We have 4" of EPS on our exterior wall and a meticulously taped sheathing and attention to detail as an air barrier. We will also be blowing dense pack cellulose into 8" wall cavities. We were hoping to avoid a vapour retarder but are we being foolish?
We are in zone 7 (I think) - Ottawa, Ontario.

Zone 4?

I think your biggest issue is the building inspector!

I've always thought it was zone 7 but I certainly may be wrong.
We are in Quebec actually, just north of Ottawa. No inspectors of the building elements. Just inspections for tax purposes...

Read this.

Yes, Ottawa with ~4500 HDD (Celcius)  is pretty similar to the cold edge of US zone 6. With 4" of EPS you're looking at R16-R17 on the exterior, and with 8" of dense pack (or is that a 2x8 studwall and thus only 7.25"?), you're looking at R28-R29 in the wall cavites, so the exterior R is about 22% of the total R.

IRC 2012 specs for zone 6 are insulated sheathing of  R7.5 or greater for a 2x4 wall (with R15 batts that would make the exterior foam 33% of the total R), or greater than R11.25 for a 2x6 wall (with R23 in the 2x6 wall that would also mean 33% of the total R in exterior foam.)

So, for Ottawa you'd need about 50% more exterior foam to skip interior vapor retarders completely, but with a smart vapor retarder such as MemBrain it should be fine.

OK thanks Dana. We bought Insulweb to install our cellulose but I imagine Membraine or the SIGA Majpell would work too. Anyone interested in buying our Insulweb?!

Just realized that we have already closed up the walls of our walkout basement. So the knee walls (the basement is stepped) have roxul in 8" stud bays and 4" of EPS on the exterior. Are we at serious risk of having condensation on our sheating?

edith, i would think so, I had frost issues just recently in winnipeg albeit with only 1.5" of foam on the outside. I ended up drying the place out and putting up membrain

So what about latex paint? It is a calss III vapour retarder..

Dana, our wall would be 7.25" dense pack cellulose. And the basement is 7.25" roxul

I agree, use MemBrain and detail it as carefully as your exterior air barrier.

Edith, you can do the same calculation Dana did to check: 4" of EPS (~R16) + 7.25" of dense pack (~R26) = R42 total. R16/R42 = 38%, so you have an acceptably large proportion of exterior insulation to omit the vapor retarder. With Roxul, the R-value for 7.25" is 28-30, so your ratio could decrease to about 35%, which is still acceptable per IRC 2012. The reason tjetson had trouble with frost, assuming he has the same stud wall thickness and insulation as you, is that he doesn't have enough exterior insulation to keep the temperature of the condensing surface between the foam and the studwall above the dewpoint temperature.

I believe that a coat of vapor-retardant latex paint on the interior is a good plan with that type of wall stackup. Feel free to correct me if I've misunderstood your wall stackup.

Great news. Thanks.
Our original plan was to used 5" Roxul board on the exterior but found it to be too expensive. We felt confident with that stackup to omit the vapour retarder.
I think we will probably use Membrain or Majpell on the rest of the house...

Actually Edith, before you jump for joy I'd like Dana to comment on this again. I am second guessing myself because he's really the expert but I don't see how he came up with his estimate that you're at only 22% exterior R. His calculation should have come up with nearly the same number as mine (a little lower since he was basing his off of a full 8" of dense pack). Dana?

I get the same calculation as you. I am comfortable with latex paint in the basement, or even a vapor retarder paint. But I will use a vapor retarder other than latex paint on the rest of the house. The other details, such as moisture management outside, attention to detail re. air barrier also give me confidence. And I will be keeping a close eye on the RH indoors.

Very good. Maybe Dana was thinking it was a double stud wall, which would get you R56 in the wall with a 22% exterior R.

maxvdh has it absolutely right- mine was an arithmetic error- MEA CULPA!!

I apologize for that- (need more coffee, and no more doing it in my head at the end of the day...)

With 7.25" of cellulose you're looking at at worst R26-R27 for cellulose layer, and if it's Type-II (1.5lb per cubic foot nominal density) EPS you'd be looking at about ~R18 performance or better during the weather that matters. So assuming R45 total you're in the 40% range, which is just fine.

Thanks for catching that maxvdh!

No thanks needed Dana, I owe you a huge debt for all the knowledge I've managed to accumulate from reading your posts. If you're anywhere near Andover I feel like I owe you and your family a dinner out for all the help with my future home design.

And Edith, I'm glad we've confirmed that your walls are in good shape!

The latex paint only works if you use air tight drywall techniques include locking in around electrical boxes.

Thanks to you both!
FBBP, this brings up a point that I have often wondered about. We have dedicated our sheathing as the air barrier. I also like to think of it as a pressure boundary. So vapor will not be drawn through our walls because there shouldn't be holes drawing air into our walls (through our sheathing). So in a sense the vapor retarder is not needed at all, but in my case I am using it as a precautionary measure. I have no doubt that we have an imperfect air barrier but we've done the best we can.