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Foil faced polyiso (R 13) on exterior in Zone 5

doughpat | Posted in General Questions on

Hi all –

My goal is to reduce heat gain on the south wall of a small bedroom.  The room will not have its own method of cooling (hence the extra effort to block heat).  It does have heat. 

The wall is 2 x 6, 16″ OC, with OSB sheathing.  The OSB sheathing has been caulked (air sealed).  The drywall will also be carefully air sealed.  Roof is an unvented (sealed) filled with 3.5″ of closed cell spray foam, and the remainder (~8.5″) with blown in dense pack fiberglass. 

I am planning to install exterior foam insulation on the south wall to help reduce heat gain (and of course, I’ll take the heat retention as well, though that is much less critical since the room has heat).  I plan on use RMax 2″ foil-faced polyiso insulation.  I’ll tape the seams, and use 1/4″ nailing strips to anchor the foam and provide a surface for hardy-plank siding. I plan to use the appropriate tape to seal the seams.  I’ll also integrate the windows into the tape assembly, being aware that this foam is the WRB.

My concern is that water-laden air migrates through the drywall, through the insulation, and condenses on the inside surface of the (relatively cold) OSB.  I believe 2″ (R13) should be sufficient, but I wanted to check.   Zone 5 (Central Oregon).  Cold and dry winters, hot dry summers. 

One additional layer of protection could be to spray foam a thin layer on the inside of the OSB, but electrical is already in and the cost would be substantial.

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  1. brendanalbano | | #1

    In zone 5, with R-20 cavity insulation, the recommendation is for a minimum of R-7.5 exterior continuous insulation to keep your sheathing warm. 2" of Polyiso is greater than R-7.5, so you're good to go on that front. Here's an easy resource for looking up the ratios:

    Regarding attaching your siding, take a look at James Hardie's instructions to make sure you are compliant:

    If I'm understanding your plan correctly, with 2" of rigid foam, a 1/4" nailing strip will be insufficient for you. You will likely need to use more substantial furring. 1x4 is a common choice.

  2. Expert Member
    BILL WICHERS | | #2

    +1 for using something beefier for a furring strip. 1/4” wont reliably hold nails, won’t meet Hardi’s specs (and will thus void their warranty), and is in general just not substantial enough to do the job. 1x4s are more commonly used, and a bit less likely to split than 1x3s, ripped strips of 3/4” CDX plywood are another good choice.

    The 2” of polyiso itself will be fine. Just be sure to get the details right so that you don’t have any leaks or issues with flashing.


    1. doughpat | | #4

      Hi Bill -

      3/4" CDX strips would be better, as my window bucks are only 2" thick (I should have made them thicker to accomodate for the furring -- I'll have to improvise). Also I have a lot of 3/4" CDX leftover!

      Though I need to make sure that would satisfy Hardy's warranty. Unfortunately that document that was linked above is pretty detailed/meaty, so I'll have to spend some time combing through it to figure it out.

  3. doughpat | | #3

    Brendan -- that is super helpful and confirms my plan is more than sufficient. The R-13.1, even with "real world" conditions (as well as the slight penalty of polyiso in cold weather) will still be well above the required R7.5.

    Thank you for pointing out the issue with the 2" furring strip. Unfortunately adding a 1x material makes the total thickness closer to 3"....going to give me some issues with the trim, but I suppose I'll just have to figure that out.

    1. user-723121 | | #5


      Then go with 1 1/2" R-10 polyiso. It is a bit more commonly available so you may get a better price per R.

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