Reading an NFRC Label: U-Factor, SHGC & Air Leakage Explained

That small NFRC label can tell you a lot about how a window will perform in a real home. U-Factor shows how well it resists heat transfer, SHGC indicates how much solar heat comes through, and Air Leakage measures how much air can pass around the window. Once you know what those numbers mean, comparing windows gets much easier.

What is an NFRC label?

An NFRC label is a standardized performance label that shows how a window, door, or skylight performs for energy efficiency and comfort.

The label is created through the National Fenestration Rating Council (NFRC) certification program, which uses standardized testing and rating procedures so products can be compared on the same basis. Instead of relying only on a manufacturer’s claims about glass, frame, or insulation, you can use the NFRC label to see how the complete fenestration product performs.

That last point is important. The NFRC rating is about the whole window or door, not just the glass. The frame, glazing, and other components all contribute to the final performance numbers.

What information appears on an NFRC label for windows and doors

An NFRC label displays five core, independently verified data points for every qualifying window and door: U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, Air Leakage, and optional Condensation Resistance.

Think of the label as a quick performance snapshot of the complete window or door. Each rating answers a different question about heat, sunlight, daylight, air movement, or condensation. Here is what the numbers mean.

• U-Factor measures the rate of heat transfer through the entire window or door assembly, including the frame and glazing. The rating ranges from 0.20 to 1.20, with lower values indicating less heat transfer.

For OKNOPLAST US, the PAVA triple-pane system carries NFRC-certified U-Factors from 0.14 to 0.26, depending on configuration. The 0.14 result comes from an optimized triple-pane configuration; standard argon-filled configurations achieve other values within the certified range. 

• Solar Heat Gain Coefficient (SHGC) measures how much solar heat passes through the product. It is rated from 0 to 1: lower values allow less solar heat in, while higher values allow more.
• Visible Transmittance (VT) measures how much visible light passes through the glazing. It is also rated from 0 to 1, with higher values allowing more natural light indoors.
• Air Leakage (AL) measures how much outside air enters through a closed window or door. NFRC lists Air Leakage as ≤0.3; lower values indicate a tighter product and less air infiltration.
• Condensation Resistance is an optional rating from 0 to 100 that shows how well a product resists surface condensation. Higher values indicate greater resistance.

Details beyond the glass can affect thermal performance, too. Warm-edge spacers help reduce heat transfer at the edge of the insulating glass unit. OKNOPLAST US also manufactures windows and doors to exact project dimensions as a made-to-measure standard, with no upcharge, according to the brand guidance.

How reading an NFRC label helps compare product energy performance

Because the National Fenestration Rating Council uses uniform testing through accredited independent labs, NFRC ratings let you compare the energy performance of different fenestration products on an apples-to-apples basis.

That sounds simple, but there is an important detail: compare the same type of rating on both products. NFRC ratings cover the complete window or door assembly, including the frame and glazing. A center-of-glass measurement, by contrast, describes the glass alone.

This matters when comparing energy-efficient products. For example, an NFRC whole-unit U-Factor measures thermal transmittance through the complete product, while a manufacturer’s glass-only specification does not. The two numbers may look similar, but they are measuring different things.

The same rule applies to insulating glass configurations. Changes in glass, coatings, gas fill, spacers, or frame construction can affect the final NFRC rating. Compare the ratings for the actual configurations you are considering, rather than assuming one number represents every version of a product.

A useful extra metric: Light-to-Solar Gain

Light-to-Solar Gain (LSG) is calculated by dividing Visible Transmittance (VT) by Solar Heat Gain Coefficient (SHGC). LSG is not an NFRC label rating, but it can help compare glazing options by showing how much visible light they provide relative to the solar heat they admit.

This can be useful when evaluating OKNOPLAST US high-performance glass. A glazing option with higher VT and similar SHGC will have a higher LSG, indicating more visible light relative to solar heat gain. The ideal balance can also vary by climate zone and project goals.

Field note: Compare equivalent NFRC ratings for comparable product configurations.

Understanding ENERGY STAR climate zone requirements versus standard NFRC ratings

ENERGY STAR is a separate program from standard NFRC ratings: it applies climate-zone-specific U-Factor and SHGC requirements to NFRC-certified performance values to determine whether a product qualifies.

Standard NFRC ratings provide independently verified performance data for fenestration products. ENERGY STAR uses these certified values to determine whether a product meets its requirements in a particular climate zone. The thresholds reflect different heating and cooling priorities across the United States.

For residential windows, ENERGY STAR Version 7.0 sets the following requirements:

*The Northern zone also allows equivalent-energy-performance pathways for certain higher U-Factors with specified minimum SHGC values.

For energy-efficient products, the right combination of U-Factor and SHGC depends on the climate zone. Colder regions generally benefit from lower U-Factors, while warmer climates often place greater emphasis on limiting solar heat gain.

An NFRC label reports the performance of the complete fenestration product, including insulating glass. ENERGY STAR then applies its qualification criteria to the relevant NFRC-certified values. U-Factor measures thermal transmittance, but a strong NFRC rating alone does not make a product ENERGY STAR certified.

OKNOPLAST US PAVA and PIXEL systems are currently undergoing ENERGY STAR certification review.

How low-emissivity coatings and glass options affect NFRC label ratings

Low-emissivity coatings help control heat transfer through the glass, directly affecting the U-Factor, SHGC, and Visible Transmittance (VT) shown on an NFRC label.

