# Minimum thickness of roof foam

Posted Aug 2, 2014 4:51 PM ET

1.

Jeremy,
In Climate Zone 3, the IRC and IECC require that (for the type of roof you are describing -- an unvented cathedral ceiling with rigid foam above the roof sheathing) the rigid foam layer have a minimum R-value of R-5. For more information on this code requirement, see How to Build an Insulated Cathedral Ceiling.

If you want to perform a more sophisticated analysis -- a dew-point calculation -- you can do so if you want to. Here is a link to an article that explains what you need to do: Are Dew-Point Calculations Really Necessary?

If you follow the principles outlined in either of these articles, you should be OK.

Posted Aug 3, 2014 7:16 AM ET

2.

The RATIO is what determines the average wintertime temperature (= average moisture loading) of the roof deck. The R5 prescription is based on the minimum total R for zone 3 is R38:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

That means as long as you have a foam to total -R ratio of R5 / R38 or higher you'll be pretty much good to go.

With 2" of polyiso you're looking at about R13 foam on the outside of R30 for a total center-cavity R of R43. But 13/42 is quite a bit greater than 5 / 30 (more than twice) so you're in pretty good shape.

Posted Aug 4, 2014 6:22 PM ET

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