How to Read the NFRC Window Label
Every window sold for residential installation in the U.S. carries an NFRC (National Fenestration Rating Council) label. This is the label an inspector reads to verify code compliance. Knowing how to read it before purchasing can prevent costly surprises at inspection.
What's on the Label
| Metric | What It Measures | Lower Is Better? | Code Regulated? |
|---|---|---|---|
| U-Factor | Rate of heat transfer through entire window assembly | Yes — lower = better insulation | Yes — maximum set by IECC |
| SHGC | Solar heat gain through glass | In hot climates yes; cold climates no | Yes in Zones 1–3 |
| VT (Visible Transmittance) | Fraction of visible light that passes through | Higher = more natural light | No |
| AL (Air Leakage) | Rate of air infiltration through window | Yes — lower = less drafts | Maximum 0.3 cfm/ft² in most codes |
| CR (Condensation Resistance) | Resistance to condensation on interior surface | Higher = better | No |
Whole-Window vs. Center-of-Glass
This is the most important distinction to understand. The NFRC label reports whole-window performance — including the frame, sash, spacer, and glass. Center-of-glass U-factor (a number manufacturers sometimes highlight in marketing) is always lower (better) than the whole-window number because the frame and spacer degrade performance. Energy codes regulate the whole-window NFRC U-factor. Always compare whole-window numbers when evaluating windows for code compliance.
What Inspectors Check
The inspector will look at the NFRC label on the installed window and compare the U-factor and SHGC to the requirements in your building permit. Keep the labels on the windows until after the final inspection — removing them before the inspector sees them is the single most common reason for unnecessary re-inspections.
Finding NFRC Data Without the Label
If you need to verify window performance before purchase, NFRC publishes a certified product directory at nfrc.org where you can search by manufacturer and product line. This is useful for confirming that the windows your contractor is specifying meet your local code requirements before they are ordered.