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Community and Q&A

Mold in the attic

GBA Editor | Posted in General Questions on

I live in Oregon. In Oct of 2009 I had a new roof installed. Existing roof had 2/12 pitch and two additions on each side of the house with shed roofs. Making the house U shaped. existing roofing and sheeting was removed. We decide to use sizer trusses to put all under one roof. New roof has one ridge line; south side is 6/12 pitch and north side has 8/12 a 19/12 and 6/12 pitch. In Jan of 2010 I discovered mold on the underside of plywood. But not all plywood is affected. In some cases bottom sheet is clean, middle moldy and top sheet clean. On the South side witch is approx. 50’x 26′ first 24’x50′ of plywood is covered in mold and last 2′ at the ridge is clean.
My house is approx. 2000 sf ranch style. I have moisture barrier on my ceiling. Attic is R-38 baffle insulation. My contractor is telling me that moisture is coming from crawl space and inside of my house. I had 2 different Industrial hygienist to come out and both told me that this is due to plywood being wet when installed. I also removed some shingles and discovered mold on top side of the plywood(north side of the house only). Top of plywood on south side seems to be clean. Also when roofing was installed I ask them to install ridge went and my roofer talk me out of it and installed 10 box looking vents at 50 sq/in net-free area. Intake ventilation on the South side is approx. 250 sq/in and north side approx. 480 sq/in.
Is it possible that plywood was wet
Is it possible that ventilation is not balanced and that I do not have good air flow
How much of air leak you need from the house to affect entire attic even above bedrooms and office where there is no penetration in the ceiling except light fixture
Thank you

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  1. GBA Editor
    Martin Holladay | | #1

    Installing plywood roof sheathing when it was wet does not lead to the kind of mold you are describing.

    The source of the moisture is most likely from the interior of your house and your crawl space. The only way to determine the leakiness of your ceiling is to hire an energy auditor to visit your home and conduct a blower-door test. If the blower-door test reveals that your ceiling is leaky, as I predict it will, be prepared with a plan to address the leaks -- ideally on the same day that the blower-door is on site. This procedure is called "blower-door-directed air sealing."

    As you correctly guessed, light fixtures in the ceiling are often sites of air leakage.

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