Mold, Mildew Attic

Published on February 6th, 2018 | by Henri


Mold on attic sheathing

Q.  I need major help. I have always read your column and your postings, but now it’s my turn to ask for help from someone I can totally trust.

I had an attic fan replaced a couple of weeks ago and while the installer was working I climbed the ladder to check the attic and saw a lot of black patches on the sheathing of the roof and around the nails; also on both sides of the wall of the attic.

I had a mold remedial person come out and give me a quote. My question is what is the correct process and how can I find somebody I can trust. I live in the northern suburbs of Chicago. Sorry I have no photos to share with you.

My house is 30 years old. The guy said the plywood on one side has a hole and that is causing the moisture, but this has always been there all these years (I am the original owner). Please help. Sincerely.

A.  If you used the attic fan during the winter to ventilate the roof, it may have contributed to the moisture condition responsible for the mold. Very seldom, if ever, is there enough net free ventilation area (NFVA) in attic vents to satisfy the CFM requirements of attic fans, so the fans rob the conditioned spaces of warm, moist air through the many cracks and crevasses found in most construction. This moisture condenses on the cold roof sheathing and gable walls.

If you used the fan solely to cool the attic during the summer, and didn’t thoroughly weatherstrip and insulate any access to the attic, the same warm, moist air convected into the attic by means of what is known as the stack effect.

There may also be other convective paths from the living spaces into the attic, such as recessed ceiling lights, bathroom and kitchen fans, etc.

There is usually no need to have costly remediation performed to remove mold. Closing all convective paths, not using the attic fan in the winter (or better yet, removing it) and providing natural ventilation by means of continuous soffit and ridge venting should dry up any living organisms in the moldy patches. The black patches may remain or peel off as they dry up, but they will have become inactive as the relative humidity (RH) in the attic drops and the moisture content of the framing members and the sheathing dries up.

The ridge vent must be externally baffled to direct any wind over its top in order to prevent most rain and snow penetration. A very popular one is Shinglevent II; there are others, but this has been my favorite for decades.

Soffit vents should be continuous and not added piecemeal along the soffits in order to provide what is called an air wash in all rafter spaces. And a free flow of air must be provided between the soffits and the ridge.

You didn’t say where the plywood is missing; is it on the roof sheathing or one of the gables? How big is the hole and do you know why it’s there? My guess is that the hole is on a gable wall and may have had a gable vent, which was removed if new siding was installed.

Whatever the reason for it, it is not what is causing the excess moisture in the attic since it is covered with roofing material or siding.

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