Because glass-reinforced gypsum has been such a familiar item for more than two decades, not a lot of education about it goes on anymore. Consequently, many people who have come into the building industry recently don't understand exactly how the material works mechanically or appreciate why it is a standard for exterior building sheathing. Here are some useful facts about glass-reinforced gypsum board.

Georgia-Pacific introduced its DensGlass Gold reinforced gypsum sheathing in the mid-80s with a formulated, moisture-resistant core that is quite different from the gypsum core of traditional paper-faced gypsum board. Two factors quickly made the glass-reinforced gypsum board popular with architects and contractors: It can withstand exposure to weather during construction; and its ease of handling and cutting contributes to rapid installation.

Since its introduction, millions of square feet of the distinctively gold-colored board have been installed and it is highly recognizable among the skylines of cities throughout North America.

Different function than paper

To begin with, the function of the glass mat in a glass-reinforced gypsum panel is quite different from the function of the paper covering on a paper-faced board. With paper-faced boards that meet the requirements of ASTM C79, the paper is bonded to the surface of a sheet of gypsum.

In a glass-reinforced gypsum board that complies with ASTM C1177, a matrix of reinforcing glass fibers is embedded directly into the gypsum. Although the texture of the mesh is visible on the face of the board, the majority of the thickness of the mat is buried in the gypsum core. The embedded reinforcing glass fibers add strength in a manner similar to the effect of reinforcing steel in a concrete slab.

Nail-pull tests

One common measure of strength for building sheathing is called the "nail-pull" test. If you have hurricane-force winds around a building, on the downwind side of the structure you may get as much as 100 pounds per square foot of negative pressure trying to pull the sheathing away from the studs. How much negative wind pressure will the exposed sheathing withstand? The nail-pull test gives an indication. It measures the amount of force it takes to break the gypsum sheet and pull it over the head of a fastener. As it turns out, this is primarily a function of the core composition working with the embedded reinforcing glass fibers.

Here's how the nail-pull test works: A 1/4-inch (6.35mm) round shank is pressed against the surface of a sheet of sheathing, guided by a 3/32-inch (2.5mm) pin in a pilot hole. The panel is supported at a standard radius of 11⁄2 inches (38.1mm) from the pilot hole. A press applies pressure to the shank until the panel breaks. This simulates negative wind forces pulling the sheet away from the fastener. With fiberglass-reinforced gypsum, even after the outer surface of the glass mat fractures, a much heavier load must still be applied to break through the gypsum. By far the largest percentage of the nail-pull test resistance is provided by the reinforced core. Similarly, fasteners driven flush with the surface of panels may fracture the surface layer of the glass mat but the major thickness of the mat is under the surface where maximum nail holding occurs.

Backing up the nail-pull numbers are tests conducted by independent testing organizations that test wall panel assemblies for wind loads according to ASTM E330. These organizations test actual wall assemblies and expose them to negative air pressures. Assemblies using some manufacturers' sheathing have been tested to values in excess of 140 psf.

Moisture resistance comes from the core

The glass mat in sheathing has to be a porous, open mesh in order to embed itself securely in the gypsum compound. That means the water-resistance board is almost entirely a function of the core. The formulated core of glass mat sheathing is significantly different from the gypsum core in paper-faced boards. The core of glass mat sheathing is highly resistant to moisture absorption and it retains its mechanical properties through repeated cycles of water exposure and drying.

Moisture-resistant gypsum board reinforced with embedded glass mats has been used for commercial building sheathing for decades-and with good reason. The sheathing retains its mechanical strength through dampness and drying cycles during installation and while cladding is being installed. That's important for resisting wind forces, both positive and negative.

Besides the obvious merit of preserving mechanical integrity, when it's exposed to weather, the moisture resistance of sheathing brings another benefit: It helps compress construction schedules.

"It speeds up the ability to dry-in a building," says Eddie Redmon, a specifier with the architectural firm Howell, Rusk and Dodson, which handles high-profile healthcare projects like Emory University Hospital and Children's Healthcare of Atlanta. "You never have to worry about weather or anything else forcing you to take it down and do it over."

The schedule compression that paperless, moisture-resistant gypsum board allows may produce construction cost reductions. Earlier move-in may also bring owners faster cash flow.

Reduce moisture and potential food sources to fight mold

The first big problem with construction is that it's done outdoors. The second problem is that you can't control the weather. Construction schedules may force a contractor to work under damp conditions, and rain and dew can be trapped in sheathing materials, holding water that can foster the growth of mold.

That's one of the reasons architects and contractors specify glass-reinforced gypsum board sheathing. Its unique water-resistant core and non-organic fiberglass facings resist mold growth. Here's why: Mold needs four key factors to grow:

• Mold spores
  
  
• The right temperature range
  
  
• Moisture
  
  
• A food source

If any of these four are missing, fungi can't grow. But these factors can be difficult to control. Mold spores are always present in both indoor and outdoor air, and most molds can flourish in any ambient temperature from 40 to 100 degrees F (4 to 38 degrees C).

You should exercise control over moisture but building sheathing still may get wet, either during installation, from weather before dry-in, or when warm, moist interior air condenses inside a wall assembly. A sheathing's resistance to moisture absorption contributes to mold resistance, because it reduces retained water.

You also can exercise control over potential food sources for mold. Replacing the paper facings used on traditional gypsum sheathing with the inorganic reinforcing glass fibers eliminates a potential food source for mold.

When tested, as manufactured, in accordance with ASTM D3273, DensGlass Gold sheathing scored a 10, the highest level of performance for mold resistance under the ASTM D3273 test method.

It may take years for the true cost of not using mold-resistant materials to surface. That's why it makes sense to specify building sheathing that handles tough moisture conditions gracefully. Glass-reinforced gypsum provides strong resistance to moisture absorption and delivers superior mechanical performance in cycles of dampness and drying. When you factor in potential construction defect claims and insurance costs, the mold-resistance features of sheathing are very cost-effective.

Reinforcement near the surface adds to beam strength

It's important to understand the mechanical benefits of having reinforcing material near the surface of a gypsum sheet. Think of the gypsum sheet like a wide, thin, box beam or truss. Bending forces on the sheet are translated into tension on one surface of the sheet and compression on the other. Reinforcement near the surface of the sheet contributes significantly to the beam strength that resists bending forces. Reinforcement at the center of the sheet contributes very little to beam strength. Beam strength becomes important when handling sheets, and for resisting positive wind pressures on the windward side of a building.

The heavy glass fibers used in glass-reinforced gypsum are in the same size category as the thick fibers used to reinforce boat hulls, golf-club shafts and bathtub surrounds. These are much larger than the fibers used historically in insulation. These reinforcement fibers are not listed as a respiratory hazard and never have been. They are simply too thick to penetrate the lower respiratory passages.

Contractors who have used sheathing for years appreciate its performance, relative lightweight and ability to score it with a utility knife and snap off a clean cut. The combination of strength, weather resistance and ease of installation promises to keep glass-mat gypsum sheathing as an industry standard for a long time to come. For further information, always consult with the manufacturers and/or the Gypsum Association on proper installation techniques, product specifications and other inquiries.