Low-E coatings reflect infrared radiation rather than allowing it to pass freely through the insulating glass unit. In colder conditions, this helps keep indoor heat inside; in warmer conditions, the right coating can limit solar heat gain while still allowing natural light to enter.

The coating is only one part of the system. Argon or krypton gas fill and warm-edge spacers also reduce heat transfer, while the frame contributes to the whole-window rating. That is why NFRC energy performance ratings reflect the combined thermal transmittance of the complete assembly.

Different Low-E technologies can change the balance between daylight and solar heat. High-performance glass can be configured differently depending on the glazing system, project goals, and climate.

Hard-coat and soft-coat Low-E technologies offer different performance characteristics. The appropriate option depends on the glazing configuration, desired solar heat gain, and project requirements. 

For example, OKNOPLAST US PAVA uses triple-pane Low-E glazing with argon fill as its standard configuration. Its NFRC-certified U-Factor ranges from 0.14 to 0.26, depending on the specific configuration. The 0.14 result comes from an optimized triple-pane configuration using a warm-edge spacer and krypton fill.

This balance can also be expressed through Light-to-Solar Gain (LSG), calculated by dividing VT by SHGC. A higher LSG generally indicates more visible light relative to the solar heat admitted through the glazing.

Field note: An NFRC rating reflects the complete window assembly, so Low-E glass should be evaluated together with the other components.

How homeowners use NFRC labels to reduce energy bills and improve comfort

NFRC ratings help homeowners connect a window’s tested performance with everyday concerns such as heat loss, solar heat, drafts, and condensation. They also provide objective data for comparing energy efficiency, rather than relying on general claims about a product. 

Start with the ratings that have the most obvious impact on heating and cooling. A lower U-Factor means lower thermal transmittance and less heat transfer through the complete window, while SHGC tells you how much solar heat enters through the glazing. In a cold climate, limiting heat loss may be the priority; in a hot climate, controlling solar heat gain can matter more. Visible Transmittance (VT) is useful when natural light is also an important part of the design. 

But energy performance is not the whole story, and home energy bills depend on more than window ratings alone. 

• Air Leakage (AL) can affect how comfortable a room feels even when the window has a good U-Factor. A lower AL rating indicates less air passing through a closed window, supporting better draft prevention and limiting unwanted infiltration.
• Condensation Resistance is another rating worth checking, especially in rooms where indoor humidity is high. A higher rating indicates that the product is better able to resist condensation forming on interior surfaces.

For OKNOPLAST US, the PAVA system has NFRC-certified U-Factors ranging from 0.14 to 0.26, depending on configuration. The rating describes the tested product performance; it does not predict a specific reduction in an individual home’s energy bills.

Field note: Think of the NFRC label as a performance snapshot. It cannot tell you exactly what your energy bill will be, but it gives you objective numbers to use when choosing a window.

What is the NFRC label’s role in ENERGY STAR certification?

The NFRC label provides the independently tested performance data that ENERGY STAR uses to evaluate a product, but an NFRC rating alone does not make a window ENERGY STAR certified.

When checking a window for ENERGY STAR qualification, start with its U-Factor and SHGC and compare them with the requirements for the applicable climate zone. ENERGY STAR Version 7.0 uses NFRC test methods for these ratings, so the numbers on the NFRC label provide the verified performance data used in the qualification process.

• Air Leakage (AL) also has a separate ENERGY STAR requirement for qualifying windows, sliding doors, skylights, and swinging doors.
• Visible Transmittance (VT) and Condensation Resistance, meanwhile, remain useful NFRC ratings but are not ENERGY STAR qualification criteria. 

So, if you are checking whether a fenestration product may qualify for ENERGY STAR, the NFRC label gives you the numbers to check—but you still need to verify the product’s actual ENERGY STAR certification and applicable climate zone.

Fied note: The NFRC label tells you how the product performed; ENERGY STAR determines whether that performance meets its certification criteria.

What U-Factor and SHGC Requirements Do ENERGY STAR Window Ratings Set for Each US Climate Zone?

ENERGY STAR sets different U-Factor and SHGC thresholds for windows in each U.S. climate zone, so the right NFRC ratings depend on where the window will be installed.

For residential windows, the current ENERGY STAR Version 7.0 criteria use four climate zones. In general, colder zones place more emphasis on limiting heat transfer through a lower U-Factor, while warmer zones place more emphasis on limiting solar heat gain through a lower SHGC.

These are the prescriptive criteria for windows. The Northern zone also has an equivalent-energy-performance pathway: windows with U-Factors of 0.23 or 0.24 can qualify with SHGC ≥ 0.35, while U-Factors of 0.25 or 0.26 can qualify with SHGC ≥ 0.40.

That means a lower U-Factor is not automatically the only goal. The best combination depends on the climate zone and the way the window will perform in that climate. ENERGY STAR also provides a Climate Zone Finder so homeowners can check the applicable zone by state and county.

When comparing energy-efficient products, check the exact NFRC-certified U-Factor and SHGC for the window configuration you are considering. Then compare those values with the requirements for your climate zone rather than relying on a generic claim such as “high performance.”

Field note: Find your ENERGY STAR climate zone first, then use the NFRC label to check whether the window’s U-Factor and SHGC meet the applicable thresholds.

Ready to compare your window options?

NFRC ratings make it easier to compare window performance and choose the right solution for your home, climate, and project goals. Let’s talk about your project.