Increasingly, homeowners in the Pacific Northwest are noticing mold on the underside of their roof sheathing. This phenomenon occurs in vented attics, and is more common in new homes than old homes. One puzzling aspect of the phenomenon is that the mold is found in well-built homes that comply with all relevant building codes. In other words, most of the homes with this problem have ceilings that are fairly airtight; they have adequate levels of insulation on the attic floor; and they have adequate levels of ventilation at the soffits and ridge. So what’s going on?
A variety of factors contribute to this problem, including recent changes in construction practices and the region’s damp climate. The most surprising factor may be the role of night sky radiation—a phenomenon which can cool roof sheathing to a temperature lower than the air temperature. In most cases, increasing the rate of attic ventilation—for example, by enlarging the soffit vents—doesn’t help; in fact, the greater the ventilation rate, the worse the mold.
Roof leaks? Inadequate ventilation? Indoor air? No, no, and no.
Researchers from RDH Building Science, a consulting company in Vancouver, BC, have investigated cases of sheathing mold in vented attics and have conducted field research on the issue. In a 2019 report published by the BC Housing Research Centre, “Asphalt Shingle Sloped Roofing Research Study,” the RDH researchers noted, “Moisture problems in sloped wood-frame attic roof assemblies are often attributed to rain water ingress, inadequate ventilation of the attic space, and air leakage condensation from interior spaces or ductwork. However, detailed investigation and testing of many of these attics has found that these factors cannot wholly account for the observed problems. … These roofs are generally constructed to meet current building code and third-party roofing warranty requirements.”