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Community and Q&A

Do I need to cover XPS in an attic?

Tyrone Powell | Posted in Building Code Questions on

I know codes vary from town to town but I’m looking for the general code requirement/interpretation for this. I have used R-10 2″ XPS for the interior of basement walls. I know that it must be covered with drywall or another similarly accepted material for compliance and I have done so. I also have learned that the exception is between floor joists at the rim joist and have benefitted from this exception…wouldn’t that be a pain of a detail! I now have to build an enclosure that had never been built for a fire box that protrudes into the attic over the garage. My construction starts with cavity blocking, steel studs, then drywall on top and sides (sealed of course) and then I’d like to cap the top off with 1/2″ ply & mineral wool. Before anyone freaks I am going to put in a collar around the flue. For the sides however I want to use two layers of 2″ xps on the sides and want to know if, being in an attic, does it have to be covered with drywall or ply?

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Replies

  1. D Dorsett | | #1

    Some jurisdictions would allow intumescent paint, others an ignition barrier or thermal barrier.

    If you used fire-rated Thermax polyiso instead of XPS it would fly without additional measures in most areas, and you'd be nicer to the planet since it's blown with pentane (with a global warming potential of ~7x CO2) instead of HFC-soup, the predominant ingredient of which is HFC134a (~1400x CO2.)

    Better yet would be rigid rock wool, which is not combustible, and low-carb, low GWP.

  2. GBA Editor
    Martin Holladay | | #2

    Tyrone,
    The answer often depends on the access to the attic -- whether the access hatch is large (in which case gypsum drywall is more likely to be required) or tiny (in which case you may get away with exposed rigid foam).

    In all cases, opinions expressed on this web site don't matter at all. The only code interpretation that matters is the one expressed by your local code official.

  3. Tyrone Powell | | #3

    Thanks for the input.
    Mr.Dorsett, though your reply was very detailed and appreciated, I did not find an easy-to-get-to seller of Thermax polyiso board or rigid rock wool, but both sound like great options. Had I more time to wait I may have been able to order it. The install likely would have been easier with these in comparison to what I used
    Mr. Holladay, thanks for the reminder of the moving target that is the code official. It pushed me over to the option of wrapping the whole assembly in R23 Roxul batts retained using duct strapping with two layers on top of the enclosure.
    Again, to both, thanks for the willingness to share your knowledge. It's always helpful to have someone to bounce ideas around when you're not decided on a course of action.

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