For decades we have used gypsum sheathing in commercial construction. Standard gypsum sheathing can be traced back to pre-World War II. In the 1940s, water-repellant face paper was added to give a good product more value. The sheathing would come in square or tongue-and-groove ends for a snug fit. Gypsum sheathing saw the next improvement when the gypsum core could be blended with a petroleum-based product, thus making it water resistant through and through. The ATSM that covered the manufacturer of this product was C-79 and has since been replaced by C-1396.
But times continue to change and our industry has a history of stepping up to meet challenges. Mold and mildew became a problem that no one was able to completely escape. While the gypsum core of the sheathing does not readily support the growth of mold, the face paper can. The invention of fiberglass-faced gypsum sheathing was developed to solve that problem. It took a little time to catch on, but when it did, it seems we never looked back. This new sheathing was more than just a new facing material—it also has a silicone infused gypsum core. The new sheathing could be left exposed to inclement weather for several weeks longer than anyone had previously thought possible. This created a need for new ATSM, and so ATSM C-1178 was developed.
It would seem this was enough, as we had the problem handled but adaptations and/or improvements just kept coming. Now designers can opt for sheathing with no facing material at all. These fiber-reinforced gypsum panels created the need for even another ASTM, and C-1278 was born. Some exterior sheathings are now even made from magnesium. The only downside to all the selections is the challenge for the designers and contractors to pick a product.
Decisions, Decisions
There are so many choices. The first item to consider is the fire resistance of the assembly. Virtually all the sheathings are fire resistant to a degree. However, if a particular fire assembly test is specified, the gypsum sheathing listed in that test must be used to comply with that test. Do not mix and match sheathings without approval from the local building department. For improved water resistance, you will likely have to do your homework. This may even be a bit of preference as all have to pass various other ASTM standards to prove resistance to water, humidity and absorption.
Oops
The common thread for all these sheathings may be the application of the panel, regardless of its composition. This generally falls under the standard of ASTM C-1280. This was developed from the traditional Gypsum Association document GA 253 and has been around a long time. This particular document provides common sense, as well as specific caveats on installing gypsum sheathing. The not well known issue of control joints in gypsum sheathing could become problematic. GA 253 states install wherever specified, and that was rarely, if ever, done or needed. However, ASTM C-1280 is not so optional. The ASTM has the statement that control shall be installed not to exceed a distance of 30 feet. This became an issue when we have moved away from our more prescriptive codes toward the more performance/referenced based code. This makes ASTM C-1280 more powerful and could enforce the more restrictive language. Industry experts are gathering to work this out, but changes at ASTM can take time and construction projects do not stop and wait for ASTM to catch up. The issue is becoming more complicated with the greater use of fluid-applied style air barriers. These manufacturers are typically experts at building science and energy savings, but not necessarily on the history or expertise of exterior sheathing installations.
The good news is that most experts seem to agree that one-piece control joints are not, nor have ever been, common with exterior sheathing installations. Conversely, they agree that if the structure or framing has an expansion joint or other designed deflection, the need for a similar joint is needed in the sheathing and the exterior cladding at the same location. This would typically be a metal-framed, head-of-wall location with deflection at the floor line. The industry, manufacturers, designers, code authorities, contractors and bureaus will continue to work together and get this sorted out. Contractors should read specifications carefully, talk to the air barrier manufacturer and ensure they are in compliance for the recommended installation of their product over the exterior sheathing that is used.
