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Thoughts on this Wall Assembly

OldMAHousing | Posted in Green Building Techniques on

Due to cost considerations, we will be limited in how much exterior insulation we are able to add to our home.

Current wall construction looks like this:  2×4 –> 3/4″ planks –> tar paper –> cedar shingles.

The interior is down to the studs.

I am curious if anyone has concerns about a wall construction along these lines:

3/4″ drywall –> 1″ EPS foam continuous –> 2×4 with 3.5″ Rockwool Mineral wool batts –> 3/4″ planks –> 1″ EPS foam continuous –> blueskin house wrap –> cedar shingles

Questions:

1. Is it silly to have the two layers of continuous foam?  I have the room to add it on the interior so I figured why not, and the exterior I understand it is best practice to reduce condensation on the planks.

2. Is blueskin a good choice?  If I go that route I need to ensure the house can dry to the interior, correct?

Climate zone: 5A

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Replies

  1. Expert Member
    Akos | | #1

    You don't want rigid on both sides as it creates a vapor barrier sandwich. Pick one and install all of it there. Usually the simplest is on the exterior as you don't loose interior space and don't have to worry about extending electrical outlets.

    Generally for walls you want a vapor permeable peel and stick, henry VP100 or VP160 will work. The peel and stick needs to go over the plank sheathing since it will be your main air barrier.

    1. OldMAHousing | | #2

      Akos, appreciate your response. Will you actually create a vapor sandwich if you're using such a thin piece of EPS?

      I read that EPS is fairly vapor permeable, especially at only 1", is "not a good vapor retarder".

      1. davgonz90 | | #3

        Yes, it will create a vapor sandwich. It’s advisable to put foam on one side or the other, but never both. For verification, you can look up article archives on this site and the experts mention this. Good luck.

      2. Expert Member
        Deleted | | #4

        Deleted

  2. Expert Member
    Akos | | #5

    Most 1" EPS I've seen is foil faced so a vapor barrier, there is unfaced but usually a special order.

    If you do go with 1" unfaced eps on the outside and 1" faced EPS on the inside than it should work as it will allow the wall to dry towards the outside.

    I still don't see the benefit to do the sandwich. This makes doing both your drywall and siding complicated. I would just pick one and put all the foam there. 2" of exterior rigid is easy to work with.

  3. Expert Member
    BILL WICHERS | | #6

    A lot of the EPS facers are actually aluminized mylar, so plastic film but still a vapor barrier. There ARE some types of EPS with perforated facers out there that are NOT vapor barriers though, so if you need it to actually be a vapor barrier, it's something worth checking.

    All that fun stuff said, I agree with Akos here that there is no benefit to putting foam on both sides of the wall. I would put both layers on the exterior, with the seams staggered to limit any air movement at the edges. This avoids the potential drying issues (assuming you just have drywall on the inside), and won't have the issue of drywalling over a squishy material that will make it harder to get the screws to set correctly.

    Bill

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