U-Factor and SHGC, Explained

U-Factor: The Heat-Loss Number

U-factor is the rate of heat transfer through the whole window assembly, glass and frame together.

It is the inverse of R-value — where R-value measures resistance (higher = better), U-factor measures conductance (lower = better). A U-factor of 0.30 means the window loses heat about twice as fast as one at 0.15. For a Massachusetts winter, U-factor is the number that matters most because the dominant energy load is keeping heat in. The Stretch Code's ≤0.27 ceiling effectively requires a quality double-pane low-E window with a warm-edge spacer; triple-pane reaches 0.15–0.20.

SHGC: The Solar Gain Number

SHGC is the fraction of the sun's heat that passes through the window, from 0 (none) to 1 (all).

A low SHGC (0.25) blocks summer sun and suits cooling-dominated climates; a higher SHGC (0.40–0.50) admits free winter solar heat. Massachusetts is heating-dominated, so a moderate-to-higher SHGC on south-facing windows can offset winter heat loss — free heat when the sun is low. East and west windows, which catch low summer sun, often benefit from a lower SHGC to limit overheating. The right SHGC is therefore orientation-dependent, not one-size-fits-all.

Reading the NFRC Label

The label standardizes four ratings so brands compare directly.

Because the ratings are independently certified by the NFRC, a 0.27 U-factor from one brand means the same as 0.27 from another — the label removes marketing from the comparison. When a salesperson quotes a window, the NFRC number is the claim that can be verified.

What the Stretch Code Requires

Adopting municipalities enforce tighter window numbers than the base code.

The Massachusetts Stretch Energy Code, in force in the large majority of municipalities, requires replacement windows to hit a U-factor of 0.27 or lower — tighter than the base 780 CMR allowance. For a window replacement to pass inspection in a Stretch Code town, the NFRC label must show that number. We confirm the rating against the town's adopted code before ordering, so the window passes the first time.

Where the Numbers Stop Mattering

A great label on a bad installation still performs poorly.

The NFRC rating describes the window unit, not the hole it goes in. A 0.27 window installed without proper flashing, air sealing, and insulation around the frame leaks heat and water around its perimeter regardless of its rating. Installation quality — square, plumb, sealed, flashed — is what turns the label number into real-world performance, which is why the install detail matters as much as the spec on the sticker.