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Certainteed Membrain

christian_330 | Posted in General Questions on

Hey guys. Climate zone 5/6 in  Northeast Ohio. I decided to air seal all gaps from studs, sheathing (60’s-70’s fibery type board with cedar siding), and plates with beads of canned foam and caulk. I’m using r-15 rockwool unfaced batts with the Certainteed Membrain on the inside as the vapor retarder. I used styrofoam soffit baffles in the eaves but had to staple them to the top plate due to the pitch of my roofs. I’m putting a lot of effort into air sealing and was wondering if  the space  under the soffit baffles, curving around the top plate under the staples, and getting behind the certainteed would be a problem. Thanks.

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  1. Expert Member
    Akos | | #1

    Typically you run a bead of acoustical sealant around the permiter of the walls. This partitionas the air barrier into smaller section and less chance of leaking across transitions.

    You can cut the VB along the middle of the top plate lay a bead of sealant underneath and tape over the seam with sheathing tape (ie Tuctape, Tyvek tape, 3M 8088 etc).

    For a typical VB install, you put acoustical sealant around any rough opening, sides and top+bottom of each wall.

  2. Expert Member


    It's especially important do as Akos suggested, as the membrane is primarily acting as a vapour-barrier where it is covered by the interior drywall, and as an air-barrier at transitions.

    Running the bead of sealant at the base right in the joint between the bottom plate and subfloor helps seal what is a common source of air leaks.

  3. christian_330 | | #3

    Damn. I wish they had shown that in the certainteed installation video haha. I feel like I did a good job sealing all gaps in the cavity, between the double top plate, and between the bottom plate and plywood subfloor. Could I just tape it to the plates and sides?

  4. Expert Member
    BILL WICHERS | | #4

    CertainTeed only gives basic info in their instructions. You need things like GBA articles to get all the fancy details to optimize everything :-)

    If you need to retrofit a bead of sealant, I'd run another row of staples about an inch down from the top of that header, then use a knife to slit the MemBrain between that new row of staples and your original row. Run your bead of sealant inside that flap along the edge of the top plate, and run a strip of flashing tape over the cut line when you're done. Now you have the sealant in there the way Akos suggested, but you don't have to strip everything down to add it.

    BTW, I like using urethane caulk here instead of acoustical sealant because it's less messy. Both work. The downside to urethane caulk is that it will set up and stop being easily squishable by drywall down the road. You can deal with this by rolling over the bead of sealant immediately after replacing the MemBrain over it. I did exactly this in my home office and it worked out fine.


  5. christian_330 | | #5

    OK thanks. Should I squish the caulk once it’s behind the barrier soooooo drywall can be flat against the studs? Also is it necessary to draw a bead Around the window if I have sealed it with tape between the jamb extensions and barrier?

    1. Expert Member
      BILL WICHERS | | #7

      I squished it. On some walls, I quickly put up the MemBrain and then drywalled right over it (not something you can easily manage on a larger project), so the drywall itself took care of the squishing. In other areas, I used whatever was handy, usually a short piece of scrape 2x4. A tape roller would probably do a great job but I never seem to have one handy when I'm caulking stuff.

      Regaring fumes, a respirator can filter this stuff out. If you're working a lot in a confined space, a respirator is a good idea. Keep in mind that you will sometimes perceive fumes in strange ways. One of the last steps in my relatively recent office renovation project was to prime the walls, which I did with oil based Coverstain since I had a lot of that on hand. I didn't notice anything in the room, but when I went OUT of the room everything smelled weird. I DID notice the headache later, and it lasted for hours. Unforunately for me, I didn't have a useable respirator at the time, and this was early in the pandemic when respirators were unobtanium. I wouldn't make that mistake again, and I have a nice new 3M respirator now since they're available again.


      1. christian_330 | | #8

        Luckily I have new 3m filter cartridges at the ready. Do you think I can skip drawing a bead around the windows and outlet/switch boxes. Any protrusions like that I put half the tyvek tape on the Membrain, tucked it into the corner and made the 90 degree angle and the other half adhered to the box or window jamb wood so I feel like it’s sealed really well and caulking isn’t necessary. I was hoping I could just caulk along the top bottom and sides and basically just make a big square.

