In part four, we took a generic look at gypsum-based sheathing; one of the most popular non-combustible types. The two basic types of gypsum-based sheathing, paper-faced and glass-mat faced, were discussed as to their performance characteristics, advantages and disadvantages. Also discussed was the latest entry into the gypsum-based sheathing category: gypsum/cellulose.

This month, we'll continue this on-going series all about sheathing with an in-depth look at USG's gypsum/cellulose sheathing product: Fiberock Aqua-Tough Sheathing; the first gypsum/cellulose sheathing product.

What's old is new again

In early 2000, U.S. Gypsum Corp. began distribution of its new sheathing product Fiberock Aqua-Tough Sheathing. The Fiberock family of gypsum-fiber products includes:
  • Aqua-Tough Sheathing
  • Fiberock Underlayment
  • VH1 Abuse-Resistant Interior Wallboard Panels
  • XL Interior Wallboard Panels

USG uses one manufacturing facility for the production of Fiberock in Gypsum, Ohio. In 1995, the plant began producing Aqua-Tough on a "demonstration line."

Panels of Fiberock are formed of gypsum fiber cores having a high density of 75 pounds per cubic foot. To reduce the weight of the panel, a lightweight perlite core is used and the panels are formed under pressure. Since Fiberock underlayment is only 1/4-inch thick, no perlite is used in its core, making it the only Fiberock product without perlite. Because of the eco-disruptive effects to tropical forests, gypsum-core underlayment is used widely as an alternative to Lauan plywood underlayment.

Alike but different

Though gypsum wallboard and underlayment share many things in common, with Aqua-Tough sheathing there are significant differences, as well. Besides the lack of perlite, Aqua-Tough sheathing is produced via a chemical process that incorporates a sealer throughout. The sealer is not used for moisture/weather resistance but rather to improve bonding strength. Fiberock gypsum wallboard was found to be "thirsty" when painted. When used as a substrate for EIFS and direct-applied exterior finish systems, the sealer prevents the premature drying-out of adhesives (commonly referred to as glue suction) that would undermine the bond between the sheathing and EPS insulation (aka bead board).

As mentioned in part four, the back-face of Aqua-Tough sheathing has embossed, diamond-pattern, rough-textured drainage channels that will allow for gravity drainage of any moisture trapped behind the sheathing (plus or minus 1/10 inch drainage plane). With a full one-year warranty (if properly installed), for weather exposure and 10 years against manufacturer defects, Aqua-Tough was intended to compete directly with glass-mat faced sheathing (offering a six-month warranty) as a substrate for water-managed EIFS, such as drainage and/or rainscreen EIFS (not barrier/face seal).

You get what you pay for

Though it has a cost premium of about 40 percent when compared to faced or surface-reinforced gypsum-based sheathing, gypsum/cellulose has several advantages over them. From a performance point-of-view, Aqua-Tough has significant advantages insofar as:
  • Durability (wind pull-off resistance)
  • Strength (flexural and nail-pullout resistance)
  • Water/moisture resistance (throughout cross-section)

Since it has no paper or glass-mat facing, Aqua-Tough cannot delaminate and the strength of the panel is uniform throughout its cross-section making it core reinforced rather than surface reinforced. The Achilles' heel of paper/glass-mat faced sheathing is its reliance for strength on the paper facing which can be undermined by simply overdriving fasteners and the face acts as the primary water-barrier; there is no redundancy built in. Being the only gypsum-based sheathing that can be installed over framing spaced at 24 inches o.c. (typical for metal framing), it meets the 30-psf wind-load criteria for one- and two-story commercial construction with EIFS (L/240 design criteria with fasteners spaced 8 inches o.c.).

Being stiffer and flatter than paper/glass-mat faced gypsum sheathing, the panel's uniform strength throughout provides immunity from damage to the surface and/or from cutting the panel and will not weaken Aqua-Tough in any way. Paper-/glass-mat-faced sheathing is scored and snapped in the field by a simple utility knife, making cutting them as easy as standard gypsum wallboard. Aqua-Tough is more difficult to cut in the field. Typically, a circular saw is used to cut Aqua-Tough but it doesn't cut as cleanly as paper/glass-mat sheathing. On the plus side, it will not release airborne glass-fiber particles when cut.

Green perspective

The cellulose fibers within the patented Aqua-Tough formulation are made from recycled corrugated containers. The gypsum used in the mix with the cellulose is synthetic gypsum (see past article entitled "Gypsum Synthesis"). Made from a process known as flu gas desulfurization, the process produces the purest form of gypsum. A high-sulfur content coal burning power plant nearby to the Gypsum, Ohio, plant provides all the calcium sulfate (gypsum) for use in making Aqua-Tough.

In 1990, the Clean Air & Water Act mandated that coal burning power plants clean up the emissions from their chimneys. Sulfur emissions were then and are now the main cause of acid rain. Through the FGD process, a mist of water and limestone (calcium) chemically combines with the sulfur effluent to form calcium sulfate (CaSO4)-synthetic gypsum. Not only does this process provide the gypsum industry with a limitless supply of non-eco-disruptive gypsum, it also cleaned up 96 percent of sulfur emissions into the atmosphere. Thus, Aqua-Tough has a very high recycled-content of 95 percent and earned Scientific Certification Systems' Cross + Globe certification.

Per ASTM D3273-00, "Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber," Aqua-Tough scored a 10-the highest score possible for mold resistance. Aqua-Tough is approved by all three model building codes for use in non-combustible construction achieving, per ASTM E84, a Flame Spread/Smoke Developed rating of 5/0 respectively.

Available in thicknesses of both 1/2 inch (2.2 pounds psf) and 5/8 inch (3.0 pounds psf), a type X core (per ASTM C79/C1278) and square-edged panel sizes of 36 inches and/or 48 inches wide by 96 inches long, a 1/2 inch by 4-foot-by-8-foot Aqua-Tough panel weighs about 70 pound, making it more difficult to handle and distribute than standard gypsum sheathing. Unlike wood-based sheathing (plywood/OSB), Aqua-Tough cannot be used as a nailing base between framing members. However, it has good thermal resistance (R value): at 1/2 inch ~ 0.5; at 5/8 inch ~ 0.6; and good permeability: at 1/2 inch ~ 35; at 5/8 inch ~ 28. The combination of a sealer and no facing provides for superior attachment of adhesives. In "hurricane alley," Aqua-Tough passed the stringent protocols of Miami-Dade County including:
  • Missile impact
  • Water/air leakage
  • Cyclical air pressure

USG's Aqua-Tough sheathing, though heavier, more difficult to field-cut and costlier than standard surface-reinforced gypsum sheathings, has demonstrated an excellent cost/performance value. With a better overall performance than glass-mat faced sheathing, it often has a lower installed cost, as well. On one- and two-story projects, about a $0.22 psf savings was realized when Aqua-Tough was used in lieu of glass-mat sheathing.

In part six, we'll complete our discussion of gypsum-based sheathing with a look at two new products: one paper faced and the other glass-mat reinforced.

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