Band Joist Insulation Between House Levels
I have a new construction project in coastal Delaware. The project will use 16 inch floor trusses for the second and third levels. Much of the second level will be elevated on piles for flood protection. Our plan has been to use spray foam on the underside of the exposed portion of the Level 2 floor both for thermal and air sealing purposes. Since we are already using spray foam, the initial plan was to spray foam the band joist areas between levels.
Exterior wall construction will include a rain screen, a layer of polyiso continuous insulation, and sheathing nailed directly to our studs to meet our hurricane wind load requirements.
The insulation contractor has proposed to use an HFO-blown closed cell spray foam. There are plenty of articles or user comments on this site that note the risk of sandwiching OSB sheathing between layers of vapor impermeable foam so I want to proceed cautiously here.
The architect and structural engineer have called for the sheathing panels to be installed horizontally, and for sheathing panels to span completely across the band joist area of the wall.
I guess I have a couple questions:
(1) Do I really need to be concerned about sandwiching the middle 16 inches of a 48 inch wide panel between layers of foam? Perhaps I’m making a mountain out of a mole hill here, and those panels would have plenty of drying potential above and below the joists (we’re using blown-in fiberglass for the wall cavities).
(2) If I use plywood instead of OSB as the sheathing panel that spans the joist area, does that adequately address this issue? Can I feel safe sandwiching a 16 inch wide swath of plywood between closed cell spray foam on the interior and polyiso on the exterior? I’m reading Lstiburek’s “Mind the Gap, Eh!” article as implying this problem is particular to OSB sheathing but perhaps that is incorrect.
(3) Is there enough uncertainty here that the best course of action is to avoid the risk entirely by insisting on not using closed cell spray foam in the band joist areas?
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Replies
I think you have the right approach. Your Architect and Engineer, are correct, about installing the sheathing horizontally and covering the rim joist for higher structural integrity. If your budget allows, is better to use plywood over OSB, especially in coastal areas.
Your plan on using exterior foam (iso) over sheathing, and rainscreen, is good, but the rim joist needs to be insulated on the inside with open cell foam, as you said, you don’t want to sandwich the OSB/Plwd with two closed cell foams. Installing 3” R20 ccSPF min. under the Level 2 floor, it’s a good option, the code requires R19 min. Those 3” ccSPF on the rim joist is not a problem, since there is plenty of room to dry-out to within the floor truss space. I would install plywood under the trusses.
If you want more insulation on the Level 2 floor trusses, you could fill the cavity with permeable insulation, install plywood under the trusses, and then, 2” polyiso foam board, taped and sealed.
Thanks, I appreciate the comments and advice.