Here are the terms you need to know when doing your window shopping.
U-Factor measures how quickly a window (or sunlight or even a door) loses heat. Lower is better as it means less heat loss.
A good window for a colder climate might have a number under 0.30 and it will also have an Energy Star approval.
Keep in mind that the U-Factor, as redefined by the NFRC, is for the entire window, not just the glass component. This includes the frame and any spacer between the glass, so the framing build and materials can affect the score.
This determines if a glass is insulated, which generally means it has two (or more) glass panels, with an air buffer between them. This separation of the inside and outside by an air layer makes for improved insulation.
Based on a rating between 0 and 1, this number rates the amount of visible sunlight that makes its way through the glass.
Higher numbers will let more light through, which is generally what people want, but you can get a lower rated glass in order to keep the light out and help your shades/blinds. This might be useful for a west-facing window where you expect to get intense afternoon sun.
Solar Heat Gain
Solar Heat Gain or the Solar Heat Gain Coefficient (SHGC) measures how much of the heat from sunlight the window is able to block. The lower the number, the better a window is at blocking solar heat gains. So you might want lower numbers if a window is south or west facing and you expect a lot of light, but a number number on the east to heat your home in the morning.
People living in the south and other warm areas will feel this number more intensely than those in the northern climes.
Light-to-solar gain (LSG) is a bit obscure, but it comes up now and then. This is the relationship between Solar Heat Gain and Visible Transmittance. It’s actually a ration from one to the other.
This metric explains how much light gets through relative to how much heat is absorbed. The high the number the more light you get with less heat. It’s like a measure of efficiency of the glass and its coating(s).
This measures how well the window blocks moisture from outside. The higher the number the better, particularly in humid areas.
This measure how much air can escape from the window when closed. You want a relatively low number, like 0.10 in order to prevent unwanted air exchange with the outside.
Most windows have something called a “Low-E Coating” which means there is a clear coating on the window that reflect heat but let’s light into your home. Typically you’ll see the outside coated in warm areas so heat is kept out. In colder zones you’d have both sides coated so you keep the home’s heat inside but don’t overheat in the summer or just on the inside so you let in all the heat possible without letting any out.
A great resource for a lot of this information or getting more details is the National Fenestration Rating Council (NFRC). This is the organization that does the ratings labels you see on most windows.