I am trying to create an R-50+ wall in the most cost efficient way. Here's my idea starting from the outside-wood siding, strapping, 2" R-board (R-12) with taped seams, Zip wall sheathing (overlapping R-board joints) with taped seams, double 2x4" staggered wall (5" space between) with blown-in cellulose (R-42), not sure about a vapor barrier here, 5/8" sheetrock. Any thoughts about the order of material, potential moisture problems and/or cost? Thanks.

5" of 2# foam will yield over R-40 and can be blown in a standard 2x6 wall. This also exceeds all but Passivhaus standards in the coldest climes. If you value your labor, I don't think you can build a better wall, save adding a single sheet of XPS to the interior or exterior. You may stagger studs to avoid the evil "thermal bridging" but it all comes at a cost.

5" of 2# foam will yield over R-40 and can be blown in a standard 2x6 wall.

Or maybe R-30.

2# foam at the low estimate of 6.5 x 5.5 gets me to R35.75 any insulated sheathing will push us over 40. I have used Thermax 1.5" (R9.8) and easily exceeded the minimum R19 using 2x4 studs, filled with 2# foam total construct a conservative R-28. Unless you live in the Arctic Circle it is hard to justify a better wall.

I am creating a passive solar home in the most northern part of Maine. 8,300 HDD, Zone 7. Not uncommon to have -30 to -40 F on several January nights. R-40 is too low for what I need. Considering that, is my wall assembly correct?

Posted By BadgerBoilerMN on 15 Dec 2012 10:39 AM
2# foam at the low estimate of 6.5 x 5.5 gets me to R35.75 any insulated sheathing will push us over 40. I have used Thermax 1.5" (R9.8) and easily exceeded the minimum R19 using 2x4 studs, filled with 2# foam total construct a conservative R-28. Unless you live in the Arctic Circle it is hard to justify a better wall.

Not likely.  You won't get a whole wall R value anywhere near those numbers with those wall stack ups and just R10 exterior sheathing.    The only way you can arrive at those numbers is to ignore the not insignificant thermal bridging of the wall assembly by the wood studs, plates, and headers.

Also, it is somewhat rare to completely fill a 3.5" or 5.5" stud cavity with 2# closed cell foam because it is difficult to trim the uneven surface of the foam back to the face of the studs for drywall installation.  Most foam sprayers will leave it at least 1" shy of the stud face to avoid having to trim that hard foam.  0.5# open cell foam is a different story and easily trimmed flush with the studs.

Using your 2x4 wall as an example and assuming a typical 25% wall framing fraction for 2x4 @ 16" oc and R1.4/inch for the wood and assuming a full cavity fill of foam for this example:

Cavity:  3.5" x R6.5 = R22.75.  U value = 1/R22.75 = 0.044.   Cavity fraction of U value = 75% x 0.044 = 0.033

Framing:  3.5" x R1.4 = R4.9.  U value = 1/R4.9 = 0.204.  Framing fraction of U value = 25% x 0.204 = 0.051

Whole Wall U value = Cavity U value + Framing U value = 0.033 + 0.051 = 0.084

Whole Wall R value = 1 / Wall U value = 1 / 0.084 = R11.9

That is not much better than ~R10 Whole Wall R value you get doing the calculation for fiberglass, cellulose, or 0.5# open cell foam to fill the cavity. 

The above R11.9 assumes you completed filled the cavity with 2# foam which is unusual.  If only filled to ~2.5", then there is essentially no R value benefit to using 2# foam over the other lower cost options. 

Adding R9.8 sheathing to this example brings the Whole Wall R value up to R21.7 which is quite a bit shy of a conservative R28.

Posted By Myrtleboone on 14 Dec 2012 10:44 PM
I am trying to create an R-50+ wall in the most cost efficient way. Here's my idea starting from the outside-wood siding, strapping, 2" R-board (R-12) with taped seams, Zip wall sheathing (overlapping R-board joints) with taped seams, double 2x4" staggered wall (5" space between) with blown-in cellulose (R-42), not sure about a vapor barrier here, 5/8" sheetrock. Any thoughts about the order of material, potential moisture problems and/or cost? Thanks.

Why bother with the extra labor and expense of Zip sheathing if you are going to cover it with R-board with taped seams?  Why not just use standard 7/16" OSB sheathing at ~1/2 the cost of Zip panels plus tape?

Assuming ~R3.7 for the blown cellulose and accounting for the lower R value of the wood framing - even though it doesn't connect the exterior to the interior in the staggered arrangement, you still have R1.4 studs and plates displacing R3.7 insulation.  In this case, the outer 3.5" stud wall will be ~R10 and the inner stud wall will be approximately the same, maybe a little higher if considered non-structural and you go with 2x4 studs 2' on center and only a single top plate and 2x4 on edge headers over door and window openings - assume R11.  Total R value will be as follows starting from the outside going to the inside and assigning ~R0.5 for sheathing and drywall.

