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Insulation Education

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Old 06-03-2019, 09:02 PM   #1
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Insulation Education

Framed a new roof onto a commercial building in Ann Arbor, MI this past week or so. Got two quotes for insulating the underside of the roof deck, for the same square footage. I need some education here. If it is at all relevant, owners are not yet sure how the insulation will be covered up (painted? sheetrocked? Not my gig, anyway)...but there will be no attic getting built. Idea is to leave ceilings vaulted.

One quote, roughly 4k, for 6" of open cell, which he insists is enough for R-36 plus (with a further claim that "after r21 of either foam you stop 97% of all thermal movement the rest is to keep inspector happy").

2nd quote, over 10k, for 6" of closed cell; this 2nd gentlemen insists that you'd have to get nearly 12" of open cell in order to get the R-38 that our inspectors are going to require here.

What's the straight story, here?
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Old 06-03-2019, 09:44 PM   #2
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Re: Insulation Education

6" of open cell isn't anywhere close to R38. It would take 10-12" to hit that. 6" of closed cell will meet that R38. As to the other claim, there's a lot of debate about it. More R value is better but the big gains per dollar are hit early on then they really taper off the higher the R-value gets. With cheap insulation like blown in cellulose, knock yourself out going from R38 to R60+. With closed cell foam you'll probably never see that money again. Either way, the inspector is going to require a certain number be met for code.


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Old 06-04-2019, 11:22 AM   #3
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Re: Insulation Education

quick primer
Eric nailed a lot but I would opt for at least 4 if not 6" of closed cell to meet codes on vapor issues (which OC doesn't) & finish off with cellulose to get you to the R49 or whatever is required in your area (plus cellulose or a roxul product helps take care of thermal / smoke protection requirements)
What is the snow load factor in your area because that can throw a wrench into what I wrote above if 50# or over
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