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Community and Q&A

Exterior Insulation Permeability

Y_Knot | Posted in General Questions on

After reviewing many GBA Q&A’s, I have decided not to proceed with ZipR due to structural integrity concerns but instead to go with insulation on the outside. I’m still leaning heavily towards Zip; however, plywood is a possibility. My primary question, though, is whether Rockwool Comfortboard is a better choice than rigid foam due to it’s increased permeability. 
Additional Info: We will be putting a smart barrier on the interior and are currently planning on Rockwool batting in our 2×6 walls. We will also have a rainscreen. Last but not least, we are in Deshler, NE so CZ5; however, 15 miles form CZ4.

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  1. Expert Member


    The permeability of the exterior insulation matters primarily in two situations:

    - Where you don't intend to use a thick enough layer to keep the sheathing warm enough to avoid moisture accumulating. Then the walls need to be able dry to the outside.

    - Where the sheathing is permeable enough to allow drying to the outside. Zip is less than 1 perm, so it doesn't benefit from a permeable layer outside it the way plywood or OSB would.

  2. Expert Member
    BILL WICHERS | | #2

    More exterior drying ability is always a plus, so comfortboard's very much increase vapor permeability is it's primary advantage here. The downside is that it's also much more expensive than other materials. If you take the money saved not using Comfortboard and put it towards more R value worth of something like polyiso or EPS, you end up with a similarly safe assembly (the more exterior R value you have, the less important exterior drying ability becomes), but also a much better wall from an energy efficiency standpoint.

    Note that Zip-R has going through testing to ensure it's structurally sound, but all those nails in shear make me nervous too :-)

    BTW, the interior side smart vapor retarder is extra insurance, and also makes that exterior drying ability of the wall less important. What I would do here is go with regular plywood (my preference over OSB, especially considering costs have been pretty close lately), then exterior polyiso to AT LEAST code minimum levels, and ideally more. I'd go with an interior smart vapor retarder too, but I'd use a relatively cheap one like MemBrain since you don't really need the performance of the better ones if you have high levels of exterior insulation in your assembly.


  3. Y_Knot | | #3

    Thank you! I was thinking 2” of exterior insulation. Mainly because of implications of having thicker insulation on exterior and how that impacts trim, siding fasteners, etc. I’ve seen a few different thoughts on the proper ratio for exterior insulation thickness. What are your thoughts on adequate thickness for my situation?
    Thanks again!

  4. seabornman | | #4

    I'm in zone 5. I used 3" of nonpermeable foam: 2 layers of 1-1/2". Once you're past 1", you're into strapping and window bucks, so why scrimp. I found 3" allowed me to replace one layer with 2x wood blocking for support of door sills and tying in of porches/roofs. It also means an interior vapor retarder is not required and insulation in studs can be installed at any time.

    1. jollygreenshortguy | | #5

      This sounds interesting. You wouldn't happen to have any photos or drawing details to share?

      1. seabornman | | #6

        Attached are 2 photos: one showing insulation down to footing, one showing a wall with insulation and strapping. I used peel and stick for WRB on the addition, and changed to zip sheathing on the renovation. Ignore the paint on the insulation; it took so long for me to get siding up, I threw some old latex paint over it to keep sunlight from deteriorating. I'll have to find drawings on another computer.

      2. seabornman | | #11

        Here's a sample drawing. It may not have been built exactly as it shows, but it gives you the gist.

  5. jberks | | #7

    Another point to consider is the fire resistance benefit of Rockwool.

    CI Rockwool + a non combustible siding (like hardie) provides good protection from exterior fires that can extend to your structure. For example, your neighbours house catching on fire, a vehicle parked in your driveway or simply a BBQ fire on your deck. So consider your surroundings and design appropriately. Moreover, CI rockwool on the roof deck, steel roofing and non combustible soffits are even better. Flat roofs with no overhangs are even better for this purpose. But these are all just my opinions. I'm just some guy on the internet.

    I believe protection from forest fire sparks require some more detailing as well, if that's an issue for you.

    ya I know, fires will never happen to you (sarcasm), But these are the things I think about.


    1. Expert Member
      BILL WICHERS | | #8

      Mineral wool does help with fire resistance, but you'd need to be very careful about any ventilation openings to really take advantage of that. A few hot embers going up into an attic vent and setting the internal structure of the attic on fire would bypass all your carefully installed mineral wool and fiber cement siding after all...


      1. jberks | | #10

        Hence the comment on non-combustible soffiting, and flat roofs.

  6. Expert Member
    Akos | | #9

    If you can build a wall with sufficient exterior insulation for condensation control for you climate zone, than permeability doesn't matter. Sure, a bit of extra drying doesn't hurt but it is not needed. The extra bit of exterior rigid also avoids needing a warm side vapor retarder other than drywall which does save a bit of labor and cost.

    If you want to build with less rigid, this is also not a problem. These types of walls have been built around me for a long time and they work just fine.

    For this type of wall, all codes require a class I or class II warm side vapor retarder. There is still a min insulation limit but usually much less than for full condensation control. This type of wall can work with non-permeable exterior insulation but has minimal drying capacity so something with some, no matter how minimal permeability is better (ie unfaced EPS,GPS, permeable of fiber faced polyiso). Using a crinkled house wrap under the foam and not taping the foam seams also helps with vapor diffusion and adds a bit of extra drying capacity.

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