What numbers are on an NFRC label?

U-factor (heat-loss rate, lower is better), Solar Heat Gain Coefficient (fraction of solar heat admitted), Visible Transmittance (fraction of visible light), Air Leakage where shown, and Condensation Resistance. U-factor and SHGC are the two that drive energy decisions.

What is the difference between whole-window and center-of-glass ratings?

The NFRC label rates the whole assembly — frame, glass and spacer — which is the honest number. Center-of-glass figures quoted in brochures look better because they exclude the frame. Always compare whole-window to whole-window.

What are default versus certified NFRC ratings?

Certified ratings are tested for that exact product and are the ones to trust for comparison. Default ratings are conservative stand-ins the code allows when a product has not been individually tested.

Why does Visible Transmittance matter?

Some aggressive low-SHGC coatings also cut visible light, so a window tuned hard for a hot climate can leave a room dimmer. If daylight matters, check VT alongside SHGC instead of chasing the lowest SHGC alone.

How to read an NFRC label: U-factor, SHGC, VT and CR

The white NFRC sticker is the one truly comparable, third-party spec you get. Learn the five numbers — U-factor, SHGC, VT, air leakage and condensation resistance — and how to read them like for like.

By Francesco Zinghinì

Every certified window carries a National Fenestration Rating Council (NFRC) label — the white sticker with a grid of numbers. It is the one truly comparable, third-party spec you get, because every manufacturer measures the same way. Learning to read it turns a showroom full of marketing into an apples-to-apples comparison. There are five numbers that matter, and each answers a different question.

The five numbers

U-Factor — how fast the window loses heat by conduction. Lower is better. Typical whole-window values run about 1.20 (bare aluminum single pane) down to 0.15–0.18 (triple-pane low-E). This is the number codes and ENERGY STAR regulate.
Solar Heat Gain Coefficient (SHGC) — the fraction of solar heat that gets through, 0 to 1. Lower keeps summer heat out; higher lets winter sun in. The right value depends on your climate.
Visible Transmittance (VT) — the fraction of visible light that passes through, 0 to 1. Higher means a brighter room.
Air Leakage (AL) — how much air passes through the assembly, lower being tighter. Not every label shows it, and it is capped by standards.
Condensation Resistance (CR) — how well the window resists interior fogging, roughly 0 to 100, higher being better.

Reading U-factor and SHGC together

The first two numbers are the heart of the label. U-factor is a winter number, SHGC is a summer number (mostly), and a good window balances them for your climate. Convert U-factor to the R-value you may be more used to with R = 1 ÷ U: a U 0.30 window is R 3.33, using the converter. Then read SHGC against your climate goal with the SHGC reference. A South-Central buyer wants both numbers low — say U 0.28 and SHGC 0.23 — to fight cooling load. A Northern buyer wants a low U (0.22 or below) but is happy with a higher SHGC to harvest winter sun. The ENERGY STAR climate-zone checker tells you whether a given U and SHGC pair actually qualifies where you live.

Certified versus default ratings

One subtlety trips people up: an NFRC label can show either certified ratings, tested for that exact product, or default ratings, which are conservative stand-ins the code allows when a product has not been individually tested. Certified numbers are the ones you trust for comparison. Also beware center-of-glass figures quoted in brochures — the NFRC label is the honest whole-window number, which includes the frame and spacer and is always a bit worse than center-of-glass. Compare label to label, whole-window to whole-window.

VT, AL and CR — the supporting cast

VT matters more than people expect. Some aggressive low-SHGC coatings also cut visible light, so a window tuned hard for a hot climate can leave a room noticeably dimmer. If daylight matters to you, check VT alongside SHGC rather than chasing the lowest SHGC blindly — the VT and condensation reference shows typical ranges. Condensation Resistance is worth a look if you have fought foggy windows before; a higher CR and a warm-edge spacer help keep the glass edges above the dew point in winter. Air Leakage, where shown, is a tightness spec; casements and awnings, which seal by compression, generally beat sliders and double-hungs here.

How the label connects to the rest of your decision

The label rates performance, not value. Two windows with identical U and SHGC can cost very differently depending on frame material and construction — that is a separate axis covered in the frame material guide. And the label says nothing about whether the window will save you money, which depends on your loads and prices; the energy-savings estimator and the payback guide handle that. Use the NFRC label for what it is uniquely good at: a neutral, standardized spec that lets you rank products on performance before price and marketing enter the room.

Confirm the label on the unit you buy

Finally, read the label on the actual product line and glass package you are quoting — not a floor sample with a different coating. Glass options change U-factor and SHGC substantially, and a salesperson quoting the best-case package while pricing a base one is an easy way to be misled. The values in our tables are labeled planning typicals; the numbers that count are the certified ones on the window that shows up on the truck.