light transmission Low-E films (70% visible transmission) are more expensive, with installed costs of $12 to $15 per square foot. Payback figures shown do not include these high light transmission low-E films.
Low-E double-pane windows
Insulating Performance
Low-e windows have U-value and emissivity values that range from 0.02 to 0.20, which means that 80 to 98% of the room’s heat is reflected back into the room in the winter. Very low-emissivity windows offer the best improvement you can obtain in window insulating performance. Low-e windows offer the warmest glass temperatures in winter, which offers highest occupant comfort.
ROI and Initial Cost
Based on national averages from RSMeans data, removing existing dual-pane windows and replacing them with dual-pane Low-E glass costs between $40 and $55 per square foot. Adding Low-E dual pane glass offers somewhat greater energy savings than either type of window film, but at a much higher initial investment. As such, the ROI for these products tends to be in the 20- to 30-year range. Note: This is for glass replacement only, and does not include replacing window frames. If frame replacement is necessary, costs will increase and paybacks will be longer.