Structural Wood Sheathing
Structural wood sheathing consists of plywood or oriented strand board (OSB) that is used to cover the exterior frame of a building. Exterior wood sheathing provides walls with structural resistance to lateral loads like those from wind and earthquakes, and wind uplift resistance if installed across assemblies. Structural sheathing also provides a nailing substrate for cladding materials including various sidings. A 2014 report by NRC also showed that walls with wood sheathing are less likely to have moisture issues than walls without due to the higher moisture storage capacity of the wood sheathing layer compared to that of a plastic insulation layer.
Plywood is an engineered wood-based panel building material made from thin sheets of softwood veneer bonded under heat and pressure with strong, water-resistant adhesives. These layers of wood are cross-laminated, meaning each layer of the wood grain is perpendicular to the previous layer. Due to this cross-lamination, plywood is a resilient material that is dimensionally stable and has two-way strength properties. Dimensionally stable means that there is little expansion and contraction across the length or width of the panel due to changes in moisture content, although the code requires small gaps between panels to accommodate this minor dimensional change. The thickness, on the other hand, will swell particularly at the edges of the panels (although less than some OSBs), if not protected and instead is left exposed to continuous wetting. Plywood used for sheathing can provide excellent lateral resistance and is an integral part of the building envelope. It is available in different grades and thicknesses. These grades imply the strength and appearance of the plywood and its appropriate use for different applications. The Effective R Tool includes the most commonly used plywood sheathing thicknesses of 1/2 inches (12.5mm) and 3/8 inches (9.5mm). You can read more about plywood here. For an example of how a wall assembly with plywood sheathing, and its corresponding information, is displayed on the Effective R Tool, please see Wall ID 13114.
Oriented Strand Board or OSB, is an engineered wood-based panel building material most commonly made from aspen or poplar (Canada) or southern yellow pine (US). It is produced by bonding together layers of thin, rectangular-shaped wood strands with strong, water-resistant adhesives under heat and pressure. The surface layer strands are aligned in the long direction of the panel, and the inner layers are oriented randomly or cross aligned perpendicular to the surface layers. OSB is a dimensionally stable building material, as there is little expansion and contraction across the length or width of the panel, although the code requires small gaps between panels to accommodate this minor dimensional change. The thickness, on the other hand, may swell for some OSBs (compared to plywood) particularly at the edges of the panels, if not protected and instead is left exposed to continuous wetting. OSB is commonly used in structural sheathing applications to resist lateral loads. OSB is manufactured to meet the requirements of CSA O325 for floor, roof, and wall sheathing, and is typically span-rated for each of these applications. It is available in a variety of thicknesses; the most common sheathing thicknesses of 3/8 inches (9.5mm) and 7/16 inches (11mm) are represented on the Effective R Tool. You can read more about OSB here. For an example of how a wall assembly with OSB sheathing, and its corresponding information, is displayed on the Effective R Tool, please see Wall ID 13904.
References
OSB. (2013). In Glossary of housing terms (Rev. ed. of: A glossary of house-building and site-development terms, 1982 ed.). Ottawa: Canada Mortgage and Housing Corporation.
Oriented strand Board (OSB) - The CANADIAN Wood council. (2019, March 29). Retrieved April 27, 2021, from https://cwc.ca/how-to-build-with-wood/wood-products/panel-products/oriented-strand-board-osb/
Plywood and OSB panel “Moisture-related dimensional stability” APA TT-028 Revised February 2021 APA Publication Search - APA – The Engineered Wood Association (apawood.org)
Plywood. (2013). In Glossary of housing terms (Rev. ed. of: A glossary of house-building and site-development terms, 1982 ed.). Ottawa: Canada Mortgage and Housing Corporation.
Plywood - sizes - Grades -The CANADIAN Wood council. (2020, April 24). Retrieved April 27, 2021, from https://cwc.ca/how-to-build-with-wood/wood-products/panel-products/plywood/
Sheathing. (2013). In Glossary of housing terms (Rev. ed. of: A glossary of house-building and site-development terms, 1982 ed.). Ottawa: Canada Mortgage and Housing Corporation.