Plywood is the most common type of wood used underneath the siding. Plywood is layers of wood cut on a large machine, pressed together under pressure and glued together to form a sheet of wood. Plywood is made from maple, pine, oak, cedar and poplar. Most commonly, plywood is made of pine. It is also made from exotic hardwoods harvested in rain forests. This type of plywood is called luan.
Oriented Strand Board (OSB)
OSB is a manufactured plywood made from chips of wood. The chips are pressed into flat panels with a special glue. OSB comes in many varieties, including 1/2 inch and 7/16 inch. The 7/16 inch variety is the type most often used under siding. Advantages come with using OSB. It costs less than regular plywood, and it dries faster. It will rot when exposed to water and retains water, however. Plywood also retains water but innovations make plywood more water-resistant today.
If you want a more weather- and water-resistant type of wood under your siding, pressure-treated plywood is the answer. This plywood is treated to resist water with several chemicals. The plywood will not retain water as long, so it resists swelling and rot. Plywood made with exterior grade glue is another type of outdoors plywood suitable for installation under siding. This is simply referred to as exterior grade plywood. It is always a good idea to install this type of plywood under siding even if you take steps to reduce water infiltration.
Installing the Wood Sheathing
Installing sheathing is a lengthy process. The plywood or OSB is first anchored to the wall framing with galvanized nails. It is cut to shape with a table saw, mounted and nailed in place. Building paper is then installed over the plywood and tacked in place with galvanized nails. After the building paper, foam board insulation is mounted to the exterior of the plywood or OSB, which provides insulation value. The vinyl siding is installed over the foam board.