Insulating unvented roof assemblies
I had read the article here…. http://www.finehomebuilding.com/how-to/departments/energy-smart-details/insulating-unvented-roof-assemblies.aspx
and since 1 and 2 require foam above the roof sheathing which I can’t do without redoing the roof which is high expense it leaves 3 and 4. The comment seems to be under the “how much insulation” that “code requires the [closed cell] foam layer to meet specific minimum R-values” in the table. I assume the closed cell based on the air-permeable comment.
My question is, there are 2 columns, the air impermeable insulation and total, however in the quoted section about utilizes a flash and batt strategy the reference is to minimum total only. So, I live in zone 4a. if my only choice is options 3 or 4, do I need to put down an R-10 value of closed cell foam or an R-38 value of closed cell foam? The costs are very different.
and second, if I meet that minimum, whatever it is, do I I not have to worry about roof rot?
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Replies
Michael,
In Climate Zone 4A, if you want to use a flash-and-batt strategy for an unvented sloped roof assembly, the building code requires that the foam layer have a minimum R-value of R-15. This code requirement is based on the assumption that the total R-value of the assembly (foam plus fluffy insulation) will be R-49. [This value has been corrected due to Dana's comment. Thanks, Dana.]
To achieve R-15 with closed-cell spray foam requires about 2.5 inches of foam. You would then need to install at least R-34 of fluffy insulation on the interior side of the cured spray foam; this could be achieved with about 9 or 10 inches of fiberglass, cellulose, or mineral wool.
For more information on these issues, see How to Build an Insulated Cathedral Ceiling.
Michael,
Q. "If I meet that minimum [= code minimum R-value requirements], whatever it is, do I not have to worry about roof rot?"
A. It's always possible for roof sheathing to rot. The most common causes of roof sheathing rot are roof leaks or roof flashing problems. But I wouldn't worry about these issues unless you have reason to believe that your roofing contractor made mistakes.
If you insulate a roof according to the recommendations in my article, and if your spray foam contractors (and other insulation workers) do a good job, there is no reason to worry about moisture problems or sheathing rot due to the insulation details.
The IRC prescriptive presumption of an exterior R15 for zone 4 is a total of R49, not R38. (or about 30% of the total R being on the exterior).
R15 out of R38 would be nearly 40%, which is adequate dew point control for zone 5 roofs.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
If I exceed R-38 does that change the minimum R-value of R-15 for the spray foam or is that a constant?
Michael,
Q. "If I exceed R-38 does that change the minimum R-value of R-15 for the spray foam or is that a constant?"
A. First of all, Dana is correct: the total R-value required in Climate Zone 4A is R-49, not R-38. I have edited my first response to correct my error.
To answer your latest question: If the total R-value of your assembly is R-49, you'll be fine with R-15 foam. If you add more fluffy insulation -- so that your total R-value exceeds R-49 -- then you would need to add more foam so that the percentage of the foam layer R-value is at least 30% of the total R-value of the assembly.