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Community and Q&A

Siga Majrex as ceiling air barrier

dpilot83 | Posted in Green Building Techniques on

I have watched some build show videos where they have installed Siga Majrex onto the bottom of the ceiling joists and then added 2×4’s as firring strips perpendicular to the ceiling joists. So the Majrex was sandwiched between the ceiling joists and the firring strips.

2×4’s run perpendicular to the ceiling joists. Then they end up with drywall fastened to the firring strips.

This means the Majrex is 1.5” away from the drywall which means the Majrex is expected to support the full weight if the insulation. Does this work well in practice?


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  1. Expert Member
    Michael Maines | | #1

    Yes it works very well. Majrex is tough stuff and insulation isn't very heavy on a square foot basis.

    1. dpilot83 | | #2

      Thanks. It sounds like you might use this process yourself?

      Mind critiquing my order of operations?

      1. Build walls (we’re planning on zip as air barrier on outside of wall)

      2. Drape sheets of Majrex over the top of the walls (including interior walls).

      3. Set roof trusses

      3a) Get the roof on to protect majrex from weather? Not sure how durable it is…

      4. Tape the majrex that’s draped over exterior walls to the zip

      5. Put double sided tape on bottom side of ceiling joists near all walls internally and the. Bring majrex that is draped over walls up to ceiling joists so it’s no longer draped down. Also roll the majrex so it adheres to the ceiling joists well

      6. Put double sided tape on remaining sections of ceiling joists one area at a time (only do as much as you can quickly complete rolling majrex onto)

      7. Roll out Majrex between walls and use a roller to adhere the Majrex to the double sided tape

      8. Tape seams

      9. Install firring strips

      10. Anywhere you’ll have penetrations in the ceiling air barrier, create a ZIP sheet to go between the ceiling joists and connect the Majrex to the ZIP. Then drill individual holes for individual penetrations and seal them once the penetration is complete.

      1. Expert Member
        Akos | | #4

        Take a look at the install manual from Siga as they cover a lot of the details for this.

        I find it simpler to install blocking between the rafters/trusses by the exterior walls and staple the strip over the wall plates to that. You now have solid backing when it comes to attaching the strip to your ceiling air barrier.

        For any connections I use tape (ie 3m8067 or Tuck tape) and acoustic sealant. Any pipe penetrations are taped the the ceiling air barrier. Backing doesn't hurt but no need to go overboard.

      2. Expert Member
        Michael Maines | | #5

        Yes I've designed similar systems many times, and installed them a few times. I recommend doing the air control membrane before standing interior walls to make continuity easier but otherwise your plan sounds good to me.

        1. dpilot83 | | #8

          That makes sense. I guess I should have specified structural or load bearing interior walls.


      3. Expert Member
        MALCOLM TAYLOR | | #7


        When we use strips of poly on interior walls, we sandwich them between the two top plates. This means you can walk the walls and set trusses without displacing or damaging them. It also makes them less prone to tears than you are drilling for wire runs.

  2. dpilot83 | | #3

    Along those same lines, I guess an alternative to Majrex would be ZIP or even taped plywood before the roof trusses go on?

    What are the pros and cons of taped ZIP, taped plywood or OSB, or something like Majrex? Seems like they are all similar concepts in terms of the order of operation.

    With the rigid options would you still install firring strips under them or would you just fasten drywall directly to the ZIP/plywood/OSB?

    If you fastened directly to it, how would you handle air sealing light fixtures and so on?

    Are there any other viable methods for creating a reliable air barrier at the ceiling when you want a traditional vented attic? I think I have heard of people making their walls 12 or 18” taller. Then there is room for more like a 1 foot or larger gap between the air barrier and the drywall. I have a hard time visualizing how to create the drywall supports in that scenario but it sounds like it would be really nice for making anything electrical that you desire work just fine.

    1. Expert Member
      Michael Maines | | #6

      Vented attics are pretty forgiving so the vapor permeance of the ceiling membrane is not as important as it is in most other locations, but it still matters to some degree.

      I usually spec Majrex or Intello for this. Certainteed Membrain would probably work but it's not as tough as its competitors. We always strap ceilings in New England anyway, so changing to 2X strapping is easy, and allows just enough space to run wires inside the air barrier in a code-compliant manner (min. 1.25" from the face of framing) and to install LED slim recessed lights.

      Zip sheathing works well for this, and may be cost-competitive. Working overhead, I prefer installing a lightweight membrane but if you have a drywall lift or a helper with strong shoulders, Zip works fine. Plywood or OSB would also work, though they are somewhat harder to make airtight because of their rough surfaces. (And on rare occasions, plain OSB isn't airtight, but I don't want to open the can of worms that statement always elicits.) With rigid material, you can install 2X strapping or if you won't have many penetrations, you can just carefully seal them. For that matter, if you are really careful, you can use drywall alone as the air barrier--drywall's airtightness literally defines what an air barrier is--but it's fussy work.

      I'm designing a new home now where my client had an interesting idea: frame a box with open web joists as ceiling joists, install Zip sheathing over that, then a trussed gable roof on top of that. It requires extra material but it will be easy to make the house airtight, easy to set trusses and there will be a generously sized, uninsulated service cavity in the ceiling.

      1. dpilot83 | | #9

        Really good info, thanks.

        Would you have time to elaborate on your 2x strapping if you use ZIP? Are you saying you would need a deeper cavity with ZIP so you double the strapping? If that is what you meant do you need a deeper cavity because the low profile lights push into the ZIP a little and that’s not a problem with the Majrex because it moves out of the way a little?

        I’m probably totally reading it wrong…

        1. Expert Member
          Michael Maines | | #10

          When possible, it's nice to be able to run wires for recessed lighting on the interior of the air barrier. By strapping/furring the ceiling with 2X material (2x3, 2x4, etc.) you leave enough space for the wires and LED recessed fixtures and their transformers to fit, all on the interior side of the air barrier.

          The transformers for LED recessed lights are about 1" to 1.25" thick so they can usually be forced to fit in a cavity with 3/4" strapping and a fabric membrane, but they simply would not fit if the air barrier was Zip with 3/4" strapping.

          Does that make sense?

          1. dpilot83 | | #11

            Hah! Yes, that makes sense. For the majrex aspect of this thread we were talking about it.

            When we switched to ZIP and you said 2x I took it as “two ex” as in two times as much as was being done with Majrex.

            You meant “two by” which refers to lumber dimensions and makes a lot more sense than what my mind went to for some reason.


        2. Expert Member
          Michael Maines | | #12

          Haha, yes, by 2X I meant two-by. I can see where that would cause confusion!

      2. Tim_O | | #13

        Let us know how that goes! Interesting concept. The open web floor joists would add considerable cost I'd think? And a decent bit of labor in themselves.

        1. Expert Member
          Michael Maines | | #14

          Yes, it will add cost, but the owners are building the house themselves and really like the idea of having a safe place to stand while setting trusses. We'll probably use T-Studs; I've talked with the owner and technical person there about it and they think it will work. When you have a small crew and you're building where it rains and snows a lot, sometimes you need to make concessions.

          1. Expert Member
            MALCOLM TAYLOR | | #15


            When this idea has come up I've been a bit skeptical about whether a largely redundant extra floor is worth it to save a few holes in the ceiling air-barrier. But an attic floor system that is deep enough to act as a service cavity for mechanicals makes a lot of sense.

            One downside is that it introduces another rim-joist, with all the complications around air-sealing and insulation that come with them.

            For an owner-builder that stable platform for setting trusses is great. The advantage disappears if you are using professional framers. Setting trusses takes about a day, and I've never found the task very onerous.

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