What’s your number? Window Performance

Screen Shot 2020-04-13 at 4.05.25 PM.png

By Dan Kolbert

This month’s column is about multiple numbers. That’s because it’s about windows, and there are several measurements that are important to understand when choosing windows or designing for them. They relate to how well the window insulates and how much UV radiation and visible light passes through the window.

U-factor. If you know what R-value is, you know what U-factor is, too. It’s the measure of heat flow through a material or assembly. U-factor is the inverse of R-value (if something is R-10, that means the U-factor is 1/10th or 0.1). U-factor was the original measure, but it made more intuitive sense to have a higher number be better, so R-value was introduced.

We still use U-factor for glass. The important thing when comparing windows is to understand what is being measured. Typically, the best-performing part of the window is the center of glass. At the edges, the spacers needed to hold the panes apart are more conductive and lose more heat, and the frames around the sashes and windows can be better or worse than the glass. Ask your distributor or check the manufacturer’s website for the whole-window U-factor, not just the center of glass. This will give you a much better idea of how the window will actually perform and make it easier to compare units. Double-glazed windows usually have a U-factor of around 0.3, and a good triple-glazed window can be 0.15 or lower.

Solar Heat Gain Coefficient. SHGC measures how much of the sun’s heat passes through the window (including the frame). At 0, a window lets in no solar radiation, and at 1 it lets in 100%. So, all windows have a SHGC between those two. A higher number is not necessarily better — you may overheat a room, especially on the south or west side, with too much solar heat gain. By varying the Low-E coatings, the windows for a house can be ordered with different SHGCs on different walls to maximize heat gain where it’s beneficial, without overheating.

SANDY AGRAFIOTIS PHOTOGRAPHY, COURTESY OF LOGIC WINDOWS & DOORS

Visible Transmittance.

VT is perhaps the simplest measurement — it is the amount of visible light that passes through a window (including the frame). As with SHGC, the numbers go from 0 to 1. Unlike SHGC, though, there isn’t a downside to a higher number. Since the frame is part of the measurement, the number will never be 1. The high end is around 0.7 typically.

There are other measures as well, measuring air leakage, sound attenuation, condensation resistance, design pressure, impact resistance, etc. But these are less critical, and a quality window will typically perform well in these categories.

This information should be available for any window that is rated by the National Fenestration Rating Council (NFRC). In fact, the windows themselves will have a sticker with some or all of the numbers right on them. ENERGY STAR also rates windows and qualifies different units depending on location. In the north, where heating dominates, they require lower U-values and higher SHGC Window performance numbers. In the south, where cooling dominates, it is the opposite.

A note about European windows: It is important to know that European windows may not use the same units. Your seller/ distributor should be able to help with any conversions necessary.

An excellent resource is the Efficient Windows Collaborative. Its website says, “EWC is a nonprofit, 501(c)3 organization that partners with window, door, skylight and component manufacturers, research organizations, federal, state and local government agencies, and others interested in expanding the market for high-efficiency fenestration products. Its goals are to increase market penetration of efficient window technologies and to make NFRC labeling a near-universal practice in U.S. markets.” Much of the information for this article came from their very informative website. Another excellent resource is the Department of Energy’s Lawrence Berkeley Lab’s Window & Daylighting section.