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Need advice on EPS insulation inside and out

GBA Editor | Posted in Energy Efficiency and Durability on

I have access to 4″ EPS sheets 2’x4′. I would like to use this insulation in my floor, walls and ceiling of a workshop that I will build with 2×6 on 24″ center. I am thinking of sheathing with OSB and using foil-faced 1″ EPS on exterior with wood siding. What vapor barrier issues will I have? Location: Halifax, Nova Scotia.

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  1. GBA Editor
    Martin Holladay | | #1

    I assume you are in climate zone 6 or 7. If you have 2x6 walls, and if you want to put rigid foam insulation on the outside of your OSB wall sheathing, then your foam needs a minimum R-value of R-15 (climate zone 7 — in other words, at least 4 inches of EPS) or R-11.5 (climate zone 6 — in other words, at least 3 inches of EPS). One inch isn't enough.

    If you're using exterior foam sheathing, then the interior of your wall assembly should be vapor-open. In other words, don't install interior polyethylene.

  2. MIKE | | #2

    Thanks for the reply Martin.I should clarify what i want to do .1"eps w/foil on outside,tyvek wrap,then OSB,2x6 on 24 centers,then 4"eps in interior walls,ceiling and under floor. If i build in this manner ,then i do not put the poly barrier on the inside wall is that correct?

  3. GBA Editor
    Martin Holladay | | #3

    No poly. But you're still putting too little EPS on the outside of your OSB, putting the inside surface of your OSB at risk for condensation. That means you have to do a flawless job of air-sealing the insulation between the studs.

    Really, it makes more sense to put 4 inches of EPS outside of the OSB, and 1 inch of EPS between the studs (or directly under the drywall).

  4. MIKE | | #4

    Thank you for the clairification Martin.I now have to figure how to attach the foam to the outer wall and then attach the siding ,any suggestions? We have high winds here at times ,especially in late summer and fall.

  5. GBA Editor
    Martin Holladay | | #5

    Glue the foam. Screw the furring strips to the studs.

  6. MIKE | | #6

    Thanks again ,I have sourced out stucco suplies and will be looking at application techniques.

    You have been great help, Much appreciated

  7. Brett Moyer | | #7

    Are you suggesting to glue the foam sheathing to the OSB? What about EIFS failures and hydrostatic pressures between the OSB and the foam?
    I would do as Dr Joe suggests and "provide a small gap between the exterior face of the OSB and the back surface of the foam sheathing to provide for some hygric redistribution."
    This can be done with back-faced grooves on certain foam boards or crinkled house wraps (Tyvek Drainwrap) installed between the foam and the OSB.
    OR if you are really bold....
    Think about the REMOTE approach ( and install a self-adhered membrane (Ice and Water Shield) over the entire wall assembly with enough exterior foam to prevent the membrane from turning into a condensing plane (Riversong will surely disagree with this approach). :)

  8. Brett Moyer | | #8

    I should also mention that you can always avoid the hydrostatic issues of exterior foam and OSB, and fastening issues of cladding through thick foam sheathings, by changing your wall assembly to a double wall or Larsen Truss.

  9. GBA Editor
    Martin Holladay | | #9

    Good advice on providing a mechanism for hygric redistribution. Your suggested detail (via Lstiburek) is better (more robust) than mine, since it helps protect the vulnerable OSB.

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