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Q.Can I lay wood clapboards directly over plywood sheathing, or do I need some kind of air barrier such as a plastic housewrap, rosin paper, or 15 pound felt? Will rosin paper absorb and hold moisture? Will the felt create a cold-side vapor barrier?

A.Wood siding can be nailed directly over plywood sheathing without a house wrap, rosin paper, or building felt, but I would not recommend it.

Primarily, an exterior air barrier, whether it is a plastic housewrap, rosin paper, or asphalt impregnated felt, prevents air infiltration through cracks in the building shell that aren’t usually sealed, such as the joints between the plate and the subfloor, around the band joist, and between the sheathing. If all the joints in the sheathing are taped, or if every joint in the sheathing is over a stud or plate and the sheathing extends past the mud sill (not just to the bottom of the wall plate), you don’t need an extra air barrier. In these cases, the sheathing is the air barrier. But these conditions seldom occur.

Secondly, an air barrier provides some protection for the sheathing from water which may be driven through the laps in the clapboards by the wind or capillary action.

Finally, both rosin paper and building felt will absorb and safely release small amounts of moisture that move through the siding because of the daily heating and cooling cycle. Without an absorptive layer, this moisture can condense on the backside of the siding, and might cause the siding boards to cup and the paint to peel. Since plywood is somewhat absorbent, the temporary moisture storage provided by building paper is usually not necessary, but is desirable when siding over impervious materials such as foil-faced foam.

Plastic housewrap is not absorptive, but since it is moisture permeable, water vapor can diffuse through it, where it can be absorbed into the face of the plywood.

Asphalt-impregnated felt isn’t an especially good vapor barrier, so you don’t have to worry much about a cold-side vapor barrier. The glue line in the plywood is a much better vapor retarder, so if there is a danger of a cold-side vapor retarder, it is the plywood, not the paper or felt, that is the problem.