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Exterior insulation thickness with SilvrBoard

jbrys | Posted in Green Building Techniques on


We are currently just finishing up our ICF basement of our new cabin build. We live in Northern Saskatchewan so we are the high climate code. The basement is going to have an insulated slab (above due to some miscommunication – is 2 inch EPS for R value of 10 enough? Then two layers of 1/2 inch osb tongue and groove screwed together) and R30 ICF with 8 inch core. It is walkout on one side to lakefront.

We are currently getting ready to do the floor system and then begin the walls. Walls are 2×6 with R24 bat going in. I had originally planned on doing 2 inch SilvrBoard but my builder recommended against it due to using heavy Hardie Plank requiring strapping to install it. He suggested just a half inch of the same product to give a thermal barrier. I know the calculators have a much higher thickness but this was all with previous products that did not allow the water to exit – SilvrBoard is supposed to allow this. 

Side note: do you suggest making the OSB sheathing the airtight layer? Interior vapour barrier still if you do that? Do most now apply a caulk/glue product to all plates and what not when installing exterior walls to make better seal? 

With exterior rigid insulation – can rock veneer be applied? Does it require a rain screed or drip layer? 

Thanks in advance! Fantastic information on this site. 

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Jared.

    R-10 is a reasonable amount of insulation for the slab and it is not problem that it will be installed above the slab.

    When it comes to your walls, the more R-value you put into the continuous exterior insulation, the more you will slow thermal bridging and the more durable the assembly will be. Also, exterior insulation can change the need for an interior vapor retarder, but that depends on the type of insulation and its permeability, and the R-value and its ability to keep the sheathing warm. You probably should detail your sheathing as an air barrier because it is a straightforward way to cover a lot of air sealing, but this doesn't have an effect on the requirement of interior vapor control. Again, that about the permeability of the exterior and dew point temperature control of the assembly.

    Check out these articles:
    The Four Control Layers of a Wall
    Walls that Work

    And yes, you should have a drainage space behind stone veneer.

    1. jbrys | | #4

      This is the product:

      R-value : 4.5 per inch
      Perm : 2.74

      I would for sure go thicker - up to even 2 inches. The concern was for siding on top of this. Are we still able to put stone veneer and cement siding like Hardie Plank.

      I believe if taped it removes the need for a wrap.

      With this perm value - it should allow the sheathing to dry to the outside - is that correct? This means there is no minimum requirement for R value due to this ability to have water vapour escape?

      If sealing/taping the OSB this does not change the vapour permeance but instead just air sealing of building. Is this correct?

      It is my understanding that RockWool exterior insulation would have very similar properties in regard to R value and permeance - allowing it be essentially substituted in this case.

  2. Expert Member
    BILL WICHERS | | #2

    Note that 2” of EPS is only a little over R8, not R10. If you need to hit R10, you need at least 2.5” of EPS.


    1. jbrys | | #3

      This is the product that we are going to use. We have Amvic ICF which is the same brand as Silvrboard. It is listed as R10 for the 2 inch product.

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