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.
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