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Community and Q&A

Insulating and Detailing Cabin Roof and Walls

Varmega | Posted in Energy Efficiency and Durability on

Hi GBA community – I’ve spent the past year reading here about insulation assemblies and I feel like the more I read, the more questions I have!
We are building a cabin in West Virginia, climate zone 5B.  We are doing most of the work ourselves.  Here’s a brief description…

800 SF cabin built on concrete piers, 2×6 walls with a single pitch roof built with 2×10 joists.  The underside of the cabin will remain exposed (not going to build any skirting)  The exterior will be zip sheathing with a rain gap (it was $25 a sheet CHEAPER than plywood, go figure).  I was going to install rockwool comfortboard on the exterior, but ordering is now 3-6 months out.  The cabin will be winterized and unoccupied from December to March every year.  Keep in mind West Virginia has almost zero code enforcement.

I am currently interested in doing flash and batt (2″ closed cell + R+13 fiberglass batt) in the walls, R-30 in the floor joists and covering with either sheathing or some type of WRB that can seal out critters,etc.  What I’m confused about is what to do at the roof and floor, insulation-wise.

If I want an airtight and economic system, how should the flash and batt walls meet with the roof insulation assembly?
From what I’ve read, flash and batt is not recommended in roof assemblies.  So, how do I create an airtight seal, but also allow the roof joists to breath?
Can I stuff the entire roof joist with fiberglass batt, or does there need to be a gap between bottom of roof sheathing and top of insulation?
Would it be better, R-value wise, to install 2-3″ of XPS on the roof joists, then my zip sheathing + roofing?
Do I need to put a vent at the apex of the roof sheathing to air out through the ridge vent (standing seam metal roofing) as well as vents at the eaves on both sides of the roof?
Do I need to worry about condensation on sheathing if the cabin is unoccupied and left cold from Dec-March?

Thanks for the help!

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Replies

  1. Stephen Sheehy | | #1

    If you haven't built it yet, you could do better and cheaper with 2x4 wall studs, strips of 2" thick foam 1 1/2" wide applied to the studs ("Bonfiglio Wall"), fill the resulting 5 1/2" space with R19 batts. If you want more insulation, do the same, but with 2x6 studs. The closed cell foam doesn't help with thermal bridging through the studs. And it's expensive. And not easy to DIY.

    1. Varmega | | #6

      I like the sound of that idea….I guess I just worry about not getting as much shear strength out of my wall sheathing if it’s interrupted by 2” of foam and screws will need to be at least 3”-3” 1/2” long. I could be wrong but, It seems to me that that could make it easier for the walls to rack or shift.

      1. Expert Member
        Malcolm Taylor | | #7

        Varmega,

        With the Bonfigiio wall Stephen is suggesting the foam strips are added to the interior of the studs, so the shear resistance isn't affected.

  2. Tim Janson | | #2

    Is this just going to be heated, or cooled as well? What will the heating/cooling system be? Will it be conditioned 24/7 April-October? 800 sq ft isn't much to heat in shoulder seasons. If you a using a wood stove, it will be easy to overheat the space.

    I don't see any reason to use spray foam on taped Zip panel construction, which forms the air barrier.

    For the roof... The lower one I would probably build an unvented assembly with exterior rigid foam (4-6") and batts between the rafters. Upper roof has a good place for a "ridge" vent so I would build a vented assembly with a ventilation baffles eave to eave between the rafters and insulate with batts below those.

    1. Varmega | | #4

      Yes, we’ll have mini splits as well as a small wood stove. It will be conditioned at least 8 months out of the year.
      I just found the GBA article on all things cathedral ceilings and it sounds like trying to create an completely unvented set up is problematic.
      By eave to eave ventilation, do you mean from the low side to the high side?

      1. Tim Janson | | #9

        OK, because you'll be conditioning for 8 month I understand more why you want to do a great job of insulating, as opposed to just weekends or no cooling in the hotter months.

        I did mean ventilation channels from the lower edge of the upper roof to the top edge.
        The reason I said unventilated for the lower roof is because there isn't a great place to put a "ridge" vent. Theoretically you could vent the roof into the siding rainscreen but we've reached the limit of what I'm able to BS about. :)

  3. Walter Ahlgrim | | #3

    It sounds to me like you are trying to put too much insulation in the walls and not enough in the ceiling.

    If you must have the sloped ceilings you may want to read this article.

    https://www.greenbuildingadvisor.com/article/building-a-budget-friendly-unvented-cathedral-roof

    I say make the walls taller under a vented attic with a flat ceiling with lots of room for R60 cheap fluffy insulation. My guess is it would cost less to both build and operate.

    Note spray foam is the most expensive way to get an R of insulation and is generally avoidable with thoughtful planning.

    Walt

    1. Varmega | | #5

      ‘It sounds to me like you are trying to put too much insulation in the walls and not enough in the ceiling’

      I am realizing this now…unfortunately I already bought all my 2x10s for the ceiling. I was in a bit of a panic to by my lumber before prices kept going up.
      Another option is to add 2x4 strapping or just fur out the studs an extra few inches on the inside to make room for maximum insulation + air gap above.
      I do have a spray foam guy who’s a friend so it might not be quite as expensive but I am all for cheaper options if there are some better ones.

  4. Walter Ahlgrim | | #8

    Given the modern style of the house it almost seems inevitable the joist spaces will be home to many pot lights displacing the pitifully small amount of insulation you have room to fit in that space if so you may want to consider putting R38 of foam insulation above the joists.

    Walta

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