Roof assemblies – minimum insulation value
ururk
| Posted in Green Building Techniques on
In my state our code specifies a ceiling R-Value of 38 (climate zone 5A) – but in the 2012 IRC it requires R-Value of 49. For an unvented ceiling with no attic space, solid continuous insulation, will R-40 be sufficient?
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Replies
This is with exterior insulation, solid foam.
John,
There is no standard definition of "sufficient."
Here are GBA, we certainly advise readers to install insulation that (at least) meets the minimum requirements of the latest International code (IRC or IECC).
In your case, the local code is more lenient than the 2012 IRC. So installing only R-38 satisfies your local code.
Is that "sufficient"? You decide. If you install R-40 roof insulation, the rate of heat flow through your roof will be 23% faster than it would be if you installed R-49 insulation.
By "sufficient" I mean enough that it won't pose a rot problem - although I am installing a continuous air/water/vapor barrier. As to the R-40, I use some basic heat loss calculations for the roof assembly, and walls, in deciding on whether I should go with code-minimum or higher. I found that the money I would save in cost over time did not equal the extra cost for another 2" of insulation over the roof (which means longer screws, more expense). Hopefully I didn't do my calculations wrong! This is a small building (20x35), and will be used for a workshop, not a living space. My original plan was for 10" on the roof and 6" on the sides, but I was unable to get used insulation in the quantity and size I needed and had to buynew.
John,
R-40 rigid foam on your roof will not cause any rot problems.
Oh - and it wan't just screws:
longer screws (2" longer)
more screws (added thickness would require higher density)
deeper window boxes (and more peel+stick membrane)
I would have loved to be more energy efficient, but I have limited funds and what I'm building hopefully is better than standard 2x4 - or pole barn - construction. You can see my slow and painstaking progress here.
John,
Thanks for the link. My favorite photo was the one below.
.
Thanks! That frog was 9' up in the corner where my rolling door is. It surprised me, to say the least.
IRC 2012 code compliance on a U-factor basis in "TABLE 1102.1.4 (R401.2.4 EQUVALENT U-FACTORS" calls out U0.026 for zone 5. That is a "whole assembly-R" of about R38, but also includes the R-value of the sheathing, roofing, and the air films.
Suffice to say, R40 of continuous insulation (or it's calculated U-factor equivalent) is more than adequate from a code compliance point of view.