- Q. I plan to install new cedar shingle siding over existing T&G board siding. Should I install housewrap under the new shingles? Since there is already a layer of housewrap between the plywood sheathing and the board siding, I’m worried that two layers of housewrap may lead to moisture problems.
A.Corresponding Editor Paul Fisette responds: I would recommend that you use a layer of #15 asphalt felt to cover the T&G boards. The new siding will not keep all rainwater out, especially in a heavy soaker of a storm, and the T&G boards need to be protected from water that penetrates the siding.
If you installed a second layer of plastic housewrap instead of asphalt felt, and liquid water were to leak into the region between the two layers of housewrap, the T&G boards could have a difficult time drying out. Asphalt felt is forgiving. Initially it blocks vapor and liquid water, but if it does get wet (under extreme exposures) it first stores water, then allows liquid and vapor to pass, and ultimately dries out. The perm rating of asphalt felt is about 5 (fairly low) when it is dry, but rises to 60 (fairly high) as the felt nears saturation.
Fussy installation and detailing is important. Carefully lap the asphalt felt so it sheds water, and be sure the flashing at the window heads is tucked under the felt.
