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Wall assembly questions to determine exterior insulation approach

mountaincabin | Posted in Energy Efficiency and Durability on

Hi everyone,

I have question regarding the picture I posted.  My home is in climate zone 5B near the central cascade mountains in Eastern Washington State.  The cabin was built in 1986 by a said builder in a mountain environment and will be our full time house in a couple years after we continue to fix it up.  The home had one add on over it’s years: (1) living room, kitchen area, loft, bedroom and bathroom all are in original construction which includes a 2×6 wall with what I believe is fiberglass batt insulation (I haven’t confirmed).  The back portion of the home has a different insulation construction (which also makes me wonder what really is in old portion). 

As you can see by the photo (from inside house to outside), it is (1) 3/4″ thick tongue, (2) 1″ rigid foam insulation, (3) wall cavity??, (4) 2″ rigid foam insulation, (5) 3/4″ plywood, (6) current cedar siding.  Now I’m sure if this foam continues through the whole wall or not so first response to me by you all may be “find out.”  

My plan is to install 2″ mineral wool on the exterior of the whole house along with a 3/4″ furring strip with 4×8′ hardie panel attached to the furring strips with a batten board finish.  This photo was taken while I had new  windows installed.  I had them furr out the windows to anticipate a 2″ exterior insulation and 3/4″ furring strip.  Again, this wall assembly in the newer portion of the home is much thicker and as it shows with the photo.  

My question, is my approach of installing a 2″ exterior mineral wool, (comfortbatt 80) still OK?  Is it OK for both wall assemblies?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Mountaincabin,

    Mineral wool insulation is vapor open and so it should not cause any problems when added over your existing walls--it will only benefit their thermal performance. That said, it is also air permeable. Before installing it, you should consider how you are going to air seal the walls. You could use tape and sealants to make the plywood sheathing your air barrier, or install a fully-adhered or fluid-applied water-resistive barrier (WRB) over the sheathing. Also, make sure your WRB is vapor open (most are).

    1. mountaincabin | | #3

      Thanks Brian. Yes I plan to remove old cedar siding then replacing and liquid flash all plywood seams. Then put a tyvek home wrap over the plywood with taped seams. Obviously then put 2” mineral with a 3/4 x 3” furring strip completed with hardie panel siding. Last question is that I see most people use lap siding hardie vs. the hardie panel sheets. Is there any issues with my installing 4x8’ hardie panel on top of 3x4” x 3” furring strips 16” OC? One contractor was worried about the panel having a soft middle which could break under weight.

      1. GBA Editor
        Brian Pontolilo | | #4

        Hi Mountaincabin.

        I would look at Hardie's specifications for that. I'm not sure what "weight" your siding will be under. Also, some builders prefer thicker furring strips over exterior mineral wool, because it compresses, 3/4 inch material may get wavy. At a minimum, find a system for making sure that your furring is in plane before installing siding. But if you do a search for exterior mineral wool, you may find some helpful feedback on this. I know some builders have gone to as much as 2x4s for furring strips over mineral wool.

  2. Deleted | | #2


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