Would R-15 rigid insulation used above roof sheathing over conditioned attic be part of R-38 required for zone 4 by IRC or ..
thomaslop
| Posted in Building Code Questions on
… has to be in addition to required R-38 by IRC ?
Thank you,
Tomasz
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Replies
Tomasz,
The R-15 of rigid foam insulation above the roof sheathing can be considered as part of the total R-value of the roof assembly. But in Zone 4, the 2012 IRC requires a total R-value of R-49, not R-38.
(Perhaps your jurisdiction is operating under an older version of the IRC.)
For more information on this issue, see How to Install Rigid Foam On Top of Roof Sheathing.
It's only considered part of the R38 (or R49) if the rest of the insulation is installed in contact with the under side of the roof deck. You can't add R15 to the roof deck and meet code with R30 between the floor joists of the attic floor, even though it adds up to R45.
But with R23 rock wool snugged up to the underside of the roof deck and R15 above it meets code under IRC 2009, but not IRC 2012 and later.
Thank you.