Like many other building components, curtain walls do not last forever. “These installations have a finite service life,” says Thomas Schwartz, president of the Waltham, Mass., engineering firm of Simpson, Gumpertz & Heger (SGH), which has a major curtain wall consulting practice.
Schwartz says many building owners are shocked to learn that a major component of the exterior wall, insulated and laminated glass units,may require replacement well before the remainder of the curtain wall. He finds that the durability of major exterior wall components is often insufficiently evaluated as part of the due diligence in buying a property.
Numerous noninvasive testing methods that do not require removal of the glass are available for determining the condition of insulating glass (IG) units, which consist of two or more lites of glass separated by a hermetically sealed air space. The most basic is ASTM E576, “Standard Test Method for Frost Point of Sealed Insulating Glass Units in a Vertical Position.”
As part of ongoing maintenance, this noninvasive test can help a building owner decide whether future glass replacement needs are likely to be minor, and handled as maintenance, or if a wholesale replacement is probable.
ASTM E576, which has been in use for about 20 years, evaluates the durability of the hermetic seals. It determines the moisture content of the air within the insulating unit, which indicates the general progress toward visible “fogging” of the unit, a condition known as seal failure.
ASTM E1392, adopted in 1998, is a non-destructive test that focuses specifically on silicone structural-glazed installations and provides, for the first time, a procedure for field evaluation, says Schwartz. The importance of this analysis is becoming more important because some structural silicone glazing installations are now more than 20 years old. He says ASTM 1392 is not widely known among design professionals, owners, or contractors.
SGH’s investigation of the spontaneous breakage a skylight glazing at a retail center in Cambridge, Mass., underscored a problem that can be averted by proper glass specification. Fully tempered glass was used to glaze the skylight, and about 10% of it failed in the first year after installation (an inner lite of laminated glass prevented the fractured glass from falling on shoppers).
Schwartz says this spontaneous breakage, which was triggered by nickel sulfide inclusions in the glass, rarely happens, but when it does, it can affect an entire batch of glass.
Glass manufacturer PPG issued an advisory in 1988 stating that heat-strengthened glass is the preferred heat-treated product for street wall façades, and that it would no longer support the use of fully tempered glass for such applications. Despite this warning, many producers and specifiers still promote the use of fully tempered glass in locations would pose a threat to people nearby.