2 by 6 framing with 3 plus inches of closed cell urethane (ncfi)… i want to run dow super-tuff 1/2 rigid foam (foil on one side, 3 poly on the other) across the inside before sheetrock…..any problems with this?? any better ideas? urethane is done and siding is on, so no foam on the outside of osb….thanks for any thoughts or ideas….south central alaska,never much more than 70 in summer and avg. 20 in winter…if the dow product foil toward sheetrock? or not…thanks
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Presumably you want to use the foam board to prevent thermal bridging between your studs and your drywall. With that foam sprayed in, you already have a pretty effective insulator - like 95% of your job is done. Full sheets of that expensive foam board aren't worth the remaining 5%.
Instead, think about cutting down 1.5" wide strips of regular urethane or XPS foam board, stapling that to the studs, then attching your drywall over that. No double vapor barrier issues to worry about, and much more economical to do the same job.
Rebuilding my home in Cypress, CA
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that dosen't sound like a bad idea....do you think, if i did put the full sheets on, that the double vapor barrier is an issue...since the air space between the urethane and full sheets(or sheetrock) would never get cold enough to create a dewpoint scenario...thanks for your ideas...john
From other posts I've read - likely you wont have an issue. But that assumes that the interior studs never go below dew point... it's possible they won't, but this is something you need to be sure about. The easiest way to be sure about it is to have foam on the outside of the studs... otherwise you will actually have to do the math.
I don't know the math myself, but others here doRebuilding my home in Cypress, CA
Also a CRX fanatic!
If your hair looks funny, it's because God likes to scratch his nuts. You nut, you.
I don't see a double VB issue there
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why?
with 2x6 walls, you'd have been better to fill them with foam if you thought you needed that much.
But tests show a decline in practical effectiveness on your investment by using more than 4" and your temps are not very extreme.
Adding the Thermax won't hurt anything, but I don't see a good reason for it myself. If I felt I needed more in your house, I'd have it sprayed in
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Taunton University of Knowledge FHB Campus at Breaktime.
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Assuming your spray foam doesn't have any voids or thin spots, I don't see any vapor barrier issues. The only thing that will be sandwiched between the two vapor barriers is the 1/2" of foam. Be sure to inspect the spray foam for any voids or cracks where cold air can penetrate and hit the panels though. With the spray foam in the stud bays, moisture can't dry to the outside, so in the event of vapor migration into the wall, I'd give it a fighting chance by placing the poly to the inside (although in a heating situation, I think the direction of air migration wants to be outward).
You'll have to consider how to deal with penetrations in the Thermax, running your electrical and plumbing for example.