The NFRC Label: U-Factor, SHGC, VLT, Air Leakage

The NFRC label is a third-party rating of a whole window or door, reporting U-factor, solar heat gain coefficient (SHGC), visible transmittance (VT), air leakage and condensation resistance. NFRC, the National Fenestration Rating Council, is a nonprofit that certifies those ratings so that products can be compared on the same basis. The label compares performance. It does not say whether a product is good enough for a given house.

What the label shows

The door label carries U-factor and SHGC for a specific glass configuration. The ENERGY STAR program states that every ENERGY STAR certified product carries an NFRC label with five ratings, and that U-factor and SHGC must appear on it.

U-factor

U-factor is whole-product heat flow in Btu/h-ft2-F. ENERGY STAR notes that values generally run from 0.25 to 1.25. The full explanation, including the R-value relationship and the conversion from European Uw, is in U-factor for windows.

SHGC

SHGC runs from 0 to 1; ENERGY STAR gives typical values from 0.25 to 0.80. A lower SHGC blocks more solar heat. That helps in cooling-dominated climates and costs useful free heat in cold ones, so the target depends on climate and orientation.

Visible transmittance

VT is the share of visible light that passes through, from 0 to 1. A low-SHGC coating can also lower VT, so the two numbers should be read together on glass with tints or multiple coatings.

Air leakage

Air leakage is a physical test result for the closed product, in cubic feet per minute per square foot of product. ENERGY STAR requires 0.3 cfm/ft2 or less for windows and sliding doors, and 0.5 cfm/ft2 or less for swinging doors. Air leakage is always measured, not simulated.

Condensation resistance

CR describes how well the interior surface of the product resists condensation. It is a comparative index and a higher number is better. It does not replace room humidity control; see window condensation.

How a product gets rated

NFRC's residential process runs in a fixed order:

1. The manufacturer starts the process and chooses an accredited laboratory and an inspection agency.
2. NFRC-based computer simulation determines U-factor, SHGC and VT for the product line.
3. A laboratory accredited under NFRC 701 runs physical tests under NFRC 102 that validate the simulation.
4. Air leakage is tested physically under ANSI/NFRC 400.
5. An NFRC-licensed, independent inspection agency (IA) reviews the test reports, inspects the plant and recommends certification.
6. NFRC certifies the product, lists it in the NFRC Certified Products Directory, and the product carries the label.

Who can certify

Only NFRC certifies the rating. The manufacturer initiates the process, the accredited laboratory tests, and the IA verifies that production matches what was tested. NFRC does not rank products or set minimum performance levels. ENERGY STAR then sets criteria on top of NFRC-certified ratings.

Label, label certificate and directory

The label is the sticker on a factory-built unit. The label certificate is a signed document with the same ratings, used where a sticker is impractical. Seattle's building department, for example, requires an NFRC label certificate to reach the building inspector before site-fabricated fenestration such as storefront or curtain wall is installed, and it states that it will not accept a manufacturer's own simulation reports or product literature in place of an NFRC listing number. The NFRC Certified Products Directory lists each certified product, and many jurisdictions ask for that listing number on plans.

A rating applies to the product configuration that was simulated and tested, so the label on the delivered unit should match the frame and glass package on the schedule.

What the label does not say

• It sets no minimum. A product can be certified at any performance level.
• It is not a structural rating. Wind load, water penetration and operation belong to the North American Fenestration Standard performance grade, covered in European windows and US building code.
• It covers energy and light, not durability. Hardware life, finish and glass seal durability are outside it.
• ENERGY STAR uses two numbers. Qualification rests on U-factor and SHGC only, plus an air leakage threshold.

Why European numbers are not interchangeable

A European data sheet gives Uw and Ug in W/m2K, a g-value, and often a CE declaration of performance. None of these is an NFRC rating. The two systems assess frame, spacer and glass under different environmental conditions and reference sizes, and NFRC rates the complete product in a standard size. Because the differences vary by product, a converted number can be higher or lower than the NFRC result. The comparison is explained further in European vs American windows.

The practical consequence for a US project is simple: an imported window is rated for energy code and ENERGY STAR purposes only when it appears in the NFRC directory or arrives with an NFRC label or label certificate. Where the order chain involves a profile system provider, a fabricator and an importer, the party responsible for the label is the manufacturer of the finished unit; see profile system, fabricator and installer.

How to check a label

• Read the U-factor and SHGC first, then compare them with the code or program limit for the climate zone.
• Match the label's configuration to the product on the schedule: frame, glass, spacer.
• Look up the product in the NFRC Certified Products Directory and compare the listing with the label.
• Ask for the label certificate when the product is site-built or custom.
• Reject a manufacturer's own simulation report or a CE declaration as a substitute where the local official requires NFRC ratings.