Wall sheathing

Sheathing

View from the southwest corner showing the wall sheathing going up over the framing. Structural wall sheathing is part of a building’s air barrier and provides a base for water barriers (such as Tyvek) and cladding. Structural wall sheathing also adds “racking strength” to a framed house, which prevents it from rocking back and forth due to earthquakes or wind. In cold climates, most building designs dictate that wall sheathing should be airtight but “vapor permeable,” so that water that enters a wall cavity can diffuse through the sheathing to the outside of the building. That’s a lot of responsibilities for one material!

There are many structural sheathing products, but the most common on residential projects are OSB and plywood. OSB (oriented strand board) is made of wood flakes glued and pressed together into a uniform panel. OSB is usually less expensive than plywood, but also less permeable and more prone to water damage.

For my house I used plywood. Plywood is made from layers of thin veneers peeled off of a single rotating log with a large, sharp knife (As building scientist Martin Holladay puts it, “these veneers come off the log like sheets being pulled from a roll of paper towels.”) The veneers are stacked at different angles, glued, and pressed into a panel. Plywood is a bit more expensive than OSB, but it’s more resistant to water damage and more permeable to water vapor.

Most plywood used for structural wall sheathing has three plies (layers) and totals 7/16″ thick. I decided to use five ply, 3/4″ plywood instead. This is a thick, strong material. In fact, 3/4″ plywood is used for building “shear walls,” special internal walls designed to resist severe wind and earthquake shaking. This upgrade cost about $2k over standard plywood, but I believe it’s money well spent. After waking up to the magnitude 7.1 earthquake last November, I’d much rather have an extra $2k in my sheathing than in fancy finishes. Nobody I’ve talked to thinks using 3/4″ is worth it, but when I asked my framer about it he said, “even if the whole mountain collapses, your house is just gonna roll downhill in one piece.”

Sounds good to me.

Photo taken December 24, 2019. Posted January 7, 2019.

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