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Window ratings measure how well a window performs in areas like insulation, solar heat gain, natural light and air leakage. Understanding these numbers can help you compare replacement windows and choose the right performance for your home.
- NFRC labels make comparison easier: Look at ratings for the entire window and compare similar window types and glass packages.
- U-factor measures insulation: Lower numbers mean less heat transfer through the window.
- SHGC measures solar heat gain: Lower numbers block more of the sun’s heat, an important consideration for Southern homes.
Two windows can look alike but offer very different performance benefits for your home. Ever notice that one room is colder than others, simply due to older (but otherwise identical) windows? This comes down to their energy ratings.
Window ratings measure how well windows perform, with each rating accounting for a different feature like heat transference or air leakage. Two windows can look alike, but offer very different performance benefits. Below, we’ve outlined the most common types of window ratings relevant for homes in the South. Take some time to learn more about how your windows keep your home comfortable and efficient.
Window Ratings At A Glance
| Rating | What It Measures | How to Read It | How It Helps You |
|---|---|---|---|
| U-factor | How fast heat moves through the whole window, including the frame, glass edge and glass center. | Lower rating is better. Typical range is roughly 0.20 to 1.20. | Helps balance summer heat gain and winter heat loss. |
| Solar Heat Gain Coefficient (SHGC) | How much of the sun’s heat passes through the window. | Scale of 0 to 1. Lower ratings block more solar heat. | Crucial for homes in the South that rely on HVAC cooling. |
| Visible Transmittance (VT) | How much visible light passes through the glass. | Scale of 0 to 1. Higher ratings let in more daylight. | Balance against SHGC, since low-SHGC glass can reduce light. |
| Air Leakage (AL) | How much air passes through a closed window, in CFM (cubic feet per minute) per square foot. | Lower ratings mean fewer drafts. May not appear on every label. | Influences comfort and HVAC workload. |
| Condensation Resistance (CR) | How well the window resists moisture forming on its interior surface. | Higher ratings are better. May not appear on every label. | Helps keep panes clear, especially on cold mornings and humid rooms. |
What Is A Window Rating? (And Who Sets the Numbers)
Window ratings are figures set by an independent nonprofit organization, the National Fenestration Rating Council (NFRC). These ratings offer measurements that help you determine the performance of a specific window. Ratings represent the entire window’s effectiveness, not just the glass. Every element, including the frame, spacer, glass and coating, plays a part in the score. Windows with NFRC ratings have passed a stringent set of evaluations and generally qualify for ENERGY STARⓇ certification.
How can you find these ratings? It’s simple: the NFRC rating is usually located on a label on the window, in product spec sheets and, sometimes, in quotes from window installers. We’ll go into more detail about the information on these labels below.
How to Read NFRC Window Labels
NFRC window labels are typically laid out in the same way for each label: the manufacturer and product line on top, window type and glass details—such as glass coatings or gas inserts—in the description and then the ratings themselves. Let’s go over some of the ratings you will probably see.
U-Factor: How Well the Window Holds Heat
U-factor ratings measure insulation levels in windows. The lower the U-factor rating, the better your insulation is.
A lower U-factor means less heat flow through the entire window (not just the glass) and less chance of heat leaking out in the winter or creeping in during the summer. Most U-factor ratings fall in the range of 0.20 to 1.20. The best way to compare U-factors is to only look between the same type of window. In other words, the U-factor of a bay window is not comparable to the U-factor of a double-hung window. They’re made differently, and will vary widely. Even different models of double-hung windows will have varying U-factors.
You may also hear about R-value if you’re researching U-factor measurements. R-values are not part of NFRC labels, so you should focus mostly on U-factors, as those will be more easily accessible and comparable when you’re debating different window options.
Solar Heat Gain Coefficient (SHGC): How Much Solar Heat Gets In
SHGC measures how much solar heat is allowed through glass. Unlike U-factor, SHGC is applicable to glass only. The rest of the window will not matter as much for this measurement. Lower ratings mean more of the sun’s heat is blocked, and you’ll commonly see values from 0 to 1. Low-E glass coatings are the main tool used to lower SHGC values on glass panes.
The right SHGC depends on the climate outside and which direction your windows face. A higher SHGC rating lets in more warmth, which is useful in cooler climates but works against HVAC needs for most of the year in the South. For Muscle Shoals, west- and south-facing windows will need a lower SHGC value to pay off in a more comfortable home.
Visible Transmittance (VT): How Much Daylight Comes Through
VT, as the name suggests, measures how much natural sunlight passes through window panes. The higher the rating, the more natural light shines into your home.
