How to Read an NFRC Window Label

Every energy-rated window carries an NFRC label—a standardized report card that lets you compare products objectively. Here's how to decode it.

What Is the NFRC Label?

According to the National Fenestration Rating Council, the NFRC label is the only reliable way to compare window energy performance across different manufacturers. The ratings are determined through standardized testing in NFRC-accredited laboratories.

The Key Numbers

The NFRC label displays several ratings:

U-Factor

• **What it measures:** Heat loss (insulating ability)
• **Range:** 0.20 to 1.20
• **What you want:** Lower is better
• **Idaho target:** 0.27 or lower for ENERGY STAR

SHGC (Solar Heat Gain Coefficient)

• **What it measures:** Solar heat entering the home
• **Range:** 0 to 1
• **What you want:** Depends on orientation and climate
• **Idaho consideration:** No ENERGY STAR restriction (solar gain can help)

VT (Visible Transmittance)

• **What it measures:** How much visible light passes through
• **Range:** 0 to 1
• **What you want:** Higher = more natural light

Air Leakage (if shown)

• **What it measures:** Air passing through the window
• **Range:** Typically 0.1 to 0.3
• **What you want:** Lower is better

Which Numbers Matter Most?

According to ENERGY STAR guidance for northern climates like Idaho:

1. U-Factor: Most important—prioritize this
2. SHGC: Secondary consideration
3. VT: Personal preference for lighting
4. Air Leakage: Quality indicator

Beware Marketing Claims

The NFRC warns that manufacturers sometimes advertise "center-of-glass" values, which look better than whole-window ratings. Always compare NFRC label values, not marketing specifications.

The Bottom Line

The NFRC label is your objective comparison tool. For Idaho windows, focus on U-factor first, verify ENERGY STAR certification, and don't be swayed by marketing numbers that differ from the official label.

*For complete details on all ratings, see: [Understanding Window Energy Ratings](/guides/understanding-window-energy-ratings)*