How important is it to tape both layers of foam?
I am installing two layers of 1-1/2″ fiberglass faced poliso. I purchased some cheap tape that I had thought would make a good seal on the seams. However, now that we’ve put a layer on, it is obvious the tape isn’t going to do the job. So now I have a lot of left over tape and facing the fact of buying more expensive tape to do the job right.
How important that the interior layer of foam be taped? One thought I had was to put the cheaper tape on the inside layer and the good stuff on the outside layer. If the inside layer is tape just for redundancy, the cheap tape might be good enough?
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Cheap tape on the inner layer is still better than no tape. Sandwiched between layers it'll still be pretty effective even if the adhesive gives up, so you might as well use it. You will of course want to use something that hangs in for the long haul over the outer layer.
Is it worth the extra expense to get the more expensive tape for both layers?
What specific tape should one use for taping off foam?
You didn't tell us whether you have any sheathing under the rigid foam. If you have plywood or OSB sheathing, that's where I would use the high-quality tape. Of course, if your house has board sheathing, you can't tape the seams.
If you can't tape the sheathing, tape at least one layer of rigid foam. Whether it's "worth it" to tape both layers of rigid foam depends on your airtightness goal, but I doubt whether the cost of the second layer of tape will be recouped in energy savings. In all cases, pay attention to air-sealing at penetrations and transitions (for example, the foundation-to-wall transition).
Q. "What specific tape should one use for taping off foam?"
A. The answer depends on the type of rigid foam. For XPS or EPS, I would use Siga Sicrall, Siga Wigluv, or 3M All Weather Flashing tape. For foil-faced polyiso, I would use almost any foil-faced tape or housewrap tape. For more information, see Return to the Backyard Tape Test.
Ah, I see. Yes, there is OSB sheathing. I guess the articles I saw that said to tape the foam weren't using sheathing. THat makes sense...tape where you want to keep the air out of, not a couple layers out from that. Who cares if the air gets into the foam layers, I want it out of the house.
I talked with my builder and he was concerned about not taping the rigid foam and whether that would allow too much moisture back to the osb. And with the wet osb against the rigid foam, there would be no way for it to dry. However, I responded that it would be able to dry to the interior. Right?
Every wall needs a water-resistive barrier (WRB). In many cases, this will be a plastic housewrap like Tyvek or Typar. If you have housewrap on the exterior side of your foam -- or, in some cases, between your rigid foam and your OSB -- I wouldn't worry about water penetrating into the OSB layer.
If you want to try to use rigid foam as your WRB, make sure that you understand the implications of that decision. (For one thing, the rigid foam has to be taped if you expect it to act as a WRB.) More information here: Using Rigid Foam As a Water-Resistive Barrier.
Martin, Thanks. We are using Typar on the outside of the rigid foam. I guess he was concerned about water vapor.
Also, I've been re-reading your backyard tape test results now that I am focusing on taping OSB instead of rigid foam. In your original article you conclude that Siga Wigluv is the obvious choice, and 3M All Weather flashing tape performed poorly. But in your "Return" article, you said both the Siga and 3M tapes performed very well. Is the 3M tape acceptable to use on OSB?
Q. "Is the 3M tape acceptable to use on OSB?"
A. Yes. You might also consider Zip System tape -- I've heard good reports from others who have tested Zip System tape on OSB.
I used 3m flashing tape on our new house in a variety of places, to the tune of three cases. It is insanely sticky. It's cheaper than Siga tapes, which we also used and which is also insanely sticky. As I recall, Martin's tape test found both to be very good.
Is primer still required to get a good bond on osb, and if so, are there alternatives that I could pick up at the big box stores?
Clay: We used the 3M tape on Advantech sheathing without a primer and it stuck fine.
Look on line for the best price for the 3M tape. The price varies a lot, unlike the Siga tapes, which were never discounted, as far as I could determine.
The 3M worked well on my OSB and treated plywood (bottom two feet sheathing) during this damp winter in January and February. I expect that if the foam is relatively clean it will stick well to it.
I just thought of something... I am going to have a lot of penetrations through the OSB from fastening the 2 layers of foam and then the house wrap. How much air infiltration is that goingto allow? Is that going to nullify all my air sealing efforts around the edges if I have a bunch of screws through the middle?
Q. "I am going to have a lot of penetrations through the OSB from fastening the 2 layers of foam and then the house wrap. How much air infiltration is that going to allow?"
A. Almost none.
Q. "Is that going to nullify all my air sealing efforts around the edges if I have a bunch of screws through the middle?"
Typar over the foam seems odd. Shouldn't it be applied over the sheathing, or does that not matter?
Hey Steve. We're going with "outie" windows, so the housewrap goes outside the exterior insulation. A good article about it is here... https://www.greenbuildingadvisor.com/blogs/dept/musings/where-does-housewrap-go