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Community and Q&A

Smart MemBrain with mineral wool

Nicholas C | Posted in Green Products and Materials on

Using Thermafiber Mineral Wool in 2×6 stud cavities. Planning on using CertainTeed’s Smart Membrain before hanging drywall.

1.) Is this the best idea in terms of proper product use and installation?

2.) Will stapling the Membrain to my studs cause any issue?

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  1. User avatar
    Dana Dorsett | | #1

    Read the instructions for installing MemBrain- they're fairly straightforward & complete:

    Be sure to read up on how to install batts properly too. Too many real-world installations are rife with gaps and voids that didn't need to be there.

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Q. "Is this the best idea in terms of proper product use and installation?"

    A. I'm not quite sure what you mean by "the best idea." There are a lot of factors here:

    1. What is your geographical location or climate zone?

    2. Is this new construction, or a renovation?

    3. Do you have any interest in reducing thermal bridging through your studs?

    4. What are your goals?

    5. Do you have a fat budget or a tight budget?

  3. Nicholas C | | #3

    Dana, thanks for the PDF. I don't have a membrain for my ceiling. That's just going to be 5/8" rock and cellulose in the attic. The walls are the mineral wool which recommends I have some kind of membrane since the material breathes. I don't understand the sealant required at top and bottom plates but not along each stud. Each staple pokes two tiny holes.

    In addition, when drywalling, the screws will penetrate the membrain many times per stud. Will I get the benefit of the membrain or does it not matter to have those staple and screw holes?

    Martin, Climate Zone 5, New Construction but exterior is already sided (so no foam for thermal bridge reduction), my goals are to make sure 1.) My walls do not have moisture problems from moisture ladden air and 2.) ensure my walls are energy efficient. My budget at this stage is fairly tight. I already spent three times as much on mineral wool insulation (compared to fiberglass) hoping that would be a huge benefit, but I question if paper faced fiberglass with taped seams would have been faster, cheaper, and more beneficial.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    The R-value per inch of mineral wool is only slightly higher than the R-value per inch of high quality fiberglass batts. But that's water under the bridge.

    Concerning screw penetrations: I think you are confusing vapor retarders with air barriers.

    The MemBrain is your vapor retarder. It doesn't need to be airtight. Holes in the MemBrain layer don't change the effectiveness of the MemBrain very much. It's still a good vapor retarder, even if it is riddled with holes.

    The drywall is your air barrier. Pay attention to airtightness when installing your drywall (and your electrical boxes) and you'll be fine.

  5. Nicholas C | | #5

    Martin, thanks for the re-clarification. Yes I was very concerned with each penetration. So if I just staple the membrain, it sounds like that is okay and I won't need to worry about sealant along edges, etc. I will plan on overlapping and taping seams, but not spend so much time trying to seal around windows and doors and electrical boxes, but instead just go for tight clean cuts in the membrain.

    As far as the drywall goes, I was thinking I would install that with sealant along the edges on my exterior walls. Edit: I do have one concern with this method since I have the membrain. If I use sealant, the sealant will only adhere to the membrain and not the wood. This means air could still go behind the membrain itself and into the conditioned space, yes? My electrical boxes will get some caulking between them and the drywall roto-zipped cut. I believe my drywall seams will be okay because of the tape and layers of mud (plus primer/paint).

  6. User avatar GBA Editor
    Martin Holladay | | #6

    If you care about air leakage, it's best to specify airtight electrical boxes. If it's too late for that, there are two areas of concern:

    1. The crack between the drywall and the electrical box (which you intend to caulk), and

    2. The holes in the back of each electrical box where the electrical cable enters the box. These are hard to seal, but you should do the best you can with caulk. (Needless to say, turn off the circuit breaker before attempting this work.)

  7. Lance Peters | | #7

    Martin, unless specific planning and detailing are done, relying on drywall to be an air barrier sounds very risky to me?

  8. User avatar GBA Editor
    Martin Holladay | | #8

    The drywall will make a more dependable air barrier than the MemBrain.

    Airtightness details depend on the owner's goals. On this job, the siding is already on, so options are limited. Of course, there's always flash-and-batt -- but in Nicholas's case, I think tightening up the drywall layer is a good approach.

  9. Nicholas C | | #9

    Hey Martin, I am not sure if you saw my second concern because I edited it a couple minutes after typing the reply.

    Do you think I should cut my membrain about 1" short at top and bottom plates to allow the caulking on those plates to seal against the drywall? Or should I instead caulk the plates before the membrane so that seals tightly, and once again caulk the plates before drywall? Option 1 seems to be the most efficient, besides taking time to make that cut straight and careful not to cut too much.

  10. User avatar GBA Editor
    Martin Holladay | | #10

    I think that you can seal the perimeter of the drywall by installing the caulk on the interior side of the MemBrain. That approach will work fine. There is no need to cut the MemBrain back.

    For more information, see Airtight Drywall.

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