condensation stains

[John Dirks Jr]

The first thing that caught my attention was staining on the sheathing and sidewall. At first I was thinking leaking flashing. A closer look revealed the gas furnace flue was loose at the collar where it passes through the roof.

I'm thinking the stains are from condensation caused by warm flue gasses getting into the cold attic in the winter.

What's you opinion on that idea?

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[Jim Katen]

Sure.

[John Kogel]

That's a reasonable assumption. The drywall would be a different temp than the OSB, so there would be less condensation there. But there's not much doubt that the leaky vent pipe is to blame.