U-factor, SHGC, visible transmittance and air leakage: what each number on an NFRC label means, and how to check it against your energy code.
Key takeaways
- NFRC ratings describe the whole product, frame and glass together, at a standard size, so they allow fair comparisons between manufacturers.
- Lower U-factor means less heat flow; lower SHGC means less solar heat gain; higher VT means more daylight.
- Air leakage and condensation resistance are optional label items, but codes commonly set air leakage limits.
- Match ratings to the energy code and climate zone that apply to your project, and confirm the label covers the exact configuration ordered.
An NFRC label reports the energy performance of a complete window, door or skylight, frame and glass together, as rated under the National Fenestration Rating Council's procedures. The key numbers are U-factor (heat loss), Solar Heat Gain Coefficient (solar heat gain) and Visible Transmittance (daylight), with air leakage and condensation resistance shown when rated. For imported products, the label is how inspectors, energy raters and buyers in the United States verify that a unit meets the energy code.
What is the NFRC and why does it matter for imports?
The NFRC is a non-profit organization that sets uniform procedures for rating fenestration energy performance. It does not set minimum performance levels. Building energy codes, such as the International Energy Conservation Code (IECC) and state codes, and programs such as ENERGY STAR use NFRC ratings to define requirements. Because the procedures are standardized, a U-factor from one certified manufacturer can be compared with another's, regardless of where the product is built.
Under the NFRC certification program, a manufacturer works with an accredited simulation laboratory, an accredited test laboratory and an independent certification and inspection agency. Ratings are calculated by computer simulation and validated by physical testing, and the manufacturer's production is subject to inspection. Products are then labeled with a temporary label on the glass and, in many cases, a permanent mark.
Keep the temporary label on the glass until the building inspector has seen it; removing labels early is a common cause of failed energy inspections. If a label is lost, the certification agency's directory and the manufacturer's certificate can help, but your inspector decides what is acceptable. For large projects, provide the energy consultant with a list of product lines and rated configurations at submittal stage, so questions are resolved before installation rather than at final inspection.
U-factor: how much heat does the window lose?
U-factor measures the rate of heat transfer through the whole product. In the US it is expressed in Btu/hr·ft²·°F. Lower is better. Typical modern double glazed units with Low-E and gas fill, in a thermally broken frame, generally rate well below older single glazed products, and triple glazing goes lower still.
Key points:
- NFRC U-factor is a whole-product value, including frame, edge-of-glass and center-of-glass effects. It will always be higher than the center-of-glass value often quoted on glass data sheets.
- To convert to the SI unit used in Canada, Europe, Australia and New Zealand, multiply by about 5.678 to get W/m²·K. A U-factor of 0.30 is about 1.70 W/m²·K.
- Different standards use different boundary conditions and standard sizes, so a European Uw value and an NFRC U-factor for the same window are not directly interchangeable.
Solar Heat Gain Coefficient (SHGC)
SHGC is the fraction of incident solar radiation admitted through the window, both transmitted directly and absorbed and re-radiated inward. It ranges from 0 to 1. A lower SHGC means less solar heat gain, which reduces cooling loads in hot climates. In heating-dominated climates, a moderate SHGC on south-facing glazing can provide useful passive solar gain. SHGC is controlled mainly by the Low-E coating type and its position in the insulating glass unit.
Visible Transmittance (VT)
VT is the fraction of visible light transmitted through the whole product, from 0 to 1. Higher VT means more daylight. Because VT is a whole-product value, it is reduced by the frame area as well as by the glass. Solar control coatings that cut SHGC also tend to reduce VT. The ratio of VT to SHGC, sometimes called the light-to-solar-gain ratio, indicates how selectively a coating admits light while rejecting heat.
Air leakage and condensation resistance
Air leakage (AL) is reported in cubic feet per minute per square foot of product area (cfm/ft²). Lower means a tighter product. Many US energy codes cap air leakage for windows and sliding doors at 0.3 cfm/ft² and for swinging doors at 0.5 cfm/ft², tested in accordance with recognized standards; check the edition your jurisdiction has adopted.
Condensation Resistance (CR) is a rating from 1 to 100 that indicates how well a product resists interior condensation. Higher is better. It is optional on the label, but useful when comparing products for cold climates or humid interiors.
