No need to tape outdoor sheathing if walls are all spray foamed?
fresnoboy
| Posted in Green Building Techniques on
Hi. I asked my GC about if he was planning to tape the gaps between the outdoor plywood panels for air sealing. He said there was no need to do that because he’ll be spray foaming (open cell) all the walls on the inside.
Note that he’s using Tyvek Drainwrap on the outside, and then a 2 ply layer of asphalt felt paper on top of that, with cedar shingles as the exterior cladding.
What do you think? Taping the sheathing doesn’t sound like that much work, but if it doesn’t buy you anything no point in doing that.
thanks!
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Replies
Open cell foam can store moisture. It might not be critical to tape the seams, but it makes sense from a belt and suspenders point of view.
Thanks Steve. It doesn't sound all that expensive to do, but it a costly thing to seal with tape? If its not expensive the belt and suspenders approach sounds like a good idea.
Zip tape is $25 for a 90' roll at Home Depot.
Mike,
Unlike Steve, I wouldn't worry about the open-cell spray foam storing moisture. In some climate zones, you may want to install vapor-retarder paint on your drywall to meet code requirements -- but even if you skip this step, your wall shouldn't have moisture problems unless there's something else funny going on (like extremely high indoor relative humidity plus a big air leak).
Walls insulated with spray foam can leak air -- for example, where sheathing seams align with double studs. The way to guarantee that your wall doesn't leak is to conduct a blower door test.
If for some reason your contractor isn't planning a blower door test, you would need to caulk all the doubled studs and double plates on the interior, as well as the seam between the bottom plate and the subfloor, and detail every window, door, and penetration carefully to limit air leakage. Taping the sheathing joints would help, but it's not the only way to seal the wall.
But you'll never know how leaky your wall is without a blower door test.
Spray foam in the cavities only air seals the cavities. It doesn't air seal the seams between doubled up studwall framing such as top plates, headers, jack studs, etc, or between the bottom plate and the subfloor. The combined cross sectional area of those seams are much bigger than they appear at first glance, and change seasonally with moisture content. With new construction they usually grow bigger as the house becomes heated/cooled and the moisture content drops.
Polyurethane caulk or purpose made sealants applied to all of those seams is a good idea, even if the plywood seams are taped. This is especially true for doubled up top plates and bottom plates, and any doubled-up window framing.
Additional air barriers always buys you something.
Consider the better performance of a larger ventilated rain screen (vs just Drainwrap).
In response to your builder, spray foam is nowhere near perfect, and does not always result in perfect air sealing (unlike common belief). The studs will shrink; the spray foam won't; it might still work or you may end up with a crack in the foam which leaks air. Tape the sheathing with a good long lasting non-butyl tape* - it will be worth it. It's also smart to caulk the top and bottom stud/sheathing joint where it cannot be taped.
*one is 3M-8067
What is a good caulk for the top stud (assuming double top plate) sheathing joint?