FGIA Analysis July/August 2022

July 28th, 2022 by Nathan Hobbs

Handling the Pressure: With Gas-Filled Insulating Glass, There Are Numerous Options

By Bill Lingnell

Insulating glass units (IGUs) are at the heart of today’s energy-efficient windows, but they are not rigid assemblies. As ambient air pressure increases or decreases compared to the pressure of internal gas, the glass lites on either side of the central cavity can bow in or out. This can be prevented, however, through custom design that anticipates jobsite conditions or through the use of special devices. Some IGUs are manufactured or installed with a pressure compensation scheme that adjusts internal pressure based upon the intended final installation site. There are several methods for accomplishing this, each requiring certain considerations during installation (glazing).

Capillary or breather tubes: Permanent air tubes are inserted in air-filled IGUs to allow for pressure equalization at some point after fabrication. Stainless steel or aluminum capillary tubes are quite small with inside diameters of a few hundredths of an inch. Breather tubes, made of metal or plastic, are similar but significantly larger in diameter. Neither capillary or breather tubes are applicable for gas-filled (argon or krypton) units. Usually, the tube is sealed on-site as part of the glazing operation.

Desiccant adsorption/desorption: A sacrificial gas mixed with the insulating gas within an IGU cavity is adsorbed into a desiccant material. This reduces the cavity pressure relative to the atmospheric pressure at the time of manufacture. It allows sealing at the atmospheric pressure of the building site, without puncturing the spacer or glass for pressure adjustment.

Pre-inflating/pre-deflating: To mimic the effect of the atmospheric pressure typical at the installation site, IGUs can be fabricated with preset positive or negative pressures by removing or injecting gas. This method does have some limitations.

Temperature manipulation: The entire IGU is heated or cooled prior to sealing and/or gas filling, affecting the gas volume. Note that temperature manipulation can also cause convex or concave conditions in the IGU at the time of installation.

Bladder relief system: The use of a flexible foil or coated plastic container (bladder) system can relieve pressure differentials due to elevation changes and variations in solar heating between the points of manufacture and installation. A bladder system can be temporary, for use during transport, or permanent upon installation. A temporary bladder is removed and access tubes sealed once the IGU reaches the jobsite. A permanent bladder remains in place to occasionally equalize pressure as needed.

Post fabrication valving: A secondary fabrication process is required at or near the installed location involving the use of a one-way valve installed into either the IGU spacer or one of the panes of glass before gas filling occurs. Such valves can be depressed mechanically to adjust the pressure in the cavity relative to the atmospheric pressure at the place of manufacture.

Each of these methods and their associated application guidelines is covered in thorough detail in the newly published FGIA manual, IGMA TM-3200-21, Design Considerations for IGU Cavity Pressure Compensation. Commercially available software programs that can predict the glass deflection under the applicable variables are also listed in the manual.

Bill Lingnell is technical consultant for the Fenestration and Glazing Industry Alliance (FGIA).
blingnell@fgiaonline.org

To view the laid-in version of this article in our digital edition, CLICK HERE.

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