Which layer lies immediately inside the exterior siding in the wood frame wall build-up?

• A. Exterior siding
• B. Plywood sheathing
• C. Building paper
• D. Wood frame

Answer: (C)

In a wood-frame wall, moisture management layers sit directly behind the exterior siding. The layer immediately inside the siding is building paper (house wrap). Its role is to act as a moisture/vapor barrier and drainage plane, helping shed water that penetrates the siding and allowing any vapor to escape, which protects the wall cavity. Behind this barrier lies the plywood or OSB sheathing, which provides structural support, and then the wood framing.

So the layer directly inside the exterior siding is building paper. Exterior siding is the outermost layer; plywood sheathing comes next, behind the building paper; and the wood frame is the innermost structural element.