Window energy ratings, U-factor and solar heat gain

Windows give a house views, daylight, fresh air and the sun's warmth in winter, but heat also moves in and out through them, which raises heating and cooling bills. Every ENERGY STAR certified window carries a label from the National Fenestration Rating Council, or NFRC, with ratings that measure its energy performance the same way for every product.

The two numbers that matter most

• U-factor. The rate at which a window lets heat flow through it, apart from sunlight. The lower the U-factor, the better the window holds heat in. It matters most in cold climates.
• Solar heat gain coefficient, or SHGC. How much of the sun's heat a window lets in. A low SHGC keeps a house cooler, which matters most in warm climates.

In a climate with both cold winters and hot summers, a window with a low U-factor and a low SHGC saves the most energy.

Whole window, not just the glass

Some ratings describe only the center of the glass. Whole-unit U-factors and SHGCs include the frame and edges too, so they reflect how the entire window performs.

What's inside an efficient window

• Two or more panes. Double-pane windows, often with high-performance glass, in place of a single pane.
• Gas fill. The space between panes can be filled with an insulating gas, which cuts heat loss in cold climates.
• Low-e coatings. Thin coatings that keep heat from radiating out in winter, or reduce the sun's heat coming in where it's warm.

Storm windows, low-e films and insulating shades are rated too. Films carry NFRC ratings, and storm windows and other window attachments are rated by the Attachments Energy Rating Council.

An annotated example

What to do next

When new windows go in, photograph the NFRC label before it's peeled off, and note each window's U-factor and SHGC in your home records. Which energy upgrades to do first covers where windows fit among other upgrades.