Doing the math, putting 5.5 inch roxul batts over the inch of closed cell foam will cost about $550. Another 5.5 inches of open cell will be around $1200. At this point the outer wall is 2x4 construction on 16 inches on center. The inner wall is 2x3 at 24 inches on center. So, the stuff would be 24inch sections of roxul with notches gut for wiring and outer 2x4 studs. I am wondering if the better air sealing properties would make the 2x cost of the open cell worth it?

What is the chance of getting stiff roxul batts to sit tight to the foam without creating a cavity for thermal currents?

Posted By FBBP on 02 May 2014 07:40 PM
What is the chance of getting stiff roxul batts to sit tight to the foam without creating a cavity for thermal currents?

I would say that it wouldnt be very high unless I compressed the batts quite a bit. I think with open cell being twice the cost it would be worth it! If there were any small gaps near the drywall I could see putting in small amounts of fiberglass.