- Q.We build timber frame homes
and wrap them with insulated panels made with skins of
OSB on the outhouse and rigid foam insulation on the
inside. We've been pleased with the results except for
one thing: the swelling of the OSB at the edges. We've
seen no damage to the roof structures, but the ridging
is unsightly when the roofs are covered with asphalt
shingles. So far, we haven't been able to find a
solution, other than to make sure that the roofer
covers the roof immediately with felt and uses a
good-quality shingle. Have you seen any instance where
this swelling has contributed to roof
failure?
A.Contributing editor Paul
Fisette responds: The problem, caused by
moisture adsorption, is fairly common with
stressed-skin panels installed on roofs. The panel
edges take on moisture more quickly than the rest
of the panel and swell as a result -- one of the
shortcomings of OSB. What can happen with roof
panels is that warm, moist air from inside the
house leaks through the seams. As the indoor air
cools, the moisture condenses and wets the panel
edges. The moisture may also collect on the
underside of the impermeable asphalt roof felt and
shingles, compounding the problem. The solution is
to meticulously seal the seams of adjoining panels
with a product like spray urethane. Inject
foam-in-place urethane into the seams before
assembly or drill holes and fill the seams after
the panels have been installed. Be careful about
using spray-in-place urethane to seal panels in
cold weather. The necessary heat of reaction is
wicked away by the cold, which can interfere with
the curing process and leave an unprotected seam.
Check with the spray-foam manufacturer for
recommendations on cold weather installations.