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Disappointing Air Infiltration Results

geoff_frood | Posted in General Questions on

We’re in the process of building a “as good as Passive House” and just did our first air infiltration test. The building is north of Montreal, Quebec (Zone 6 almost 7). The final build will be a double wall with dense pack cellulose, with OSB or Ply on the outside on the interior wall giving me a service cavity. I treated the exterior ply sheathing as my air barrier with full calking and taping of all penetrations, as well as house wrap. It has a cathedral ceiling, also dense pack with site made ridge baffles. At this point the interior is still open studs, which brings us to the air infiltration test. We got a result of 2.27 ACH50 which I’ll admit is quite disappointing. I suspect the ply that I used for the ridge baffles are to blame as it was recycled and the glue seemed spotty.
My question is this, should I continue to work on getting the outer sheathing air tight or should I just put my efforts on getting the interior sheathing as tight as possible and let the dense pack bridge the gap as far as tightness goes.

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  1. Trevor_Lambert | | #1

    I would have put the air barrier on the inside sheathing to begin with. All the tape and caulking is going to go through many temperature cycles over the years, whereas inside the conditioned envelope it's protected from most of that. I suspect that two partial air barriers buffered by dense packed cellulose isn't going to be all that effective, but I'm sure the experts will weigh in. I also expect some to say that 2.27ACH is not all that bad, and knocking yourself out trying to get it below 0.6ACH won't be worth the effort. And they may be right, but it's sometimes hard when you have expectations that go unmet. It seems ridiculous now, but my house came in at 0.22ACH and I was mildly disappointed because I expected to be below 0.2ACH. I determined that most of that leakage was through the OSB sheathing, and briefly considered painting it. Luckily I came to my senses and moved on.

  2. Expert Member


    Houses which rely on the exterior wall sheathing for their air-barrier usually do one of two things at the roof: If they are going to continue the same strategy and use the roof sheathing as the primary air-barrier they go with an unvented assembly, or they transition to the interior surface of the roof assembly and vent the cavities. Relying on the sealed baffles isn't a good strategy. I bet you will find a huge improvement with a warm side air-barrier there. What are you planning to do for an vapour-barrier?

    1. geoff_frood | | #3

      Seeing as its a cathedral roof, I wasn't comfortable with an unvented assembly. It was always part of the plan to have a solid continuous layer of ply/OSB under the trusses to support the dense pack, so that could be sealed up quite easily. By my understanding, OSB meets the canadian standard as a vapour barrier, so that's the current plan. Originally I had planned on using MemBrain on the walls but I'm not convinced that it would hold up to the dense pack. Also OSB comes out on par cost wise. The exterior sheathing is ply.

      1. Expert Member
        MALCOLM TAYLOR | | #4


        You should be fine. I bet the sealed OSB will tighten your place up a lot. Might be worth checking with your building inspector before relying on the OSB alone as your vapour barrier. It may require a coating to get down to 1 perm.

  3. Expert Member
    Akos | | #5

    I think its next to impossible to properly air seal site built baffles for a vented roof. GBA is partly responsible for pushing for this, never made sense to me. With a vented roof, your air barrier should be on the interior. This is simple to detail properly, easy to visually verify, can be tested and easy to fix any leaks before the drywall goes on.

    I would finish the ceiling air barrier and test again. This will tell you where your issues are which can be fixed while things are open.

    As Malcolm pointed out, be careful with OSB as a vapor barrier. My inspector did not accept it.

  4. blacksturgeon | | #6

    To Malcom’s point, regarding the OSB being questioned as a legit class II vapor barrier by the building inspectors here in Canada, my recent experience was along those lines. I believe Malcom has noted here on GBA in the past that the NBC specifically lists OSB and 1/2” exterior grade plywood along with other “sheet goods” as acceptable class II vapor retarders. Over the past year, I have been building our house with double stud walls......a “Lstiburek” double wall style with 1/2” fir plywood on the exterior side of the interior wall, detailed as my air and vapour barrier. After receiving my building permit, and then subsequently discussing some of the specifics of the build right on site with the inspector, he then later questioned the use of the plywood as the vapour barrier. This happened when he came to do the structural inspection. He messaged our building design person (who provided our drawings) questioning the use of the plywood, and fortunately, with her input, he ultimately accepted it. I’m in NW Ontario, climate zone 7.

    1. geoff_frood | | #9

      Our municipality is a bit of a wild west zone when it comes to building inspections, basically there are none. I'm looking in to hiring an independent inspector to give me some peace of mind and also to give me some backing should there ever be a problem with insurance. I imagine that the idea of using sheets goods as barriers is going to be a novel concept for whoever I bring in.

  5. Jon_R | | #7

    Keep in mind Lstiburek's comment - "The significant disadvantage of exterior air barrier systems is their inability to control the entry of air-transported moisture into insulated cavities from the interior."

  6. tallpinescabin | | #8

    Out of curiousity, what's the size of the building? Might not be so bad in a 1000 SF building vs a 4000 SF 2 story... (Surface area to volume ratio...)

    1. geoff_frood | | #10

      Its a 950 SF foot print with 3 stories. He used a volume of 29882 cubic feet for his calculations. No unusual building details, just a cube with a 12/12 roof.

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