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Community and Q&A

Smart vapor barrier for finished basement ceiling?

onejeremias | Posted in General Questions on

I am finishing a 1300sqft basement in a house in climate zone 6. The basement is mostly underground on all sides, with 2″ XPS foam on exterior concrete walls. We insulated with 1″ XPS foam, 1 – 1.5″ spray foam, then the 2×4 metal stud wall with R13 Rockwool. We ended up with this configuration because I only learned about the need for continuous insulation after installing the 1″ XPS and the stud wall with 1-1.5″ space from XPS. A very knowledgeable guy on another site advised adding the spray foam and using the rockwool, which I have done. He also advised adding a smart vapor retarder (Membrain), which I have ready to install, but his instructions for using it only seemed to apply to the walls, he never mentioned the ceiling. Half of the basement is planned to use Ceilingmax drop ceiling, the other half drywall. We have R23 rockwool in the basement ceiling, and I have insulated the rim joists with 2″ XPS and air sealed, along with sealing the gap between the 1″ foam and the concrete at the top of the wall. It is a new contruction house with no moisture issues (yet). So my question is whether I should be installing the Membrain product in the ceiling as well? The purpose of the drop ceiling is to be able to access the pipes, junction boxes, etc, so seems like we’d eventually have to poke holes in the thing anyway.

On a side note, I’ve really appreciated the valuable info this site has already offered on this project, so thanks to all of you experts out there for offering your expertise so freely!

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  1. Expert Member


    Vapour-barriers, smart or otherwise, are used to control the movement of vapour, usually between conditioned and unconditioned spaces. If you have now conditioned your basement, there should be no vapour movement to worry about between it and the rest of the house, and no need for a vapour-barrier on the ceiling.

    With the foam insulation you have added to your basement walls, there isn't much use in installing a smart vapour-barrier there either, although it probably won't do any harm.

  2. Expert Member
    BILL WICHERS | | #2

    There is no point in putting a vapor retarder/barrier in the basement ceiling if you have a conditioned basement. As Malcolm pointed out, there won't be any vapor drive between two sides of a wall or floor that have conditioned space on both sides. It's the same as for insulation: insulating a wall that has the same temperature on both sides doesn't accomplish anything, except maybe for some sound reduction. Whatever is doing air circulation in your home will be making sure that the vapor levels on either side of that floor are pretty much the same.

    A smart vapor retarder in the basement wall isn't really doing anything either, because you already have a vapor barrier, or very close to one, against the masonry wall with all that XPS and spray foam. With the amount of rigid foam insulation you have in those basement walls, you shouldn't have any issues with interior moisture condensing inside those walls, so the vapor retarder doesn't gain you anything. This is the same for "regular" exterior walls that have enouhg exterior insulation -- walls like that don't really need faced batts, since the moisture won't condesne out inside the walls anyway. I don't see the vapor retarder hurting you, but it doesn't help, either, so I wouldn't bother installing it.

    BTW, adding spray foam to that assembly was probably over kill. The XPS layer should be enough as long as you seal the seams with caulk, canned foam, or tape. The spray foam doesn't hurt you though, so don't worry about it causing any problems.


    1. Expert Member
      PETER G ENGLE PE | | #3

      The spray foam added more continuous R-value to the walls and it also anchors the steel studs against vibration, so its not at all wasted. Getting the continuous foundation insulation up above R-20 is very good in zone 5. OTOH, because of the thermal shorts caused by the steel studs, the mineral wool inside the studs probably IS wasted. Oh, well. Good intentions....

  3. onejeremias | | #4

    Thank you all for the quick replies! To clarify, here is what the gentleman on the other site said:

    In Chapter 11 of the IRC 2021 (see TABLE N1102.1.3) the code minimum R value for basement walls for your location (climate zone 6A) is R15 continuous insulation (c.i.) or R19 between wood stud (bad solution) , or R5 c.i. + R13 between wood studs. The 2" of XPS is labeled R10 but will perform at about R8.5 (0r less, if waterlogged) after 20 years.

    If you can afford it, add 1-2" of CLOSED CELL spray polyurethane directly on to the existing polyiso, sealing it to the steel studs, covering the interior side of exterior stud edge. That would bring the foam R up to R12, which would then be good enough to fill up the remaining ~3" of stud bay with rock wool batt insulation without concerns of excess moisture/frost accumulation.

