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dueling vapor barriers


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Anyone want to see what happens when a goofy interior designer is entrusted to design details?

Little 2.4 mil house in the City; when the fancy new siding was put on the exterior, the designer thought laying 1/2" drywall as sheathing and covering the exterior w/WR Grace was a good idea. He also must have read a book somewhere, because he has a vapor barrier on the interior too. Add a busted duct boot in the attic/plenum to drive conditioned humidified air into the plenum, mix, & wait. It took about 2 years to rot out the parapet framing.

Condensation can be a real beyotch.....

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Yes and yes.

The busted duct was pressurizing the "attic" plenum, forcing conditioned air up into the parapet, & dew point was taking care of the rest. You would not have believed the amount of moisture we wrung out of this joint; it was saturated.

Flashing wouldn't have helped because the dodo put the WR Grace outside the frame & inside the brick; the water never got to the brick, it just condensed & soaked into the framing & insulation. This was about vapor barrier confusion more than anything else, at least that's what I think. It was an object lesson of what happens when one "sandwiches" building materials between 2 vapor barriers.

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