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Should I call for rigid mineral wool sheathing to be installed in two layers?

getmeoutofgba | Posted in Energy Efficiency and Durability on

I’m planning to use rigid mineral wool sheathing on a retrofit project. In the past when I’ve done this with XPS or polyiso rigid foam sheathing, I’ve called for 2 layers of foam, with staggered seams, presumably because rigid foam shrinks and installing in 2 layers reduces the likelihood of “cold lines” in the continuous insulation layer. Mineral wood doesn’t appear to be a product that shrinks and the photos I see suggest that the product can be joined at each edge quite tightly.

The overall assembly planned is like this: new engineered lap siding over 1 x 4 furring strips anchored through insulating sheathing to the stud wall. Stud wall has 7/16″ plywood sheathing; all joints will be taped/sealed to create air barrier. Water resistive barrier will go over insulated sheathing, so it can be tied to the window and door flashings.

So, 2 layers of 1 1/2″ with staggered seams, or 1 layer of 3″ insulation?

Rachel Wagner

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  1. GBA Editor
    Martin Holladay | | #1

    I don't think there is a single answer to this question. It's a judgment call. I'm guessing that the drawbacks of a single layer of insulation are fewer with mineral wool than with rigid foam, based on the fact that the material is fibrous and doesn't have a distinct, linear cut line at the perimeter.

    GBA has reported on two jobs with exterior Roxul on walls. One used two layers of insulation with staggered seams (you can read about that job here: Wrapping an Older House with Rock Wool Insulation), while the other used a single layer of mineral wool (you can read about that job here: Installing Roxul Mineral Wool on Exterior Walls).

    After you have read those articles, you can check out a few more if you are hungry for more details:

    Mineral Wool Boardstock Insulation Gains Ground

    Installing Mineral Wool Insulation Over Exterior Wall Sheathing

  2. getmeoutofgba | | #2

    Thank you Martin. The two case study articles are very helpful. I'll use the articles to discuss the approach with the builder. I want 3" of rock wool, so there will be time to discuss whether it goes on in one layer or two. I agree that there isn't a single answer.

    The part about the birds nesting in the rock wool was a surprise. I've decided to put the WRB over the rock wool, as I like the WRB to be the "back" of the rainscreen assembly, plus it is easier to design good flashing details for windows, especially in a way that allows a window to be more readily replaced in the future.

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