Friday, February 26, 2010

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


Anonymous said...

Where is it written that gas filling wider than 16mm "reduces the efficiency and durability by approximately 35 percent"?
We fill our units to 95% (100%) using gas sensors, not timers. There may be a drop off in added performance beyond a certain gas space, but it's not reducing the performance.
Glazing pockets of fixed width have to allow 2 or 3 layers of glass from 3mm to 6mm thick. Spacers change as a result.

S.K.MARWAH said...

Architectural glass and Aluminum Systems

Project management consultants for glazing works
Tel – 91-11-26165311, 09810329925, 09310630855
E mail-
1. Argon gas has a closed valence shell ns2np6 configuration. All the orbitals in the valence shell of the noble gases are completely filled by electrons and it is very difficult to alter this arrangement by the addition or removal of electrons. Thus Argon exhibit very low chemical reactivity. With reference to its chemical inactivity,it is called Argon , the name which has been derived from the Greek word ‘Argos’ which means ‘the lazy’ . It is highly used in certain industrial applications to create inert atmosphere due to its high level of chemical inactivity/ inertness. Due to this property of inertness, it is preferred to be filled in between the glasses of a double glazing unit to create an inert atmosphere and is used for the filling of double glazing enclosures for high performance sound/ thermal insulation by creating one of the insulating barriers in the form of inert atmosphere in double ,triple or multi glazed insulating unit.
2. Argon gas is one of the factors of the various others which contribute to the /thermal control.

9. One or more design parameters can be stipulated while designing double insulated glass unit. If two of the Design parameters like 20 mm gap and use of Argon are stipulated, target stipulated to achieve a certain /thermal insulation can be achieved by enhancing/reducing the properties of other insulation barriers, properties of glass and others . As per design formulae the insulation properties are directly proportional to the gap between the glasses .,the more the gap, more the barrier. Target values for sound/thermal control in Double glass unit can also be achieved by reducing the gap to even smaller than 12 mm due to limitation of the glass panel thickness by enhancing some other properties of glass. Accordingly the total thickness of the glass panel is obtained to match the available seating space in the frame where it is to be fitted as per the architectural requirement. The decrease in property value of one parameter can be compensated by the increase in property value of the other available design parameters. The designer is at liberty to prepare various models of glass units to achieve the target values as per the standard codes and ultimately decide on the basis of architectural requirement and other limitations.
10. There is nothing like a partially filled or fully filled double glazed unit or the optimum gap. In any gas, we have a very large number of molecules that are only weakly attracted to each other and are free to move about in space. For a given volume or mass of gas the equivalent in gaseous or liquid phase is calculated and accordingly the gas is filled.
11. Argon filling is one of the property enhancers for the insulation barrier .If the insulated glass unit is designed to fill the gap in between the glasses with Argon, the volume of the gas shall commensurate with the volume of the gap otherwise the glass unit doesn’t achieve the target design value. It will be like designing a glass unit with Argon fill but instead using more of dry air, which will be an under specification unit. There is no fixed criteria like 20 mm space in between glasses, there is no optimum gap like 14 mm for Argon as described above.

S.K. Marwah

Anonymous said...

How do you determine the Overall thickness of an IG UNIT?

The measurment is 28 x 56 1/4

1/8 clear
1/4 mill air