Why labels matter
Two windows can look identical and perform very differently. Performance labels let you compare windows on the same terms. In the U.S., many windows carry a label from the National Fenestration Rating Council, an independent organization that sets the procedures for rating window performance. The label rates the whole window, frame and glass together, not just the glass.
U-factor
U-factor measures how well the window keeps heat from passing through it. Lower numbers mean less heat loss. In cold climates, where most of the year is about keeping heat in, U-factor is usually the most important number on the label. In hot climates it still matters, because heat flows in both directions.
Solar heat gain coefficient (SHGC)
SHGC measures how much of the sun's heat comes through the window, on a scale from 0 to 1. Lower numbers mean less solar heat gets in. In hot climates, and on west- and south-facing windows that take afternoon sun, a lower SHGC can help keep rooms cooler. In cold climates, some homeowners prefer a higher SHGC on south-facing windows to let in winter sun. The right number depends on climate, orientation and shading.
Visible transmittance (VT)
VT measures how much visible light comes through, also on a scale from 0 to 1. Higher numbers mean more daylight. Some low-E coatings that block a lot of solar heat also reduce visible light slightly; if daylight matters to you, check VT alongside SHGC.
Air leakage
Air leakage measures how much air passes through the joints of the window under test conditions. Lower is better. It's an optional rating and not every label shows it. Remember it rates the window itself; air leaks around the window, between the frame and the wall, depend on the installation.
Condensation resistance
Condensation resistance is an optional rating, on a scale from 1 to 100, that indicates how well the window resists condensation forming on the inside surface. Higher numbers mean better resistance. It matters most in cold climates and in homes with high indoor humidity.
Which numbers matter most where
- Cold-winter climates. Focus on U-factor first, then condensation resistance.
- Hot-summer climates. Focus on SHGC first, especially for sun-facing windows, then U-factor.
- Mixed climates. Balance both, and consider choosing different glass for different sides of the house.
The ENERGY STAR program publishes window criteria by climate zone, which is a useful starting point.
Comparing proposals
When two contractors propose different windows, ask each for the label ratings for each window type, and compare them side by side. Ask whether the ratings are for the exact size and glass package you're buying, since ratings can vary with configuration.
What the label doesn't tell you
The label doesn't rate installation quality, durability over decades, hardware, or how the window looks. And it doesn't tell you how much energy you'll save in your house. Those depend on the contractor, the product line and your home.
Ratings and real rooms
Label ratings are measured under standard test conditions, so that every window is rated the same way. Your rooms aren't standard: shading from trees and overhangs, which way each wall faces, and how you use the room all change how a window feels. Use the ratings to compare windows with each other, and talk with the contractor about how each would work in the room you're thinking of.
Keep the labels
Contractors often remove labels during installation. Ask for them, or for the product specifications, and keep them with your warranty documents.
Questions to ask
- What are the U-factor and SHGC for each window you're proposing?
- Do the ratings match the exact glass package and size I'm buying?
- Would you choose different glass for the sunny sides of the house?
- Can I have the labels or specification sheets?