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Minimum foam on the exterior of 2×6 wall…has it changed recently?

kenorakq | Posted in Energy Efficiency and Durability on

My daughter lives in a wood framed condo. It has 2×6 exterior walls with fiberglass bat insulation in the joist bays. There is 6 mil poly on the interior side immediately behind the drywall.

She lives in Winnipeg Canada (zone 7).

The management company hired an architect to reside and insulate the exterior walls.  To my surprise the NEW wall has had the old wood siding removed (it was in bad shape ) and then…

The new assembly is.. from the inside..

5/8 drywall… 6 mil poly… R20 fiberglass batts… 3/4 plywood… SOPREMA Stick VP WRB… 2″ of styrofoam SM (R10)… 3/4 (1×4)  vertical furring… finally LP horizontal siding.

This supposedly made it past an architect and engineer…. 

my question is related to the MINIMUM R value outside the sheathing in zone 7.. I thought/have read than 2/3 of the insulation value must be outside of the sheathing… I’m adding 4 inched aof foam to the exterior of my 2×6 wall because I thought it was needed!

Has the rule been amended?

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  1. Jon_R | | #1

    > MINIMUM R value outside the sheathing

    More like that such a rule (without additional qualifiers) never existed, neither in code nor from a "needed for a good wall" building science standpoint. If you understood that from somewhere, you should get the author to improve the writing.

    On the other hand, don't think that you can use any amount of any kind of foam and have it work - perms become critically important if you are going to use less than Class III enabling amounts of external foam. See here.

  2. GBA Editor
    Martin Holladay | | #3

    Actually, the best article on this question is a different one. Here is a link to the relevant article: "Rethinking the Rules on Minimum Foam Thickness."

  3. kenorakq | | #4

    I "thought" the the rule was just that, "a rule" ...the written in stone kind... not the ..of thumb kind. I will stop worrying about the condo based on this info.

    As an associated point and an even more personal point I have been standing on my head for a couple of years (too long) planning on adding 4" of foam (EPS) to the exterior of my home expansion and renovation in Kenora NW Ontario Canada (zone 7).

    I have been torn between knowing that I need to improve my exterior wall design to include some exterior foam and learning that *any* really meant "a lot".

    In my case 4 inches.

    I'm already committed to the foam but... now have a clear(er) understanding of what's nice to have vs need to have.

    I'm going to inventory my foam stash (yup,,I've got a couple of portable garages full of a variety of reclaimed foam, both EPS and XPS in an assortment of thicknesses) and see if I can cover the house with 2 1/2" or 3" foam instead of 4".

    While I know the thinner foam compromises the thermal performance, a reduction to 2.5 or 3 makes installing and flashing details around doors and windows easier and as I'm learning sometimes good enough is good enuf!

    I would like to see an article that speaks to this...opps...there is one ... reference made by Martin (thanks)

  4. kenorakq | | #5

    I've got 270 1 1/2 " by 2x8 XPS SM roofmate and 400ish EPS 4x4 slabs in a variety of thicknesses... most (250) are 1 1/2" with the rest being a near even split of 2" and 2 1/2".

    I am considering changing my exterior insulation to 3" from 4".

    I have enough 1 1/2" XPS to do two layers... this would simplify my framing and trim and (I now believe) won't significantly compromise the durability of the wall.The use of the XPS will made a firmer nailing surface as well since its more dense... the EPS will go under the concrete floor. Looks like I'll have some to spare.

    Thanks for clearing up the misconception about exterior foam.

  5. charlie_sullivan | | #6

    My biggest concern about this is actually the use of XPS, which, in North America, is made with an extremely high global warming potential blowing agent. The best alternative would be graphite infused EPS (e.g. "neopor" brand), but regular EPS could also be used. As a bonus, either of those can dry to the exterior a little more than XPS.

    1. Jon_R | | #7

      > either of those can dry to the exterior a little more than XPS

      About 4x as much, which can make a big difference.

      Excellent interior side air sealing always helps, more so as sheathing spends more hours below condensation temperatures.

  6. Expert Member
    Peter Engle | | #8


    Tim mentions above in #4 that the foam is reclaimed and already on hand. It's GWP was spent by the original user.

  7. charlie_sullivan | | #9

    Thanks Peter--I didn't notice that the name of that commenter was the same as the original questioner.

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