Posted By kenjlap on 25 Jan 2017 08:52 PM
Any advice would be helpful. Building in northern ny, st Lawrence county. trying to insulate my walls better without putting foam on outside of sheathing as I want to use cultured stone on exterior.
I thought about building with 2x8's 2 feet on center and then cutting 2" insulation to fit between studs. I would cut them a little small and use some cans of sprayfoam to fill. I would then stick r21 bats in to finish the job. spray foam here is $1 per aquare foot per inch. So it would cost me about 6k for 3 inches. I could do it my way for $3500 and a few days of my time. Not sure what my overall r value would end up or the condensation factors, but figure it would be better than the 2x6 16 on center r21 bat house I live in now. any thoughts and thanks.
A cut'n'cobbled foam approach is risky, since maintaining the perfect air seal over time is next to impossible.
At 2" of foam you don't even have sufficient R-value of the air-impermeable foam to meet IRC prescriptives on the 5.5" batts. The IRC calls out R11.25 minimum on the exterior side of wall cavities for US climate zone 6. See TABLE R702.7.1 CLASS III VAPOR RETARDERS:
http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%207.html
If you used polyisocycnurate it would meet the letter of the code per it's labeled R12, but on a performance basis not so much. Polyisocynurate has to be derated to reflect it's actual performance on the cold side of the assembly. In your county & stackup don't assume more than R5/inch, R4.5/inch may be more realistic during the months that count if it's in the Adirondacks at altitude. You're looking at a minmum of 3" of foam to make it work even from an IRC minimums perspective.
If you're going to cheat the R-ratio you might still be OK if you used a smart vapor retarder such as 2-mil nylon (Certainteed MemBrain) under the sheet rock, but making that air tight for the long haul isn't exactly assured either. Using 2" of closed cell spray polyurethane would deliver more assured air tightness, and would just squeeze by the IRC minimums in the table. But it comes with a substantial environmental hit from both the high polymer per R, and the HFC245fa blowing agent (a powerful greenhouse gas).
Using 2x6 framing with cut-in bracing for the structural strength, and 3" of any type of rigid foam on the exterior (seams taped, caulked to the framing inside each stud bay)and NO structural sheathing would be a better approach- cheaper too. EPS and polyisocyanurate are both blown with pentane, which is far more benign than the HFCs used for XPS or closed cell polyurethane. Using reclaimed roofing foam from commercial building demolition or re-roofing is about as green as it gets for foam insulation (the environmental hit has already been taken, you're just loading up the "benefit" side of the cost-benefit scale by extending it's lifecycle.) There are a number of vendors trading in reclaimed roofing foam in New England.