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Exterior Insulation Under Rainscreen

atgambleassoc | Posted in Energy Efficiency and Durability on

Hi All,
My exterior wall assembly detailing was originally as follows (Exterior to Interior):

– Vented Cladding
– 1/2″ Battens
– 2″ Rigid Min. Wool Board CI (R8ish)
– Solitex Mento or Fronta Quattro WRB
– Plywood Sheathing
– 2×4 Framing with Min Wool Batts in stud cavity (R15)
– Intelligent Vapor Retarder
– Painted Interior Finish (GWB or Nickelgap)

The GC tells me he can’t source 2″ min wool or even EPS rigid insulation due to supply chain issues and is asking if XPS is okay instead.  I’d like to avoid it because a) we’re trying to avoid foam on the project for environmental/human health reasons and b) I think vapor-open insulation offers better insurance against moisture issues against the sheathing.  But am I being overly cautious, given that the assembly can dry to the interior?  If so, should I specify a textured WRB between the sheathing and the XPS?  I’ve followed other posts on the matter but feel like I’ve talked myself in circles on this and could really use help with clarity here.  Thanks!
Alyson

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Replies

  1. Expert Member
    Zephyr7 | | #1

    You won't have any health issues using XPS or EPS. The issues you usually hear about with "foam" are related to bad spray foam installations. Factory made foam products such as any of the common rigid foam board materials aren't an issue.

    You shouldn't have any moisture issues with your assembly. You can dry to the interior, and you'll have enough rigid foam with your assembly that you should be safe in all but the very northernmost climate zones. You could put wrinkled housewrap (Drainwrap, etc.) against the sheathing prior to the XPS going up for a little extra insurance, but I wouldn't worry about it. Spend time doing all your flashing details correctly -- that's really more important.

    Bill

  2. atgambleassoc | | #2

    Thanks Bill--and sorry I neglected to include that were in CZ5 here in Belmont, MA!

    If we do go with XPS, should the seams be taped?

    1. Expert Member
      Zephyr7 | | #3

      I would tape the seams. Just be careful what tape you use and make sure it stays stuck. A lot of tapes will appear to stick to XPS, but they'll start to peel off in a short amount of time. Try to put one of your battens over each taped seam to help hold the tape in place for the long term regardless. All you need is for something to be pressing down on the tape over the same to keep it from peeling off.

      Bill

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