Glazing 101 - Selecting the right glass for your project
When selecting glass for a project, where do you begin? There are several characteristics of glass that will guide you to a decision. The aspects of appearance and performance are at the top of the list as well as code and local city mandated regulatory requirements.
First, what gives glass it’s appearance? More importantly, what are you seeing when you look through a one-inch insulated glazing unit (IGU)? Typically, there are two pieces of 1/4 inch thick glass, separated by an aluminum spacer that allows 1/2 inch of airspace between the pieces of glass. Even clear glass is not completely clear. As you look through an empty glass soda bottle, there is a slightly greenish cast. This is a result of the iron that is in the glass. The other appearance factor with clear glass is the low-E coating that is typically applied to surface #2 of a one-inch insulated glass unit. Other appearance factors include the percentage of visible light (VLT) that passes through the glass and the exterior reflectance of the glass.
Performance factors will also play a role in the glass selection process. In order the meet energy code requirements, the entire exterior building envelope must pass minimum performance criteria levels or energy modeling. The glass in today’s buildings play a major role. In southern states, the major performance number we start with is the solar heat gain coefficient (SHGC). This is a measure of how well an IGU blocks the heat from the sun. It is expressed as a number between zero and one, with the lower numbers blocking the most heat. Another performance number is the U-factor measuring how well a glazing product prevents heat from escaping a building. U-value is a major performance influencer in northern states, whereas SHGC is the larger performance factor for southern climates. The diagram below illustrates the major environmental factor differences between cold weather and warm weather glass selection.
The current trend in selecting building glazing is to be “aesthetically neutral”, with the least amount of color perception. In decades’ past, we had to rely on glass tinting to get the performance values needed. However, today's technology of low E coatings has allowed clear glass to equal and surpass that of tinted glass. The low E coating, while not a color in and of itself, affects the perceived color that the eye sees. This can vary from slightly silver, green, blue or brownish hues depending on the chemical makeup of the coating.
Another type of glass from “clear” glass is low iron glass. This removes the iron that produces the “Coke bottle effect” of slightly greenish tint that is visible in clear glass. However, low iron glass lowers the performance values and is a higher cost than clear glass. An entire building of low iron glass could not meet the energy modeling requirements which is why low iron glass is usually reserved for grade level glazing or use in entry lobbies and atriums. Eventually, the science of creating Low-E coatings may allow for entire building facades to use low iron glass, and still meet energy code requirements, but that is not today’s reality.
When selecting glass for a building the best option is to view a building that already has the glass installed. If this is not possible, or if the glass is too new to be on an existing building, a mock up can be constructed to view and compare similar glazing options. Below is an example of an onsite glazing mock up:
In the end, the glazing selection process should result in a building that the Owner, Contractor and Architect will be proud of.
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