SYMPOSIA PAPER Published: 01 January 1989

Special Considerations in the Design and Use of Small Insulating Glass Units


The use of small insulating glass units (IGUs) in residential construction is growing in popularity. Architects and window manufacturers use small units to recreate colonial and other historic window designs, while maintaining the energy efficiency associated with insulating glass. Small insulating glass units have unique characteristics resulting from the large interpane air-space as compared to glass thickness and the relative stiffness of the glass sheets and their inability to deform to relieve pressure differences between the ambient air and the air within the sealed interpane air-space. This research explores the relationship between glass size, glass thickness, air-space thickness, sealing conditions, exposure conditions and the resulting stresses in the glass, and performance of the units. We explore the significance of glass surface damage, spacer breather tubes, and glazing conditions as they relate to small IGUs.

Small insulating glass units without pressure relieving breather tubes are subjected to prolonged pressure differences between the interpane air-space and the ambient air. The high stiffness of the glass does not permit the glass to deflect in response to these pressure differences as do lites of glass in larger units. Stress in the glass can be significantly increased if the glass is held in place by a rigid glazing method, as is often the case with wood frames. These sustained stresses may be of sufficient magnitude to cause moderate to high rates of fracture in glass of good surface quality and extremely high rates of breakage in glass of poor surface quality. Residential glass often has moderate to poor surface and edge quality owing to residential glazing and maintenance methods, has rigid edge restraint, and is assembled into small insulating units. The combination of these factors results in the high breakage rates of some residential IGUs.

Author Information

Schwartz, TA
Simpson Gumpertz & Heger Inc., Arlington, MA
Zarghamee, MS
Simpson Gumpertz & Heger Inc., Arlington, MA
Cole, GG
Simpson Gumpertz & Heger Inc., Arlington, MA
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Developed by Committee: C24
Pages: 121–134
DOI: 10.1520/STP22995S
ISBN-EB: 978-0-8031-5102-4
ISBN-13: 978-0-8031-1286-5