Northwest attic mold can grow with no leak, no bath fan venting into the attic, and plenty of airflow. More vents will not fix it.
So if an inspection finds mold on the underside of your roof deck, the first question is where the water is coming from. The answer decides the fix, and the wrong answer pays for a repair that does not hold.
The study
Graham Finch, Robert Lepage, Lorne Ricketts, James Higgins and Marcus Dell of RDH Building Engineering in Vancouver, British Columbia, reported this work at the 30th RCI International Convention and Trade Show in March 2015. It is a building-science field study. It measured wood, not people.
The starting point was a pattern the firm kept seeing in newer houses, townhouses and multifamily buildings: wet, stained, moldy roof sheathing in attics built to code, with airtight ceilings and code-required ventilation, where the usual explanations did not add up.
To test it, they built four small roof huts, each 6 feet by 6 feet, with half-inch plywood decks and dark three-tab asphalt shingles. Three roofs were pitched at 3:12, 4:12 and 6:12 over standard roofing felt. A fourth control roof, at 3:12, sat under a fully adhered, vapor-impermeable membrane meant to shut out every source of wetting from above.
The huts went on the open roof of a two-story commercial building. That was the point of the design. There was no heated attic below them, no indoor air leaking up, no duct leakage and no bath fans, and the underside of each deck had unrestricted ventilation. Every usual suspect was removed on purpose. Each roof had one slope facing due north and one facing due south.
Moisture pins and temperature sensors logged the plywood from September 2012 through April 2014, two full winters.
What they found
15% to 30% Average deck moisture started early fall at 15%, passed 20% in October, and reached up to 30% at worst in December and January.
The authors treat wood below about 20% moisture content as generally safe from decay, and about 28%, fiber saturation, as the level where fungal growth and decay become a real risk. Between the two is a cautionary zone. They also note that air at the wood surface above about 80% relative humidity can support surface mold even when a moisture pin does not read high.
These roof decks stayed above 20% for the entire winter, up to six months. Every slope, both orientations. The north sides generally ran wetter than the south, most clearly on the steeper roofs.
Visual checks confirmed fungal growth on the sheathing at every slope and orientation. When tested, it was mostly Cladosporium, a genus the authors describe as commonly found outdoors and known mainly for breaking down dead plant matter.
Where the water came from
The outdoor air. On a clear night, a roof radiates heat to the much colder sky. The deck can drop below the dew point of the surrounding air, and water condenses on its underside, or freezes there as frost. The team photographed exactly that on the morning of January 2, 2013.
14% to 23% of hours In December 2013 and January 2014, the decks spent between 14% and 23% of logged hours cold enough for condensation. The 3:12 north slope logged 193 such hours out of 1,394; the 6:12 north slope, 316 out of 1,396.
Sun is the counterweight. Over a few days in October 2013, sunshine dropped the surface moisture of a north-facing deck by about 10%, and each night’s condensation put nearly as much back. In a climate with long stretches of mild, humid, overcast weather, the wetting outweighs the drying.
The authors’ own conclusion is blunt: the ventilation air is the moisture source, so the usual preventive step of attic ventilation, in their words, “only exacerbates the problem.”
One oddity they flag themselves. The steeper 6:12 roof logged the most hours of possible condensation, yet it showed the least visible mold. They say the discrepancy needs more investigation, and so do we.
Why this is showing up more now
The physics are old. What changed is the house underneath. Older attics leaked warm, humid house air and a lot of heat. The humidity caused obvious problems, but the heat also kept the roof deck warm and drying. Newer attics are airtight and insulated to R-40 and up. That is a real improvement, and it also removed the heat that used to mask this slower wetting from the outdoor air. The authors also suspect changes in plywood itself, such as the mix of heartwood and sapwood in plywood cut from later-growth timber.
That last point showed up on the test roofs. Condensation beaded on the denser heartwood areas, while the sapwood soaked it in, and visible mold largely stayed off the heartwood.
The limits, stated plainly
Four small huts in one city is not a survey of real attics. They had no house under them on purpose, so they prove a mechanism exists, not how often it drives the mold in any particular attic.
This is Vancouver, British Columbia, not Vancouver, Washington. RDH’s own technical bulletin says that in the Portland metropolitan area, vented roofs “currently do not appear to exhibit these problems to the same extent” as further north, while describing the local risk as elevated and advising designers and builders to plan for it.
No health outcome was measured. The authors themselves note that in many cases attic sheathing mold does not necessarily damage the structure or create health problems. The concerns they name are perceived health risk and property value.
The mold index figures are modeled, not observed. Their mold growth model predicted mold on the 3:12 and 6:12 roofs they modeled would reach the most the conditions could support within 2 years. The visual observations agree that mold grew, but the model is a prediction.
What survives: a well-built, well-vented Pacific Northwest roof can wet and grow mold from outdoor air alone.
The intervention evidence
Right after construction, before any mold, the team applied treatments to test sections on the underside of the plywood, then added two more after the first winter. They checked them visually over roughly 1.5 years.
- Bleach and a consumer water sealer did not prevent fungal growth. In some spots, the authors write, bleach appears to have “primed the surface for a new wave of fungal colonization”, leaving heavier growth than the untreated wood beside it.
- Products containing a mold-killing ingredient, one wood treatment and one paint, performed best. Two copper and zinc wood preservatives also held up well so far.
- None of it is proven long term. The authors say longer field studies are needed to know whether any treatment has to be reapplied.
The paper also describes real repairs where mold came back a few years after full remediation and air sealing. That is the practical cost of fixing the wrong cause.
One result on underlayment is worth knowing. The control roof, under a sealed membrane, ran wetter in the top of the plywood than the felt roof, because it could not dry upward. The authors are careful here: on real, older roofs with penetrations, a membrane’s better leak protection likely outweighs that drying penalty.
What gets lost in the attic mold marketing
The usual pitch after attic mold is found is more vents, a bleach spray, or air sealing. Air sealing and proper ventilation still come first; the RDH bulletin calls them the first method for any attic moisture problem, and a leaking bath fan duct is a genuine and fixable cause. They just do nothing about condensation from outdoor air. And bleach, the cheapest and most common answer, failed in this test.
What the data supports
Read the pattern before paying for a fix. The paper’s own photographs show what different causes look like. Staining concentrated around a bathroom exhaust duct or a dryer duct points to a leak you can repair. Staining and dampness spread across the north slope of a well-ventilated attic, with no leak nearby, is what the authors hypothesize comes from night-sky condensation.
Confirm that bath and dryer fans vent outdoors, not into the attic or into a soffit where the humid air can be pulled back in.
Ask any remediation bid what it does about the moisture source, not only what it does to the mold, and whether bleach is the plan.
Bottom line
Northwest attic mold can come from the outdoor air alone, so venting harder and spraying bleach are not fixes on their own. Find the water source first.
First step: the next time someone is up in your attic, note which slope is stained and where. That one observation narrows the cause.
Talk to illumenair
illumenair does mold and indoor air testing in Portland and Vancouver homes.
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References
Finch G, Lepage R, Ricketts L, Higgins J, Dell M. “Why Wood Frame Attics Get Wet & Moldy in the Pacific Northwest.” 30th RCI International Convention & Trade Show, San Antonio, Texas, March 2015.
Ricketts L, Finch G. “Mold in Vented Wood-Frame Roofs in the Coastal Pacific Northwest.” RDH Technical Bulletin No. 010, September 2015.