Shiplap is enjoying a newfound wave of popularity due to home improvement reality shows, but its appeal goes beyond aesthetics. In building construction, the primary role of structural sheathing is to withstand wind loads, transferring them to the backup wall framing and the building structure, says Greta Eckhardt, AIA, senior specifications writer for Boston-based Payette. Sheathing also provides a substrate for insulation and air, water, and vapor barriers. Common sheathing types for commercial projects include plywood, OSB, cement board, and glass-faced gypsum panel. More recent introductions include hybrid products that integrate weather barriers, insulation, or both. Here are some factors to consider when specifying structural sheathing.
Strength
Shear strength should be top of mind when selecting sheathing. The required load capacity, measured in pounds force per linear foot in the U.S., should be provided by an engineer because “building codes are notoriously weak on shear wall requirements,” says Jordan Smith, assistant producer for Risinger&Co.’s The Build Show, a YouTube series produced in Austin, Texas. Getting true values of a product’s strength can be difficult, Smith says, so an engineer should review the selected sheathing type for the project.
Potential products, in order of ascending shear strength, are glass-faced gypsum sheathing, reinforced cement board, plywood or wood (including shiplap), OSB, and then premium-grade OSB, Smith says. One workability consideration: Reinforced cement board, which integrates a glass-fiber mesh, is dense and can be difficult to cut in the field.
Weather- and Water-Resistance
In general use cases, glass-faced gypsum sheathing’s core and glass-fiber face will resist water and air infiltration. The wall still requires a water-resistive barrier (WRB) unless the sheathing explicitly includes one. Cement board also resists moisture, but will also benefit from a distinct WRB.
Plywood and OSB panels come in Exposure 1 or Exterior grades. Exposure 1 sheathing can be exposed to the elements on a temporary basis, say during construction, but not permanently. Exterior-rated sheathing is water resistant, although all OSB requires a distinct WRB installed between the sheathing and cladding. Smith notes that plywood, though more expensive than OSB, “can absorb and let off more water than OSB because it doesn’t have as many adhesives.” Several recent OSB products integrate a WRB facer to help reduce installation time and cost.
Thickness
The local building code will prescribe the minimum sheathing thickness based on several factors, including project type, building height (or number of floors), and wind zone. Typical OSB and plywood panels thickness range from ¼ inch to ¾ inch, the latter of which Risinger&Co. prefers for its wood sheathing of choice, plywood. “The thicker, the more rigid … and more durable,” Smith says.
Hybrid panel products are inherently thicker in order to account for their components, while glass-faced gypsum board should be ½ inch to 1 inch thick, depending on application.
Insulation
Sheathing with insulating capabilities—made possible with plastic, foam, cellulose fiber, paper-faced and foil-faced boards—can alone offer thermal resistance values ranging from R3 to R15, Smith says. In general, a higher R-value means a greater panel thicknesses.
Installation
Following a manufacturer’s guidelines—including the recommended nailing schedule—is critical to ensure the sheathing product performs as expected. As such, the expertise of regional contractors may also be a consideration if the project schedule does not allow for a long learning curve. For example, Eckhardt is comfortable specifying fiberglass mat–faced gypsum sheathing—with its moisture-resistant facing—for local healthcare, university, and other commercial projects because of its prevalence in the region. “Our contractors here in Boston are used to working with it,” she says.
Panels for all sheathing types, Eckhardt continues, should butt together so that the WRB and air barrier are fully supported and not spanning gaps.
Cost Comparisons
Of the sheathing options discussed, wood products are likely the most expensive, particularly if they integrate other envelope components, such as a WRB. That premium is justified, Smith says: “The fact that integrated systems take care of that in one layer is a huge benefit to a lot of people.”
Still, Risinger&Co. mostly uses plywood and commercial-grade waterproofing because integrated systems are relatively new to the market. As hybrid products attract more attention from architects and develop a track record in the field, their price may decrease with the economies of scale.
In particular, Eckhardt is keeping an eye out for more integrated gypsum sheathing products. “It’s wonderful to see manufacturers collaborating, so that you can buy a product that has an air barrier on it. It simplifies the construction out in the field.”
