Gypsum exterior sheathing increasingly substitutes for plywood or OSB.
Gypsum, a common mineral (hydrated calcum sulphate) has several sources. Often mined, it originates as a by-product of flue gas, or as an evaporate of salt marshes and other tidal areas. As the world supply of wood diminishes and the price for wood products increases, gypsum has become an important substitute for many wood products in the building industry. Historically found most often in interior wallboard, gypsum has more recently become a substitute for plywood as a component of a fire-rated exterior, structural sheathing board. Other forms of gypsum sheathing qualify as a fire-rated non-structural sheathing product.
Non-structural Gypsum Sheathing
Often called "dry wall," or "gyp board," interior gypsum sheathing in its simplest form consists simply of a layer of gypsum bonded between two heavy building papers. It comes most often with an overall thickness of 1/2 or 5/8 inch. But more complex versions abound. One popular combination has a specially formulated waterproof gypsum core between two reinforcing fiberglass mats, with an acrylic exterior coating bonded to a polymer-modified gypsum layer. These high-tech, non-structural gypsum boards often have fire ratings, which essentially guarantees that the product will resist a controlled fire for a certain length of time, usually from one to four hours. Other premium forms of non-structural gypsum sheathing, such as fiberglass-faced gypsum, offer similar, but somewhat different advantages over plain gyp board, at a variety of price points. For residential building however, the simpler, common gyp board remains most popular.
Exterior Gypsum Sheating
Most gypsum sheathing designed for exterior use has some structural strength, but in order to qualify as a substitute for structural sheathing, the product must carry a structural rating stamp. Structural versions will be fire-resistant and will provide code-required bracing support. As of summer 2010, structural gypsum sheathing must have a printed acknowledgment that it conforms to IRC R702.3 and meets ASTM C1396. Exterior gypsum sheathing comes in either 1/2 inch or 5/8 inch thicknesses, usually as a four-foot-by-eight-foot panel, less often as a 2-foot-by-8 foot tongue and grooved panel.
A Potential Mold Problem
When contemplating structural gypsum sheathing as a substitute for plywood or OSB exterior sheathing, note that some sheathing and sheathing applications can present a mold problem, because structural gypsum sheathing is so impermeable to moisture. While it functions very well to keep external moisture out, it also prevents internal moisture from evacuating the building. Humidity becomes trapped in the walls and mold can result. Good building design and the proper selection and application of sheathing will avoid this problem. Extensive literature exists on this important subject. In addition to the Resources cited below, do an Internet search for both "rain-screen building technology," and "mold problems airtight houses."
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