Which is a better option for wall construction in climate 6?

Option 1:
2x6" 16" o.c. frame with R24 batts plus 1" of polyiso (R6.2) on the outside
Nominal ~R30

Option 2:
2x8" double staggered 2x4" wall with cavity filled with 1/2lb. open cell spray foam (R28)
Nominal ~R28

Thank you.

Coming from me I say that the 2x6 wall is better option. I am currently building a home and that's how I built it. 2x6 with 1.5" polyiso on the outside of the sheathing. Structurally strong, plenty of insulation, easy to airseal and honestly, everyone knows how to do it correctly.

Thank you.
That is what I am leaning towards as well. The 2x8" wall sounds attractive, but when I considered that it's easier (and likely cheaper) to get the same R-value with a standard 2x6 with sheathing, it is loosing its appeal. As well, the 2x8" requires either a 2" wider foundation on all sides or extra 2" lost from interior space on each wall compared to adding exterior rigid foam.
Although it is not necessarily a deciding concern of me, batts also avoid any speculation of spray foam VOCs in the house in case of a careless installation procedure.

In a cold climate, the vapor retarder is best on the inside. So either use unfaced EPS foam (which is also better for the environment) or put the polyiso on the inside. I'd use cellulose to fill the cavity.

One could use enough external polyiso to probably avoid most condensation problems in a dry-to-the-interior design, but why add expense/risk when it's easy to use a dry-to-the-exterior design?

Small amounts of polyiso on the exterior in cold climates is specifically not recommended.

jonr
I won't steal your thread eljay but i will take the chance to say to jonr that each wall stack up has risk associated with it if it's not done correctly. Polyiso foam can be had through several recycling businesses for a fraction of the cost new. It offers excellent R value per inch and makes an excellent rain screen because of the facing it has and it doesn't absorb water. Exterior mounting of polyiso is an excellent idea. NOW, if you want to use foam and all that inside, polyiso is not the best choice, but for a cellulose design or a roxul batt design...its a great fit. My personal approach to this was keep all the stuff I don't want in my house OUTSIDE THE HOUSE....so that meant vapor/air/rain/kritters all staying OUTSIDE the envelope, so that's what I did.

Posted By eljay on 11 Aug 2014 10:07 PM
Which is a better option for wall construction in climate 6?

Option 1:
2x6" 16" o.c. frame with R24 batts plus 1" of polyiso (R6.2) on the outside
Nominal ~R30

Option 2:
2x8" double staggered 2x4" wall with cavity filled with 1/2lb. open cell spray foam (R28)
Nominal ~R28

Thank you.

Either of those options would require vapor-barrier latex or a smart vapor retarder like MemBrain or Intello Plus on the interior side to protect the sheathing from excessive wintertime moisture accumulation.

In a zone 6 climate with only an inch of polyiso on the exterior you need to de-rate the performance of that polyiso to about R5, or even R4.5, since during most of the heating season the average temperature through the foam will be well under 30F, which is below the severe knee in the performance curves.  A better/more resilient assembly of the same thickness would be a 2x4 wall w/ R15 batts, with 1"/R6 polyiso outside the sheathing, and 1"/R4.2 Type-II EPS outside the polyiso (seams staggerd with those of the polyiso). The EPS will be performing at R4.7+ during most of the winter, and keeping the polyiso warm enough to run about R5.5. 

Better still would be to use a dual layering of 1.5" of EPS (R6.3) + 1.5" of polyiso (R10), outside a 2x4/R15 OR an 2x6/R23 wall, which would be sufficient exterior R for dew point control at the sheathing layer without interior side vapor retarders (standard latex paint would be good enough.)

The thermal conductivity curves across mean tempertures through the insulation layers for different materials look approximately like this:



You'll note that whenever the mean temperature through an inch of polyiso is -5C/+23F or colder (as it would be for much of the winter) it's thermal conductivity is about twice what it is at the vertical dotted line, which is the temp at which it is tested for labeling.  That means during the coldest weather instead of R6.2 or whatever, it's true performance is less than R3 (!) Putting an inch of EPS outside makes the mean temp through the polyiso layer warmer moving it down the conductivity curve toward it's highest-performance inflection point (about +15C).  In climate zone 6 if you're only going to put an inch of foam out there, use EPS- it will outperform polyiso during the winter months, and will allow a reasonable amount of drying toward the exterior.  You have to be in climate zone 3 or warmer for an inch of exterior polyiso to dramatically outperform an inch of EPS.

