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How can I remedy a cathedral ceiling with fiberglass batts in unvented rafter bays that now emits a strong musty odor?

miroland | Posted in GBA Pro Help on

Investigation revealed 3 problems: 1) no soffit vent; 2) insulation extending beyond the end of plastic rafter vents closing off the air channel up to the ridge vent; 3) no blocking above the wall to seal up the fiberglass. I’ve corrected the first two and caulked the ridge on the inside, but am unable to fix the blocking. The odor persists and I fear the fiberglass is contaminated with mold that the revived air flow won’t be able to dissipate. Would pulling out the fiberglass (through the soffit to avoid major deconstruction) and filling the bays with foam work? Would open cell work since access for closed cell is not practical? The assembly already has 1/2″ foil-faced polyiso on the interior under the sheetrock. –Michael Roland

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  1. Expert Member
    Dana Dorsett | | #1

    With foil faced goods on the interior you have no real choice but to stick with a vented roof. But the "real solution is to re-build the ceilings (and maybe the roof), converting it into an unvented roof, complying with IRC 2012 presriptive methodologies:

    Note, the foam-R minimums presume the total R to be the full IRC code minimums for those climate zones:

    If you're in zone 4 and you don't have enough rafter-depth to actually hit R49, you don't really need the full R15 as foam above (or directly under, and in contact with) the roof deck- you only need the comparative percentage. R15 is about 30% of R49, and if you only have 5.5" deep rafters, you can get away with R10 above the roof deck and R23 rock wool batts between the rafters (R10 is 30% of the R33 total) or 1" of closed cell polyurethane on the underside of the roof deck (R6-R7), and 4.5" of damp sprayed cellulose (~R16) or a compressed R19 kraft-faced batt (~R15) and no interior side vapor barriers.

    A full cavity fill of open cell foam works in US climate zones 1 & 2, but becomes iffy in zone 3 (especially if applied to OSB roof decks that have "cool roof" shingles). But it's fine even up to zone 6 if you use a "smart" vapor retarder such as Certanteed MemBrain or Intello Plus, or (2nd best), a "vapor barrier latex" primer on the new ceiling gypsum.

  2. GBA Editor
    Martin Holladay | | #2

    It sounds like you have a badly constructed insulated cathedral ceiling, with several possible problems. I don't think there are any half-way solutions that are likely to work. To do it right, you need to plan for a major rebuild -- one that probably includes opening up the rafter bays, either from the interior or the exterior.

    For information on doing it right, you might want to read this article: How to Build an Insulated Cathedral Ceiling.

  3. bsandersga | | #3

    I chose to build a new roof on top of my old roof. I retrofitted my fiberglass batt filled cathedral ceiling by completely encapsulating the original rafter bays. I cut off the rafter tails and sealed them, and I sealed over the old ridge vent. Then I built up on top of the old roof deck with 3 inches of rigid foam and a half inch of foil faced polyisocyanurate to reflect the heat, then a 1.5 inch vented gap, new deck and ridge vent thus keeping the old ceiling structure inside the house. I was careful to seal everything above the old roof deck. Now, the house is a lot less drafty in winter and is much cooler in summer.

    BUT, I didn't have to deal with any polyiso on the interior and I am in Georgia (climate zone 3).

  4. miroland | | #4

    Thank you for your responses. Brief follow up. How crucial is it that fibrous insulation in vented bays be sealed from outside air? Some details in Fine Homebuilding/GBA show seals, others do not, particularly when using pre-formed plastic rafter vents. Also, when using blanket insulation on an attic floor, isn't that area open to outside air via soffit vents, ridge vent, and/or gable end vents? --MR

  5. GBA Editor
    Martin Holladay | | #5

    Wind-washing of the top layer of fibrous insulation reduces the thermal performance of the insulation. If you use a ventilation baffle (for example, a polystyrene baffle like ProperVent), you will reduce the effect of this wind-washing. Even better is a site-built baffle that is installed with attention to airtightness.

    If your baffle isn't airtight, the flaw is not fatal. But if you are trying to do a conscientious job, it's always a good idea to try to enclose fibrous insulation with an air barrier on all sides.

    Why isn't this done on attic floors? The main reason is that insulation contractors have discovered that it's less expensive to just blow in a few more inches of cellulose to counteract the effect of wind-washing than it is to try to cover the top of the insulation layer with Tyvek.

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