Understanding Replacement Window Ratings Will Help You Get the Right Window For Your Climate

Replacement Window ratings are essential for you as a homeowner when shopping for replacement windows. By using the published performance ratings, it will allow you to make an apples to apples comparison between the different replacement windows you are deciding upon for your project.

As a consumer it is important to understand exactly what replacement window ratings are important, what the replacement window ratings mean, and how to determine the best replacement window ratings for your project. The information below will give you a better understanding of each element of the rating process.

Who Determines the Replacement Window Ratings?

The National Fenestration Rating Council (NFRC) is an organization created by the companies within the window, door and skylight community. The council relies on input from suppliers, builders, architects, manufacturers, government agencies, and many other entities to help in the window ratings creation process. The replacement window ratings system developed by the NFRC is based on total product performance.

Every window that is certified to the NFRC standards will include an NFRC label on the product. This label provides the only certain way to determine a window's energy properties and make product comparisons between windows. The NFRC label will also be found on all products which are part of the ENERGY STAR program.

There are four primary replacement window ratings that the NFRC uses to determine the window performance, U-factor, Solar Heat Gain Coefficient, Visible Light Transmittance, and Air Leakage. In the near future a fifth window rating will be included: Condensation Resistance.

Replacement Window Ratings Definitions

The following sections define in greater detail each of the window ratings that the NFRC uses to measure the performance of windows.

U-factor

The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. Because it is a measure of heat loss through the window, the lower the U-value, the better the window will perform. When you are shopping for replacement windows be sure to talk in terms of the U-Value and not the R- Value of the windows.

R-Values are a measure of how well something insulates and is typically used to judge the performance of insulation in your walls. The insulating value is indicated by the R-value which is the inverse of the U-value. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.

Solar Heat Gain Coefficient (SHGC)

The official definition of the Solar Heat Gain Coefficient is as follows: The SHGC is the fraction of incident solar radiation admitted through a window, both admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.

While that is a very detailed definition, you are probably sitting there wondering what the heck it means! In layman's terms solar heat gain is the same feeling you get when you stand in the sun for an extended period of time. The suns radiant heat hits your body and begins to warm your skin. After time your body has absorbed the sun's radiant heat and you have in essence "gained" the sun's heat. This results in your body temperature rising and you get hot and want to get out of the sun.

The same principle applies to the windows in your house. As the sun beats down on your windows, the windows will begin to absorb heat gain. If the SHGC is high on your window, the heat passes right on through and starts to raise the "body temperature" of your home.

By having a window with a low SHGC, you prevent the radiant heat from being able to pass through the window keeping the inside of the house cooler in the warm summer months. SHGC is the more important in Southern climates than it is in Northern because of the sun's brutal heat.

Visible Transmittance (VT)

The visible transmittance (VT) is an optical property that indicates the amount of visible light transmitted. The NFRC's VT is a whole window rating and includes the impact of the frame which does not transmit any visible light. While VT theoretically varies between 0 and 1, most values are between 0.3 and 0.8. The higher the VT, the more light is transmitted. A high VT is desirable to maximize daylight.

Select windows with a higher VT to maximize daylight and view.

Air Leakage (AL)
Heat loss and gain occur by infiltration through cracks in the window assembly. It is indicated by an air leakage rating (AL) expressed as the equivalent cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks in the window assembly.

At this time, the AL is optional. It is good to choose replacement windows that have a very low air infiltration rating. Windows with a higher air leakage window rating will let the heating or cooling out of the house. This will result in a "drafty" window and less energy efficiency. Select windows with an AL of 0.30 or less (units are cfm/sq ft).

Understanding replacement window ratings is just the beginning of your research. Where you live will depend will effect which rating you want to focus on to maximize the energy efficiency of your windows. For more information on what ratings you should select depending upon your climate, feel free to find out more at the window ratings page.