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Community and Q&A

Site built ventilation baffle moisture issue

walldocmp | Posted in General Questions on

I installed site built rigid foam ventilation baffles per GBA article

I installed roxul under the baffles and then got pulled away for a month to do other work. My plan was to come back and add Membrane plastic sheeting and 2″ rigid foam under the joists. When I got back today to do the sheeting and rigid foam I stuck my hand up between the roxul and baffles and it was soaking wet. I am assuming this is because i didn’t get the plastic sheeting up right away. Any suggestions on how to move forward? Should I be pulling out all the insulation?


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  1. walldocmp | | #1

    The joist bays are open to the vented attic. Will they dry to the attic over time?

  2. NormanWB | | #2

    It sounds like you have an air sealing issue that is allowing moist air from your living space to get into your attic. Is this a cathedral ceiling or a flat one?

  3. walldocmp | | #3

    There is no plastic sheeting up yet so all of the moist air from the living space can reach the baffles. The house is not lived in and I assumed it wouldn't matter if I waited to put up the plastic sheeting. But it did. There was enough moisture in the unoccupied house to condense between the insulation and baffle. My question is now that the insulation is wet do I need to tear it all out or will it dry out to the vented attic once the plastic sheeting is up. The cathedral ceiling only runs about 8' and ends in a vented attic.

  4. user-2310254 | | #4

    Just to be clear, MemBrain is a smart vapor retarder. It and poly are not equivalent.

    It sounds like the home could use additional air sealing. (Here is one article on this topic: Also, what kind of sheathing is above the baffles?

  5. Expert Member
    Dana Dorsett | | #5

    In most US locations you don't really need a vapor retarder on the conditioned space side of a vented attic, but it does have to be reasonably air tight prior to insulating. If rock wool on the attic floor next to the vent chute is getting wet, you have an air leak, probably a pretty big one if it's already wet this early in the heating season.

    Where is this house located?

    Is it being heated, despite not being occupied?

    Is this new construction, or a rehab on older construction?

    Yes, it will eventually dry into the vented attic, but if it's accumulating moisture that rapidly you may see water damage in the ceiling below before the winter is out.

  6. walldocmp | | #6

    Is a vapor barrier under rafters of a cathedral ceiling combined with vapor impermeable site built rafter vents a bad idea? Is that effectively a double vapor barrier? In my case the cathedral ceiling is only 8 feet and ends in a vented attic.

  7. walldocmp | | #7

    Dana, It is a new construction in Maine zone 6. The house is heated although unoccupied. The ceiling is not up yet so there is effectively no air barrier from the conditioned space. Is there a risk using a vapor retarder in combination with vapor impermeable site built rafter baffles in a cathedral ceiling?

  8. Expert Member
    Dana Dorsett | | #8

    Impermeable chutes are potentially an issue if they're thick. If it's half inch foam it doesn't much matter what type it is but
    if it's an inch or more (particularly if it has foil or plastic facers) it may need an interior side vapor retarder other than standard latex paint on gypsum.

    An inch of unfaced EPS has about the same vapor retardency as standard latex paint, and still retains some drying capacity into the channel through the foam, and even the rafters will pass moisture through the 1" path around the edges.

    An inch of XPS has about 1/3 the vapor permeance of latex paint, which is still way better than nothing, and shouldn't be a problem even at 1".

    A 1" path through the rafters may even be good enough for 1" foil-faced polyiso, but it's half the drying rate you'd get around 1/2' foil-faced polyiso, and NONE would be passing through the foil.

    A continuous layer of rigid foam under the rafters already a pretty good vapor retarder, even if it's EPS (about 1.2-1.5 perms, if Type-II unfaced EPS). If it's foil-faced polyiso (recommended, for the higher R) it's a true vapor barrier, and all drying would have to be toward the exterior, so keeping the site-built baffle foam on the thin (and more vapor open) side would be important. Using MemBrain won't do much for you anywhere in that stackup.

    In the mean time, pull the rock wool out of the cathedralized ceiling portion until you're ready to at least add the interior side rigid foam, and when that interior side foam is installed, tape the seams and seal the edges right away, don't wait. The baffles and rafters will dry reasonably quickly if the space is being heated, but it's probably worth spot checking the moisture content of the rafters close to the baffles with a 2-pronged meter before putting the insulation back and sealing it all up.

  9. walldocmp | | #9

    Dana, Thanks for the help. What you are saying makes sense. Ill get it all dried up. Check the rafters with a moisture meter, skip the Membrain and get the polyiso up and sealed immediately. I used 3/4" plastic faced EPS for the baffles. Maybe not the best choice. Big lesson here is to get the ceiling air sealed as soon as possible after insulating. I really appreciate you taking the time to explain it all.


  10. GBA Editor
    Martin Holladay | | #10

    The main reason for the phenomenon that you observed is that there is no air barrier on the interior side of the mineral wool insulation. Even ordinary taped drywall would have been enough to stop the phenomenon you observed. What happens is that your warm, moist interior air has unimpeded access to a cold surface -- the fibers of the insulation do nothing to slow down the movement of air through the insulation.

    The phenomenon has been reported here on the Q&A pages of GBA many times -- often in stud bays of homes being built during the winter. Owner-builders put their hands behind the fiberglass insulation in their walls and notice that the exterior sheathing is soaked. That's because there was a long delay between insulating and drywalling.

  11. walldocmp | | #11

    That was exactly my situation. A long delay between insulating and dry walling. Lesson learned. Thanks for the help.

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