Moldy roof sheathing at overhang
NikoFL
| Posted in General Questions on
Hello,
I converted the attic to a non vented configuration in a 1980 house in Climate Zone 2A. I blocked the area between the top plate and the underside of the roof sheathing between the rafters with non permeable foam. A year later I see mold growth developing on the underside of the plywood roof sheathing in the 2 foot overhang. Any idea why the roof sheathing is wet enough to show mold growth?
Thank you
Niko
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Replies
It sounds like you have exposed plywood sheathing at the roof overhang. You never covered it with a finish material? And it’s not painted or otherwise sealed? I have 2 ideas.
1. Could this be spreading from the attic? You don’t describe your air sealing and insulation strategy at the roof, except for the foam blocking at the top plate. Since the mold started after you converted your vented attic to an unvented attic, this could be a possibility. Please describe your air sealing and insulation strategy in more detail.
2. You live in a hot humid climate, and the wood is outside and will have a moisture content on the high side, especially if it’s not exposed to direct sunlight or has other means to dry out. Is it possibly that by adding the blocking, you eliminated some air flow that was previously helping to dry this wood after it rained, and keeping the moisture content in check? And now it’s staying wetter for longer?
Thank you! I think your second idea is spot on. Missing airflow due to the blocking and staying wetter longer. And yes the overhang is still exposed at the underside. Do you think covering it up with aluminum or similar type of soffit will help? Or should I include the overhang into the sealed attic - basically having the air control layer follow the bottom of the overhang around the facia to the fully adhered roof underlayment. I airseal with visconn fiber from 475.
Thanks again for the reply.
Niko