Sealing under-layers of built-up exterior foam panel insulation
Hello again GBA’ers.
Making progress on the details of my 6″ graphite EPS over Zip panel exterior wall assembly, and coming down to the final few questions, I think.
At this point I want to use ThermalStar GX EPS, and the assembly will three layers of 2″ to get to the final design depth, the outermost face of the outermost layer will be faced, but none of the others will be, which leaves two (I think) questions:
1) Is there any compelling reason to use can foam in the seams to eliminate any tiny gaps between panels? I’m hoping ‘no’ because I’ve done that before and it is _very_ time consuming.
2) Do the seams of the ‘under’ layers need to be taped? Again, hoping ‘no’- taping _and_ staggering the seams of the built up layers would be fairly complicated.
Again, thanks very much in advance for any thoughts or guidance.
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Replies
Gerald,
With three layers of rigid foam, the most important detail is to stagger the seams of successive layers (so that none of the seams -- either vertical or horizontal seams -- line up). If you do that, most builders would agree that taping the outermost layer of foam is enough -- you probably don't need to tape the seams in the other layers of foam.
That said, plenty of GBA readers have taped two or more layers of rigid foam. Some people get obsessive.
Ultimately, here's what you want:
1. You want to handle exterior moisture. That means that you need a ventilated rainscreen gap, good flashing details, and a WRB.
2. You want a good blower-door test result. How good? That depends on your goals. If you get the number you hoped to get, you succeeded.
Good luck.
-- Martin Holladay
As usual, thanks very much Martin.
1) The board foam I'm looking at right ThermalStar LCi with facing:
"ThermalStar LCi is 3rd party evaluated for code prescribed water resistant barrier and fire code approvals by UL (UL ER16529.1)."
Also, I'll wrap the bottom of the foam assembly with one edge of a strip of WRB taped to the face of the WRB on the outer foam board and the other edge taped to the rim joist.
2) The air barrier is Zip sheathing, carefully taped, with OSI SC170 at certain points- particularly across the top. The attic spray foam will tie into the air barrier and external EPS board assembly by another application of spray foam.
Per design, I'm shooting for ACH 0.60.