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Community and Q&A

Air Barrier and Vapor Retarder for Drywall Ceiling

dan7210329 | Posted in General Questions on

What should I do to the drywall at the attic floor to make it a better air barrier, and to assure if moisture is introduced, it can dry out?  I assume the previous owners’ painters probably used latex primer/paint everywhere, so wall and ceiling assembly permeances are around 5.  If I want to get drywall permeance to .5, I should repaint the ceilings with a vapor retardant primer?  I read Martin’s “All About Vapor Diffusion” here:, and am left wondering if my drywall is sufficiently impermeable, and what steps I should take to lower its permeance.  (Wouldn’t it be nice if there was a gadget to obtain a reading of an assembly’s permeance in situ?)

It seems a far easier task to paint the ceiling with a vapor retardant primer, but would a sprayed or brushed liquid vapor retardant applied to the attic side of the ceiling drywall be tighter?  Assume I’ve properly sealed all the larger holes & gaps: removing or enclosing light cans; using sprayfoam and caulk around fans and electric boxes; install wind-washing dams at soffits; and other measures – following Matt Bath’s nicely detailed air sealing efforts described here:; and Martin Holladay’s “How to Insulate an Attic Floor” here:

It makes sense to me to get attic floor/ceiling drywall air-leak secure before I have the insulation contractor add loose fill insulation into the attic space – to R49.  Original attic fiberglass loose-fill insulation has subsided.

This house is in a small 55+ community in a western suburb of Philadelphia.  Philly is in a mixed humid 4a Zone with large swings in temp and humidity: muggy late summer periods of highs above 90, summer nights around 80; January days can stay in 20s for a week.

The semi detached house built in 2003 has a poured concrete basement walls, unfinished; typical timber-framed construction with vinyl siding & asphalt roof.  All but 1 room is on the first floor; a “loft” room with dormer: rest of house has peaked roof with one gable.

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  1. Expert Member
    BILL WICHERS | | #1

    Air sealing is relatively easy for the drywall ceiling itself: make sure you have properly taped and mudded joints between sheets, and make sure you seal any penetrations (electrical boxes and lights, primarily). The trick part is sealing the tops of the WALLS. To do that, you usually have to go up in the attic and seal any penetrations in the top plates, usually with a combination of canned foam for large holes and caulk for small gaps.

    If you have a vented attic, you don't really need to worry too much about moisture -- just paint the ceiling and call it good. The ventilation in the attic will take care of any moisture that makes it's way up there.

    Is there any particular reason you're concerned with vapor migration through the attic floor?


  2. Deleted | | #2


  3. dan7210329 | | #3

    Thank you for you thoughts, tips and assurances. OK, I don't need to worry about water vapor during the winter if I have a tight air-barrier.
    Recently I spent many hours air-sealing gaps and joints in the attic floor assembly: between studs, drywall, around light fixtures (replaced protruding ceiling cans with flat LEDs), bathroom fans, electrical wiring penetrations into top plates, etc. I'm somewhat more at ease that condensate won't form in the attic in winter, when the house is 70F and the attic is maybe 40-50F: with a decent air-barrier in place, there will be less warm, moist air from the house to rise up through the attic floor and hit cold attic surfaces, condense and degrade materials. And installed AccuVents assure air can move from soffits to ridge vents, helping to keep roof sheathing dry.

    While I think I understand the function of a tight air-barrier, I am still confused about how to handle water vapor in the summer. I worry that my new R30 of Rockwool batts laid on the attic floor - without a semi-permeable water vapor - won't stop the hot (120F+) humid attic air moving through and condensing somewhere in the Rockwool or on the cool attic drywall floor. Should I be concerned here?
    Maybe hot humid summer attics experience continuous cycles of wetting and drying? I have not read of such a thing, but I can imagine it might work that way, even when there is no leaky HVAC systems in the attic to help things dry out and stay cooler.

    1. Expert Member
      BILL WICHERS | | #4

      Optimal walls for the winter are not optimal for the summer. That's why there is the concept of the "perfect wall" out there. Everything involves some tradeoffs though, so the general rule is to build the best assembly for your climate zone, and accept that it's not perfect all the time. Most of us on GBA are in heating-dominated climates, so most of the discussions tend to be around walls that work in the Northern climate zones, which means vapor barrier/retarder on the "warm in winter" side, which is the interior side of a wall. In the summer, if you're running air conditioning, it would be a little better for things to be the other way around, but you're stuck. In practice, this isn't usually a problem -- things can dry out during seasonal changes as long as you don't have any double vapor barriers.

