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Community and Q&A

Minimum thickness of roof foam

jshima | Posted in Energy Efficiency and Durability on


I am building a 12’x18′ room in climate zone 3c in Sonoma County Ca. The roof on the rest of the house is an unvented cathedral ceiling that has 3″ of closed cell spray foam under the roof deck with dense pack cellulose filling the remainder of the 2×8 rafter bays. I have since learned that with the standing seam metal roof, moisture could get trapped in the roof sheeting if it gets wet. It has been 3 years and I have not noticed any problems. On the new roof I plan on putting 2″ of polyiso on the roof deck. Two layers of 1″ with staggered and taped seams. The 2×8 rafter bays will be filled with R30 Roxul. The metal roof will be attached with 3″ screws through the foam and into the roof deck. I have read that all I need is R5 to meet code but then I read that I need a certain percentage of foam based on the total R value. Which is correct? Will this roof have a condensation/ moisture problem?

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    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.

  2. Expert Member
    Dana Dorsett | | #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:

    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.

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