Last updated: October 7, 2026.

Window shopping gets confusing when every product has multiple numbers. The NFRC label is designed to make performance easier to compare across products using standardized ratings.

U-Factor

U-Factor measures heat transfer through the window. Lower numbers indicate better insulating performance. It is especially useful when comparing how well different windows resist heat loss.

Solar Heat Gain Coefficient

SHGC describes how much solar radiation passes through the window as heat. Lower values reduce solar heat gain; higher values allow more solar heat through. Climate, orientation, and shading all matter when deciding what is appropriate.

Visible Transmittance

VT measures how much visible light passes through the window. Higher values generally mean more daylight, though glass coatings and other performance goals can affect the balance.

Air Leakage

Air Leakage is another piece of the comfort equation. A strong glass package cannot compensate for a poor installation, so the product rating and installation details both matter.

Verify the Product You Were Quoted

NFRC provides a certified-products directory that can be used to verify rating data. Ask for the exact manufacturer, series, glass package, and product code rather than relying only on a showroom sample.

Related reading: How to shop for efficient windows after the federal tax credit ended · Are high-performance windows worth it? · Double-pane vs. triple-pane windows

The Bottom Line

No single rating tells the whole story. Read the label as a group of performance measurements, match them to your climate and room orientation, and compare the complete installed product.

Sources: NFRC — Window Energy Performance Ratings; NFRC Certified Products Directory.