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Community and Q&A

Siding with inherent rainscreen

joenorm | Posted in General Questions on

Planning a new build and would like to install Roxul comfortboard on the exterior in combination with blown in cellulose in the wall cavity.

Being someone who enjoys killing two birds with one stone I wonder your opinion on two siding options that seem to offer “built-in” rain screens.

First is corrugated metal siding oriented vertically. the only thing I wonder is how fastening it through the Roxul would be?

Second option would be reverse board and batt. Laying the battens on first creates an air gap behind the board.

These seem like good options and would eliminate the need for furing strips.

Anyone see an issue with this idea?

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  1. Expert Member
    Peter Yost | | #1

    Hi Joe -

    First, you did not mention your weather-resistive barrier. Will that be located to the interior of the exterior rigid insulation? That is where I would locate the WRB in this assembly.

    Second, I would want the space behind my cladding--corrugated metal or reverse board and batten--to be a true dedicated vent and free draining space. For these claddings, that means horizontally-oriented spacers. That means notched furring strips, or criss-crossed furring strips or a "honeycombed" manufactured spacer.


    1. joenorm | | #4

      Thanks Peter,

      The WRB, as you suggested, would be to the interior of rigid insulation. Am I understanding you correctly in that you would suggest horizontal strips applied to the Roxul(in this case) and the cladding attached to that? Wouldn't the horizontal strips catch and soak up water worse that free draining vertical space of a corrugated panel? It seems like a huge extra step that can be avoided, not to mention less depth to build out to meet windows and doors.

    2. Expert Member
      MALCOLM TAYLOR | | #9

      Both corrugated metal siding and reverse board & batten function as rain-screens, and meet our code requirements by having less than 8% of the cavity they create in contact with the substrate behind. That's without (as Martin says) the inherent drying and drainage capacity of the mineral wool.

      Reverse board & batten is more prone to moisture getting into the cavity than many other types of siding.

  2. GBA Editor
    Martin Holladay | | #2

    I disagree with Peter on this one. Semi-rigid mineral wool is free-draining -- this is an inherently well-draining and dry approach. The problems don't involve the need for a bigger air space -- the problems involve the difficulty of installing anything on mineral wool, which is squishy.

    If your installers have the right touch, it will work just fine.

  3. joenorm | | #3


    I wonder just how "squishy" it is? Around here the minimum order for comfortboard is a pallet, so I better be sure it will work well.

    I imagine corrugated metal over this product is ideal, the large surface area of the metal panel might help it resist deflection into the Roxul. I would be using a heavy gauge deep corrugation so the panels are quite rigid.

    1. Expert Member
      BILL WICHERS | | #5

      Are the corrugated panels rigid in both axes? The corrugated panels I’ve seen are always stiff in the axis parallel to the corrugations, and pretty floppy the other way. You don’t want a ripply look to your wall if the fastened parts of the corrugated panels squish into the mineral wool more than the rest of the panel.

      I’ve cut pieces of small galvanized pipe to make standoffs in the past. You put a fender washer on each end and thread a bolt all the way through with the pipe acting as a sleeve in the middle. The pipe then takes the compressive force of the bolt so that the soft material doesn’t squish. It works well, I’ve done this many times to mount heavy things (masts and signage) on soffits that don’t have enough structure to support anything else. The downside is it’s a lot of extra labor to install.


      1. joenorm | | #7

        Yes, floppy one direction but I would think the stiffness in the other direction would distribute the force enough to resist squishing the board. Hard saying not knowing. No way would I ever create pipe standoffs, waaaay too labor intensive.

    2. Expert Member
      Akos | | #6


      The problem isn't only that they are squishy (they are squishy enough to be compressed by 2x3 strapping) but also that they are not uniformly squishy. This is the bigger pain, need to adjust every screw to get things level or make standoff (like Bill suggests). Both labor intensive.

      I would suggest you get some of the rigid mineral wool insulation that is stock at the box stores and try out building a wall section.

      Rigid works great in commercial walls where the wall panels are installed over Z grits not directly over the rigid insulation.

      Until the mineral wool folks get they product more consistent and cheaper, I would stick with other types of rigid insulation or go with a wall assembly that doesn't use exterior insulation.

  4. keithhoffman22 | | #8


    DIYer here:
    I've used mineral wool board in two applications.

    First, in a crawl space with PMFs. Because the mineral wool is air permeable and not subject to moisture damage, I used it without any air or water barriers (other than the concrete of course). Others here might disapprove but I'm very confident of this approach in my negative latent load climate zone in a conditioned crawl space. It worked GREAT with PMFs for this application. The small amount of squish was perfect for covering small remnant form lines etc. I recommend a 2 layer approach for really harsh climates.

    Second, and more relevant to your situation, I've used it to backfill stucco repairs on an existing structure that had a ~3.25" buildup (1" xps and crazy strapping and air gapping). I was able to use the 2 3/8" R-10 product (hard to obtain, now I have lots of excess) with a thin strap (some strapping is more like 5/8" than 3/4"). It was some work to get the straps to sit relatively flat. I can't imagine installing lap siding over it. It was awful to stucco over that as it was impossible to pull the stuccowrap tight enough and I had to use stupid amounts of stucco on the patch. The straps telegraph. I don't think there is a drain gap there anymore basically. Thankfully it was just a small patch. If I was doing that kind of work again (for now, I'm using scrap foam insulation in the places I've had to deal with the darn flickers without a gap), I would place the stucco wrap directly on top of the mineral wool and attach the wire directly over the stucco wrap (no gap). It would probably stucco nicely then. I didn't consider making standoffs as it wouldn't have helped with stucco particularly. If I was going to make standoffs, I'd personally consider trying PVC piping (1/2"??) and shell out for the milwaukee one handed battery powered small diameter piping/tubing cutter (<$200). You could make pvc standoffs at a brisk pace that way.

    Personally (and this is just a DYI opinion tempered by experience with Roxul products and receiving opinions of carpenters etc about drainage gap style installs over comfortboard), if I was ever pursuing a complete siding system replacement instead of small repairs, especially in the context of the need to increase overall wall R value, I would probably pursue look into TJI style larsens using comfortbatt with thin retaining straps or ledges every 8'-12' (since two stories of stacked comfortbatt is probably pushing it). Cheaper, easier to work with, better install, off the shelf, less stinky, easier to work around penetrations, standard nailing surface for siding etc. I know there are folks out there (this site?, internet blogs?) who have reported on using comfortbatt in larsen trusses.

    I don't think corrugated is going to hide non-flatness too much. Your manufacturer can probably guide you on what deviation is covered well. I'd expect you'd have to shim the straps to flat if they aren't.

    Good luck! I love the Roxul batts but the boards seem like they offer challenges for siding.

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