- 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.
