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Can kneewall polyiso boards be left exposed on the attic side?

John Sean | Posted in General Questions on

Can kneewall polyiso boards be left exposed on the attic side?

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  1. Expert Member
    Dana Dorsett | | #1

    In many/most areas if it has fire rated facer it's fine to leave it uncovered. Currently that narrows the product offerings down to Dow Thermax line of polyiso.

  2. GBA Editor
    Martin Holladay | | #2

    It usually depends on whether the attic is accessible -- for example, via a door.

    If the attic is inaccessible, most building departments will rule that a single layer of drywall (on the interior side of the kneewall) provides adequate fire protection. But code interpretations vary from jurisdiction to jurisdiction -- so it's always a good idea to call up your local building department to discover what they think.

  3. John Sean | | #3

    Instead of polyiso, what specific housewrap can I use?

  4. GBA Editor
    Martin Holladay | | #4

    Housewrap and polyiso are both air barriers, so in that sense they are comparable. However, unlike housewrap, polyiso has R-value (usually between R-6 and R-6.5 per inch). So if you switch to housewrap, you won't be getting the benefit of a continuous insulation layer that interrupts thermal bridging through the studs.

    As with polyiso, it's important to contact your local building department to ask whether they will allow housewrap to be left exposed in this location.

  5. John Sean | | #5

    What brand polyiso do you use?

  6. GBA Editor
    Martin Holladay | | #6

    The brand of polyiso isn't too important (unless your building department tells you that Thermax is required for code reasons). Buy whatever brand is sold locally.

    Many green builders prefer to buy used (recycled or reclaimed) polyiso. Look on Craigslist.

  7. Expert Member
    Dana Dorsett | | #7

    I tend to use whatever brand my local foam reclaimer and factory-seconds foam outlet vendors have on hand, which varies. YMMV.

    Where it's cost sensitive and higher-R, a layer of ultra-cheap reclaimed foam with a layer of 1/2" Thermax facing the attic air works.

  8. John Sean | | #8

    This is actually for a bonus room above a garage.

    There will be no opening after completion.

    I plan on affixing the polyiso to the outside stud edge.

    There are already fiberglass batts.

  9. John Sean | | #9

    What kind of nail to attach 2 inch thick polyiso board to studs?

  10. Expert Member
    Malcolm Taylor | | #10

    Cap nails

  11. John Sean | | #11

    I don't see cap nails longer than 2 inches

  12. Mike M | | #12

    You can find plastic washers in varies sizes from 1" diameter to 3" and possibly more. I was able to order a box of 1000 of the 2.5" diameter for pretty cheap. Do a quick search for plastic foamboard washers and you should get a ton of options to look through.

  13. Expert Member
    Malcolm Taylor | | #13

    They are also available at all our local lumberyards here.

  14. John Sean | | #14

    I bought those nails from Lowes.

    Do you need to use adhesive too, or are they good enough?

  15. Expert Member
    Malcolm Taylor | | #15

    They have very good holding power. I wouldn't worry about adhesive.

  16. John Sean | | #16

    Do you tape or foam spray in the gaps?

  17. GBA Editor
    Martin Holladay | | #17

    To seal the seams between panels of foil-faced polyiso, you can use either a high-quality tape (housewrap tape or foil tape) or canned spray foam. Tape is probably the easier approach.

  18. John Sean | | #18

    When I need to use foam, for example to block the floor joists, is this fireblock foam fine?

  19. GBA Editor
    Martin Holladay | | #19

    That type of canned spray foam is fine. But if there is no code reason to use fireblock foam, you can use ordinary canned foam instead.

  20. GBA Editor
    Martin Holladay | | #21

    You're overthinking this. Either type will work.

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