Understanding Window Energy Efficiency Ratings — What Do They Mean for Your Home?

Understanding Window Energy Efficiency Ratings

Nearly 30% of your home’s energy use can be attributed to heat gain and loss through your windows. That means more than a quarter of your monthly energy bill can be attributed to air coming in and out of your house without your knowledge.

No one wants to pay for heating and cooling they aren’t using, which is why energy-efficient windows are a must-have for today’s homeowners.

If you’re in the market for new energy-efficient windows, then you may have encountered the rating system used to describe these windows. Created by the National Fenestration Rating Council (NFRC), window ratings are numbers designed to provide consumers with information about a specific window’s potential for heat loss and light transmission.

Understanding the system the NFRC has developed allows homeowners to understand the benefits of energy-efficient windows and helps them select the type of window that will be best for their home and their budget. Window energy efficiency ratings can also help homeowners select windows to reduce or increase natural light in certain areas of their home, sometimes with dramatic changes to a room’s appearance or temperature.

Why Window Energy Efficiency Matters

Doors, windows and skylights are significant points for the transfer of heat in and out of your home. The transfer of heat is what determines the temperature inside your home at any given time. In other words, your windows and doors can help keep you warm in the winter and cool in the summer.

If you live in an older home or your windows are original to the house, then there’s a good chance that your windows are single-pane windows. Single-pane windows rely on just one pane of glass to block outside temperatures from making their way indoors. And they aren’t very good at keeping all the air your HVAC unit produces inside either.

If you’re considering upgrading to energy-efficient windows, you need to know not all windows labeled as “energy-efficient” are equal. ENERGY STAR®-certified windows — those with NFRC ratings — span a range of options. All meet a base-level requirement for energy efficiency, but some use features like double- or triple-panes plus coatings to increase their ability to keep temperatures as they should be.

Windows that have a higher rating will tend to do a better job of keeping you comfortable indoors. They also offer additional savings on utility costs because they prevent your HVAC system from having to work overtime to keep those indoor temperatures steady.

Window Energy Efficiency Ratings Explained

Window Energy Efficiency Ratings Explained

Window energy ratings are typically found on a sticker on the window itself or listed among the specs of the window. When you look at a window’s energy efficiency ratings, there are four different rating numbers that combine to tell you what to expect from that particular window. Here’s what those different window energy ratings mean.

1. U-Factor

One of the first numbers you’ll notice when looking at efficiency ratings for windows is their U-Factor. A window’s U-Factor rating shows how effective the window is at keeping heat from transferring through the glass. This particular rating refers to the air temperature, not the window’s ability to block or absorb solar heat, which we’ll talk about later. Expressed in British thermal units (Btu) per hour times the area of the window and the temperature in Fahrenheit (Btu/hr*ft2*°F), the NFRC formulates a window’s U-Factor by considering both the glass itself, as well as the frame and materials used for spacing. In this case, the lower a window’s U-Factor, the more efficient it is.

2. Air Leakage

A window’s air leakage rating refers to the window’s ability to prevent the movement of air — and therefore a change in pressure. The lower the number, the tighter the fit. This one can get a little tricky because a window’s air leakage assumes that the window will be installed correctly, and that’s not always the case. To ensure a window fits tightly and lives up to its low air leakage rating, choose an experienced window installer you can trust to install the windows correctly.

3. Visible Transmittance

Visible transmittance ratings refer to how much light a specific window allows to pass through the glass. If you’re looking to increase the amount of natural light in your home, look for a window with a higher transmittance rating. If you want to reduce the light coming in, consider choosing a window with a lower transmittance rating.

4. Solar Heat Gain Coefficient

Also referred to as SHGC, this rating refers to a window’s ability to regulate the heat coming into your home. This is different than the U-Factor rating because it refers to the heat a window lets in or blocks from the sun’s rays. A window with a lower SHGC rating will allow less radiation from the sun and, therefore, less heat. This can be important in rooms that tend to be warm, preventing them from becoming uncomfortable when you’re in them. Many of today’s energy-efficient windows have a Low-E coating on the glass that can help to prevent more of the sun’s radiation from coming through the glass and altering your home’s temperature.

Window Energy Certification Labels

In addition to rating labels, windows also come with certain energy certification labels. These labels are designed to help you understand what ratings and testings a window has gone through to receive its energy efficiency label. There are four primary labels you may encounter as you shop:

1. ENERGY STAR® Certification

ENERGY STAR® labels are perhaps the most commonly recognized because they can also be found on various household appliances. But what exactly does an ENERGY STAR® certification mean? This particular label means that the window meets certain standards for SHGC and U-Factor ratings. It also indicates that those ratings meet the criteria for what type of window is needed in your geographic area.

2. National Accreditation and Management Institute (NAMI) Structural Certification

A window with this certification has been passed independent tests from the National Accreditation and Management Institute. Its results provide energy performance ratings that can be used to indicate the window’s overall efficiency.

3. NFRC Certification

Although other independent agencies use their rating system, the NFRC provides its own certification based on the four criteria mentioned before: U-Factor, air leakage, visible transmittance and SHGC. They also evaluate windows for condensation resistance, which is a window’s ability to resist forming condensation on its inside surface.

4. American Architectural Manufacturers Association (AAMA) Certification

This fourth certification steps away from NFRC ratings to test for some of the other things consumers look for: structural strength, air leakage, thermal performance and water leakage. Some windows are only tested for thermal performance — these can only receive a Silver Certification Label. Windows that have been tested in all four areas can earn a Gold Certification Label.

Contact Sunshine Contracting for Personalized Window Service

Understanding Window Energy Performance Ratings

Need help understanding how a window’s ratings will impact you down the road? Let us help!

At Sunshine Contracting, we pride ourselves on providing personalized service every step of the way. We can help you navigate all the technical information that comes with window ratings to find the right window for your home. Once you purchase your windows, trust Sunshine Contracting to install them in your home. We don’t contract your installation out to someone else. Our experienced team of installers will properly install your windows to ensure that they are operating at optimal efficiency. Request a free estimate today!


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