In last month's part three of this ongoing series, we put sheathing in its historical context, discussed the four basic types of sheathing products and selection criteria for choosing sheathing materials. In the first two parts, we discussed broadly the environmental impacts of wood-based sheathing products (plywood and oriented strand board) and the general use and purpose of sheathing.

This month, we'll take an in-depth look at one of the most widely used sheathing product types: Gypsum-based.

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As mentioned in part three, gypsum-based sheathing is one of the four basic sheathing types inclusive of:
  • Wood-based
  • Cement-based
  • Plastic/foam-based
  • Gypsum-based

First introduced as a non-combustible sheathing product in the '60s, gypsum-based sheathing is widely used in a variety of building types and sizes. With Georgia-Pacific's introduction of DensGlass Gold fiberglass-mat faced gypsum sheathing in the '80s, the market-share for gypsum-based sheathing was transformed and expanded exponentially. Gypsum-based sheathing provides excellent fire-resistance characteristics, in much the same way standard Type-X gypsum wallboard does.

The gypsum core serves as a natural fire suppressant due to the mineral (incombustible) nature of gypsum rock and its high water content which, when subjected to the heat of a fire, releases its integral water content as steam, thus acting much like a sprinkler system to retard the flames. For its fire performance characteristics, gypsum-based sheathing is widely used in non-combustible construction where it is required by code.

Standard Type-X gypsum typically has chopped glass fibers and/or vermiculite added to the "recipe" of ingredients (i.e. starch as a binder) that makes up the gypsum core and provides the gypsum panel's integrity under fire conditions. With proper fastening, gypsum sheathing is strong enough to transfer lateral wind loads-both positive and negative, to the structural framing for a building's complete lifecycle. However, gypsum sheathing does not have the same ability to resist shear forces, as does wood-based sheathing. Typically, where gypsum sheathing is used over exterior wall framing, internal and/or diagonal "X-Bracing" is required to supplement the wall assembly's shear (racking) resistance. In general, there are two types of gypsum-based sheathing products:
  • Paper-faced
  • Glass-fiber mat faced

Paper-Faced Gypsum Sheathing

The original gypsum-based sheathing is also known as surface-reinforced sheathing, whereby the water-repellent paper faces (on both sides and long edges) provide the panel's strength and fastener holding power. Supplemental core reinforcing may include cellulose fiber added to the Type-X gypsum core. Paper-faced gypsum sheathing meets all ASTM C70 requirements and has the advantage of installing quickly and easily-just like its interior grade cousin gypsum wallboard.

When installed properly, it will shed water for several months but is typically only warranted for 30 days exposure to the elements. If and when exposed to moisture for extended periods, the paper faces delaminate and the gypsum core can/will dissolve. Since the gypsum core provides no significant resistance to fastener pullout, the paper face alone provides only marginal fastener pullout resistance. If the fastener is overdriven and breaks the paper face, the holding power of the fastener is completely compromised.

Glass-Mat Faced Gypsum Sheathing

Like paper-faced gypsum sheathing, glass-mat faced gypsum sheathing is of the surface-reinforced variety, therefore it relies on its face, not its core, for water and pullout resistance. However, because of its compliance with ASTM C177 requirements, improved water resistance, lightweight and ease of cutting and installation in the field, it has become the most popular sheathing product choice for commercial, non-combustible construction projects-particularly as a substrate for EIFS. Though the core can/will degrade under extended periods whereby it is exposed to moisture, it is typically warranted for up to six months exposure. In effect, the glass-mat facing acts as an effective moisture/weather barrier superior to the water-repellent paper on standard gypsum sheathing.

Like gypsum sheathing, overdriven fasteners that break the glass-mat face compromise the holding power of the fastener subjecting the panel to "pull-off" by lateral wind loads. Since the facing is made of embedded glass fibers, these fibers will become airborne when cut and handled. Like fiberglass insulation, these fibers can cause skin irritation and are suspected of causing the same kind of damage to lung tissue as asbestos fibers when inhaled. For this reason, the same precautions taken when installing fiberglass insulation should be taken when installing glass-mat faced gypsum sheathing.

New Kid on the Block

The newest category of gypsum-based sheathing is known as gypsum/cellulose. Unlike its predecessors paper and glass-mat faced gypsum sheathing, gypsum/cellulose sheathing is core-reinforced rather than surface-reinforced. Made of cellulose (wood) fibers, gypsum and water-resistant additives, it has a uniform, homogenous cross-section without a facing of any kind. As such, it does not rely on a surface layer (paper or fiberglass) for:
  • Moisture resistance
  • Strength
  • Holding power

It will not lose strength if a fastener is overdriven or if the face is cut. If a cut-end is exposed to moisture, it will not decompose and it provides a stiffer, flatter, smoother surface capable of resisting shear forces. A diamond-shaped drainage pattern embossed on the back of each panel provides a "drainage plane" for any water that gets behind the sheathing to drain down and out by gravity flow. For these and other reasons, gypsum/cellulose sheathing has gained in popularity and market share since its introduction in 2000. Despite a cost premium of about 40 percent (as compared to standard gypsum-based sheathing), it is being specified with increasing frequency for all types of construction projects.

In part five, we'll take an in-depth look at USG's Fiberock Aqua-Tough Sheathing-the first gypsum/cellulose sheathing product offered/available in the exterior sheathing market.

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