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Replacement Windows Mpls: What Do Window Ratings Really Mean?

If you’ve shopped for replacement windows lately, you’ve seen the variety of ratings on windows, but perhaps you’re confused about what these numbers mean.  Here’s a basic primer on window ratings.

The National Fenestration Rating Council (NFRC) is an organization created by the many companies within the window, door and skylight community. These businesses came together in order to create a standard rating system for their products. Information is taken from suppliers, builders, architects, manufacturers, government agencies, and many other entities in order to provide the most current and accurate information.  Every window that is certified to the NFRC standards will include an NFRC label on each product. Because the NFRC standards are now higher than those of Energy Star, you will often see their label on all Energy Star products as well.

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. There are also plans to include a fifth factor in the near future – Condensation Resistance.  The ratings are defined as follows:

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 insulate. 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 describe the insulating properties of walls, not windows.

Solar Heat Gain Coefficient (SHGC):  The official definition of the Solar Heat Gain Coefficient 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. Just remember that the lower a window’s solar heat gain coefficient, the less solar heat it transmits.  Solar heat is comparable to standing in the sun. After a while, you will begin to absorb this heat and need to move. As the sun beats on the windows of your home, the SHGC rating will measure the amount of heat that is absorbed. If the SHGC is high on your window, the heat passes right through and starts to raise the “body temperature” of your home. This is good if you live in colder climates, but if you live in hot climates, you will be looking for a low SHGC rating to prevent the radiant heat from being able to pass through the window. The lower SHGC number, the better your chances of keeping the inside of the house cooler in the warm summer months.

Visible Transmittance (VT):  The Visible Transmittance (VT) rating measures the amount of visible light passing through the window. 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 if you want to maximize daylight in a darker room.

Air Leakage (AL):  Heat loss and gain will take place in any window; the question is how much. Air Leakage is measured by the amount of cubic feet that have passed through 1 sq ft of window surface area. The lower the AL, the less air will pass through cracks in the window assembly.

By taking the time to learn a few basics about windows, you will be able to make a smart decision about what you are looking for in your home. A little knowledge goes a long way. Call Window Outfitters to learn more about how to replace your windows or doors this year. You will be glad you did. 952-746-6661


Window Outfitters is a premier Window Replacement, doors, siding contractor and installer. As Contractor in the St Paul, Minneapolis, (Twin Cities) Minnesota (MN) metro, we proudly serve, but are not limited to, the following areas: Minneapolis Energy Efficient Vinyl Windows, Replacement Window Contractors Minnesota, Burnsville, Apple Valley, Lakeville, Savage, Bloomington, Edina, Richfield, Eagan, St Paul, Hastings, Minnetonka, Plymouth, Maple Grove, Eden Prairie, Farmington MN, Chaska, Shakopee, Chanhassen, Victoria, Mendota Heights Anderson Windows Minneapolis, Marvin Windows Minneapolis.