Q. I'm planning to reroof an expanded Cape Cod on a barrier island off the East Coast, where strong winds and moisture from the ocean are common. Not only are the shingles stained with mold, but the underside of the roof sheathing above the knee-wall area is also moldy, although we found no roof leaks. We also had some water and mold issues in the crawlspace, which have been corrected. We've removed the fiberglass batts a previous owner had stapled between the rafters and plan to replace the sheathing. Because the rear shed roof is almost flat, the roofing contractor advises against using a ridge vent for fear wind-blown rain will enter. There is no room for gable vents. Should I keep the soffit vents even without ridge or gable vents? How can I ventilate the knee-wall area, prevent mold growth, and help cool the second floor?
A. Contributing editor Paul Fisette responds: You have an interesting set of challenges, all related to your coastal climate's extended periods of high moisture and relative humidity. The mold inside the knee space indicates that the relative humidity there reaches 70 percent, the threshold above which mold and mildew will grow. In your case, I don't think ventilation is a good solution. In fact, venting could make the situation worse. The outside air contains a lot of moisture, so it's unlikely to dry out your damp interior spaces.
I think the smartest approach would be to redefine the thermal envelope by installing a continuous air barrier and insulation along the outermost boundary of the shell. With this strategy, everything inside the structural shell will stay close to the same temperature and relative humidity as your living space. I would advise filling the stud and rafter bays with a spray-foam insulation; this will minimize air leakage through the sheathing and help keep the interior dryer and cooler in the summer. (While dense-pack cellulose would typically also work fine, I might opt for spray foam given the damp climate.) You can close off the soffit vents.
It's also important to keep the crawlspace as dry as possible and to make sure there are no air leakage paths coming from it. Otherwise, moisture will be transported into the living space through plumbing and wiring penetrations and other pathways.
Replacing the shingles may not ultimately prevent future mold growth on the roof surface, given the damp outdoor air. But using mold- and algae-resistant shingles would be a good precaution.