NFRC label at a glance
| Rating | Measures | Range or units | Better is |
|---|---|---|---|
| U-factor | Heat transfer through the whole product | Btu/hr·ft²·°F | Lower |
| SHGC | Solar heat admitted | 0 to 1 | Lower in hot climates; depends on design in cold ones |
| VT | Visible light transmitted | 0 to 1 | Higher for daylight |
| Air leakage | Air infiltration | cfm/ft² | Lower |
| Condensation resistance | Resistance to interior condensation | 1 to 100 | Higher |
How to check a label against your project
- Identify the code and climate zone. The IECC and state codes set maximum U-factor and, in many zones, maximum SHGC. Your energy consultant or plan reviewer will confirm the applicable edition.
- Check the product line and configuration. The label identifies the manufacturer, product series and the specific configuration rated, such as frame type, glazing and gas fill. Make sure it matches what you ordered.
- Compare whole-product numbers. Do not compare an NFRC whole-product U-factor with a center-of-glass number from another supplier.
- Look beyond energy. The NFRC label does not show structural performance. Design pressure and water resistance come from separate testing, such as to AAMA/WDMA/CSA 101/I.S.2/A440.
- Consider ENERGY STAR. If your project or incentive program requires ENERGY STAR, check the current version's criteria for your climate zone.
Example: comparing two window quotes
Suppose two suppliers quote a casement window for a mixed climate. Supplier A states "U-value 1.1" from a glass data sheet. Supplier B provides an NFRC-rated whole-product U-factor of 0.28 for the casement configuration with a specified Low-E coating and argon fill. These numbers cannot be compared. Supplier A's figure is a center-of-glass value in W/m²·K; converted, it is roughly 0.19 Btu/hr·ft²·°F, but the whole window, including the aluminum frame and spacer, will perform worse than that. Ask Supplier A for a whole-product rating under the same procedure before making a decision.
The same principle applies to SHGC and VT. A glass-only SHGC is higher than the whole-product SHGC because the frame blocks some solar radiation. Compare like with like, ideally whole-product ratings from certified sources.
Doors, custom sizes and mulled units
NFRC ratings are produced at a standard size for each product type so that comparisons are consistent. A project unit of a different size will perform slightly differently, but codes generally accept the rated value for the product line and configuration. For doors, opaque door leaves and doors with glass inserts have their own rating approach, and the proportion of glass affects the result. For mulled combinations, check whether the individual units are rated and how your code treats combined assemblies. Where products are site-built, such as some storefront and curtain wall systems, alternative rating paths may apply; your energy consultant can confirm what documentation the plan reviewer expects.
What to ask an overseas supplier
For imported windows, ask for the NFRC certification information for the specific product line, including the certification agency and the rated configurations. A factory may hold NFRC ratings for some series but not others. Several of our aluminum and wood series are documented as NFRC certified or tested in accordance with NFRC procedures; others are documented for CE or Australian AS2047 compliance. When you request a quote, tell us the energy targets and we will confirm which aluminum window or wood window configurations have supporting documentation.
Canada uses NFRC 100 procedures for U-factor through CSA A440.2, and ENERGY STAR Canada uses its own criteria and SI units. Projects in Europe, Australia and New Zealand rely on different rating systems, such as CE marking under EN 14351-1, WERS in Australia, and NZS 4211 and H1 compliance pathways in New Zealand. If a project spans markets, plan the documentation for each.
Common misreadings
- Treating the center-of-glass U-value from a glass supplier as the window U-factor.
- Assuming the label from one size or configuration applies to every unit in the order.
- Choosing the lowest SHGC everywhere, which can cut useful daylight and winter solar gain.
- Forgetting that labels do not indicate impact or structural ratings.
For more on how glass choices drive these numbers, see our guide to double vs triple glazing, Low-E and argon.
Frequently asked questions
What is a good U-factor for windows?
It depends on your climate zone and the code your project follows. Colder zones require lower U-factors. Check the energy code edition adopted locally, or the ENERGY STAR criteria for your zone if required, then choose a product rated at or below that maximum for the exact configuration.
How do I convert NFRC U-factor to W/m²K?
Multiply the NFRC U-factor in Btu/hr·ft²·°F by about 5.678. For example, 0.30 equals roughly 1.70 W/m²·K. Note that European Uw values use different standard sizes and conditions, so converted numbers are indicative and not a direct equivalence between rating systems.
Does an NFRC label show hurricane or structural ratings?
No. The NFRC label covers energy performance: U-factor, SHGC, visible transmittance and, optionally, air leakage and condensation resistance. Design pressure, water resistance and impact ratings come from separate structural and impact testing, documented on other labels, test reports or product approvals.
Can windows made in China carry NFRC certification?
Yes. NFRC certification depends on the product line's simulation, testing and inspection by accredited bodies, not on country of manufacture. Ask the supplier which specific series and configurations are certified, and confirm the rated configuration matches the units you order.
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