This rating is often affected by SHGC ratings, since glass coatings and tints impact the VT score implicitly. Glass that blocks more solar heat naturally blocks more light, even if it’s largely imperceptible to the human eye. The goal for these two ratings is balance. You may value more daylight, which reduces the need for additional indoor light, or favor efficiency with a lower SHGC value. There is no one right answer.
Optional Ratings: Air Leakage, Condensation Resistance and Sound
Air leakage (AL), condensation resistance (CR) and sound (STC) ratings may be left off the window label depending on the manufacturer. Nevertheless, they can be key to your home’s comfort, especially during humid summers.
AL shows how much air passes through gaps or seams in your window. Lower AL values show fewer air leaks. You’ll see AL values measured in CFM per square foot, or cubic feet per minute per square foot.
CR measures how well your windows prevent moisture from seeping in. Higher values are optimal here, since that means your home is more tightly sealed. Although this most often applies to moisture in humid air, higher ratings can also help prevent frost buildup during the winter months.
The sound transmission class rating is a rarely included but important rating that shows how much sound can get through the window. The higher the value, the less sound carries from outside the window.
What Are Good Window Ratings for Muscle Shoals Homes?
Now that you’ve got a feel for the ratings on individual windows, let’s determine what score is optimal for homes in Alabama. According to the ENERGY STAR Climate Finder, Fulton and DeKalb County are in the South-Central Climate Zone. With this in mind, here are some appropriate U-factor and SHGC ratings to shoot for:
| Maximum U-factor | Maximum SHGC |
|---|---|
| ≤0.28 | ≤0.23 |
For Alabama homes, U-factor and SHGC values matter the most, since they keep heat in or, more importantly for Southern summers, out. Hunting for lower U-factor and SHGC ratings on your windows will pay off in a more comfortable home year-round. It’s not a guarantee of more savings on your energy bills, though ENERGY STAR-rated products do help with that.
Ratings compare products under standard test conditions, so real-world results also depend on installation, shading and how the home is used. Still, these ratings are a good starting point that will help you prioritize lasting energy efficiency and comfort.
What Is Behind Good Window Ratings?
Different windows will have different ratings because of the components that make up each window. Some common components that create better window ratings include:
- Low-E coating: This microscopically thin coating filters heat and UV light from glass panes. It can have an incredible impact on both SHGC and U-factor values.
- Argon gas inserts: Colorless, odorless and denser than oxygen, argon gas slows heat transfer through the glass, creating extremely effective long-term insulation.
- Warm-edge spacers: Spacers insulate the edge of the sealed glass unit, where heat loss tends to concentrate.
- The frame: The frame helps determine the U-factor’s rating, so frame material and design matter.
ENERGY STAR vs. NFRC: What’s the Difference?
We’ve dropped mentions to both ENERGY STAR and NFRC throughout this blog. Is one more important than the other when you’re shopping for replacement windows? No—in fact, they rely on each other.
NFRC supplies the ratings and ENERGY STAR, an EPA and DoE program, uses those ratings to set minimum performance standards for each climate zone. A window can carry an NFRC rating without necessarily qualifying for ENERGY STAR certification.
Window World is proud to carry energy-efficient, ENERGY STAR-certified products with ratings that back up our windows’ performance.
How to Use Ratings When Comparing Window Quotes
Now that you know how to interpret window ratings, let’s use them to make sure you get the best value for your investment. Here’s some practical ways to use this information:
- Compare the same window type and size on every quote.
- When obtaining estimates, ask for the U-factor, SHGC and VT of the exact glass package.
- Double-check ENERGY STAR limits for the South-Central zone before comparing numbers.
- Weigh installation and warranty terms alongside window ratings, since a rating only matters if installation goes well.
Why Alabama Homeowners Choose Window World
Window World prioritizes quality and honest pricing, and it shows in our free estimate and installation processes. Our free estimate appointments are designed to make sure you feel confident in the product you’re reviewing, and our window experts are happy to answer any questions you may have about the window ratings. Ready to learn more? Reach out for a free estimate appointment and experience the Window World difference for yourself.
Frequently Asked Questions
What is the difference between U-factor and R-value?
U-factor and R-value are two sides of the same coin. U-factor measures thermal conductivity, or how well a window holds heat. R-value measures thermal resistance, or how well a window resists heat. A low U-factor score is better, while a higher R-value score is better.
Are Window World windows ENERGY STAR-certified?
Window World is an ENERGY STAR partner, and many of our products, windows included, are ENERGY STAR-certified.
What is a good SHGC for windows in Alabama?
Since Alabama is in ENERGY STAR’s South-Central zone, they recommend a maximum rating of 0.23 for SHGC. Lower SHGC ratings block more solar heat, keeping your home cooler.