Paul Fisette responds: Clapboards should be
installed tight against the trim boards, and the
joints should be left uncaulked. Omitting the caulk
allows any water that penetrates the joint to drain
reasonably well and promotes drying.
Although the use of caulk does not cause any
disasters, it can elevate the moisture level of the
sheathing. I have taken moisture readings of
sheathing on many homes, and on average the
sheathing underneath caulked corners has a higher
moisture content than that under uncaulked corners,
and a higher moisture content than the sheathing a
couple of feet away from the corner. I recommend
that vulnerable areas of the sheathing (like
corners) should have a double layer of asphalt felt
or house-wrap under the trim boards.
Hot southern exposures, wide temperature swings,
and wet-dry cycling take a toll on caulked wood
joints. In most cases, caulk quickly loses its bond
from one of the wood surfaces and leaves a crack
where water can enter. Yet even cracked caulk can
slow the drying process.
Well-detailed caulk joints require several fussy
details. For caulk to work well, the end grain of
the clapboards should be painted, to prevent
absorption of the caulk solvent. Moreover, a bond
breaker (backer rod) should be installed in the gap
against the sheathing to avoid three-sided
adhesion. This backer rod ensures that the caulk
will adhere only to the siding and the trim board,
so that the caulk can stretch without tearing.
A caulked joint works best when the gap measures
a uniform 1/4 inch. But technical specs for most
building sealants indicate that joint movement
should not exceed 50%. Since a 6-inch trim board
can shrink and swell by 1/4 inch as a result of
normal exposure to the elements, you would have to
leave a 1/2-inch gap between the trim board and
clapboard ends to accommodate 50% movement.
Who’s going to do that?
Finally, uncaulked joints require less
maintenance. Few homeowners have the time to dig
out old cracked caulking on a regular basis.