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Community and Q&A

No Exterior Insulation, Exterior Air Barrier?

NickJVR | Posted in General Questions on

I’m building a tiny house on wheels (a moveable cabin; not to be moved frequently) in New Hampshire, on a very tight budget. So far, the house is framed with 2×4 walls (2×6 roof, 2×6 floor), sheathed with plywood, wrapped in Home Depot Tyvek (shingle-lapped but untaped except for vertical seems), and 1x3s over the studs and Tyvek all the way around. I plan to install T1-11 siding, paper-faced fiberglass batts facing in, and drywall with latex paint.

Question:
Since I do not have any exterior insulation, I’m worried about the plywood being a condensing surface for water, so I’m planning on doing the airtight drywall approach to stop air leakage into the cavities. Is it still a good idea to seal up the joints in the plywood (joints run sideways across the cavities because it was hard to install vertically by myself) and tape all the seams of the Tyvek, or could that contribute to condensation issues were the interior air barrier (drywall) to fail? I have asthma and my girlfriend is very allergic to mold/mildew, so we are more concerned about condensing surfaces than energy efficiency in an 8’x20′ tiny house. Should we install a dedicated air and vapor barrier between the studs and the drywall, or tape the batt vapor barriers together, then only use the back of the cabinets and interior framing (loft floor, bathroom wall) as service cavities to avoid penetrating the interior air barrier?
Is drywall cracking a serious concern? The interior air temperatures will NOT be consistently warm; will extreme temperature changes affect drywall?

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Replies

  1. Expert Member
    Akos | | #1

    A well detailed air tight drywall should be good enough. You have a nice vapor open exterior especially with the T111 on a rainscreen, which provides plenty of drying.

    I would still tape up with a quality tape (Zip or 3m 8067) the horizontal seams. A bit of belts and suspenders approach never hurts.

    Also order in air/vapor tight device boxes (plastic ones with a flange) for anything that is going into exterior walls/roof. These provide a much better seal and cost just a bit more than regular device boxes.

    Detailing craft facer as an air barrier is next to impossible as it is very fragile. You can detail poly or one of the smart vapor retarders. In your case, as long as you are careful with the drywall install and caulk around the perimeter, it should be good enough.

    Make sure to figure out your air barrier transitions from floor to wall and wall to ceiling. These are usually the biggest sources of air leaks in most construction.

  2. Expert Member
    Zephyr7 | | #3

    A vapor barrier won't necassarily do anything to prevent wood rot, and may even make rot more likely. An exterior side vapor barrier will do nothing useful at all in the winter, since the moisture drive is from the inside towards the outside, and in the case of the OP, the exterior side vapor barrier removes the ability for his wall to dry to the exterior which is a negative.

    Bill

  3. NickJVR | | #2

    I live in NH and don't use air conditioning. A vapor barrier on the outside of my sheathing would definitely not be beneficial. That's one reason I chose Tyvek, besides it being cheap.

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