Wall insulation and integrity question
This question is for Zone 5a at 8,500ft in Colorado. I can get recycled Polyiso and EPS for much less than any other insulation and want to use it, but not on the exterior.
I have been soundly talked out of cut and cobble by the experts here. For the exterior wall, from the outside in, siding, 1×4″ rain-screen, Tyvek commercial D house wrap, CDX properly taped and rolled. For the insulation, it has been well recommended that I install dense-pack cellulose and I agree.
However, it is much more expensive than the rigid foam (for me). Will it make a good wall to dense-pack the 2×6″ load-bearing wall and then run polyiso in the thermal break, lap it, tape it, and seal it as detailed as possible, then build the interior 2×4″ wall? The theory is that the dense-pack can dry to the outside. The polyiso will act as insulation and as an interior vapor barrier. That will give me a pretty good R42 wall and allow me to leave it mostly untouched because electrical and mechanical can run in the 2×4″ wall that won’t have insulation in it, the cost will be much less because the Polyiso is so inexpensive compared to the dense-pack cellulose.
The concerns are:
Will 5.5 inches of dense-pack cellulose have trouble drying to the outside?
Will any vapor/air that makes it through the polyiso to the dense-pack create a problem?
Is it safe to assume the drywall will be fine without insulation in the stud bays? I assume it will because fiberglass bats don’t offer much if any wall reinforcement, but don’t want to assume.
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Replies
mhenson,
That's a very well performing wall. When it has been suggested in the past, the objections are having to buy the foam, when you could just use more cellulose or batts. But in your case that isn't a consideration. You don't say how thick the foam board will be. One option would be to use less and fill the interior wall with batts.
The cellulose will be fine drying to the outside. Like any other wall, the better the air-sealing, the less moisture the insulation has to deal with.
The drywall will be fine, just as it in on interior walls with no insulation.
Thank you, Malcolm. The thermal gap measures 3 inches, so I'm planning to use two layers of insulation that are 1.5 inches each and lapped over each other. My aunt in Texas can provide me with a semi-load of recycled polyiso for the cost of fuel and her driver's time, which is almost free."
mhenson,
What a great situation! A 4'x8' sheet of 1 1/2" XPS is $90CDN here.