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
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 feasible, start
from the bottom and overlap the upper sheets by several inches,
then tape the joints.
The second problem with your assembly is that you are
placing the rafters directly over the T&G deck and laying
the rigid insulation between them. It would be much more
efficient to lay the rigid insulation directly over the
decking, thus minimizing the joints between materials and
increasing energy efficiency by eliminating the short circuits
provided by the rafters. Good-quality extruded polystyrene foam
has plenty of compressive strength to handle a roof load.
On future jobs, instead of rafters, use 2x3 sleepers on edge
24 inches on-center, and fasten them to the deck with long
screws. If you mark the location of the beams below, the screws
can be driven without concern about coming through the ceiling.
With shallower-pitch roofs, a 1-inch penetration into the roof
decking should be sufficient as long as you put enough screws
in. With steeper pitches, you should provide greater
penetration and use strapping across the ridge.
Finally, you should provide continuous ventilation on the
underside of the plywood. Extend the tails of the sleepers 3
inches beyond the existing fascia, install a new fascia, and
provide soffit ventilation by means of an off-the-shelf
ventilation strip. You should also install an externally
baffled ridge vent such as Shinglevent II at the ridge.