Window Facts

Energy Performance Ratings And How They Work

Energy efficient replacement windows have been growing in popularity over recent years. Everybody wants to get their hands on these windows for a variety of reasons. These windows save you money and help the environment which is why the demand for these windows has sky rocketed.

Energy efficiency ratings vary with different climates across the United States. Windows that perform well in mild and dry California, may not perform as well in hot and humid Florida. It is important to understand how these ratings work, and the way they vary depending on the climate. This article will inform you on the 4 most common ratings on an NFRC label (National Fenestration Rating Council).


U-factor measures heat loss through the window. In other words, it measures how well a window will prevent the heat inside the home from escaping through it. Typically the U-factor ranges between .2 and 1.2 for windows. In this case, the lower the score the better the window is at insulating the home. This is important for people who live in colder climates. Windows that have a good U-Factor score keep the home warmer during the winter. It also puts less strain of the HVAC system. The reason being is that with more heat being retained in the room, less energy is being used to keep the room at the ideal temperature.

Solar Heat Gain Coefficient (SHGC)

This rating measures heat gain. Essentially what this number is determining is how well a window will prevent exterior heat from entering the home. SHGC is scored on a scale of 0-1, with a lower score meaning that less heat will be able to enter the home. This heat is usually transferred through UV rays, which means that preventing them from entering through the window will prevent the home from heating up. The transfer of UV rays is especially combated with windows that have Heat Mirror Technology associated with them. This technology blocks over 99% of UV rays while also insulating the home 40% more effectively in some cases.

Visibility Transmittance (VT)

Visibility Transmittance measures how much daylight passes through a windows glass. This measurement is based on factors such as Low-E coatings, tints, colored glass, and a few other variables. This factor is important because it shows how much light enters the home through the window. This is measured on a scale of 0-1 with the higher score meaning more natural light passes through your windows. Personal preference is important when determining what VT score you want your windows to have. Some people like to have a lot of daylight in the home while other’s may like it to be darker in the home. The environment and region also play a role. For instance, if you live in the cloudy and rainy pacific northwest, you may want to have windows that have a higher VT score since natural light is more scarce in that area than say the southwest desert region of the country.

Air Leakage

Air leakage measures how much air will be able to enter the home through the window. This is the opposite of U-Factor which measures how much air will leak from the home. Air leakage is measured on a range of .1 to .3 with a lower score meaning that the windows allow less air into the home. This is an important metric for any climates that have temperatures that stray far away from the ideal temperature in your home. If your window has a poor air leakage score, you could be subject to higher energy bills as your heating/cooling systems need to work harder to achieve the ideal temperature in the home.

At the end of the day, these 4 energy performance ratings should point you in the right direction when it comes to choosing the perfect windows for your home. While there are definitely other factors that go into finding the right window, these metrics should give you a good place to start. Find a professional with experience in the window industry who can lay out your options for you and point you in the right direction.

Similar Posts

Leave a Reply

Your email address will not be published. Required fields are marked *