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Replacement Window Ratings

Replacement Window Ratings Will Help You Compare Replacement Windows and Select the Best for Your Project.

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.

Understanding the ratings on the window and looking for the best performance ratings when shopping for your windows will help you get the best replacement windows 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.

This means that the ratings you see on any NFRC window sticker are calculations of how the entire window unit performs under the testing guidelines determined by the NFRC. These total replacement window ratings will differ from center glass ratings. It is important that when you are comparing replacement window ratings you insure that you are using the NFRC’s total unit window ratings.

replacement window ratings 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.

When you are shopping for replacement windows be sure to have the salesperson provide you with the NFRC window ratings for the windows they sell. This will insure your windows are tested and will perform according to the replacement window ratings assigned to it.

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. When you are researching replacement windows for your project it is helpful to know what each one means so you can choose the best window for your project.

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.

It is important to know that different climates will benefit from different U-Values. The following climate zones will help you to determine what U-value to look for in your part of the country. U-values will be more important in northern climates where it is important to keep heat inside the house during the winter, while less important in the southern climates where keeping the heat gain down is of primary importance.

Northern Climate: Select windows with a U-factor of 0.35 or less. If air conditioning loads are minimal, windows with U-factors as high as 0.40 are also energy-efficient if the Solar Heat Gain Coefficient is 0.50 or higher. Some double-glazed low-e products have U-factors below 0.30. Some three-layer products have U-factors as low as 0.15.

North/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.

South/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.

Southern Climate: A low U-factor is useful during cold days when heating is needed. A low U-factor is also helpful during hot days when it is important to keep the heat out, but it is less important than SHGC in warm climates. Select windows with a U-factor lower than 0.75 and preferably lower than 0.60.

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

The following climate zones provide more detail about the SHGC window ratings you should look for when shopping for your replacement windows:

Northern Climate: To reduce heating, select the highest SHGC you can find (usually 0.30-0.60 for the U-factor ranges required in colder climates) so that winter solar gains can offset a portion of the heating energy need. If cooling is a significant concern, select windows with a SHGC less than 0.55. Use RESFEN to understand trade-offs.

North/Central Climate: If you have significant air conditioning costs or summer overheating problems, look for SHGC values of 0.40 or less. If you have moderate air conditioning requirements, select windows with a SHGC of 0.55 or less. While windows with lower SHGC values reduce summer cooling and overheating, they also reduce free winter solar heat gain. Use a computer program such as RESFEN to understand heating and cooling trade-offs.

South/Central Climate: If you have moderate air conditioning requirements, select windows with a SHGC of 0.55 or less. While windows with lower SHGC values reduce summer cooling and overheating, they also reduce free winter solar heat gain. Use a computer program such as RESFEN to understand heating and cooling trade-offs.

Southern Climate: A low SHGC is the most important window property in warm climates. Select windows with a SHGC less than 0.40.

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.

Be careful about getting window tint on your replacement windows because it will reduce your visible light coming into the house. If you are simply trying to increase the energy efficiency of the house, use the window’s low-e and gas filled airspace to provide the performance. A window tint will help to reduce the amount of heat transferred across the window, but it is not worth the loss of daylight and view inside the house.

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.

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