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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William,
As I'm sure you know, every wall needs a water-resistive barrier (WRB). If you want to use rigid foam as your WRB, you would, of course, need to tape the seams of the rigid foam.
If you will be using something else as your WRB -- for example, plastic housewrap -- then taping the seams of the rigid foam is optional.
If you choose to tape the rigid foam seams, your wall assembly may be slightly more airtight than it would if you didn't tape the rigid foam seams. By limiting air movement at the seams, the tape may also slightly improve the R-value of your wall assembly.
Whether these benefits are worth the cost and hassle depends on your goals and your budget. Plenty of GBA readers have decided that it's a good idea to tape the rigid foam as well as the wall sheathing. But you don't have to if you don't want to.
By the way, it's not at all unusual for a wall assembly to have more than one air barrier. For more information on this topic, see One Air Barrier or Two?
-- Martin Holladay
Thanks, Martin. So sounds like we might be OK skipping the tape on the foam. We'll have housewrap over the foam, so that'll be the WRB. Maybe we'll still plan on caulking or spraying foam where any gaps between panels might warrant this.
I wouldn't use caulk or foam if it was me. You've already got the r value, the foam is therefore just sealing the joints from airflow and not really providing r value if you're insulation is installed tight and done well. To me you're looking to air seal if you're going to bother using foam and or caulk, spray foam is generally not good at air sealing, better than nothing sure, but the caulk will do a much better job. However for sealing the joints of exterior insulation, it would be hard to caulk all joints and not spend a wack load of money and time becuase of needing a ton of caulk. That all sounds like a hastle. If you want to seal the joints, tape the joints. Way easier and a much better solution to air seal. Simply lapping your foam and staggering the joints does nearly a good enough job.
I did exactly this with a recent PassiveHaus project. Used ZipWall system, with 5" of Closed cell in the stud bays (24" OC) and 2" exterior XPS rigid foam which was simply used for it's thermal property not WRB as the actual WRB was the Zip system itself. Also had architectural trim system(easy trim) as the rainscreen/bulk water shedding which lifted and pitched the cladding about 1/8" off the wall. Didn't like the additional labor of taping the rigid foam plus the potential for condensation/moisture to be caught in-between sheathing and foam.