- 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
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
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.