There is some staining/discoloration and/or mold or mold-like fungal growth present on the roof sheathing in the attic.
This condition may be consistent with the effects of Night Sky Radiation.
Night Sky Radiation is when the roof sheathing temperature falls below the ambient dew point temperature as a result of net radiative heat loss to the much cooler night sky. (see link below)
This is common in the Pacific Northwest Maritime Climate in the spring, winter and fall, where moist outdoor air is introduced to the attic via normal ventilation of the roof structure and the moisture condenses on the cold roof sheathing. Repeated wetting of the roof sheathing can lead to staining/minor deterioration of the wood and even result in fungal growth in some areas when wood moisture content is high enough—especially on the north side of the roof. Non-heartwood types of roof sheathings (whether plywood or OSB commonly used in newer construction) can contribute to worsening of this condition.
It should be noted that even if you do not live in a Maritime Climate, there may be localized conditions that can duplicate this phenomenon—at least seasonally.
The condition is more common in well insulated and air-sealed homes but may be present and masked by worse conditions in poorly insulated/air-sealed homes. Night Sky Radiation and other contributing factors should be part of any evaluation of how to address the condition.
Adding more ventilation can contribute to a worsening of this condition.
Reducing ventilation to newer recommendations of 60/40 (60% lower/40% upper) may improve the condition. Metal roofs and insulating under the roof covering can help reduce the night sky radiation and result in less condensation on the roof sheathing. These options should be considered with the home is re-roofed.
Since this condition is considered to be “outside” the living environment and little structural damage is likely, minimizing its continued growth is recommended. Attempts to use surface treatments to discourage fungal growth are still in the experimental stages, with some treatments only making matters worse.
If you are trying to have your Building Performance Professional sort out what is going on in your attic, make sure they are familiar with the principles of Night Sky Radiation.
Why Wood Frame Attics Get Wet & Moldy in the Pacific Northwest
If for some reason the above link will not work you can try: Night Sky Radiation.
Post Script:
At a recent inspection, I found the following example of the apparent affects of night sky radiation on the North face of a carport roof.
As is clearly visible in the photo, the overlying roof shields part of the roof from the night sky allowing the covered portion to stay slightly warmer while the colder roof areas are cold enough to collect moisture from the air as indicated by the greying/fungal areas around the triangular shape.
By Charles Buell, Real Estate Inspections in Seattle
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