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Shiplap over open cell

Tinman77 | Posted in Green Building Techniques on

I have a cathedral ceiling that I plan on installing primed Pine shiplap planking as my finish.

The roof is constructed of 24″ O.C. 2×4 scissor trusses with 5/8 OSB zip plywood sheathing under asphalt shingles.
The way the trusses are designed I only have about 7 in at the top of the wall and don’t achieve my full 10in depth required for R-39 insulation until about 28″ up the slope.

My insulation contractor installed open cell foam, which does fill theĀ  sealed soffit which extends 12″ past the exterior wall.

At the peak the trusses are 20in deep and he only installed 10 to 12 in on average of insulation. I was expecting him to overspray and shave where it wasn’t as deep near the walls.

So I am left with A continuous a space between the insulation and the pine boards I will be installing.

My main question is vapor barrier or not. I was thinking CertainTeed membrain but any advice would be greatly appreciated.

I believe I am in zone 5 20 miles south of Boston Massachusetts

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  1. Expert Member
    Michael Maines | | #1

    I would definitely add a vapor retarder. Although open cell foam is code-compliant because it's airtight, it's vapor-permeable so in cold weather moisture can diffuse through the foam and accumulate at the sheathing. Membrain is fine; it's not a vapor barrier, it's a variable permeance vapor retarder that slows vapor from reaching the foam but allows drying to the interior if moisture accumulates.

    Did your contract say they would fill the trusses or that they would provide a specific depth or R-value?

    1. Tinman77 | | #3

      They weren't filling the entire depth throughout, as there is about 20 in at the ridge and only about 8" at the wall. I get the full 10 inch needed for a 39 at about 18 in from wall.

  2. maine_tyler | | #2

    It sounds like you essentially have a tiny unvented attic with ocSPF.
    What is the picture showing? I see fiberglass and what looks to be a flat bottom chord/tie

    1. Tinman77 | | #4

      Sorry I had attached a wrong picture previous but updated on last reply. Guess essentially I have a very small attic

  3. Deleted | | #5


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