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Understanding the airtight drywall approach (ADA)

KevinEJ | Posted in Green Building Techniques on
If drywall is installed with screws, drywall tape and joint compound alone, isn’t an air barrier created? Gaskets/adhesive seem logical wherever the continuity of drywall is interrupted (at the floor, rough openings, outlets, and partition walls), but what’s the function of putting it everywhere else? Why are wall boards glued to top plates and ceiling boards glued to rafters if the ceiling will then be taped with compound to the walls? Is this strictly a belt & suspenders approach? Or am I just not understanding how ADA works? I’ve read everything I could find on the subject.
These questions popped up when wondering how to deal with this:
In a balloon-framed rake wall with cathedral ceilings and no top plate at the ceiling plane, do I need to add solid blocking so the top of the wall sheets have something other than 24 inch o.c. studs to adhere to? Similarly, at the eaves (perpendicular to the rafters), is backing needed between rafters for the ceiling drywall edges?
Thanks!
Kevin

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Kevin,
    Q. "Gaskets/adhesive seem logical wherever the continuity of drywall is interrupted (at the floor, rough openings, outlets, and partition walls), but what’s the function of putting it everywhere else?"

    A. You're right. You only need gaskets at locations where air leakage is possible. Many articles on the airtight drywall approach -- including some in Fine Homebuilding magazine -- misunderstand this point.

    You can trust my article on this method. Here is the link: "Airtight Drywall." In my article, I suggested that the following locations need caulk or gaskets:

    • Drywall perimeter. Use a continuous bead of caulk or drywall gaskets along the bottom plates and top plates of exterior walls, along the top plates of partition walls under insulated ceilings, and around the perimeter of all rough openings.

    • Intersecting walls. On partition walls that intersect exterior walls, seal both sides of the stud nearest the intersection. With caulk, seal the crack between the first stud in a partition wall and the partition’s bottom plate and top plate.

    • Windows and doors. Seal between window frames and window rough openings using low-expanding foam, gaskets, or backer rod and caulk. If your windows have drywall returns, install gaskets on the faces of the rough-opening studs (behind the drywall jamb extension) rather than the edges of the stud. Caulk window and door casings to the drywall.

    • Penetrations. All of the penetrations through the drywall need to be sealed, including electrical boxes. It’s also important to seal plumbing and wiring penetrations through top and bottom plates in exterior walls and partition walls.

    Q. "In a balloon-framed rake wall with cathedral ceilings and no top plate at the ceiling plane, do I need to add solid blocking so the top of the wall sheets have something other than 24 inch o.c. studs to adhere to? Similarly, at the eaves (perpendicular to the rafters), is backing needed between rafters for the ceiling drywall edges?"

    A. Yes. You don't need it for drywall attachment reasons, but you need it for air barrier reasons.

  2. KevinEJ | | #2

    Thanks for the reply, Martin.
    I reread your article. You wrote "Although air can’t leak through drywall seams sealed with paper tape and drywall compound, it can easily leak through cracks wherever drywall is screwed to framing lumber. The airtight-drywall approach relies on caulk and gaskets to seal these cracks."

    To help keep things from cracking, should I float wall-wall corners or any wall-ceiling junctions? I have 2x6 walls with three stud "california corners" and TJI rafters.

    In my second question, I asked about blocking in particular areas. In this 2012 FHB video (Video Series: How to Hang Airtight Drywall), these same areas are being ignored, so are they relying on drywall tape alone? Should solid blocking be in these areas for air barrier reasons, or will the tape be enough? I've attached a few screenshots to show what I'm referring to in the FHB project.

  3. Malcolm Taylor | | #3

    Mark,
    You shouldn't haven significant cracking unless you have a trussed roof. The one or two superficial cracks you may get as the framing dries you will want to fix for aesthetic reasons anyway.

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

    Kevin,
    Malcolm appears to be talking about drywall cracking or cracks in the drywall compound.

    The reference in my article to "cracks" has nothing to do with drywall cracking. I was talking about the tiny gaps between the framing (for example, a stud) and the back of the drywall. Air can escape through these cracks -- especially if air can enter stud bays through electrical outlets.

  5. Malcolm Taylor | | #5

    Martin, I think Mark was asking about drywall cracks too:

    "To help keep things from cracking, should I float wall-wall corners or any wall-ceiling junctions?"

  6. KevinEJ | | #6

    Martin & Malcolm,
    Thanks. (I'm Kevin, who's Mark?)
    I'll attempt to clarify my question.

    I understand Martin is referring to sealing cracks that are the gaps at junctions where two building materials meet (studs and drywall, etc). That makes sense.

    My unanswered question pertains to the Fine Homebuilding video linked above (also posted here on GBA: "Video: How to Hang Airtight Drywall"). I see that Myron skips the drywall backing locations I'm asking about (pointed out in my screenshots posted above).

    Martin, despite them skipping the drywall backing we're discussing in this instructional video, does your advice stand to block in these locations for "air barrier reasons" but not for "drywall attachment reasons"?

    And my rephrased question about floating:
    If I add this extra blocking, should I be screwing the drywall to it, or could I cause excessive cracking in the drywall compound if I do so?

  7. User avatar GBA Editor
    Martin Holladay | | #7

    Kevin,
    I'm not going to try to defend Myron Ferguson's personal interpretation of airtight drywall. Suffice it to say that I don't agree with Myron.

    If your house has just one room, you can create an interior air barrier with the airtight drywall approach without many complications. As long as the walls and ceiling of this room are airtight -- achievable with drywall tape, airtight electrical boxes, and attention to penetrations -- you're good to go.

    The problems begin if your house has two or more rooms. Now you have partitions, and that's where the leaks can occur. If the electrical outlets in the partitions aren't airtight, you need gaskets at the studs at the partition intersection to make sure that things stay airtight.

    Similarly, you can make the transition from a wall to a sloped ceiling rather nonchalantly if the house has just one room. If the house has two rooms, you have to think about partition intersections and leaks at the perimeter of the walls and ceilings.

    I think that you are much more likely to stop hidden airway paths if you include blocking (or a top plate) at the intersection between a wall and a sloped ceiling.

  8. KevinEJ | | #8

    Martin,
    Hopefully no offense taken, I'm certainly not trying to pin anyone against each other here. Just noticed that job was very similar to my current one (a one room building with similar framing). Thank you for the clarification on the complexity a partition adds, and for dumbing it down for a novice. Much appreciated.

  9. User avatar GBA Editor
    Martin Holladay | | #9

    Kevin,
    You're welcome. And good luck with your project.

  10. Malcolm Taylor | | #10

    Kevin,
    Apologies. I was replying to Mark in another thread and happily started calling you that name in this one.
    I'm a fan of blocking behind drywall not only for the reasons Martin gave, but because it often proves useful for the trades that follow who they are installing trim, cabinets, built-ins etc. I've often lamented the absence of backing, but never it's presence.

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