A. Henri de Marne, a consultant on light-frame buildings in Waitsfield, Vt., responds:What is probably happening is that interior moisture is being carried by air convection through the joints in the T&G planking, the rigid insulation, and the rafters, and is condensing on the underside of the new sheathing. From there, it drips down through the joints of the same components back into the living space.
There are some changes you can make to this assembly that should prevent the problem. The first thing you should do is to install a 6-mil plastic vapor retarder over the roof side of the T&G plank ceiling. This will stop the convection of moisture-laden air into the roof cavity. This should be done preferably in one piece. If that’s not...
to read the full article.