Discussing double glazing

I got an e-mail the other day from a Mr. J.E. Swain, and he asked two questions:

1. "Why is the double-glazing industry producing 20-millimeter space partially filled argon gas units? Considering argon gas optimum gap between glass panes is 14 mm with a maximum of 16 mm, any wider, the gas reduces the efficiency and durability by approximately 35 percent, and why partially filled instead of full fill? 2. "Why is there no easy way for the consumer to determine the extent of argon fill of any double-glazed unit that is supplied?"

I forwarded his e-mail to a few experts within the industry, and this is what I got:
On the first question, Margaret Webb, executive director of the Insulating Glass Manufacturers Alliance, Ottawa, Ontario, suggested contacting the manufacturer directly. In general, she said, "Manufacturers will gas fill to the concentration required to obtain a specific rating for code compliance. This can vary from manufacturer to manufacturer as the overall thermal performance values are determined by the entire window assembly; [it] is not just a function of gas fill. Typically, gas fill will improve thermal performance of a unit by 5 percent."

Tracy Rogers, technical director, Edgetech I.G., Cambridge, Ohio, said that some manufacturers do not have optimized IG packages and best fit IG systems into existing framing systems. "A window system design must consider other dimensional requirements than just the IG unit overall dimension," he said. "Other manufacturers are designing for acoustical performance and, in this case, the greater the glazing gap the better; regardless of gas type. Lastly, some don’t understand proper design for thermal optimization."

Jeff Haberer, tech services engineer, Cardinal Corp., St Louis Park, Minn., said that introducing 20-mm spacers isn't a trend that he's seeing. "We have offered a 19-mm spacer for some time as it allows the use of two 3-mm glass lites to make a 1-inch overall thick IG unit that fits in most commercial glazing frames," he said. "This, plus a slight improvement in sound control (3dB) are the only advantages that I see."

On the second question, Webb said that the industry has not developed a non-invasive method of determining the gas fill in the field. "Calibration of any measurement device would be required for the specific field conditions, which change frequently even within one day," she said. "At the present time, the method of determining the gas fill of a unit is by removing the window from the building envelope and sending it to a lab. This need not be a destructive test but there are measurement devices that can be more easily calibrated in a lab setting where the environmental conditions can be controlled."

Rogers offered: "If a window manufacturer provides product that is certified under the National Fenestration Rating Council, the design gas fill content (argon, krypton, etc.) is listed on the temporary label (on the glass). If this label isn’t available, the product can be traced by the CPD # on the permanent label as posted on the NFRC’s Certified Product Directory on the NFRC Web site. If not NFRC certified, then they have to go by what the manufacturer provides."

Haberer said, "The difficulty in measuring argon has been pursued for decades now. There are now non-destructive ways of determining gas fill. They are relatively easy to use and determine gas content, but they cost approximately $12,000."

In the U.K., the British Fenestration Rating Council has a parallel program and requirements, Rogers added.

I'd love to get your feedback as well, and I'm sure so will Mr. Swain. Add your answers as comments or e-mail me, and I'll make sure they reach him.

—By Sahely Mukerji, Senior editor, Glass Magazine