Under Pressure: Tips for Avoiding Over-Compression in Curtainwall

By Helen Sanders

The glazing guidelines issued by industry associations recommend the edge pressure exerted by a fenestration system on the edge of an insulating glass unit (IGU) not exceed 10 pounds per linear inch (PLI). Exceeding this pressure can cause the IGU’s primary sealant to extrude out of the sealing area into the vision area, resulting in “PIB scalloping.” This causes not only an aesthetic issue, but also can shorten an IGU’s lifetime by irreversibly damaging its primary seal. In extreme cases, over-pressure can result in glass breakage.

Pressure plates secured with fasteners are used to apply pressure to the perimeter of IGUs to hold them in place on curtainwall systems and to create an air and water seal. Curtainwall manufacturers design these systems carefully, with customized gaskets and specified fastener spacing and fastener torque settings. These settings ensure that no more than 10 PLI is applied uniformly along the glass edge when installed correctly. Deviations in any of these parameters can cause IGU edge over-compression.

Tool Evolution

According to Don Pangburn, product designer at Oldcastle BuildingEnvelope®, there have been increasing cases of over-compressed IGUs, which he correlates to installation tool evolution. He remembers when installers used 120-volt corded impact drivers and made final adjustment to fasteners by hand using a regular torque wrench. Subsequently, installers began to use adjustable cordless, battery-operated 8- to 12-V drills.

In more recent years, he notes the advent of lightweight, long-lasting, lithium-ion battery powered impact drivers: “This little one-handed lightweight driver was any fabricator’s dream, easily switching out a Phillips driver to a 3/8-inch hex head in three seconds. The problem is the torque. These drivers typically are not regulated with a limiter that’s anywhere near the required torque for pressure plate installation. With current drivers, the average torque is about 125 foot-pounds, which is equivalent to 1500 inch-pounds.” Typically, pressure plate fastener torque specifications range from 50- to 100-inch-pounds. It is easy to see how fasteners can be over-tightened using these drivers. The use of limiters or a torque wrench for final adjustment is highly recommended.

Curtainwall Considerations

Adopting a “one-torque fits all” approach also can lead to problems, since not all curtainwall systems have the same torque specifications. If fasteners are too close together, even if torqued correctly, over-compression can occur. While most pressure plates come pre-drilled to control spacing, fasteners can end up too close at the ends of the horizontals, or additional holes may need to be drilled if not positioned well. Instead, Pangburn recommends considering hole position when cutting horizontals, too.

Helen Sanders is in strategic business development for Technoform North America Inc. in Twinsburg, Ohio. Read her blog each month at usglassmag.com/insights.

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