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Community and Q&A

Affordable Air Control Layer

homedesign | Posted in Energy Efficiency and Durability on

When budget is not a concern…where would Dr. Joe put the(ACL) Air Control Layer?
See Joe Lstiburek comment (5/24/09)

Why is it that only the wealthy can afford a Plywood Air Control Layer?
If it makes sense on a luxury home….why not an Affordable home?
What is wrong with ample structural sheathing?
Why should we limit our designs to the absolute minimal amount of CODE racking resistance?
Sure…plywood is not cheap….neither is gallons of “guey stuff”..and time spent applying “guey stuff”
Safely Engineering away almost all of the structure is not cheap(man hours) either.

Why not ONE very good ACL instead of hoping that one of 2 sort-of ACL’s works?

Joe Lstiburek:(5/19/09) “I typically ask for both interior and exterior air barriers and hope that I get at least one with some continuity. In commercial construction I prefer exterior air barrier sheathings by a wide margin. In residential construction I don’t have much of a favorite.

Why is commercial more important than Residential?

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  1. homedesign | | #1

    from Martin's "OK to Skimp" Blog.... Martin Holladay (2/11/10):"
    I agree. Yesterday I attended Efficiency Vermont's "Better Buildings" conference — one of the best regional conferences in the country. Marc Rosenbaum gave a great talk on air barriers. He's tried them all over the years. His favorite is now to use wall and roof sheathing as the air barrier.
    He uses either the Huber Zip system or conventional sheathing (OSB or plywood) with taped seams. If you're taping OSB seams with peel-and-stick tape, you need to use a primer first.
    Build your house with no overhangs at the rakes or eaves. At the joint between the roof sheathing and the wall sheathing, fold over your tape to connect the wall air barrier with the roof air barrier. Then you can install your exterior foam (if you intend to use any).
    The rake overhangs and the eave overhangs are built as separate units, tacked on to the walls after the air barrier is complete.
    He says he is getting Passivhaus-level airtightness results with this simple and robust air barrier system."

    I am trying to start a fresh disscussion on Structural Sheathing Air Control here.
    Is there any chance of asking Marc Rosenbaum to comment?

    I know that Kaplan Thompson Architects seem to be taking up the ZIP strategy.
    I first learned about it here at GBA from a comment by Jesse Thompson.
    It just makes so much sense to me as an alternative to spray foam.

    My twist is to add the structural sheathing Air Control Layer at the floor of the attic instead of at the roofline.

  2. Riversong | | #2

    One of the things I agree with Joe on is that PH air tightness standards are unreasonable, uneconomic, and unnecessary (and likely unhealthy).

    John, you spoke of "obsessions" on Joe's blog. One common obsession among not only "green" designers and builders, but the entire industry, is that if a little of something is a good thing a lot of it must be a better thing.

    This results in overuse of products such as self-adhering membranes and tapes; manufactured lumber materials such as plywood, OSB, MDF, TJIs, Glulams and Microlams; plastic membranes such as vapor barriers, WRBs and roofing underlayments...

    Wendell Berry, one of the wisest living agrarian philosophers, bemoans the loss of the notion of "forbearance", or doing without what is not really necessary. We use products and methods just because they're available and we can or because they market well - increase the perception of value.

    Very few homes really need structural sheathing. Diagonal bracing is good enough for a timber frame and has worked for 150 years in light framing (and is still accepted by the IRC). Materials and methods are used in luxury homes, not because they "make sense" but because they fit the budget.

    Affordable homes need to be designed smarter than luxury homes, with materials and methods that are sensible, cost-effective, and truly necessary. And green homes should be built with as many locally-produced materials as possible, to support both the local ecology and the local community-based economy.

  3. GBA Editor
    Martin Holladay | | #3

    Marc is pretty busy, but maybe he can be induced to comment. I took notes during Marc's presentation, so if you have any specific questions, I might be able to help.

  4. homedesign | | #4

    My first questions:
    Are they also making a serious effort with Airtight Drywall?
    Or is ADApproach not-so-important with Tight Sheathing Air Control.
    Is anybody using Sheathing as Air Control in conjunction with mostly Outsulation?(Joe's Perfect Wall)
    Is the liquid membrane that Joe speaks of in his "dream wall" even needed?

  5. GBA Editor
    Martin Holladay | | #5

    My notes won't necessarily answer your specific questions, but you may be interested anyhow.
    These are notes from Marc Rosenbaum's presentation at the Better Buildings by Design conference in Burlington, Vermont on February 10, 2010. All errors should be blamed on me, not Marc.

