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Community and Q&A

Insulate a cabin to last

musielski | Posted in Energy Efficiency and Durability on

Martin – I’ve learned a tremendous from you & GBA. This is a 1st asking a question. I’m also an engineer & lic Home Inspector, and I see what happens to homes.
I’m current building a cabin in northern Hayward, WI (Zone 7). I’m on a slab, 20’x26′, 1 1/2 story, 2×6 walls 16″ OC. I had a local carpenter put up the shell, and the rest is up to me. The cabin is sheathed in OSB, Tyvek, Fascia/soffit, and Anderson windows & door are in. I installed kraft faced R19 Batts in the walls (drywall & siding are not yet installed).
I went to the cabin this weekend ….cranked the space heaters, outside temp was 17, inside reached 70 & RH settled at 45. Sure enough, moisture & some ice on the sheathing when I pulled away batts. I want this to last 100 yrs for me, my kids, & next generations.
Exterior foam of 3″ is really not an option. I was going to run a experiment….pull back 3 or 4 cavities, install 2″ XPS, spray foam or snug fit back in place, compress the R19 reducing it to R13.
Granted…this is time consuming….but I never mind time at the cabin. I realize this doesn’t address thermal bridging. My concern is longevity.
1st…Are there any major flaws with this plan? moisture along edges, moisture between foam and OSB even though the surfaces touch, drying, etc?
2nd…Have you ever seen this done with a 1/2 gap between the ridged foam & OSB so theres no contact? …and possible vent holes at the top & bottom of the wall cavity?…basically a rainscreen on the opposite side of the OSB.

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  1. GBA Editor
    Martin Holladay | | #1

    This issue just came up in another thread (Site built ventilation baffle moisture issue).

    I'll copy the paragraphs I recently wrote for that thread:

    "The main reason for the phenomenon that you observed is that there is no air barrier on the interior side of the mineral wool insulation. Even ordinary taped drywall would have been enough to stop the phenomenon you observed. What happens is that your warm, moist interior air has unimpeded access to a cold surface -- the fibers of the insulation do nothing to slow down the movement of air through the insulation.

    "The phenomenon has been reported here on the Q&A pages of GBA many times -- often in stud bays of homes being built during the winter. Owner-builders put their hands behind the fiberglass insulation in their walls and notice that the exterior sheathing is soaked. That's because there was a long delay between insulating and drywalling."

    Wait until warm weather to install the drywall. Once weather is warm, make sure everything feels dry -- then install drywall.

    Alternatively, you can pull out all the fiberglass batts, crank up the heat, and dry the sheathing that way. Then quickly reinstall the batts, and quickly install drywall (in an airtight manner, of course).

  2. musielski | | #2

    Martin, that actually puts my mind more at easy. Inspecting homes day-in & day-out I see problems first hand, but I can't rip walls apart unless I want to loss my license.

    When I head back next week I have two sheets of 2" foam I'll install behind batts. It will be interesting to see the difference it makes. Even though its a Cabin, walls will be drywall(providing and air barrier) ....and the ceiling a pine T&G with a baked on poly supplied by Menards.

    What's nice is the information you provided here is ahead of what you actually see in the field.
    Thank you!

  3. GBA Editor
    Martin Holladay | | #3

    A tongue-and-groove board ceiling, even one with polyurethane on the boards, is not an air barrier. You'll have lots of moisture problems in your ceiling (and high energy bills) unless you install taped drywall on your ceiling before you install the ceiling boards.

  4. musielski | | #4

    If I install a CertainTeed MemBrain on the ceiling would that provide the necessary air-barrier instead of drywall? I used attic Trusses. Down the road 1/2 the attic will be unconditioned storage space & 1/2 will be a room for additional sleeping.

  5. GBA Editor
    Martin Holladay | | #5

    In theory, MemBrain can be installed as an air barrier. However, the material is thin and fragile, so in reality, on the job site, I'm skeptical as to the effectiveness of your plan, especially since the MemBrain will be peppered with fastener penetrations.

    A lot of cathedral ceilings have moisture problems due to exfiltration of interior air, so I'm very conservative when it comes to detailing the ceiling air barrier behind tongue-and-groove boards. I recommend that you install taped drywall. Once you have installed the drywall, you may just decide to skip the board ceiling.

  6. musielski | | #6

    Thanks for increasing my work load with ceiling drywall. Fortunately that project will wait till spring.
    As a follow-up to cut & cobbling my wall cavities to eliminate moisture on sheathing, I did consider 1" interior foam over my studs. That seems to lend it's self to the same issues as interior poly vapor barrier while doing nothing to eliminate wet/winter sheathing. I would imagine you need interior foam thick enough to stay above dew points on hot/humid summer days(to avoid all the traditional interior poly condensation problems).
    Is the cot & cobble over kill?...or am I best on waiting till spring and dry walling with a tight air barrier and skipping the cavity foam? I do fall on the conservative side.

  7. GBA Editor
    Martin Holladay | | #7

    I haven't heard of any summertime moisture problems arising from the use of interior rigid foam. As far as I know, interior rigid foam is safe. For more information on this issue, see Walls With Interior Rigid Foam.

  8. musielski | | #8

    Thanks Martin - after pouring over all this makes senses!
    The one area that I haven't wrapped my head around (or even seen talked about much) is using foam on the interior. I would think the threat of condensation build up exist the same as using 6ml poly -and that's the last thing we want. Other then having added R valve this would seem to be a risky option.

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