Construction Materials-Window Performance Labels

This post first appeared at Professional Resources for Andersen Windows & Doors| Andersen Windows.

I’ve heard it said, if we didn’t have to put windows in houses, we could build one heck of an efficient home.  But who wants to live in a house where you cannot see outside or let daylight and fresh air in.  Windows are an important part of every home.  When choosing a window, do you understand what that sticker stuck to the window glass is telling you?  You should have at least a basic understanding of the information.

Back when I was starting out as a general contractor, I relied on the salesperson at my lumberyard to determine the window performance I needed for my climate, after all, a window is a window, right?  Well, as it turns out, there are a lot of choices in window performance that should be decided based on the location of the project and specific goals of the home.  For instance, if you live in a heating dominated climate, you may want to choose a window with more solar heat gain and a lower U-factor.  If you live in a hot climate, less solar gain may be needed.  All this information, and more will be listed on the window sticker, and most new windows will have a window sticker placed on the window before they leave the factory.

Who is the National Fenestration Ratings Council?  These are the people who set the information requirements for window labels.  The NFRC is an independent, non-profit organization that establishes objective window, door and skylight energy performance ratings to help homeowners, builders, architects, and others in the construction industry make purchasing decisions based on window and door performance.  Windows and doors that are NFRC certified have been tested by an independent, third-party testing lab with the information gathered during testing displayed on the window sticker.

What information is displayed on a window sticker?  Most windows will have four metrics shown on the window sticker, U-factor, solar heat gain coefficient, visible transmittance and air leakage.  There is an optional fifth metric, condensation resistance.  Let’s dive a little deeper into each of the metrics.

U-factor denotes the insulation value of a window.  Most builders will be familiar with R-value, the higher the number, the better the insulation properties.  Another way to put it, R-value is a measurement of heat resistance.  U-factor on the other hand is the rate or speed of heat loss or gain through the window.  With U-factor, lower numbers are better.  Maximum window U-factors are noted for prescriptive codes depending on where in the country the project is located (climate zone).  Minimum code requirements for window U-factors will range from 0.50 in hot climates to 0.30 in cold climates.

Solar heat gain coefficient (SHGC) is a measurement of how much solar heat gain is allowed through a window.  This metric has a listed value between 0 and 1 with lower numbers allowing less solar heat gain through.  Much like U-factor, energy codes specify what is allowed based on climate location.  0.25 is required in hot climates, 0.40 in more moderate climates and there are no requirements for SHGC in cold climates (zones 6, 7, and 8).

Visible transmittance is the next window sticker metric.  Multiple panes of glass along with different coatings that can be used to modify insulation characteristics and the SHGC will affect the amount of visible light that enters through the window.  Visible light, or visible transmittance is listed as a range between 0 and 1 with 1 being the maximum amount of visible light and 0 being a window that you would have a hard time seeing through.  Building codes do not list a minimum or maximum value requirement.  Usually, more panes of glass and more films that help improve window performance will reduce the amount of daylight moving through the window, which results in a lower visible transmittance number.

Air leakage is another metric that may be listed on NFRC window stickers.  Air that leaks through the different parts and pieces of a window will affect comfort and the operating cost of a home.  The air leakage number is only for the air leaking through the window assembly itself, not air leaking around the window due to air sealing deficiencies during installation.  The number range is from 0.1 to 0.3, with 0.3 being the maximum air leakage allowed.

One optional metric that is seen less on window stickers is condensation resistance.  It’s common in cold climates for moisture to form on the window glass during certain times of the year.  This can be in the form of water, ice or frost.  This phenomenon occurs when the temperature of the glass moves below the dew point temperature.  The window condensation range is 1 to 100, with the higher number having better condensation control.

Other information that will be listed on window stickers is the manufacturer’s name, the model of window along with the materials used in its construction.  The number of windowpanes and any glazing films and/or gas fills used on and between the panes will also be noted.  The sticker may also identify if the window meets the requirements of any energy certification programs, such as Energy Star.

Choosing the window metrics based on what works best for a given project is much better than having a salesperson choose a window for you based on the code minimum for your location.  Hopefully this article has helped inform you what the informational sticker on your new window means.

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