Getting mixed signals about my above grade exterior wall system. Western WI - Location (MLPS/ST.PAUL MN) I am Putting 2" Exterior Foam and was planning on using cellulose for the interior wall cavity so I could "Dry/Breath" towords the interior of the wall asembly. Question - I can probably get a good deal from my spray foam contractor for the interior wall cavity; If I spray foam 3" inside my wall cavity (2 x 6 wall) will it allow enough air circulation for my wall to dry to the inside, or am I creating a "BAD FOAM SANDWHICH" senario. Thank You.

You need to fill the wall cavity - don't leave a 2.5" void between the insulation and drywall.

Thanks for the reply Arkie6, I guess I was thinking that by leaving some air space it would be able to dry to the interior. I have 2000 sq ft of above grade walls to insulate and filling 5 1/2" of foam may be more than I am willing to pay (Probably in the neighborhood of 7-9 K). Spray-in cellulose is looking much better at this point.

Around here, 3.5" or 5.5" of wet spray cellulose or R38 of loose fill cellulose costs ~$1 / sq ft installed. 1" of open cell foam costs ~$0.50 / sq ft. 1" of closed cell foam costs ~$1 / sq ft.

To fill 1 sq ft of that 5.5" cavity with cellulose costs $1.00.
To fill 1 sq ft of that 5.5" cavity with open cell foam costs $2.75.
To fill 1 sq ft of that 5.5" cavity with closed cell foam costs $5.50.

The cellulose and open cell foam would have approximately the same whole wall R value. The closed cell foam would have a moderately higher whole wall R value, but not as high as you would expect due to the parallel thermal conductance of the low R studs and plates. It is really hard to justify the cost of a full cavity fill of closed cell foam.

If you really want closed cell foam for its air sealing properties, a better approach is 1" of closed cell foam followed by lower cost cellulose or even fiberglass batts to fill out the remainder of the cavity. But if you seal the joints in the exterior foam and wood plates properly, cavity foam is probably unnecessary. Your highest benefit vs. cost will almost always be cellulose insulation.

I suggest taped rigid foam + cellulose + Membrain (ie, two air barriers).