Sheathing

Role in Building Science

• Structural Integrity: Sheathing's most important role is to add shear strength and rigidity to a building. It ties the framing members (like wall studs) together, preventing the structure from racking (racking is the lateral shifting or parallelogram effect that can happen during high winds or seismic activity). This creates a stiff, stable box that can withstand forces from all directions.

• Air Barrier: Sheathing, especially modern types like oriented strand board (OSB) or plywood, acts as a primary component of the air barrier. When the sheathing seams are properly sealed with special tape or fluid-applied membranes, it prevents uncontrolled airflow into and out of the building. This air tightness is essential for energy efficiency, as air leaks can account for a significant amount of a home's heat loss or gain.

• Moisture Management: Sheathing is the layer to which the water-resistive barrier (WRB), often called house wrap, is attached. The WRB is the actual moisture barrier that prevents liquid water from entering the wall assembly. However, the sheathing itself provides a durable, smooth surface for the WRB to be applied correctly, and some modern sheathing products (like ZIP System) have the WRB integrated into the panel itself.

• Insulation (R-Value): Traditional wood sheathing (plywood or OSB) has a very low R-value, typically around R-1.3 per inch. Its primary role is not insulation. However, a newer category of sheathing called insulated sheathing includes a layer of rigid foam board on the exterior. This adds a layer of continuous insulation to the outside of the framing, which is highly effective at reducing thermal bridging (heat loss through the wall studs), significantly increasing the overall R-value of the wall assembly.

Common Types of Sheathing

• Oriented Strand Board (OSB): The most common and cost-effective sheathing today. It's made by compressing layers of wood strands with adhesives. OSB is dimensionally stable and provides excellent structural performance.

• Plywood: A traditional sheathing material made from thin veneers of wood glued together in alternating directions. It's known for its strength and is generally more moisture-resistant than OSB, though it is not waterproof.

• Insulated Sheathing: These panels are a combination of a structural board (OSB or plywood) and a layer of continuous foam insulation. They serve as a structural panel, air barrier, and continuous insulation, all in one.

R-Value of Sheathing Types

• Plywood and Oriented Strand Board (OSB): These are the most common sheathing materials. Their R-value is minimal, typically around R-1.3 per inch of thickness. This means a standard 1/2-inch panel has an R-value of about R-0.6. This minimal R-value offers some thermal resistance but is not a significant part of the wall's overall insulation.

• Insulated Sheathing: This is a modern product that consists of a structural panel (like OSB) laminated to a continuous layer of rigid foam insulation, such as polyisocyanurate (polyiso) or extruded polystyrene (XPS). The R-value of these panels varies with the thickness of the foam, ranging from about R-3 to over R-12.

Sheathing and Thermal Bridging

Thermal bridging is the process by which heat flows through a building's more conductive components, such as wood or steel studs, bypassing the insulation in the wall cavity. This creates "cold spots" on the interior walls and roof decks during the heating season.

Traditional sheathing does little to prevent thermal bridging because its R-value is so low. It's essentially a weak thermal resistor that still allows heat to easily transfer through the framing members.

However, insulated sheathing is designed specifically to combat thermal bridging. When installed on the exterior of the framing, it creates a continuous layer of insulation that covers the entire wall and roof deck, including the studs and rafters. This breaks the thermal bridge and ensures a more consistent temperature across the entire surface.