The thicker the foam relative to the cavity-fill R, the warmer it's average temp, and the better polyiso will perform.  But the performance of that last outer inch of polyiso is pathetic at sub-zero F temperatures, in the R2-2.5 range, while an  outer inch of EPS would perform at 2x that, which is why the dual-layering with different foam materials is so much more effective.

Thank you again Dana! I didn't know that polyiso resistance changes so drastically with falling temperatures!

I learned from your other responses on the forum that I would need at least R12 on the exterior in my climate to prevent moisture formation. Unfortunately, our building code requires an interior vapour barrier and most builders use polyethylene sheet under a gypsum board. See the Canadian National Building Code (page 54): http://www.cmhc-schl.gc.ca/odpub/pd...9301531508

I wonder if I can specify a smart vapour retarder as you suggested for the inside and comply with the code and not drive the cost higher.

If I cannot get rid of the interior vapour barrier, I recall you suggested rock wool for the exterior rigid insulation. Is this Roxul ComfortBoard R6 the product you mean? With this, the wall could dry to the exterior comfortably in my climate, correct?
So, with 1.5" of Roxul and R24 batts, I'd be at R30 nominal. What would be the effective R-value of this assembly with 2x6 16"oc? I should be easily beating that 'Option 2' 2x8 assembly correct?

Are my options for higher R and moisture-resilient assembly essentially these?
1. With poly on the interior, use at least 2.5" of XPS on the exterior (to get R12.5)
2. With poly on the interior, use 3" of Roxul board (R12)
3. With smart vapour retarder (if allowed), freedom to use any rigid board thickness because of inward drying capability?

Did I get this right?

Thanks again.

Posted By jonr on 12 Aug 2014 10:53 AM
In a cold climate, the vapor retarder is best on the inside. So either use unfaced EPS foam (which is also better for the environment) or put the polyiso on the inside. I'd use cellulose to fill the cavity.

One could use enough external polyiso to probably avoid most condensation problems in a dry-to-the-interior design, but why add expense/risk when it's easy to use a dry-to-the-exterior design?

Small amounts of polyiso on the exterior in cold climates is specifically not recommended.

Thanks jonr. I'm thinking that since my code requires an interior vapour retarder, the idea of putting a polyiso on the inside of the stud cavity actually makes sense since it would allow the wall to dry to the exterior and the polyiso would perform better since it would not be subjected to the temperatures where it drops its R-value as Dana pointed above.
So, perhaps that's another choice for me on top of the three I listed above.

It's a commonly held myth that Canadian building codes require interior side polyethylene or other Class-I vapor retarders, although that is an inexpensive and common solution.  Variable-permeance class-II vapor retarders like MemBrain offer more resilience.  Yes, it is more expensive than polyethylene sheeting, but there are good arguments that it is worth the additional expense.

Roxul ComfortBoard comes in a few different thicknesses, and is highly vapor permeable to allow exterior drying toward the exterior, and would be a much safer solution at R6 than an inch of polyiso would be.

In  the order presented:

1: poly on the interior w/ 2.5"+ XPS on the exterior is a potential moisture trap. The vapor permeance of 2.5" XPS is about 0.5 perms, which makes for very slow drying rates in a cold climate.

2: poly on the interior w/ 3 of Roxul can potentially work- it has plenty of drying capacity, but may be problematic in summer if you use air conditioning.

3: Smart VR + any rigid-board thickness works, but works better if the rigid board is at least semi-permeable (say 1-2 perms) or more. Unfaced Type-II EPS could work, since it doesn't hit 1 perm until about 3" (R12.6), and doesn't hit 0.5 perms until about 6".  Fiber faced polyiso is about 1-perm at most thicknesses & densities until it's 3" or thicker (depending on density.) Foil faced foam is extremely low permeance at any thickness, and would be a riskier choice.

A 2x6 16" o.c. wall (~25% framing fraction)  w/ R24 batts and R6 rigid rock wool insulating sheathing comes in at about R21.5 whole-wall, allowing ~R1 for the combined R of the gypsum, structural sheathing, & siding.

A 2x8 24" o.c. wall (~20% framing fraction) with similar density batts (R4.35/inch) comes in at about R22.4. It's pretty much a wash from a thermal performance point of view (only an R0.9 difference) but since the sheathing runs colder and more prone to wetting being on an exterior layer, it's somewhat less resilient.

Thank you very much Dana!