What Do Window Ratings Mean?
Updated: Mar 6
TLDR: Window ratings are important. Choose the right window for your home. In Nashville, TN, your baseline will be a window with low U-factor (0.27-ish) and a low to moderate SHGC (0.35-ish), depending on your house design. Got lots of shade? Go for a larger SHGC.
Windows come in a vast variety of price and quality levels. On the low end, a vinyl single-paned window is about as simple as they come. On the high end, three or four panes of glass, heavy-duty seals between the panes, insulated frames, and tight latching come together in a very energy-efficient (and expensive) package:
Between those extremes, the variety in windows is staggering. Thankfully, every new window comes with a rating label, like this one:
This label helps us choose the right windows for our buildings. Where does the labelcome from? The National Fenestration Rating Council (NFRC…the window agency) does just what their name implies, rates windows in five categories:
U-Factor measures the rate of heat transfer and tells you how well the window insulates. U-factor values generally range from 0.25 to 1.25 and are measured in Btu/h·ft²·°F. The lower the U-factor, the better the window insulates. The U-factor is largely governed not by the glass in the window, but by how the windows are put together: two panes or three? Distance between panes? What’s the gas between the panes? What is the frame made of?
Solar Heat Gain Coefficient (SHGC) measures the fraction of solar energy transmitted and tells you how well the product blocks heat caused by sunlight. SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. The lower the SHGC, the less solar heat the window transmits. A solid wall has an SHGC of 0; no light gets through it. As you can imagine, SHGC is a characteristic of the glass itself.
Air Leakage (AL) measures the rate at which air passes through joints in the window. AL is measured in cubic feet of air passing through one square foot of window area per minute. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cf·m/ft². To meet the Passive House standard, ALL of the air leakage in the home must be lower than 0.6 times the volume of the house per hour. For example, let’s say you’ve got a 1,200 sq. ft. home with 9-foot ceilings. That’s 10,800 cubic feet of volume. To meet the Passive House standard, your maximum air leakage will need to be under 6,480 cubic feet per hour when the house is under a small amount of pressure. NOTE: It may be very unusual to find the AL specs on a window label. You may have to dig deeper into the technical specs on the website.
Visible Transmittance (VT) measures the amount of light the window lets through. VT is measured on a scale of 0 to 1; values generally range from 0.20 to 0.80. The higher the VT, the more light you see. This is another measurement you may not find on the window label.
Condensation Resistance measures how well the window resists water build-up. Condensation Resistance is scored on a scale from 0 to 100. The higher the condensation resistance factor, the less build-up the window allows. Again, the window label probably won’t have this information on it.
Here in Nashville, Tennessee, we get a lot of summer sun, and July and August can be particularly hot. We tend to spend a lot more money on cooling in the summer than on heating in the winter (I know I do). As such, the baseline window here should have a low U-Factor in the range of 0.27, which will help block the transfer of thermal energy all year round. The baseline SHGC will depend more on the design of the home. If you have huge, south-facing windows with little or no roof overhang to shade them from the sun, you should look for an SHGC as low as possible, say 0.35 or so. If you live in the woods or get lots of shade, a higher SHGC won’t hurt you at all. Give us a call at (615) 898-9115 and we can plan your next home, with windows appropriate for your site and design.