    Why do I like an exterior air barrier? Because of partition walls, floors, and wiring, and things that go in and out of the walls.

    I’ve tried interior air barriers. I’ve had Tremco acoustical sealant all over my hands. Airtight drywall is not easy. The approach takes lots of careful workmanship and you can’t really test it until you are almost done. If you have an interior air barrier, the air barrier has to weave in and out at the rim joist.

    Exterior sheathing is easier to seal. Sheathing is durable. Foam is not as durable, so I wouldn’t use it as the air barrier. I think it’s best to attach the roof overhang later -- after the air barrier is sealed.

    Ideally you want a vapor permeable sheathing. Unfortunately the perfect product doesn’t really exist.
    You can always add more insulation on the outside of the sheathing to keep it warmer.

    Cavity fill insulation must be very resistant to air movement. I wouldn’t do it with fiberglass batts. I prefer dense-packed cellulose. I am nervous about interior air going through the fiberglass. I want the whole thing to be really tight.

    I like to use plywood or OSB sheathing taped with a peel-and-stick tape, or Huber’s engineered ZIP system with a faced OSB.As a WRB, it’s possible to use a roll-on or spray-on rubberized coating such as Sto Guard applied over the sheathing.

    I’m nervous about foam and tape, because the foam moves. Or you can use a flash coat of closed-cell spray polyurethane foam against the inside face of the sheathing.

    If you use tape, it must be compatible with the sheathing, and if you need a primer, you need to be sure that the primer is used and that the temperature is right the day it’s installed. Earlier today, Paul Eldrenkamp was talking about open-cell foam falling off the rim joist because it was installed at too low a temperature.

    Preferred tapes are Vykor Plus for walls and Roof Detail Membrane (from WR Grace). Since roofs get hotter than walls, you need a tape that is rated for use on roofs.

    Huber’s Zip wall is a good system. Peel-and-stick tape on OSB doesn’t work without a primer.
    STO Guard uses a three step process: mesh tape at the seams, then Gold Fill over the mesh, and then a layer of Gold Coat over the entire surface.

    You could caulk the sheathing when you put it up. The only problem with that is I can’t tell if they did it or not.

  6. homedesign | | #6

    This is a related question ... concerning Ray Moore's Persist wall
    Bear in mind that Ray(Austin,Texas) does not insulate the 2x4 stud cavity in his wall....
    Why does he need the Expensive Vapor Barrier (Peel&Stick) layer?
    Would not The Plywood Sheathing if properly detailed take care of the Air Control Layer
    and The polyiso take care of vapor drive?

  7. homedesign | | #7

    And what about the Poliso?
    Since there is no insulation in the stud cavity,
    Latex painted drywall and ventilated cladding....Is vapor Drive even a Problem?
    Why not use XPS or EPS?

  8. Robert Cozart | | #8

    I'm currently building a house with OSB sheathing. Here is a picture of the interior Would it be wise to seal all the joints with Great Stuff? I think this is a project that I could do myself for little cost.

  9. Jesse Thompson | | #9

    We've been talking up ZIP because we've been listening to Marc Rosenbaum and John Straube a lot, at places like NESEA's conference. We're just architects, we listen to what the smart folks who see lots of projects every year are saying and try to keep up.

    I'm also a big fan of diagonal board sheathing on the exterior of buildings, and buildings that dry to the outside in cold climates, but it's enough of a break from standard practice that it's difficult to implement when working with contractors for the first time. That's the primary advantage of a taped exterior air barrier, it's almost standard practice for most stick framers and only takes minor adjustment to their practice, instead of turning everything they are used to doing upside down.

    If we had breathable high-R wood fiber sheathings like in Europe, it would be great ( I'd love to not be using as much OSB on the outside of buildings.

  10. Riversong | | #10

    I'm also a big fan of diagonal board sheathing on the exterior of buildings, and buildings that dry to the outside in cold climates, but it's enough of a break from standard practice that it's difficult to implement

    It's ironic that the method and material that built most housing in America until WWII is not only no longer considered "standard practice" but something that a modern building crew would have difficulty with.

    If there's a carpenter who can't cut and nail diagonal boards, they'd better turn in their hammer (oh yeah, no one uses hammers any more).

  11. Jesse Thompson | | #11

    Robert, I'm not disagreeing.

    In my experience, it's not that they don't know how to do it, it's that most builders don't know how much to estimate the process will cost, so it goes in the "protect myself" category and makes us look bad for specifying it.

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