        1. Expert Member
          BILL WICHERS | | #13

          I would caulk around the windows. I didn't caulk my own electrical boxes, I carefully lined the perimeter with flashing tape to tie the box into the plane of the MemBrain. I also sealed all the wire entrances to each box. I use the fiberglass "hard boxes" which don't have any holes except what you punch out for wiring, so there is far less work needed to air seal the backs of those.

          Caulk generally goes around the top, sides, and bottom of a wall, and around windows and/or doors. Those are where structural transitions are, and that's where leaks can be. Ryobi makes a powered cordless caulk gun which can really help when doing a lot of caulking -- your hand will thank you.


  6. user-723121 | | #6

    When I air sealed my attic I rolled back the existing fiberglass batts and sealed all of the bypasses with urethane caulking. I was up there quite a while and did get very light headed from the fresh caulk in a semi enclosed space. If I were to do it again I would use a more people friendly caulking.

  7. christian_330 | | #9

    Can I substitute silicone caulk for the acoustic sealant as it’s readily available and still squishy when dry. Also this is the taped joint of all protrusions. Do I still need to caulk around the window?

    1. Expert Member
      BILL WICHERS | | #10

      Yes you can, but I like urethane caulk better here. Neither silicone nor urethane is really squishy enough after curing, so you want to flatten out either after application. This isn't hard to do, just apply a bead of sealant, fold the flap of MemBrain back into place, then run something over the top with some pressure to squish the bead of sealant flat. Easy.

      You need to caulk around all perimeter locations, which includes windows. Just thing "I need to caulk around anything that transitions to something else". That means windows, doors, top plates and bottom plates. If you have any other penetrations (doggie door, etc.), caulk around those too.


      1. christian_330 | | #11

        Ok. Not to be difficult but is the reasoning behind caulking around windows and doors that there may be air leaking around the window foam sealant and framing so it will be leaking behind the Membrain to the rest of the area? I guess my reasoning was I felt confident that I sealed the window and framing well and if I bridged the Membrain to the jamb extensions with the tyvek tape it would be completely sealed. Thanks

        1. Expert Member
          BILL WICHERS | | #14

          It's parly as you suspect, and partly because windows penetrate the wall, and for completness you typically seal around ALL penetrations. Some of this is belt and suspenders. I think of it as cheap insurance, and I caulk here even if the window will be taped and foamed in later. Caulk is relatively cheap, and it goes up quickly, so there is little downside to using it here.


  8. christian_330 | | #12

    Also, I was told by certainteed I wouldn’t need a retarder on the main floor ceiling. Hope you all agree haha because that would set me back a lot. I don’t think I have to because it’s a vented attic.

    1. Expert Member
      BILL WICHERS | | #15

      You don't need a vapor retarder in the ceiling below a vented attic. You don't need a vapor retarder in interior walls or in ceilings between conditioned floors, either. You put a vapor retarder in exterior walls because that's where moisture issues can cause problems. In a vented attic, any moisture that gets up there is going to get out through the vents, so you won't have severe accumulation in the way you can in walls.

      You DO want to air seal the ceiling under the attic though -- air leaks can overwhelm even vented attics in terms of moisture.


  9. christian_330 | | #16

    Ok, that’s what I thought. This caulking in no fun man haha. I also got nervous because I read silicone can damage certain poly’s. I think Membrain being polyamine? (Nylon) it should be fine. At least so says a chemical engineer friend haha.

    1. Expert Member
      BILL WICHERS | | #17

      I like the Cole Palmer chemical compatibility database to check stuff like this:

      It says acetic acid (the solvent that is involved in the curing process for most silicones) is a problem for nylon, so silicone caulk is probably not a good idea for use with MemBrain. Looks like your chemical engineer friend was right! :-)

      I have used urethane caulk without problems (although I admit I didn't check the chemical compatibility database first :-) I'm not sure what the solvent used in polyurethane caulk is anyway...


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