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Slate Sheathing Exterior Wall – “Direct Apply”

nikojo | Posted in General Questions on

Cupaclad makes a slate product designed as a ‘rainscreen’ with slate tiles screwed to frame attached to wall with exterior insulation.  Very nice but also more intense labor, etc.

They also show a ‘direct apply’ application in which they screw directly to the sheathing and use HDPE waterproofing between courses essentially covering all the sheathing with HDPE.

We are using 2×6 framing with plywood sheathing and mineral bats inside.

I’m assuming some WRB/Tevdek would go over sheathing and this would go over that directly.

My question is…….are there concerns regarding the sheathing and rot??

I’m thinking this won’t let the sheathing breath enough with the HDPE.

What does everyone think?

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Replies

  1. Expert Member
    Akos | | #1

    That is a bad idea in cold climate. That HDPE sheet will be a cold side vapor barrier, even with the gaps between courses, it is still asking for trouble.

    If you want to go the direct apply route, use strips of house wrap.

    I think something like cedar breather would be a much better option though, this is very close to a cedar shake install.

    1. Expert Member
      MALCOLM TAYLOR | | #2

      Akos,

      I wonder how much drying you get through slate tiles, even with a substrate like cedar breather?

      1. Expert Member
        Akos | | #3

        They are still tiles, so lots of gaps.

        Metal tiles are not an issue, so I can't see why slate would be.

        1. Expert Member
          MALCOLM TAYLOR | | #8

          Akos,

          True.

      2. Expert Member
        Michael Maines | | #9

        I've worked on a few houses with slate roofs installed over skip sheathing. You can see daylight at almost every tile. They overlap enough that it's not usually a problem, but it's certainly not an airtight assembly.

  2. nikojo | | #4

    What if I had plywood then WRB then rockwool insulation (1-2") then install the slate over that as above??

    I'm thinking this isn't much different than a roof? People put down ice and snow shield all over it so how would this be different?

    1. Expert Member
      Akos | | #5

      Rigid mineral wool is pretty squishy, 2" is definitely no go. You could use foam insulation though, cement board (which I guess is pretty close) can be nailed up through 1" of rigid typically.

      Typically roofs are vented, so the sheathing under the I&W can dry to the vent space.

      Puttying an exterior vapor barrier over a stud wall means any moisture that makes its way to the cold sheathing will condense and without a drying path, it will build up until the structure fails.

  3. nikojo | | #6

    Thanks Akos.

    If i am using mineral wool batt on the inside of the wall is that enough to allow it to 'dry' to the inside?

    1. Expert Member
      MALCOLM TAYLOR | | #7

      nikojo,

      In a heat dominated climate the vapour-drive is almost always to the outside. Permeable insulation, or a vapour-open inside surface doesn't help the wall dry much.

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