Attic Moisture
bryanw511
| Posted in General Questions on
About a year ago I finished the exhaustive process of adding DIY polyiso ventilation baffles into every rafter space in my attic. I thought my attic would be in good shape and I’d never have to worry about it again. Today I peeked into my attic just to check things out and found moisture on the underside of some of the baffles near the blown fiberglass. What could be the cause of this?
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Replies
Bryan,
Warm, moist air is making it's way through the ceiling or top-plates and condensing on the first cold surface it finds - which is the foam baffle. The same air is probably accumulating on your sheathing, but is removed by the attic ventilation.
Ideally a better air-sealed ceiling below the blown in insulation would eliminate the condensation, but I doubt it matters much if you aren't see any adverse effects on the things that matter like the rafters or sheathing.
A few years ago before adding the baffles I peeked up there one freezing cold day and the entire underside of the sheathing was frosted. I'm thinking of removing the blown, spraying 1-2" of foam to seal the attic floor, then blowing again.
bryan,
I think it's well worth doing doing, one way or another.
Spraying the entire attic floor is overkill, you really only need to do the wall top plates. You could spray out around the eaves to get the exterior wall top plates, then strips of foam to cover any interior top plates. You'd basically be spray foaming a drawing of your floor plan onto the attic floor this way. This will save a lot of material, and there is no need to air seal the back of a sheet of drywall -- drywall itself is already an excellent air barrier.
Note that it's possible to do the air sealing work with caulk and canned foam, but sometimes spray foam is easier if you have limited access. Either way, you'll want to try to clean away as much of the loose fill material as possible prior to air sealing, to ensure that the foam can adhere well to the surfaces you're trying to seal.
Bill