How to Select the Right Window Glass

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With recent energy code changes and new developments in engineered glass, how is anyone to make sense of all the options and ratings? We break it down for real people so you can select the best glass options for your windows.

When purchasing windows today, you’ll find four basic ratings listed on the glass sticker:

1) U-FACTOR: similar to wall insulation, this measures the insulating quality of glass on a scale from 0 (greatest insulation) to 1 (least insulation). The smaller the number the better the glass is at insulating. Insulation acts like a coat for your house in the winter to hold warm air in, and like a cooler to hold cold air in during summer. Argon gas between two panes of glass will greatly increase the insulated quality. This all equates to savings on your utility bill no matter your climate.

2) SOLAR HEAT GAIN COEFFICIENT (SHGC): measures how well the glass blocks heat from the sun on a scale from 0 (greatest heat blocked) to 1 (least heat blocked). This has the greatest affect on the south side of your home where the sun shines directly year-round.

3) VISIBLE TRANSMITTANCE (VT): similar to sunglasses, this measures how much light passes through the glass affecting the brightness of a room. It’s measured on a scale from 0 (least light / brightness) to 1 (greatest light / brightness).

4) LOW-E COATINGS: Invisible metallic coatings on glass that reflect heat but still allow light through. This affects all the above ratings, but has the largest affect on the Solar Heat Gain Coefficient. There are several levels of Low-E: 180, 240, and 366 (and a few in between); the lower the number (180), the less heat blocked from the sun, the higher the number (366) the greater the heat blocked from the sun.

For example, if you have south facing windows that are not shaded from the summer sun, Low-E 366 will greatly reduce the summer heat gain and reduce your utility bill. However, if your home is sited for passive solar - large windows facing south to allow winter time heating from the sun but are shaded during the summer - Low-E 180 would allow the greatest heat gain in the winter, 240 would allow a mid-range heat gain in the winter, and 366 would block most of the heat gain and negate most of the benefits of passive solar heating.

The majority of residential glass in the U.S. is manufactured by Cardinal glass (their website has additional useful info) and used by most window manufacturers. Some window manufacturers have changed the nomenclature, i.e. Low-E 180 may be called “advanced” or the like. Ask your dealer for the Low-E level equivalent.

If nothing else, remember you always want a low U-Factor number, and the Solar Heat Gain Coefficient may vary depending on how much heat gain you desire from the sun on a given side of your home.

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