Window Energy Ratings1

Things feeling a little drafty in your neck of the woods? If you notice yourself shivering every time you walk through your home, it may be time to think about buying a new set of windows. But a lot has changed for windows in the past decade or so. Specifically, the products on the market now have gotten a lot more efficient, thanks to material and design improvements that help reduce drafts and keeping heating and cooling inside your home, where it belongs.

However, unlike a washer and dryer, where the end game for efficiency is pretty much the same across households, buying the right energy-efficient window depends a lot on the climate in your area. For instance, if you live in the North, you might want your windows to allow in more passive solar heat than you would if your home was in a warmer part of the country. And getting the recipe just right for your region can seriously affect your home’s energy consumption.

Window Energy Ratings2

NFRC Ratings and ENERGY STAR Certifications

To help homeowners make sense of how different windows behave in various climate conditions, the National Fenestration Ratings Council created a system for gauging their performance. The NFRC runs a voluntary program which tests different products for both their heat loss and gain, as well as the amount of visible sunlight windows allow into your home. After a product has been tested, a label displaying the ratings can be applied to the window’s packaging and product brochures.

These ratings are used to determine whether or not a window can be labeled as ENERGY STAR certified, meaning that it meets the EPA’s minimum criteria for energy efficiency. This certification is a sure sign that a window will be more efficient than one that doesn’t bear the ENERGY STAR label—however, if you want the maximum energy efficiency, you may need to dig a little bit deeper. A minimum requirement is just that—the minimum. So you may find that slightly higher or lower ratings are needed to keep your home comfortable and to truly save money on your energy bills. To help you pick out the best window for your area, let’s take a look at each individual rating and what it means.

Window Energy Ratings3

U-factor Ratings and Climate

A window’s U-factor indicates how much heat escapes through a window once it’s installed. Most windows rate somewhere between 0.15 and 1.20 for U-factor, although the lower the U-factor, the better the window is at preventing heat loss. That doesn’t necessarily mean the lowest U-factor you can find is the best, however. In hotter climates, a slightly higher rating may actually be ideal, since it will keep your home from overheating throughout the winter. For instance, the minimum U-factor requirement needed to achieve ENERGY STAR certification is 0.27 or lower in the country’s northernmost reaches, whereas it’s only 0.40 or less in the south. To view the minimum U-factor requirements for each region, check out the EPA’s program requirements.

When shopping for windows, make sure the U-factor has been determined by the NFRC, however, since the ratings council tests the whole window—including the frame and insulation—for overall performance, rather than basing its rating on just how the glass performs. A window’s frame is one of its weak points, so a product that hasn’t been tested for leaks in this area may not perform as well.

Window Energy Ratings5

Solar Heat Gain Coefficients and Extreme Temperatures

On the other side of the equation, there’s a window’s solar heat gain coefficient, or SHGC. This measure indicates how well a window’s glazing shades a home’s interior, preventing heat gain from solar radiation. You’ll want to pay particularly close attention to your window’s SHGC if you live in a warmer climate, since reducing excess heat in your home can make your AC work more effectively. A rating below 0.25 will serve you well in these areas—the lower the SHGC, the better the window is at shading your home.

However, northern homeowners who find themselves saddled with high heating expenses may also find their window’s SHGC a useful measurement. Higher heat gain coefficients can be used as a passive heating technique to lower HVAC energy consumption in the winter. To do so, it’s important to balance the SHGC with the appropriate U-factor rating to achieve a beneficial equivalent energy performance. These comparisons can also be found in the North Climate Zone table in the ENERGY STAR program requirements.

Window Energy Ratings6

Air Leakage Also Plays a Role in Energy Efficiency

While a window’s U-factor rating gives an indication of how much heated or cooled air escapes through the window, there’s also a separate measurement for how much outside air a product allows into a home. This is the Air Leakage, or AL, rating. The lower a window’s AL rating, the less air it allows inside. In order to be labeled as an ENERGY STAR window, it should measure no higher than 0.30 cubic feet per minute.

Window Energy Ratings7

Visual Transmittance Helps with Lighting Expenses

The previous measurements express how a window affects your home’s heating and cooling efficiency. But that’s not the only benefit energy-efficient windows can provide. Windows with a high Visual Transmittance (VT) rating also allow more visible daylight to filter into your home, which can be used in the daytime to offset lighting costs. Lighting a home typically makes up about 10 percent of a home’s total energy expenses, so a little relief there is helpful. To help homeowners understand the relationship between a window’s VT and SHGC ratings, the NFRC also created the Light-to-Solar-Gain rating, or LSG, which indicates how effectively a window allows daylight into your home while still shading the interior from solar heat gain. If you’re hoping to use VT to reduce your lighting expenses, it’s a good idea to look for products that display an LSG rating as well—the higher the number, the more light you’ll get without excess heat.

If you’re smart about your energy ratings, you can effectively use your windows to reduce energy costs in your home. When you see the rate of return on your energy bills, the time you invested reading and understanding window measurements will seem well worth it.

Author Bio
Erin Vaughan is a blogger, gardener and aspiring homeowner.  She currently resides in Austin, TX where she writes full time for, with the goal of empowering homeowners with the expert guidance and educational tools they need to take on big home projects with confidence.