How to Read the NFRC Window Label

Every window rated by the National Fenestration Rating Council carries a label with standardized numbers, so you can compare products directly. The NFRC lists five ratings, U-Factor, SHGC, Visible Transmittance, Air Leakage, and Condensation Resistance, each on its own scale. This guide walks through the label rating by rating, with every figure tied to its source.

U-Factor: insulation

The NFRC window ratings guide describes U-Factor as a mandatory rating that measures the amount of heat inside your home that can escape through a window, with a rating range of 0.10 to 2.00.

The same guide states that a lower number means less heat loss. U-Factor is the rating to prioritize in heating-dominated climates, where keeping heat inside is the goal.

SHGC: solar heat gain

The NFRC window ratings guide describes SHGC as a mandatory rating that measures the amount of solar heat that can enter a home through a window, with a rating range of 0.00 to 1.00.

The same guide states that a lower number means less heat gain. SHGC is the rating to prioritize in cooling-dominated climates, where blocking solar heat is the goal.

Visible Transmittance: natural light

The NFRC window ratings guide describes Visible Transmittance as a mandatory rating that measures the amount of solar light that can enter a home through a window, with a rating range of 0.00 to 1.00.

The same guide states that a higher number means more natural light. Unlike the heat ratings, a higher VT is generally desirable, though it trades off against solar heat control in very sunny climates.

Air Leakage and Condensation Resistance

Two optional ratings round out the label. The NFRC window ratings guide describes Air Leakage as an optional rating for the volume of air that can infiltrate a home through a window, with a range around 0.3, where a lower number means less air leakage.

The same guide describes Condensation Resistance as an optional rating for a window's ability to prevent moisture on the interior, rated 1 to 100, where a higher rating means less condensation. Because both are optional, they do not appear on every label.

Using the label with ENERGY STAR

The label numbers connect directly to certification. ENERGY STAR requires that windows meet U-Factor and, where applicable, SHGC requirements based on climate zone, and directs buyers to determine their climate zone first.

ENERGY STAR reports that replacing old single-pane windows with certified windows lowers household energy bills by an average of up to 13 percent nationwide. Reading the NFRC label is how you confirm a window meets the U-Factor and SHGC thresholds for your zone before you buy.