Glass standards
NFRC

NFRC ratings explained

Every manufacturer swears their window is high performance, so who do you believe? NFRC is the neutral referee: one standardized label, tested the same way for everyone, so a U-factor from Brand A actually means the same thing as a U-factor from Brand B. Learn to read that little sticker and you can compare products honestly and prove code compliance in one glance.

6 min read Foundational Energy code critical

What NFRC is

The National Fenestration Rating Council (NFRC) is a non-profit organization that administers a performance certification and labeling system for windows, doors, and skylight products. NFRC ratings are required by the IECC (International Energy Conservation Code) and referenced by ASHRAE 90.1 as the basis for energy code compliance for fenestration products sold in the US.

NFRC certification involves third-party simulation (using validated software, primarily WINDOW from Lawrence Berkeley National Laboratory) and third-party inspection to confirm that labeled products match the models. Manufacturers cannot self-certify.

NFRC label, typical certified values
Example: High-performance double-pane IGU in commercial curtainwall
U-Factor
0.28
Btu/h·ft²·°F
SHGC
0.23
dimensionless
VT
0.48
dimensionless
CR
62
condensation resistance

The five NFRC metrics

MetricWhat it measuresDirectionTypical code requirement
U-FactorRate of heat flow through the entire assembly (Btu/h·ft²·°F)Lower = better insulation≤0.30–0.40 depending on climate zone
SHGCFraction of solar energy that enters as heat (0–1)Lower = less cooling load≤0.25 in hot climates; varies by zone
VTVisible light transmittance (0–1)Higher = more daylight≥0.27 minimum in ASHRAE 90.1
ALAir leakage (cfm/ft² at 25 mph wind pressure)Lower = less infiltration≤0.3 cfm/ft² in most codes
CRCondensation resistance (0–100 index)Higher = more condensation resistantNo mandatory minimum; specifier discretion

Whole-window vs. center-of-glass

NFRC ratings are whole-product values, they include the combined effect of the glass, the frame, the edge-of-glass zone, and the spacer. This is critical to understand when comparing product specifications:

  • A glass manufacturer's data sheet might report a center-of-glass (COG) U-factor of 0.21 Btu/h·ft²·°F for an IGU.
  • The same IGU installed in an aluminum curtainwall frame with thermal break might carry a whole-window NFRC U-factor of 0.33.
  • The difference is real: aluminum frames conduct heat rapidly, and the edge of glass is thermally degraded by the spacer and frame.

Energy code compliance must always use whole-window NFRC values. Specifying a product based on COG performance data and then claiming compliance to code using those numbers is incorrect.

Curtainwall and storefront
For curtainwall and storefront systems, NFRC requires the entire system (framing + IGU) to be rated as a unit for the specific configuration tested. A glazing contractor cannot combine a high-performance IGU with an arbitrary frame and claim the rated U-factor applies, the frame thermal performance is part of the system rating.

Finding NFRC data

NFRC maintains a public Certified Products Directory (CPD) where specifiers can look up any NFRC-certified product by manufacturer, product line, and configuration. Values in the CPD are binding, they represent what the manufacturer has certified and what can be submitted to building departments for energy code compliance documentation.

Simulation tools
Architects and engineers can use WINDOW (free from Lawrence Berkeley National Laboratory) to simulate custom IGU configurations and generate COG thermal performance data. THERM (also from LBNL) extends the analysis to include frame and edge-of-glass effects. These tools are used by NFRC-accredited laboratories to generate the certified values in the CPD.
Disclaimer: NFRC procedures and requirements are updated periodically. Always verify current requirements with the NFRC and applicable energy code for your jurisdiction.