Wish to do new build in location seeming to require R30 minimum R38 recommended in roof. Want to foam.

Builders and their insulation subs all tout open cell foam, and say 5.5 inches of it is what the do, meets "code," and is what they have been doing.

But the open cell foam only yields something like R3 per inch, at the way upper end, thus 5.5 x 3 = less than R20, way less than the minimum for roofs.

Am I missing something here?

Are these guys wrong?

Posted By Willin on 27 Aug 2013 10:57 PM
Wish to do new build in location seeming to require R30 minimum R38 recommended in roof. Want to foam.

Builders and their insulation subs all tout open cell foam, and say 5.5 inches of it is what the do, meets "code," and is what they have been doing.

But the open cell foam only yields something like R3 per inch, at the way upper end, thus 5.5 x 3 = less than R20, way less than the minimum for roofs.

Am I missing something here?

Are these guys wrong?

The ASTM C 518 tested R value of open cell foam at half-pound density is about R3.3-3.5/inch, at 0.7lbs density it's a bit over R4/inch. At 2lbs density (like Icynene's MD-R-200) it's a bit over R5/inch. Density matters. 

Half-pound goods are the most common, and will exceeds the R-performance of low density R19 batts (which perform at R18 when installed in a 5.5" cavity rather than their full ~6" manufactured & tested loft.)  But it won't outperform R23 rock wool batts or high density "cathedral ceiling" fiberglass.

But at 5.5" open cell foam at any density does NOT meet code. (And yes, those guys are just wrong.)

Some of the marketing folks at Icynene (and others) have been pushing the "it's all you need" concept for awhile, using fuzzy arguments about the superior air sealing as compared to the straw-man with all the stuffing beat out of it alternative: The low density fiberglass batt.   Poorly installed low density batts installed at R30 can sometimes underperform R20 open cell foam, but that doesn't mean the poor installation met the letter of code any more than the sub-code R value of the open cell foam- they're both violations.  And while open cell foam makes air sealing easier, it's by no means assured (you still have to test), and there are other ways to get there too.

To hit R38 with half pound foam you're talking 11", not 5.5".  And it has to be installed in two 5.5" lifts not a single blast of 11", or there will be a risk of fire in the short term as it's curing, and shrinkage gaps later.  It's good stuff, but you may be able get better bang per buck with a foot of damp sprayed cellulose (or dry blown) than a foot of open cell- it just depends on the local market.  In all but the coldest locations in TX it's relatively safe to do unvented cathedralized ceilings/roof with cellulose, though technically to meet code you'd be required to put R5 in rigid insulation above the roof deck. (That's far more important with fiberglass or rock wool cavity fill than with cellulose or open cell foam.)

Thanks for the information. No matter how hard I push it, they still come back with the "we've had no problems" story.

It is a location where permitting involves no plans review and there is little to no inspection. The builders can pretty much do what they want, and a lot of corners get cut.

If the builders do what they want without listening to the boss (you) it's time to fire somebody.

Building codes sans-enforcement isn't unique to TX, a real problem for the buyers of tract homes, but on a custom build MSG79 is right- you get to call the shots, even if the contractors are calling you crazy.

The economics of taking it up to code-min with foam aren't necessarily there for a house you plan to dump in 4-8 years and move on- it's a lifecycle cost of energy issue for the code regulator. But insulating to only half of code min COMFORT issue for those who live there.

Foam installers all over are pretty big on the, "It's all you really need." theory.

But what they're really saying is, "That's all that's cost effective when insulating with this somewhat expensive material, and we'd lose too many contracts if we quoted the full code-min R."

I can't blame 'em for wanting to stay in business, but the notion that ~R20 cuts it TX from a code-compliance and comfort point of view is simply not so. In the absence of code enforcement, you get to choose the point where the pain in the wallet is excessive.

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