Windows
Why worry about windows?
Windows are wonderful, but can also be substantial energy wasters. There are two ways in which heat is lost through windows, conduction and convection. Conduction is when heat moves from the warm area of an object to the cooler area. Insulation is installed to prevent heat loss to conduction, and the glass in windows does not serve as good insulation. Convection occurs when heat is transferred through the air from warmer to cooler locations. In regards to your windows, air traveling through leaks around the glass panes and frame of your window will cause your home to lose heat.
Reducing the heat lost through your windows can be significant, and can significantly reduce your carbon footprint and energy bill.
Reducing window air leaks
It can be fairly inexpensive and simple to reduce the amount of heat your windows lose through air leaks (convection). One DIY option to improve your windows is weather stripping. There are many different materials that can be used for weather stripping, including reinforced foam, rolled or reinforced vinyl, or a tension seal. Some weather stripping materials are easier to install than others, and have varying levels of effectiveness. The U.S. Department of Energy provides a comprehensive comparison of these weather stripping materials, along with others on their website. You can also caulk windows which seals small gaps. However, caulking is generally done for stationary aspects of your home, so this is more appropriate for windows that are not regularly opened.
One other inexpensive DIY window improvement project is applying plastic over the windows. A plastic window kit can be purchased at most home improvement stores, which will include the plastic material and an adhesive to secure the plastic to your window frame. This will reduce the air leakage from these windows, but is generally not as effective as installing storm windows or weather stripping. The video below goes through a number of things you can to do yourslef to reduce air leaks, and improve its insulation.
Improving window insulation
One way that windows can lose less heat to conduction is by having better insulating properties. Most insulating materials are measured by their ability to insulate, which is signified by a numeric R-value. A higher R value means the material is a better insulator. A window’s ability to insulate, and thus not leak the heat of a home outside, is measured by its U-value, which is the inverse of an R-value. The lower the window’s U-value, also referred to as the U-factor, the more it resists heat flow and the better it insulates.
The equivalent R-value of an Energy Star® rated window is about 3, whereas the Energy Star® recommended R-value of exterior walls is 20. Energy Star® recommends a U-Factor of 0.3 for windows. However, an average single glass glazed window has a U-value of approximately 1, a double glass glazed window has a u-value of approximately 0.5, and a triple glass glazed window has a U-value of approximately 0.36. Even with efficient windows, you’re still losing a significant amount of heat to conduction when compared with most walls.
R-values and corresponding U-factors for common window types
Notice that the difference between multiple glass pane windows is fairly insubstantial. Replacing windows generally has a high upfront cost without significant improvements to the insulating qualities of your windows. Window modifications can have energy savings similar to full window replacement, and window replacement often has a payback period of over 20 years. Many affordable DIY changes to the windows are already installed in your house can be a preferable alternative. While you may want to replace your windows for other reasons, there are simpler ways to reduce heat loss.
One comparatively inexpensive alteration you can make to improve your windows is the addition of storm windows. Storm windows minimize the amount of air that moves through your windows, and can be installed on the interior or exterior of your home. In some cases, storm windows can minimally double the R-value (or halve the u-value) of single-pane and double-pane glazed windows.
Given the variety of options for improving the insulating properties of your windows, it is often a good idea to first get a free energy audit to help determine which option is best for your home. You can also visit our Smart Energy Choices site for more information on the process of a getting a free energy audit in our area.
Further Reading
“Replace Windows for Energy Savings? It’s Not That Simple” [pdf], article by Phil Cherry for Smart Energy Choices, 11/3/20.
“Windows and Doors”, from Kansas State University Extension