- Q.The house I’m
building has a cathedral ceiling framed with wood
I-joists, continuous ventilation at the soffit, and a
continuous ridge vent. The ceiling inside will be
T&G pine. I plan to use R-38 Kraft-faced
fiberglass batts (which will allow a 1 1/2-inch air
space between the roof sheathing and the batts), and
would like to apply Tyvek housewrap over the
Kraft-faced batts. However, I’m concerned that
moisture will accumulate between the batts and the
Tyvek. If moisture accumulation isn’t a
problem, does it matter which face of the Tyvek is
exposed to the room?
Q.Henri de Marne
responds: Since Tyvek is permeable to
moisture, I wouldn’t use it as you
propose. In this situation, I recommend you use
unfaced fiberglass batt insulation instead of
Kraft-faced batts, then install a 6-mil plastic
vapor retarder over the unfaced batts. Unfaced
batts allow for better quality control during
installation (you can see how snugly they fit
against the flanges of the I-joists), and 6-mil
plastic is a more effective vapor retarder because
it eliminates the joints created when Kraft-faced
batts are used.
The vapor retarder must be installed as
carefully as possible. The T&G pine ceiling
provides little or no protection against moisture
migration. If poor installation techniques are
used, condensation will occur on the underside of
the roof sheathing and on the top of the
insulation.
Henri de Marne is a building consultant in
Waitsfield, Vt., specializing in moisture-related
construction problems.