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Community and Q&A

Does this wall assembly sound good to avoid mold?

green654 | Posted in Energy Efficiency and Durability on

I’m getting so many different opinions that it’s hard to make sense of it all. I’m Building a new house and the added cost and logistics involved with external foam insulation is not an option for us. Spray foam on the inside is not an option either. We are in zone 5. Will this wall assembly do a good job keeping mold and moisture out of the wall? Any other suggestions to someone who is very committed to avoiding moisture/condensation issues in the home?

Exterior wall:
2. Advantech zip sheathing
3. Tyvec drain wrap
4. 3/8 inch air gap for rain screen
5. Hardie Board 

Interior wall:
1. Framing (2X6)
2. Intello smart membrane
3. 5.5 inches rock wool insulation
4. drywall


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  1. Expert Member
    BILL WICHERS | | #1

    The Intello should go under the drywall, before the mineral wool, from the perspective of the interior. This means drywall/intello/mineral wool/exterior sheathing. The purpose of the vapor retarder is to limit moisture getting into the wall in the winter, and that moisture comes from INSIDE the home.

    There is no point using drain wrap if you have a rain screen here. Drain wrap provides water channels, and it's usually used when there is otherwise no drainage gap. With a rainscreen, you have a pretty good sized gap for drainage already, so the drainwrap doesn't gain you anything. Zip's coating already provides you a WRB here too, so you don't even need "regular" tyvek.

    Aside from that, this is a pretty basic wall. Spray foam doesn't really gain you anything here, so I wouldn't even consider it. Exterior rigid foam would gain you a lot, but you said you don't think you can tackle that in this project. Are you sure about that? You could save money with reclaimed rigid foam, and it's not really a big deal to install unless you get into the "thick" category (which usually means over 2 inches or so). Two inches and under isn't usually very difficult to install. The advantage you get with exterior rigid foam is CONTINOUS insulation -- no thermal bridges -- which will keep your wall quite a bit warmer, which will help a lot with keeping it dryer, and that helps keep mold away. Since you're already planning on a rain screen, you'll already have a surface to put your siding on, so the rigid foam doesn't complicate that part. I would use ripped strips of 3/4" plywood to build the rainscreen.


    1. green654 | | #2

      Thanks Bill. The issue is that Advantech zip and drain wrap are already up. Builder says there is not enough room to add two inches exterior insulation plus rain screen. Do you think we will get mold on the Zip sheathing without exterior insulation?

      1. Expert Member
        BILL WICHERS | | #5

        I think you'll probably be OK, but I'd recommend detailing the interior drywall as an air barrier in addition to the vapor retarder to get you some extra protection.

        Note that with 2x6 walls, you could use R7.5 worth of exterior rigid foam. If using polyiso, that means about 1.25" worth 1.5" would be better. If you can fit that much, I'd definetely do it. Exterior rigid foam will give you more protection for your wall, and significantly increase the whole wall R value as well.


  2. user-2310254 | | #3


    Just want to confirm that your builder taped and rolled all the seams on the ZIP. Rolling is critical to getting the tape to bond with the panel membrane.

    1. green654 | | #4

      He says he did. I might have a Huber rep come out to the house site and confirm all was done correctly. We had the Tyvec rep come out and confirm WRB was installed correctly.

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