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Vapor barrier with EPS outboard, fiberglass batts in 2×4 wall?

smeldrum | Posted in Energy Efficiency and Durability on


I am designing a wall assembly and have some hesitation to commit to this assembly due to concerns about trapped moisture in the cavity. The project is in British Columbia, Climate Zone 5, Central Kootenay, Canada.

The assembly I wish to use is as follows:

Exterior Rainscreen with 1×6 Pine T&G Siding on 1×4 strapping over
*1.5″ to 2″ Rigid EPS Foam Insulation R5.6 to R7.5 (is the extra half inch worth it?)
House Wrap/Air Barrier
1/2″ Plywood on
2×4 SPF filled with R12 Fibreglass Batts
*Vapour Barrier (only if acceptable based on permeability of EPS thickness)
1/2″ Plywood/GWB Interior Finish

I understand the importance of breathable walls and ability for moisture to escape wall cavities, but semi-rigid/mineral wool is beyond my budget and not as readily available to me as EPS.

My research has led me to believe that the above assembly should not use Poly Vapour Barrier as the walls need to dry to the inside. I have read that EPS has higher permeability than other Rigid foams and if you do not exceed 1 1/2″ you can still use a Poly VB inside. Culturally, It has been drilled in me to put the poly inside so I hesitate to do so.

So my questions are if I use 1 1/2″ EPS can I still use interior Poly VB?
If I use 2″ EPS and do not install interior Poly VB is the insulation value increase really worth it?

Thanks in advance!

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  1. Expert Member
    Dana Dorsett | | #1

    Kootenay is really more like a zone 6 climate, and you really WOULD need R7.5 on the exterior for dew point control, at which point you can skip the interior side polyethylene. In the unlikely event that you are at a sufficiently low elevation that it's really a zone 5, R5.6 would be enough. The extra R value is not only "...worth it...", it's essential to making it truly resilient. In fact you may want to go one better:

    At the same 2" foam thickness you could use foil faced polyisocyanurate and get marginally higher performance, and the fact that it can't dry toward the exterior through the foam wouldn't matter at all.

    Short of that, using Type-II EPS (1.5lbs per cubic foot nominal density) you'd be at about R8.4 @ 2", which is plenty of dew point margin on R12-R15 of cavity fill, and it's much more rugged to handle and has far fewer shrinkage issues than the low density stuff you are considering. At 2" it will still be more than 1-perm, which still offers a significant seasonal drying capacity toward the interior, but still only ~1/3 of the drying rate through interior latex paint.

    If the inspector insists on seeing some plastic film, use 2-mil nylon (Certainteed MemBrain), not 4 or 6 mil polyethylene. Nylon becomes vapor open when the cavity air's humidity is high enough to support mold growth, but its less than 1 perm (and meets Canadian code definitions of "vapour barrier" ) when the cavity is relatively dry, which it will be all winter.

  2. GBA Editor
    Martin Holladay | | #2

    Dana gave you good advice. The principles behind this type of wall are explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Briefly: walls with exterior rigid foam need to be designed carefully, ensuring that the exterior rigid foam is thick enough to keep the sheathing above the dew point during the winter. Done correctly, this is a wall that stays incredibly dry. These walls are designed to dry to the interior, and should never have a layer of interior polyethylene. Some old-school Canadian building inspectors don't understand the building science behind these walls, and so some builders have to install interior MemBrain (a "smart" retarder) -- an unnecessary expense -- to satisfy ignorant building inspectors.

  3. smeldrum | | #3

    Hi Dana, Martin,

    Thanks for the prompt, informed response. I am confident now to use the 2" outboard and no internal VB.
    The information on poly isolated and type II eps is much appreciated.
    Considering the shrinkage as you mentioned, would a 2 layer approach with lapped/taped seams help to tighten this envelope? Either 1.5"+0.5"(ext) or 1"+1"?
    Thanks again, Steven

  4. Expert Member
    Dana Dorsett | | #4

    A 1" + 1" taped seams approach is pretty straighforward. Foil facer seams are easy to air-seal with temperature-rated foil tapes (eg: Nashua 324a, found in most box stores in the US, but there are many others.)

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