Low-e double-pane windows
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.
When new windows are being installed in your commercial building, building occupants in the area need to be relocated. The time it takes to install new windows depends on the condition of the existing windows being removed, how many windows you’re replacing, and what’s being replaced (a complete tear out vs. retaining window frames, etc.). For each window, you can anticipate between 20 and 60 minutes of installation time, depending on the type and size of the window. Depending on the time of year, parts of your facility may not be accessible while old windows are being removed and new windows are being replaced.
Warranties for low-e glass as part of a retrofit project are typically 10 years.
Effect on Occupants and Outdoor Views
Similar to newer low-e window films, these types of windows are designed to be spectrally selective and reflect infrared light while allowing visible light to pass through. This allows natural lighting into a space without the extra heat passing through the windows into the conditioned space. Newer low-e windows do provide the solar control of moderate to higher performance window films, but with much less visible light reflectance and more visible light transmission. Sometimes, however, the extra visible light can cause issues with too much glare.
Whether you choose to replace your building’s windows or install window film on existing windows, any of these options is a smart move toward an energy- and cost-efficient facility. After evaluating the differences in insulating performance, ROI and initial costs, installation processes, warranties, and the effect on occupants, you should be able to make an educated decision about which option is right for your building environment. Any improvement you make to the insulating performance of your windows is a step in the right direction.
Steve DeBusk has 27 years of experience in the energy-efficiency business. His primary focus has been working with ESCOs, energy management companies, consultants, and contractors in the development of energy-efficiency project opportunities. DeBusk is a Certified Energy Manager, a Certified Measurement and Verification Professional, and a Certified Sustainable Development Professional through the Association of Energy Engineers.