How to select a glass type for your windows
January 09, 2014
Are you finally replacing your drafty, ill-fitting, hard-to-operate windows? Having made that decision, you're about to be confronted with many more! One of the biggest mysteries window-buyers run into is selecting a glass type. With seemingly endless options, it's tough to know where to start let alone what to buy. Here, we'll talk about glass types in terms of heat reflective coatings and whether you'll need to add gas to an insulated window.
You'll also want to keep u-values and Solar Heat Gain Coefficient (SHGC) in mind as you peruse your options. The u-value of glass refers to how well that glass resists heat flow. The lower the u-value, the better it resists heat. SHGC measures the solar radiation through your windows that's released as heat inside your home. The lower the SHGC, the less heat you have you in your home. Your home's location and orientation will determine how high or low you want your SHGC to be.
When you buy replacement windows, you can select glass with various levels of a heat-reflective material. Here are options you have to choose from:
Clear, non-coated glass: Just like the name says, this glass has not been treated with any type of coating to help it resist the effects of the sun. This is best used in areas where you want full sunlight. If your home has a passive solar design, this type of window could work in the areas where you want to collect the sun's heat. The u-value of this glass is .50 and the SHGC is .70.
Low-e glass: This type of glazing is a barely visible coating of a metallic material - zinc tin oxide (ZTO) combined with silver. It's the first generation of low-e glasses, and it doesn't meet most Energy Star requirements. The low-e coating filters certain waves of the light spectrum, including radiant. Low-e glass also reflects heat back to its source - meaning it bounces the sun's heat back outside in the summer, and in the winter, reflects the room's heat back into itself. This coating has a u-value of .35.
Low-e2 glass: Also called low-e squared, this glass has two coatings of ZTO and silver. While this makes the glass slightly darker, it's far more efficient than a single coating. Low-e2 filters more radiant light. It also fulfills Energy Star requirements for their Northern Zone. (See Energy Star's zone-finder if you're not sure which zone your home is in.) The u-value of low-e squared glass is about .32.
Low-e 3glass: This coating was developed for the now defunct Tax Credit in 2009. As the name implies it has three coatings of ZTO and silver, and its u-value is .30. Additionally, it has a low solar heat gain coefficient. This coating is best used in areas where you get more sun and so need more sunlight filtered out. It is used in most Energy Star zones.
Low-e, low solar heat gain: These windows do exactly what they say in the name - provide a low-e coating with a low SHGC. It has a low u-value - about .30 - but also a very low solar heat gain. That means this window is excellent at blocking heat. It's best used in places like Texas, Florida, and the gulf states, which all get a high amount of heat.
Having a gas infill between the panes of your window glass can lower the u-value of your windows. You have two options to pick from:
Argon: This gas is the more common option and is also the cheaper option. However, it does not work as well as your other option.
Krypton: This gas works better to help reduce u-values, but is also more expensive and more suitable to small spaces.
With these details in mind, and with a quick visit to the Energy Star climate map, you're ready to talk to a window salesperson about your exact needs.