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Community and Q&A

Rockwool Comfortboard 80 bottom edge details

JaredWA | Posted in General Questions on

I’m installing 1.5 inches of Rockwool insulation on the outside of my house. I have been looking for details on how to finish the bottom border.

Looking at the diagrams from Rockwool, they always show a continuous layer of rockwool extending into the ground. I will be stopping my insulation at the sheathing/foundation joint. I am doing a fairly typical install, using 1×3 furring strips and Cora vent SV-5 for the bug screen on the rainscreen part. The detail I have not seen on any installation is what to to directly under the bottom boarder of the 1.5 inches of insulation. Is it ok to leave that open since no one will it as it will be hid behind the siding and bugs/rodents do not like it? or do I need to run a length of L flashing under it to protect the bottom edge?  Thanks in advance.

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  1. Expert Member


    You need some mechanical protection. The best bet is a perforated J flashing similar to this.

    If you are getting one made up, you might as well size it to cover the bottom of the rain-screen cavity as well and forego the Cor-a-Vent.

    Another alternative is a pt 2"x2" with a sloped top. This will give you a nailing base for the Cor-a-Vent

  2. JaredWA | | #2

    Malcom. I don't think they make it in the size to do what you are suggesting. It would need to be 2 1/4 thick. ( 1.5 insulation+ 3/4 furring strip cavity) to cover both. I would love to do it all in one, but I can't seem to find anyone that carries J flashing here in Spokane, let alone perforated J channel the can be custom made. I am thinking of just doing 1 1/2 zbar under the insulation and then SV-5 in the rainscreed cavity. Anyone do something similar?

    1. Expert Member
      MALCOLM TAYLOR | | #4


      Yes that will work. For it to be effective make sure the horizontal leg has a slight slope and the drip-edge is at 90 degrees for a tight fit against the Cor-a-Vent.

      1. JaredWA | | #5

        Installed it today and yes that is what I did. I just kind make sure I bump it a tad bit with the insulation when I install it to make sure its 95-100 degrees for a slight slope outword.

        1. Expert Member
          MALCOLM TAYLOR | | #7

          Good stuff - Keep those pests out. The problem we were finding here with just using bug-screen was that plants or grass often grew up into the cavity, and pulling them out wrecked the screen. Then at that p0int your options were pretty limited.

  3. Expert Member
    Peter Engle | | #3

    Any sheet metal guy could make your J-channel custom, from perforated stock. Bending that up on a brake would take no time. Even bronze or stainless steel window screen would last pretty much forever in that location and would keep most bugs/rodents out of the wall cavity.

    1. JaredWA | | #6

      I called around and didn't get anyone saying they could at a reasonable price.

  4. mountaincabin | | #8

    Hey Jared, I ended up adding 4" comfortboard to my foundation but I had a transition between the wall and the lower because I had 2" continuous on my exterior wall. So here's a few pics of what I did; hope they help. I had a 2x4 furring stip wall but did a belt and suspenders approach to the bottom wall detail (as you can see in pic). We did a perforated J metal; we used Pacific Sheet Metal in Seattle but like everyone said you can ask any metal shop. They had a lot of questions on what type of perforation I wanted so I just looked up specs from a rainscreen carrier and used them. We used roofing nails to attach the J metal into the wall (used liquid flashing over nails), but before attaching that we stapled aluminum window screen. Then when the comfortboard and furring strips were installed, we seated the furring strips into the J metal, pulled the window screen tight and stapled the window screen to the from of the furring strip to complete the assembly. It was over the top but then I used a air sealing flashing tape over the window screen + J metal portion that was attached to wall so that it was another layer of flashing and attachment point. We did this because in our area east of the cascades, we get carpenter ants so we wanted to try to do what we could to prevent them.

    Good luck! -Zach

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