‘Low-E’ Coating Cuts Heat Transfer In Windows
Q. I’m thinking of purchasing some replacement windows that are argon-filled and have a “low-e” coating. How do I know I’m getting the real McCoy? - R.W., Spokane
A. Congratulations on your choice of low-e/argon-filled windows. This type of window represents the state of the art in energy-efficiency for mass-produced windows. Both low-e and argon gas significantly reduce heat losses during the winter and heat gains during the summer through your windows.
Checking for low-e coating: The low-e coating applied to the window is a very thin coating - one or two molecules thick - of a metal oxide. As a surface coating, it can be seen by doing the following test. Hold a flame - a match or cigarette lighter will work fine - in front of the window from the inside of the house. On double-pane windows, the flame will be reflected four times. If the window has a low-e coating, one of the flame reflections will have a bluish or greenish tint to it. Typically it will be the second or third reflection, depending upon whether the low-e is on the outside of the inner pane of glass or the inside of the outer pane.
Another type of low-e window has the coating applied to a thin film of Mylar plastic suspended between the inner and outer panes of glass. To check for the presence of low-e coated mylar, look for a tiny hole in the corner of the mylar film. This hole is needed to equalize pressure between the inner and outer air spaces created by the film.
Checking for argon gas: Checking for the presence of argon gas is not as easy. Argon gas is colorless, odorless and virtually impossible to detect when sealed between the two panes of the window. The only field check is to look for two plugs on the spacer bar between the panes of glass. These plugs are left when the argon gas is injected into the window during manufacturing. The presence of these plugs does not ensure there is really argon in the window. And some windows that don’t have plugs do have argon. The manufacturer label and reputation are all you have to go on.
Labels: In Washington state, the State Energy Code requires all windows to be labeled with an energy rating. These ratings are provided by the National Fenestration Rating Council (NFRC). NFRC labels are your best guide to the thermal performance of a window. The labels show a window’s tested U-value. The smaller the U-value, the more energy-efficient the window. U-values range from about 1 to .20. Almost any window with a U-value below .40 will have both a low-E coating and argon fill.
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The following fields overflowed: BYLINE = Amy Mickelson Washington State Energy Office Bruce Carter assisted with the writing of this column.