    As for air tightness installing a carefull taped & detailed layer of 2mil nylon "smart" vapor retarder (eg: Certainteed MemBrain) under the drywall would be a pretty good "belt AND suspenders" approach.

    Not sure if that changes anything, but it at least gives you more if needed. He did mention that the metal studs end up losing 40% efficiency compared to wood, so the extra rockwool seemed to make sense, to give us as much as we could.

    My main concern was that without ceiling vapor barrier, the vapor might collect in the rim joist area and if I didn't seal those well enough, we'd end up with issues, so just wasn't sure if ceiling barrier would mitigate anything. Since I have the barrier product already, I'll probably install in the walls unless anyone here tells me that it's pointless if I don't also do the ceiling. Even a small amount of effectiveness using vapor barrier seems reasonable to me, as it doesn't take that much work to put it up.

    Edit: Forgot to add that I misspoke about the 1" XPS on the wall, it's faced poly-iso as mentioned above. Maybe that changes perspective.

    1. Expert Member
      NICK KEENAN | | #5

      When he said "under the drywall" he meant on the walls.

    2. Expert Member
      BILL WICHERS | | #6

      The hardest part to detail will probably be the top of the wall in the rim joist area. I doubt very much you'll get a perfect installation of the vapor retarder in that area since you'd likely have to "wrap" the top of the wall, which assumes a continuous top plate. I would think of the vapor retarder as primarily limiting moisture in the wall itself, not to keep the moisture from getting up to the rim joist area. If you have done a proper job of insulating the rim joist, moisture there shouldn't be a problem anyway.

      Remember that when using rigid foam, if you have a thick enough layer to keep the interior side of the foam above the dew point, you won't have condensation or moisture issues. This is true regardless of the presence of a vapor retarder or barrier. When the vapor retarder helps you is when you have a situation that allows some interior surface in the wall to drop below the dew point so that moisture CONDENSES on that surface as liquid water. Such a situation can exist with insufficient amounts of rigid foam, or if you have thermal bridges through the rigid foam. If you have continuous rigid foam of sufficient thickness, your wall is safe. Note that batts are a different animal here, because vapor can freely move through a batt which means the batt does not present an insulated interior "side" as a first condensing surface in the way that a sheet of rigid foam does.


  4. onejeremias | | #7

    Ok so it seems I'm good to skip the ceiling barrier, thank you all for confirmation on that.

    I think the only question remaining would be whether I should bother with the barrier in the wall. I am with Zephyr/Bill that it's unlikely I'll get a perfect installation, I'm not very good with this stuff and don't always know what to look for. Given that, and given the R-value of the 1" polyiso + 1.5" spray foam + thermally shorted metal stud wall with 3" rockwool, is the consensus that it's pointless to install the vapor barrier there, or will there be any benefit if I don't get it all perfect? My plan for installation is 3M Super 90 spray adhesive, apply the barrier to studs in one continuous sheet, with a small 1" wrap up and onto the bottom of the joists, all the way down and adhered to the track at the bottom. Bottom track will be gap sealed where it meets the concrete. If this is a pointless endeavor, I'll just find a way to sell what I have and be done with it.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      I think the smart vapour-barrier is at best neutral. Impeding the drying to the interior isn't useful, and you already have a pretty good air-barrier in place.

      1. onejeremias | | #12

        Thanks for the input Malcolm. Technically that makes one vote for (the other guy advising) and one against. I guess if I don't hear any more to the contrary here, I'll forgo the barrier, as it'll get me to a finished product faster :-)

  5. user-1072251 | | #9

    You’ll also get moisture migrating up through the floor, which is a huge thermal bridge and which will continually drain heat out of your space. You’d be wise to install 2” foam board with a plywood surface.

    1. onejeremias | | #10

      Hi Bob, are you talking about the subfloor now? If so, plan is for 1" XPS + .5" plywood. There is a vapor barrier under the slab, so as far as I understand it, moisture through the floor shouldn't be much of a concern. I don't have enough headroom to add any more foam, but hopefully it'll be enough.

  6. user-1072251 | | #11

    If it’s all the height you can afford, it’s certainly better than not installing anything.

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