Each window manufacturer company has to test their windows in order to be eligible for Energy Star. Each replacement window comes with the sticker, which looks like the one on the left side of this page. On the sticker it is said an important information about the energy efficiency of the replacement window.
First comes U-Factor, which is usually between 0.15 and 1.20. You want that number to be the lower the better. U-Factor shows how well the replacement window is gonna be keeping the heat inside the house, especially during the cold winter months.
Next one comes Solar Heat Gain Coefficient, which tells how much heat of the Sun is reflected from the glass of the replacement window. It is measured on the scale from 0 to 1, the lower the number the more heat is blocked by a window glass. This indicator is a little bit tricky, because it depends in what climate you live. If your house is located in mostly warm climate, then you want that number to be as low as possible closer to 0, if it is mostly freezing temperatures then you want to see that indicator closer to 1.
In our situation we are located in Central New Jersey, Mercer County, so we have both extremes of high and low temperatures, so if that indicator around 0.5, we should be ok with our replacement window.
Visible Transmittance indicator shows how much light comes through the replacement window glass, is measured from 0 to 1. The higher the number the more light is gonna penetrate the glass and get into the house.
The Air Leakage Indicator is measured from 0.10 to 0.30, the lower the number the better. That's an optional indicator, the replacement window manufacturers are not obliged to post it, so some residential windows come without that indicator, which does not mean that the window is not gonna be energy efficient. It means the replacement window has an air leakage at least below 0.30 and is made according to industry standards, but the manufacturer chose not to post that indicator.
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