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Does the underside of fiberglass need to be covered in a flash-and-batt unvented conditioned attic?

user-5525860 | Posted in Green Building Techniques on

I’m renovating my cape cod in Maryland and decided to do an unvented roof assembly since I didn’t have good ways of venting the roof valleys and I have ductwork running through the attic, so I wanted a conditioned attic. I should have read the GBA blogs before this project to install rigid foam above the roof sheathing but, in the absence of that wisdom, I’m installing R-15 closed cell spray foam directly to the underside of the roof sheathing and R-30 batts directly under that.

Here’s my question. In the areas where the batts (in a flash-and-batt situation) extend above and below the drywall (extending above the drywall flat ceiling to the ridge beam and below the drywall sloped ceiling from knee-wall to plate), should the batts be covered? I’m installing unfaced batts to avoid having a double vapor barrier (since the closed cell foam provides that) and will hold them in place – where they won’t be in direct contact with drywall – with the same fabric used in BIBS. I know the batts must be in contact with the foam. But should the underside of the batts be covered with an air barrier to prevent convection currents or any other issues?

Oh, and is it important to air seal the finished ceiling with a conditioned attic? I already air sealed the crap out of the entire second-floor/attic.

Do you see other issues with this installation I haven’t yet considered?



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  1. GBA Editor
    Martin Holladay | | #1

    It's best to enclose the fiberglass batts with an air barrier. Drywall would be best, but you could use MemBrain or plastic housewrap, I suppose, as long as your local code official doesn't raise fire safety issues. (I don't know whether housewrap should be left exposed; it may be flammable.)

    Fiberglass performs better when it is enclosed on all sides by an air barrier. If it is left exposed, it's also hard to keep in place -- it usually eventually falls out of the framing bays due to gravity.

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