Posted By Bob I on 16 Dec 2012 11:34 PM
How about build the structural house on the inner half of the foundation wall - supporting walls, floors, etc, then add the 9' or 10' exterior partition. you'd have very little thermal bridging anywhere.

You mean like this:

Bob,
        "liebler - i like some of your wall ideas. how about build the structural house on the inner half of the foundation wall - supporting walls, floors, etc, then add the 9' or 10' exterior partition. you'd have very little thermal bridging anywhere. the the question becomes do you need the foundation under it? especially if you insulate the exterior of the foundation with foamglass- inorganic & impervious to almost everything."

The reason I suggest a double wall foundation is for construction ease.  The construction sequence I envision allows, at least. the outer stud wall's framing to be constructed on the sub floor, then after the outer wall frame is erected the inner structural wall can be framed and sheathed on the sub floor then tilted into place.  I'm not familiar with foamglass, perhaps it could support the exterior stud wall.

My house will be built over a poured concrete basement which will serve as the outer foundation walls.  Inner stud walls, in the basement, set on the slab, will support the floor and really the whole house.   To account for wood shrinkage over time I'll have a "crush strip" of EPS foam between the top of the concrete and the bottom plate of the outer 'main level' wall's bottom plate. 

The concept of thermal bridging "bugs" me!  all it means is areas of higher thermal conductivity.   

Here is my suggestion for an r50 wall From the outside: siding (can even be stuco or adhered masonry) , 1/4" insulating rain screen/drainage plane ( Greenguard DC14)  (r1) , WRB (housewrap), 3/4" fiberboard (r2), 2x4 wall framing 24 OC cavities filled with rock-wool bats (r15), 5 1/2" space between double stud walls filled with rock-wool bats (r23), iinner (structural) stud wall 2x4 24" OC sheathed on outer side with 1/2" plywood (r0.5) detailed as primary air barrier with liquid vapor barrier material, cavity s filled with rock-wool (r15)

I checked several distributors of 3/4" fiberboard and it's a bit pricey @ ~$34 for a 49"x97". Could I substitute rock-wool with blown-in cellulose? What's the cost:benefit between the two different insulation types? What's a liquid vapor barrier and is it placed between plywood and stud? Why use plywood?

could i substitute the plywood sheathing with a rigid polyiso foam board such as Atlas R-board, 5/8"=r3.8, to ramp-up the r-value? (<1.0 perm rating)

BLDGTYP build and blogged about double stud airtigth r-50 wall like one being discussed here. Including the thermal bridge free detailing at the foundation (and also at the windows) they build this cabin this summer in Wi. their blog has great details and photos
http://bldgtypblog.blogspot.com/

"I checked several distributors of 3/4" fiberboard and it's a bit pricey @ ~$34 for a 49"x97". Could I substitute rock-wool with blown-in cellulose? What's the cost:benefit between the two different insulation types? What's a liquid vapor barrier and is it placed between plywood and stud? Why use plywood?"

 First you are looking at the wrong kind of fiberboard if it's that expensive!  Here is what I mean but I only found the 1/2" version in my quick search.  ww.lowes.com/pd_15380-44903-NA_0__?productId=3138497&Ntt=sheathing&pl=1¤tURL=%3FNtt%3Dsheathing&facetInfo=
The 3/4" should be under $10/sheet.

Of course you could substitute blown cellulose BUT!  It is difficult to find contractors who will blow cellulose to the proper density, it may bow the exterior sheathing, it has a lower r value/inch,  it will probably cost more, it isn't practical to DIY.
it may require fire stopping where the rock wool is a fire stop, it would require cutting holes & patching in sheathing ( it could be installed from either side) The raw material cost for dense pack cellulose (based on the best high volume pricing I could find on the net and manufacturer's "fill tables") and Roxul bats (purchased from Lowes) is almost exactly the same.   I strongly prefer the Roxul for it's inherent vermin and fire proof nature.
 
Liquid applied vapor barrier is a product like StoGuard with VaporSeal, there are several competitors .  It can be applied on either side of the plywood, my preference is the outer face before the wall is erected.  This will be the primary air barrier and attention needs to be paid to completing the barrier around corners and openings etc..

Why use Plywood?  Honestly, it's my personal prejudice, I suppose  OSB would work but I've seen lots of moldy OSB but no mouldy plywood so I think the small cost difference is worth the "sleep".

"could i substitute the plywood sheathing with a rigid polyiso foam board such as Atlas R-board, 5/8"=r3.8, to ramp-up the r-value? (<1.0 perm rating)"

NO!  The plywood is the 'bracing" and is structurally VITAL!  You could possibly substitute OSB.  At a true whole wall r of 50+ you are already where well over 1/2 of your wall heat loss will be through windows & doors, even with the VERY best and not very many!   Unless you are building a passive house inside the arctic circle and planning thermal shutters and other heroics for the windows an doors you have spent all you need on the walls.  But if you really want higher r the most cost effective change would be to change the space between stud walls from 5 1/2" to 7 1/4" which would bump the whole wall to 57+.