      From a physics standpoint, you want the vapor barrier/retarder on the "warm" side of the assembly so that it can keep moisture away from the "cold" side of the assembly where it could condense. The moisture itself isn't really the problem, the condensation part is, since that's where things get "wet" from the moisture. In the winter, the exterior sheathing is cold, so that would be the condensing surface, so you put your vapor barrier/retarder on the inside behind the drywall to keep moist air out of the wall.

      Remember that it's the dew point that has to do with when condensation will occur, not the relative humidity (RH), which just tells you how much moisture is in the air as a percentage of the capacity of the air at a given temperature to hold moisture. To really know when you'll get condensation, you need to know the temperature of the condensing surface and the dew point of the air around it. If the condensing surface will drop below the dew point, you'll get condensation.

      In practice, think about your window in the morning. If it fogs up on the exterior, that means the window was below the dew point for the outdoor air. That's probably pretty close to an indicator for condensation anywhere else too on the exterior of your home, which would include the exterior side of the drywall facing the attic spaces. Remember though that that window will clear up pretty quick on most days, which means the conditions changed enough that the condensation re-evaporated, which is "drying". As long as conditions aren't such that the moisture builds up over time and never dries out, you'll be fine. In most cases, you don't have much to worry about with a vented attic -- time has shown vented attics to be very robust. I don't think you're likely to have any trouble with your new attic insulation in terms of moisture here.

      Note that in your particular case, R30 is not to code. You could cap your mineral wool batts with blown cellulose, which would add some moisture buffering capacity. I would, at minimum, beef up the R value to at least code minimum though. If you heat with natural gas, keep in mind that rates are likely to be about triple this coming winter compared to last winter, so any extra insulation you put in now means less money you'll spend on heating. Don't go crazy though -- it is possible to put in so much insulation that you'll never recover the cost in energy savings.


      1. cldlhd | | #5

        I live in the same area as the OP ( northeast suburbs of Philly) and while I haven’t worried about my attic much as I have a fully vented soffit and a ridge vent and I can access and check it. I have thought about condensation in the wall cavities . In particular 2 bedrooms where 4 exterior walls ( 2 per room) have 5/8” zip system for sheathing which I believe is permeable and painted drywall ( 1 coat of primer and 2 coats of latex) on the inside. Inside the 2x4 wall I have about 3” deep of closed cell spray foam filling the cavity. My concern is that in the heating season warm humid air will pass through the drywall then be stopped by the foam then , since it’s colder inside the wall, I’ll get condensation. Often in winter when I close the double cell cellular shades at night I have condensation on the inside of the window in the morning. I realize that the window has a much lower R rating than 3” of closed cell spray foam but still worries me a bit.

        1. Expert Member
          BILL WICHERS | | #6

          3" of what I assume to be closed cell spray foam is about R18 or so, but assume an imperfect fill which puts you down around R15 or so. That spray foam (assuming closed cell) is not really vapor permeable, so the drying ability of the exterior sheathing doesn't really change things inside the wall. While it's true that the interior face of that spray foam layer will be slightly cooler than the temperature of the room itself, it likely won't be below the dew point, so condensation won't form. You might get a small amount of moisture from the stud edges, but that should be pretty minimal.

          When I build these types of walls (although I almost never use spray foam in a wall), I like to put a smart vapor retarder behind the drywall for some extra insurance, but that's difficult to do as a retrofit unless you're doing a "down to the studs" renovation in the room.


          1. cldlhd | | #7

            Thanks for the info, yeah if I had to do it over again I would probably do it different and just use fiberglass batts either with a poly vapor retarder or kraft faced. I believe I have read out here that the amount of vapor that actually goes through painted drywall is a very small amount though. I wasn't aware of smart vapor retarders when I did the job. But that's sort of what I was thinking that the inside of the interior wall on the surface of the spray foam is probably warmer than the dew point. I took a few Flir images of it and the only parts that look cool are where the studs need to drywall. I assume the exterior shaving on its inside surface could only be a possibility of an issue during the summer but I doubt it. There have been a few times when there was a pillow that ended up on the floor ( don't ask how ...) And it was in the corner and then in the morning when I picked it up there was a little bit of condensation on the baseboard trim and the floor. I guess it acted as enough of an insulator and it cooled down with all of the wood not acting as a very good insulator behind the trim. That did get me a little worried about what about if warm moist air goes underneath the baseboard and then cools down and condensates. But I guess the only way to really check would be to tear some of it out and it's not something I really want to do.. I included two pictures one is showing how cold the surface of the baseboard is in my bedroom compared to the walls and the other is a what I thought pretty cool shot of my mini splits compressor and line set in the winter.
            They say there is no perfect wall I guess.

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