Taping rigid foam on wall exterior when that’s not the air barrier?
We’re planning on installing 1 ½ inches of EPS over the sheathing (OSB, Zip panels, or plywood, not sure yet.) Everything I’ve read about this indicates the importance of caulking or applying spray foam in the seams, and then taping those seams. But we’re thinking we’ll make the air barrier at the sheathing layer, taping all of the sheathing panels together, possibly also caulking from inside of stud bays. So why then do we need tape on the foam? I can see the value of using some spray foam in the seams if the gaps are wide, but shouldn’t we be able to get them to fit pretty tightly together? Also, I’ve heard that rigid foam panels can shrink sometimes … If this were to happen, wouldn’t it be advantageous to leave the seams untaped, sort of like a control joint? If they were taped, wouldn’t the foam perhaps crack elsewhere to relieve stress?
A more general related question: How much performance will we sacrifice if the foam panels are tacked up to fit fairly tightly, but with no additional work on the seams?
I won’t be doing this with my own hands, so I need to give detailed instructions to contractor. We want a lot of care taken with the actual air barrier and would rather use the contractor’s time and attention on this.
(Walls will be 2×6 with damp-sprayed cellulose. Most likely will have furring strips and engineered wood lap siding exterior to the foam, but might go with vinyl.)
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