GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Exterior rigid insulation – semi-permeable EPS with poly vapour barrier

brettsichello | Posted in Energy Efficiency and Durability on

We are currently working on two residential projects where we have specified exterior rigid insulation over a 2×6″ wood framed wall. Both projects occur within Climate Zone 5 but they also have very different amounts of annual rainfall (260mm in the interior of British Columbia vs 1650mm closer to the coast). The wall assembly looks like the following in both cases with the minor difference of the insulation in the stud cavity.

Exterior Wall Assembly
– Cement Board or Cedar Cladding
– 1/2″ Plywood Strapping – Rainscreen
– 2″ EPS Rigid Insulation – Type 1 (10 psi, 5.0 perms water vapour permeance per 1 inch) – seams are not taped or staggered
– Tyvek Weather Resistant Barrier with lapped and taped seams
– 1/2″ OSB sheathing
– 2×6″ Wood Studs at 16″ o.c.
– R22 Fibreglass Batt Insulation (coastal) or R20 Densepack Cellulose (interior)
– 6 mil poly vapour barrier with taped seams
– 1/2″ Gypsum Wallboard

Technically we could get away without the vapour barrier on the interior but after researching this endlessly we believe that by using the semi-permeable, not taped, not staggered seam EPS, the wall assembly will dry to the exterior through the foam and the seams rather than being encouraged to dry to the interior where the mechanical system has to deal with the excess moisture.

Are we incorrect in our analysis of the permeability of the EPS or should we be taping or staggering the EPS seams and eliminating the interior vapour barrier? We are open to having a WUFI analysis on the different climates (rainfall/moisture) but would appreciate any input.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Dana Dorsett | | #1

    Tape the stagger seams- the tape has effectively zero effect on it's vapor permeance, but makes a fairly substantial difference in how much air gets through the foam layers. At 2" it runs about 5 perms, which is comparable to dry #15 felt, and PLENTY of drying capacity. You could also go with Type-II EPS, which would still be over 1-perm @ 2", and plenty of drying capacity into a rainscreen. (It's more rugged for handling on site too, won't be breaking off corners every time you bump into something the way you would with 1" Type-I.

    With poly on the interior side of the assembly there is ZERO drying toward the interior, even if you dessicated the place to Martian atmosphere levels, but 1 perm of drying to exterior-only is fine. The lower permeance of the higher density EPS is even a benefit if you air condition the place. With R8 on the outside of R20-R23 cavity fill you have reasonable dew point margin for winter for the house on the coast and can skip the interior poly, just make the gypsum air-tight (which includes meticulous detailing around the electrical & plumbing penetrations.)

    On the other house it depends on exactly where it is. You'd be fine at an R8/R20 ratio at the southern end of the Okanogan valley, but at higher elevations and further north or east it's more like a US zone 6B climate than a 5B climate, it might not be sufficient foam-R to go without interior side vapor retarders. If the interior house is still west of the passes of the coastal mountain ranges it's probably fine without poly, as long as you're under 500meters in altitude. Better than poly would be smart vapor retarders such as MemBrain or Intello Plus however.

    R23 rock wool is a somewhat better product than R22 fiberglass, beyond the miniscule difference in R-value. (Greater recycled content, lower moisture wicking, higher fire resistance, etc.) If it's priced at all comparably, it's worth it, but not if it's a big up-charge.

    I have a rock climbing self described "dirt bag wall-rat" niece who spends most of her time trying out different climbing routes at Squamish these days/years (when she isn't snowboarding). I originally hailed from Seattle, but have a great fondness for the "the wilds" of B.C., and plan to spend more time there. B.C. has lots of natural beauty, spectacular seascapes, gorgeous mountain ranges, and GREAT backcountry skiing ( lift-served too). It's a great place to call home! (Living in New England I've developed a case of "mountain-envy". :-) )

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Dana about taping the foam seams. Airtightness is always valuable at this layer.

    You don't want polyethylene on the interior, however. I suggest a "smart" vapor retarder like MemBrain.

  3. brettsichello | | #3

    Hi Dana and Martin,

    Thank you for your recommendations as they are really appreciated.

    Revising to Type II EPS sounds like a good idea for durability on site. I was hoping to avoid having to stagger the seams as it adds another step on site and it is going to be challenging enough to get the guys on site to do the installation properly but we’ll have to work through that and may have to lead by example/demonstration on site with multiple follow up reviews. Thanks for explaining the permeability of the tape on the seams, we’ll make sure that is done as well. I would prefer to use rock wool over fiberglass but the up charge is ridiculous in this area.

    On all of our projects we specify and require gasketed electrical junction boxes that are also spray foamed where the wire penetrates the rear of the box. All plumbing penetrations are taped and sealed to the vapour retarder. I suppose that if we didn’t use a vapour retarder the best way would be to put a piece of blocking in between the studs with a gasket on the face of the blocking to be tight to the gypsum board and spray foam the plumbing penetrations at the back of the blocking. That actually brings up another good question, any thoughts on gasketed junction boxes vs. a poly boot? NuTek manufacturers the boxes I'm referring too.

    Looking at the costs of 6 mil poly, Membrane (twice the cost) and Intello Plus (six times the cost), Membrane seems to be the winner for this particular application.

    Here is a bit information on each respective projects:

    Project 1 – Mission, BC, Climate Zone 5A (cool-humid), 186m altitude, 3050 Degree-Days Below 18 Celius (64.4 farenheit)

    Project 2 - Kelowna, BC, Climate Zone 5B (cool-dry), 351m altitude, 3600 Degree-Days Below 18 Celius (64.4 farenheit)

    I hear you on the “calling of the mountains”. I’m originally from the prairies (20 minutes north of North Dakota), spent a number of years for schooling/work in Toronto and then had enough… the mountains with splitboard in hand!

    Thanks again for your time.

  4. Expert Member
    Dana Dorsett | | #4

    Sounds like MemBrain it is then!

    I'm surprised that Kelowna's climate isn't colder- I thought they had more HDD than that, but then again I've usually gone through Kelowna in winter on the way to someplace snowy. (I like the look & feel of the Okanagon lakes region- Kelowna has a very nice vibe.) If it's truly only a 3600 HDD celcius climate, R8 would be enough dew point control on R20 cellulose (though I know the inspectors still love poly, or stuff that at least LOOKS a bit like poly.)

    I don't have any experience with the gasketed electrical boxes, can't really comment on how well they work relative to other methods.

    The Canadian midwest always feels more vast & empty to me than the US side. Driving from Vancouver to Thunder Bay (mostly on the trans-Canadian highway 1) in a marathon non-stop drive late one June, somewhere in Saskatchewan I went nearly 20 minutes without seeing a single house or car. (That's never happens on I-94 in North Dakota, which I've traversed multiple times.) It felt pretty surreal- looking to the right it felt like I should be able to see Texas from there! :-) The stretch of prairie from Calgary to Winnipeg seemed endless.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |