Volume 38, Issue 1, January 2003


Glazing 101
    Basics for Good Residential IG Unit Glazing

by Werner Lichtenberger

Proper glazing is one of the most important attributes to the long service life of a residential insulating glass (IG) unit. Glazing is necessary to secure and support the glass and prevent the infiltration of water into the glazing cavity. It also allows for movement between the glass and sash and guarantees that vented systems are designed correctly.

Protect Against Moisture
Water is a natural enemy of window units, and water inside the glazing cavity is the leading cause of IG unit failure. If the glazing is inadequate or incomplete, water may enter from either side of the glass and cause damage. In fact, a Sealed Insulating Glass Manufacturers Association study indicated that a failure rate of 15 percent over ten years can be expected if the edge seals are in contact with water.

Choose the Proper Glazing System
When choosing the appropriate glazing system for residential IG units, manufacturers should consider economics, environmental considerations and production requirements. The initial analysis should include the determination of a glazing system’s relative cost compared to its anticipated service life. Naturally, the service life is affected by how and where the system is used.

A temperature- and humidity-controlled environment will limit physical changes in a glazing system and ensure better operation. For instance, aluminum, PVC and wood sashes are all subject to dimensional changes caused by fluctuations in temperature and atmospheric conditions.

The type and design of sashes and glass are also important. Complex designs and large sash dimensions as well as large glass lites and the use of low-E glass all place great stress on the glazing system.

Achieving Good Glazing
Proper glazing is a compromise between the ideal criteria for effective glass support and waterproofing and the practical consideration of economics, tolerances and good installation. Manufacturers should heed the following methods to achieve this 

• Glaze above 40° F (4° C) — Frost and condensation can contaminate 
substrates below this temperature;

• Form a watershed — Gunnable sealants, when applied as cap beads, should form a bevel, or watershed, away from the glass;

• Use a heel bead — When applying a heel bead, lap onto the glass a minimum of 3/16-inch and make positive contact with the sash;

• Surface preparation — Clean the sash and glass edge prior to glazing;

• Setting blocks — Generally, use Neoprene or EPDM with a Shore A hardness of 80-90 durometer and placed at quarter points with a minimum length of 1 inch;

• Avoid glass damage — Handle IG units carefully to avoid edge damage and stress cracks;

• Tape compression — Keep all glazing tapes under proper compression;

• Air barrier — When glazing vented or pressure-equalized systems install an interior air barrier around the 1G unit’s perimeter;

• Clearances — Observe minimum face clearances – 1/16-inch is generally the minimum on residential windows;

• Vent holes — The glazing cavity should be vented to the exterior; size, shape and position of the vent holes is critical. Minimum sizes of 1/4-inch in diameter or 3/16-inch by 3/8- inch slots should be used;

• Insulating units — Glazing must be compatible with the IG sealant to provide a long-life exterior/interior air and water seal and prevent water from contacting the edge seals.

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Remember that a good glazing design will ensure improved service life of a residential IG unit. This data should be provided to homeowners to minimize IG unit failures. 


LICHTENBERGER Werner Lichtenberger
, P. E., serves as projects manager for TruSeal Technologies, located in Beachwood, Ohio. 


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