I live in the rainy Pacific Northwest, and I recently noticed what looks like mold on the cedar boards of my cathedral ceiling. It appears to be between a couple of rafters. Although there is good venting at the eaves, there is no venting at the ridge. What’s the probable cause of this apparent mold, and what’s a possible solution?
—ED via email
Martin Holladay, editor of Green Building Advisor, responds: Mold growth occurs under conditions of high humidity. In your climate, one possibility is that your indoor relative humidity is too high; this is especially likely if your house doesn’t have air-conditioning. Ideally, your home’s HVAC equipment will keep the indoor relative humidity below 40% during the winter and below 60% during the summer. If you don’t know your home’s indoor relative humidity, buy a few hygrometers and begin monitoring. If your indoor relative humidity is high, you need a dehumidifier or an air conditioner.
It’s also possible—though, in my experience, unlikely—that your ceiling boards are damp because of a roof leak. Roof leaks usually leave stains or reveal themselves via drips, and you don’t mention any stains on your ceiling boards.
Finally, there’s the lack of a ridge vent. Whether or not your roof assembly needs a ridge vent depends on the type of insulation installed. Unvented roof assemblies should have exterior rigid foam or spray foam between the rafters; if you have one of these types of insulation, no ridge vent is needed.
If your roof assembly is insulated with fiberglass batts, mineral wool, or cellulose, you need a ventilation channel between the top of the insulation layer and the underside of the roof sheathing. You also need a ridge vent. If you have this type of insulation, it isn’t very hard to install a ridge vent (assuming you have a gable roof).
Verifying whether or not your roof has a ventilation gap between the top of the insulation and the underside of the roof sheathing can be tricky, and probably isn’t necessary if all you care about is fixing your mold problem. You may be able to tell what’s going on by removing the soffit and looking upward with a strong light. If you decide to open up the ridge to install a ridge vent, you’ll have a chance to inspect the rafter bays from the top. If one of these inspections reveals that your insulation is tight to the roof sheathing, it’s going to be tough to create a ventilation channel under your roof sheathing. In that case, the best way to improve the performance of your roof assembly might be to install rigid foam above the roof sheathing, followed by a second layer of rigid foam with the seams staggered relative to the first layer, and to finish the job with an additional layer of sheathing and new roofing.
While the lack of a ridge vent could conceivably be causing mold on your ceiling boards, that’s not the usual symptom of poor ventilation. The usual symptom in such a case would be a dripping ceiling during cold weather (or during a warm spell immediately after a cold spell).