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mrbreadpuddin | Posted in General Questions on

I want to put EPS on top of an underinsulated/unvented roof, then plywood and asphalt shingles. Am I right that this can be done without sleepers (nailers) going through the foam? Would 5/8″ plywood be thick enough for shingle nailing? Any thoughts on how thick the foam could be? The cieling height is too low to add venting or insulation from the inside.

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  1. GBA Editor
    Martin Holladay | | #1

    1. Yes, it can be done. This is a site-built SIP.

    2. This type of roof can be vented if you want (by installing 1x4 or 2x4 strapping between the top of the rigid foam and the top layer of sheathing), or can be left unvented. If you choose to build an unvented roof assembly, most shingle manufacturers will not honor the shingle warranty. This is not really a deal-breaker, however, since shingle warranties aren't worth much.

    3. Make sure that your roof assembly meets the minimum code-required R-value for your climate zone.

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