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Community and Q&A

Siding on 1-inch Comfort Board mineral wool – REVISED

Seeking_Wisdom | Posted in GBA Pro Help on

I plan to build a new house this spring.  It will be std 2×4 construction with zip board sheathing.  I would like to install 1″ of mineral wool board (comfort board or  thermafiber rainbarrior) on the outside.  Building site will be in  St. Louis, Mo. Can I fasten LP Smart Side (Diamond Kote) 4 x 8 ft. panels with battens, straight over the comfort board (or equiv.), without furring strips?  If so, do I use screws or nails?  Do I need drain wrap between the siding and the comfort board? Thanks for your input!

Update to Question – a few more details:
This house will be on a farm with LOTS of critters that would love to take residence in a hollow rain screen cavity.  (think mice, snakes, wasps, etc).  I know about bug screens – but I’m afraid that mice will find a way to get through.

Also, the reason for this comfort board is to expand the wall 1 inch beyond the foundation.  The basement is a walk out and the vert. siding will extend to cover the foundation on the sides and back.  But you need to fur out the concrete basement walls to put siding on.  So I was going to use ripped 5/4″ deck boards as the foundation furring strips (actual thickness = 1 inch, same as the comfort board).  Then the plane from ground to roof would be even.

So how’s this for a compromise:  fur out the 2×4 walls with 5/4″ ripped deck boards, the same as is used on the foundation walls.  Except, rather than placing them over the wall studs, put them midway between the studs, attached by screw at the wall sole plate and top plate.  They would be glued to the zip board and stapled from the inside wall cavity to hold them tight as the glue drys.  Then I put comfort board between the furring strips, thereby  reducing thermal bridging, as they would “cover” the wall studs.  Thus, no “hollow” rain screen to invite nesting.

I could use 1″ wide perforated metal bug screens on the bottom to foil the mice.  Plus, they don’t like mineral wool.  Now the next question is:

Do I need a drain wrap between the siding and the comfort board or will the permanence of the mineral wool (50 perms – and hydroscopic to boot) be enough?

Thanks again for your input!

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  1. Expert Member
    Peter Yost | | #1

    Hi S_W -

    I would not recommend installing any vertical wall cladding without a ventilated rainscreen in any but the driest of climates.

    Check with the manufacturer or their online installation materials in terms of fastening requirements. I recommend screwing your furring strips to the framing and then you can attach your cladding to the furring.


  2. Expert Member
    Akos | | #2

    You'll have a hard time installing any cement panels directly over rigid mineral wool. The wool is still relatively "soft" and the screws would probably bend/crack the panels especially near the edge.

    Best bet is to strap out the wall and attache the panels to that or go with wood siding.

  3. Expert Member
    Akos | | #3

    Mounting the panels to something solid like the 5/4 would work.

    Might be simpler to rip the 5/4 with a slight bevel and use it as horizontal strapping. This would save having to deal with holding it in the middle of the stud bays and cutting the insulation. This would put the strapping a bit over 24" OC, which fine with some of the cement board panels, would need to check with the manufacturer.

    No matter which way you mount the 5/4, as long is it is not directly over the existing stud, it is the same amount of thermal bridging, no need to complicate your life.

    I would put the house wrap bellow the mineral wool, it is much easier to attach and flash plus mineral wool is self draining.

  4. Seeking_Wisdom | | #4

    1. So since the zip boards come with a barrier and the mineral wool is self draining, I take it that drain wrap is not necessary?

    2. If I use horizontal strapping (furring strips), should I route groves on the backside to allow for water drainage? (something like a 3/8" groove every 16 inches?)

    1. Expert Member
      Akos | | #6

      I would just put some thicker washers or 1/4" plywood under the strapping where you screw it into the studs.

      This also gives you a bit more height and less chance of the mineral wool thickness causing issues with the siding.

  5. Expert Member
    Peter Yost | | #5

    1. need for drainwrap - agreed, I don't see the need for it given the rest of your assembly

    2. I wish there was an easier way to accomplish drainage but since you need the furring to adequately attach your furring to y0ur structural wall and your cladding to the furring, you can't use other "stand-off" materials that are plastic honeycombed, such as Coravent.

    Maybe others have a sense of what the rout pattern should be; I think 3/8-inch for depth sounds right but 16 inches "feels" like about half of what you need but I must say I have never tested this nor seen other info on this. But check out this GBA article:


  6. brendanalbano | | #7

    What about this assembly:

    - LP SmartSide Cladding
    - Benjamin Obdyke Slicker Classic (or similar product from Tyvek, Keene, etc)
    - 1 1/2" Zip-R R6 sheathing
    - Studs
    - Gyp

    The zip-r puts your sheathing outboard of your insulation which lets you use a mesh drainage product like BO Slicker instead of furring strips, which seems like it might be more resistant to nesting. Now it also might not, bugs are tricky critters!

  7. Seeking_Wisdom | | #8

    The problem with the zip-r is that our mice love the foam. Certain bugs do too. And even though a bug screen could cover the foam, there is nothing to prevent a rodent (chipmunk, mouse, rat) from chewing through the siding to get to the foam, bypassing the perforated metal at the bottom (or corivent, etc). That is one of the reasons why I would rather use the mineral wool. Critters generally dislike it.

    Other reasons include: 1. no need to worry about the dew point. 2. Self draining. 3. Fire resistant. 4. mold resistant. 5. Affordable. My supplier can get me 1" mineral wool board for 42 cents per s.f.

    So I thought I had this assembly figured out but then Peter Yost through me a curve ball with his link. Basically, it sounds like I need to provide adequate drainage but also air flow as well.

    My question now is: If I have vertical furring strips (not over the interior studs) and mineral wool board between them, is the permanence of the mineral wool enough to allow the siding to "dry out". Or do I really need a true air gap?

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