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When you’re comparing energy-efficient windows, the NFRC label can look more like a technical specification sheet than a buying guide. Three numbers deserve your immediate attention: U-factor, solar heat gain coefficient (SHGC) and visible transmittance (VT).These ratings describe different aspects of window performance. U-factor measures heat transfer, SHGC measures solar heat entering the home and VT measures how much visible light passes through. Learning how they work together can make it easier to compare window options and understand what you’re actually buying.Comparing options now? You can get a free custom quote from Renewal by Andersen to see how certified ratings apply to your home.
What is an NFRC window energy label?
The NFRC label provides independently certified performance information for a window, door, or skylight. The National Fenestration Rating Council (NFRC) uses standardized testing and certification to evaluate whole-window performance, including the glass, frame and other components.
That whole-unit measurement matters. Comparing only the glass or a manufacturer’s advertised center-of-glass performance may not tell you how the complete window performs.
The NFRC label can include several ratings, but U-factor, SHGC and VT are among the most useful when evaluating energy-efficient windows. ENERGY STAR uses NFRC-certified performance data when qualifying residential windows.
How do you read U-factor on a window label?
U-factor measures the rate of heat transfer through the window. Lower U-factor numbers indicate better insulation, meaning less heat moves through the window assembly. NFRC describes U-factor as a measure of how well a product keeps heat from escaping a room.
For homeowners, U-factor is particularly important during cold weather. A lower value can help reduce heat loss through windows, which may contribute to a more comfortable interior and lower heating demand.
What it tells you
Less heat transfer
More heat transfer through the window
Don’t compare U-factor numbers without considering the complete product and your climate. ENERGY STAR requirements vary by climate zone, so a suitable rating in one part of the country may not be the same as the right target elsewhere.
What does SHGC mean on an NFRC label?
SHGC stands for solar heat gain coefficient. It measures the fraction of solar energy that passes through a window and contributes to indoor heat. The rating ranges from 0 to 1; lower numbers allow less solar heat to enter.
The right SHGC depends heavily on climate, window orientation, shading, and how you use the room.
In a hot climate, a lower SHGC can help limit unwanted solar heat. In colder climates, some windows may benefit from allowing more solar heat inside, particularly where passive solar heating is useful. ENERGY STAR notes that window orientation can also affect the appropriate SHGC.
U-Factor vs. SHGC
These two ratings answer different questions:
Measures
U-factor
Less heat transfer
Solar heat entering through the window
Less solar heat gain
A window with a low U-factor isn’t automatically the right choice for every room. You also need to consider how much sunlight reaches the window and whether you want to limit or capture that solar heat.
What does VT mean on a window label?
VT stands for visible transmittance. It measures how much visible light passes through the window. The rating ranges from 0 to 1, with higher numbers allowing more daylight into the home.
VT can be useful when natural light is a priority. A large window with a higher VT may allow more daylight into a room, while coatings, tints, additional glass layers, and frame dimensions can affect how much light ultimately passes through the complete window. VT is therefore a lighting-performance measurement, not an energy-efficiency score by itself.
If you’re choosing windows for a bright living room, kitchen, office, or bedroom, ask how the glass options affect both VT and solar heat gain.
What other NFRC ratings should homeowners check?
U-factor, SHGC, and VT are the three ratings most homeowners encounter when comparing energy performance, but the NFRC label can also include air leakage and condensation resistance.
Air leakage measures how much air passes through joints in the window. Lower values indicate less air leakage. Condensation resistance measures how well the window resists condensation, with higher values indicating greater resistance.
These ratings can provide additional context when you’re comparing similar products.
Schedule a free consultation with Renewal by Andersen to review energy-efficient window options and compare their certified performance ratings.
How should you compare NFRC labels?
Start by comparing the same window type and configuration. Then look at U-factor, SHGC, and VT together rather than focusing on one number.
Use this checklist when reviewing contractor proposals:
U-factor: Is the rating appropriate for your climate? SHGC: Does it make sense for the home’s climate and window orientation? VT: Will the glass provide the amount of daylight you want? Air leakage: How effectively does the window limit unwanted air movement? Condensation resistance: How does the product compare if condensation is a concern? ENERGY STAR: Is the specific product configuration certified for your climate zone?ENERGY STAR’s current residential window criteria use U-factor and SHGC requirements that vary by climate zone. The program recommends using its climate-zone information when selecting qualifying products.
Renewal by Andersen notes that its windows carry NFRC labels showing whole-window U-factor, SHGC, and visible transmittance, allowing homeowners to compare certified performance rather than relying solely on individual component claims.
What should you ask a window contractor?
Bring the NFRC label or product specifications to your consultation and ask the contractor to explain the ratings for the exact window configuration being quoted.
Ask:
What is the NFRC-certified U-factor? What is the SHGC? What is the VT? Is the specific configuration ENERGY STAR certified for my climate zone? How do the glass, frame, and installation affect overall performance?New Windows From Renewal by Andersen
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How can you use NFRC ratings when buying windows?
The NFRC label gives you a standardized way to compare window performance. U-factor tells you about heat transfer, SHGC tells you about solar heat gain, and VT tells you about daylight. None should be viewed in isolation.
Your climate, window orientation, room location, shading, and comfort priorities all affect which combination makes sense. Once you understand what each number measures, you can have a more informed conversation with a contractor and compare products using consistent performance data.
Ready to replace your inefficient windows? Schedule a free consultation with Renewal by Andersen to get a quote tailored to your home’s energy needs.
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