R12 + R0.5 + R10 + R18.5 + R11 +R0.5 = R52.5

That is a pretty thick wall.  Assuming 1/2" for siding and 3/4" for strapping, you are looking at an overall wall thickness of ~16.5"

Siding, furring, taped building wrap, OSB, double stud walls with cellulose, taped MemBrain, drywall is better/cheaper. You might even be able to skip the furring and OSB. Adjust the cellulose thickness to get any R value you want.

Thank you Arkie; I stand corrected. Passivhaus in Maine...now that is a little different. I like your plan, but there would still be Thermax under my sheathing and the vapor barrier secured.

do not include a vapor barrier. the exterior foam is a vapor barrier, so in order for the wall to work, it has to be able to dry to the interior. And you do need the ZIP wall or some other method to air seal the wall at the sheathing plane. Do not eliminate the furing or the sheathing.

"Why bother with the extra labor and expense of Zip sheathing if you are going to cover it with R-board with taped seams?  Why not just use standard 7/16" OSB sheathing at ~1/2 the cost of Zip panels plus tape?" < Okay, I was not sure about substituting Zip panels with OSB, because of losing the water barrier characteristics from the Zip panels. Price certainly favors OSB. My question that I still have is regarding vapor, air and water barriers. By using OSB instead of Zip, do I need to add a water barrier? Is my air barrier achieved simply by taping the R-board layer well? Do I need a vapor barrier between the inside stud and drywall? I still have a hard time wrapping my head around what I need for these layers.

By using OSB instead of Zip, do I need to add a water barrier?
your siding and and flashing are your main water barriers; the furing is a secondary water barrier, not the sheathing (if water makes it that far, you're already in trouble)
Is my air barrier achieved simply by taping the R-board layer well?
No. You need a dedicated air barrier all around the house at the sheathing plane
Do I need a vapor barrier between the inside stud and drywall? NO (unless you're growing mushrooms, but there are better places to do that)

Is my air barrier achieved simply by taping the R-board layer well?

No. You need a dedicated air barrier all around the house at the sheathing plane

So with well-taped ZIP panels, I have a good air barrier at the sheathing plane.

Probably a stupid question, but I am assuming that the sheetrock will need to be installed (taped?) before the wall cavity is filled with cellulose? Typically with this type of wall assembly, where is the access point for the cellulose to be blown in?

Wet spray or stabilized cellulose is blown onto the wall before it is closed up. Not sure at what thickness it can't be done.

You need a dedicated air barrier all around the house at the sheathing plane

What is the point of having a vapor barrier (insulating foam boards) outside of the ZIP sheathing air/water barrier? Aside from providing more R-value, what are the insulating boards doing for vapor movement?

In Austria I saw homes and multi story home under construction, R57 or the equivalent was the claim.
Interior finish Cement over clay blocks. Blocks were about 12" thick with vertical 1" air pockets.
The exterior side of the block gad 6" EPS blocks fastened to the clay blocks. It appeared they were going to do a stucco type finish.
Another interesting thing was w after hours walk through a lumber yard where I saw sheets of 3/4 OSB approximately 8' x 20' Forget about carring it with no wind Ha Ha.

The fit and finish of Austrian houses is awesome. You build differently when you can picture your grandkids living in the house.

the exterior foam warms the rest of the wall assembly, decreasing the chances of condensation in the wall cavity.

As I said I walked around town and the hillsides during my time off. I was there taking a class to be certified in Geothermal. I went in hardware stores,lumber yards and construction sites both in my spare time and as part of the class. Talk about detail, the options for doors, windows and fire resistant security shutters I never knew existed. Or that thermal mass building is not alternative but main stream. Lastly I later found out the town was famous for a narrow shingle that was two inches wide with about two inches showing. A round bottom that comes to a point. As I was walking to the town center with the only other American in the class I mentioned how amazing the shingling was. The other guy was just as sure the shingles came in sheets. I knew without pulling a shingle I knew I was looking at old workmanship. My Dutch Irish Great grandfather and grandfather built the house my mother was born and raised in. We use to build more homes built to last we still do, some of us. Sadly it is not the norm to build to last. Home building took the same path US cars took to last past the warranty, just barley. Auto industry turned itself around so can the building industry. I suspect most in this forum build to last. ICF and SIP both if properly weatherized and clad will be here for our grandchildren's children as will SCIP that is already clad in Cement. Concrete buildings put up by the Romans are still standing and in use.