Hello, I would like to know your opinion:

2X6 16" o.c. with interior 2X3 cross strap allow for 7" of c.c. spray foam with a 1" gap for the uneveness of the foam
Plywood sheathing on the exterior

I also tought of doing only 5.5"ccspf and adding 2" of eps on the exterior of the plywood but I have concern about sandwiching the plywood between two non breathing material.

I know it's not the greenest or the cheapest wall assembly but it's fast, allow for a perfect vapor and air barrier, and is easy for anyone to build...

Alexis

ICF walls would be better.

What climate zone is it built for?

It's good until you get to the edges - then you have thermal bridging and sealing issues. I would use cellulose and EPS.

You're right about not the greenest or cheapest. I assume the 2x3's are on the flat for a total thickness of 8"? For the money you'll spend on the cc Foam you could do a lot better by building a 2x4 bearing wall with either 2x4 or 2x3 horiz strapping (total of 6" or 7") and then apply 2" or xps over the sheathing outside and fill the cavity with cellulose. I am about to start this type of wall (with 2x4 strapping) in a Zone 4/5 house in a moist location and found this to be the highest R for the buck and also have the best moisture control (NO vapor barrier on interior side). I'll be applying vertical board and batten to the exterior with a vent rainscreen space. Some sidings are easier or harder to do over the foam (wood shingles - hard / clapboard or hardiboard - easy). Windows take some finagling.

Unless it's a water-blown cc foam 7" is anti-green at the extreme. Icynene MD-R-200 is water blown 2lb foam (but not MD-C-200, which is blown with HFC245fa, like most other ccSPF is), and Aloha has 1.8lb & 2.5lb closed cell water blown goods.

At 2" thickness Type-II EPS is about 1.5 perms, about as permeable as the plywood sheathing itself, and has VERY reasonable drying rates. Putting EPS on the exterior would provide a better thermal break as well.

The Icynene water blown foam is still about 1.3 perms @3", which would run ~0.8 perms @ 5, a class-II vapor retarder, which is fine. That allows the sheathing at least some drying capacity toward the interior. You could model that stackup for your climate with WUFI, but with lower perm foam on the exterior sheathing than the interior foam and a higher average temp for the sheathing you'd be in pretty good shape for most US climate zones. I've requested (but haven't seen) ASTM E 96 specs on the Aloha water blown foam, but it has to be somewhat similar. The higher the density, the lower the permeance, but it's unlikely to be as low-perm as HFC blown foam (and that's a GOOD thing!)

The building is in northern canada and the foam is closed cell. I looked at cellulose for a long time, but since I need a vapor barrier on the warm side of the assembly ( ADA not permitted) ccspf seems to be the only way to get a perfect vapor barrier. One of the primary concern is durability of the wall and ccspf seems to be the best in that area. If I can put 2" of eps on the exterior of plywood, I would only put 5" of ccspf and no strapping on the interior ( the strapping was for adressing thermal bridge).

I can get ccspf installed for 1$bdft so icf is not in the same price range!

I first wanted to go with a double wall dense pack cellulose system, but it still seems a bit experimental. So many concern with vapor open sheathing, condensation issues... when you read all the question regarding thick cellulose wall, it just seems like there should be some more research done before commiting in a large project. Even if it cost 5K more to do ccspf, it's the price of granite countertop and gives me a trouble free, structurally very solid wall system. No possible moisture intrusion, perfect air barrier, and like I said, it had structurally to the wall.

What town? (For weather data/moisture analysis purposes.)

Membrain on the interior side should meet code and creates a second air barrier (always a good thing).

St-Esprit, Québec

Membrain sounds great, but what about when there is high moisture on the interior because of a shower, cooking, or something else. Then the moisture gets into the wall assembly... The vapor barrier parts just seems to be an headache and impossible to be perfect. That's what the spray foam seems to solve.

Posted By Alexis on 18 Oct 2012 03:30 PM
St-Esprit, Québec

Membrain sounds great, but what about when there is high moisture on the interior because of a shower, cooking, or something else. Then the moisture gets into the wall assembly... The vapor barrier parts just seems to be an headache and impossible to be perfect. That's what the spray foam seems to solve.

Looking at the Weatherpark datasets for St-Esprit compared to the data for Montreal, your 99% design temp would be about -10F/-23C (even though it gets much colder than that on 99.6th percentile nights), with an average mid-winter temp of about +9F/-13C, which is warm enough that MemBrain would be adequate, even with the humidity from showering & cooking peaks. 

Moisture transport via vapor diffusion is a surface-area x vapor pressure x time issue, and vapor diffusion is extremely slow compared to air-transported moisture.  Even though there would be peaks where the MemBrain opens up a bit, those times are short and have little consequence- it's the AVERAGE humidity over time that counts.  And when the warmer weather arrives and the RH in the cavity rises it'll open up and pass that moisture quickly to the interior.

But with only 2" of exterior EPS you'd still need the MemBrain if using cellulose in that climate.

With a code-required 10mm rainscreen between exterior EPS the drying capacity of the sheathing should be pretty good even in winter.  Using water-blown closed cell foam would give you a sufficiently low vapor permeance to skip the interior vapor retarder, and would have a decent overall drying capacity for the assembly, if not quite as good as MemBrain.  If you used HFC-blown closed cell foam it would have a lower overall drying capacity unless you skipped the exterior EPS.  See if there isn't a local installer who handles MD-R-200 (Icynene is a Canadian company headquartered in Mississauga Ontario- I'm sure there are installers in the greater Montreal area.) 

As cavity fill the difference in whole-wall are between R5/inch and R7/inch is miniscule (about R1.5) due to the thermal bridging.  If that R1.5 feels meaningful to you, note that 2: of exterior EPS will average nearly R10 (R5/inch) during January, if only R8 (R4/inch) during the summer,  (another R2 when you need it the most), so it's doing more for improving your mid-winter performance than any differences in closed cell cavity fill product could achieve.