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Unconventional insulation assembly– will it work?

LizBrown | Posted in General Questions on

The building is an A-frame Bunkhouse with a vaulted ceiling for housing summer campers and other programs, and is not a “conditioned” space per say. There will be a woodstove in the central room to take the chill off in the spring and fall, but no heat or AC in the majority of the building, and it is intended for 3-season use and isn’t a residence with cooking or bathroom facilities. We’re in eastern WA state, climate zone 5, where it’s hot and dry in the summer, cool and wet in the spring and fall, and cold in the winter.

The insulation assembly we’re currently planning is (from ceiling to roof): pine ceiling boards, Siga Majpell vapor membrane (.16 perms), Rockwool R-23 batting, Rockwool R-6 Comfortboard 80, 2x purlins for attaching the roofing to, Siga Majvest WRB (54-68 perms), standing seam metal roofing. (In the central room with the wood stove, the insulation is R-30 batting and R-12 board.) It is intended to be an unvented “hot” roof system. Also of note is that there is no roof decking in the plan—the engineer doesn’t require it for shear or structural strength. We’re not using any rigid foam materials because in this area any kind of foam gets totally infested with ants!

Originally when I was talking with the contractor about this assembly, we were talking about using vapor-permeable membranes on both sides, so that the insulation cavity could breathe to the inside and outside, but the membrane on the ceiling side has already been installed, and when I looked into it I learned that the Majpell is not actually vapor permeable.

My main questions:

1.)    Will this insulation assembly still work, given that it can only breathe to the outside, and the outside is a standing seam metal roof?

2.)    Should we put in another layer of skip-sheathing over the Siga Majvest WRB and below the metal roofing, to provide a space for vapor diffusion, or will it still diffuse directly under the roof?

3.)    Is there likely to be an issue in the winter months with vapor diffusion into the insulation cavity from the outside through the Siga Majvest?

4.)    Is the Siga Majvest actually the right product for a roof? (16:12 pitch) I’ve found some references for using it on roofs, but more for using it on exterior walls.

Thanks so much!




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  1. Expert Member
    Akos | | #1


    You are proposing a fancy roof for something that will not be used in the winter. I have a 2x6 roof with mineral wool batts cottage in northern Ontario and it works pretty well even in -25C.

    I would skip most of the outside plan. Just put a standard synthetic roof underlayment directly over the rafters, install your 2x purlins and metal roof.

    Without a winter moisture source, your roof will work no matter how you build it. You will still get night time condensation under the metal roof, the purlins and the underlayment are important to keep to deal with that.

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