Liebler-I appreciate all your help. A couple other questions if you don't mind- 1. when it comes to the plywood air barrier, what type of tape do you prefer for air sealing? I've been looking at SIGA and looks like a nice product but also great instruction. ------http://www.siga.ch/en/product-overview/rissan-100-150-detail.html------ 2. I'll be on a slab. Between the two stud walls, I should place a strip of foam board the thickness of the cavity, butted up tight against the two sill plates? Additionally, prior to doing that, SIGA makes a nice product that seals the wall base to the foundation base (not sure if this would be done on the interior side of the inner stud wall, or the outer side of that wall). Is that something that should be done on the outer sill plate as well or does the WRB layer do that? 3. One thing I have difficulty understanding is how to adhere the WRB to the sheathing without punching thousands of staples holes through it. Again, SIGA makes a nice double sided adhesive for this application------http://www.siga.ch/en/product-overview/primur-roll-detail.html)------- 4. Along the same lines as above, nailing the outer layer you suggestion (just below the cladding) you run into the same problem, but SIGA again makes a nail sealing tape-----http://www.siga.ch/en/product-overview/nail-sealing-tape-detail.html-------- 5. Lastly, I obviously want an airtight house confirmed by a blower door. By paying close attention air sealing of the inner stud wall during construction, can this realistically be achieved? Thanks again.

since you are building on a slab, you'll need these foundation gaskets: http://conservationtechnology.com/building_gaskets.html
Siga has a liquid you can paint on concrete which helps the adhesion to concrete.
I've given up on fabric WRB (due to the staples & how hard it is to seal the top & bottom) & use ZIP & will look into the spray on liquids now available

Myrtleboone,
      You said :  "2. I'll be on a slab. Between the two stud walls. "
The concrete slab is a massive heat conductor, AKA thermal bridge.  You're wasting your money on a high r wall!!!  If you are planning slab on grade, you'll need insulation under the slab AND around it's perimeter AND a second footing outside the vertical insulation to support the outer wall. The vertical insulation around the slab needs to extend down below "frost depth" or a 2 foot minimum and should be above r30 (6" of XPS foam). Under the slab at least r30 also. Under the slab you could save substantial $ by using Perlite.
For air sealing the plywood  I'd follow the procedure of the vendor of the liquid applied vapor barrier, they have approved meshes or tapes that are placed between layers of the liquid.  A bead of acoustic sealant between the slab and the inner plate should seal that joint.  

There are several important things the DC14 does.  among them is it is a redundant WRB.  With 2 effective WRBs there is absolutely NO need to worry about the staples, you're not after an air seal on the outside, the only need is to keep most of the liquid water out.  With the DC14 serving as a drainage plane, on both it's inner and outer sides and preventing vapor that evaporates from the cladding from migrating into the rest of the wall.  I'm obviously impressed by this product and think it deserves consideration under any cladding but especially under stuco or thin masonry which is what I'm using. 
 

Dual air barriers, one on each side of the wall is always better than a single air barrier.

I would use a frost protected slab without footings.

You can put foam over the entire slab to avoid a concrete thermal bridge between the two sides of the double wall.

"Dual air barriers, one on each side of the wall is always better than a single air barrier. "
True but the inner one in this type of construction is where serious detailing is needed.  the outer WRB system is a pretty good air barrier & with out much attention it'll essentially eliminate 'wind washing' of the insulation.

"I would use a frost protected slab without footings."
That is EXACTLY what I've described for the slab itself!  STILL!  Slab Perimeter insulation is needed! AND a system of THERMALLY ISOLATED support needs to be under the outer wall, this is best & most simply accomplished by a 'conventional' footing extending below frost depth.

"You can put foam over the entire slab to avoid a concrete thermal bridge between the two sides of the double wall."
NO!   Unfortunately, foam is NOT STRONG ENOUGH to support the concentrated loads transmitted through the wall's bottom plates.
The slab is still a MAJOR HEAT CONDUCTOR to the sides or edges!  The only way to control the heat flow out the slab edges is vertical insulation!

To give a bit of a feel for the importance of slab perimeter insulation I computed the whole wall r of a 9 foot r50 wall sitting on a 4" slab with no perimeter insulation.   r 18 is the result!

If you are planning slab on grade, you'll need insulation under the slab AND around it's perimeter AND a second footing outside the vertical insulation to support the outer wall. The vertical insulation around the slab needs to extend down below "frost depth" or a 2 foot minimum and should be above r30 (6" of XPS foam). Under the slab at least r30 also. Under the slab you could save substantial $ by using Perlite.

Yes, I do plan on super insulating my slab. No numbers crunched but my "hunch" is R-35 that extends beyond the slab about 24" (not sure exactly how far beyond I should go???). What's your opinion on a continuous plastic vapor barrier underneath the concrete but above the EPS that extends up into the inside stud wall and sealed to it.------http://www.gologichomes.com/blog/2009/09/foundation-vapor-barrier-installed/------Do I need footings if I use ICF grade beams (see same link)? I envision laying the foam blocks (Type II) down beyond the perimeter, setting up my ICFs and creating the perimeter with the slab within it. Still not strong enough? Here's a an illustration of what I think you're describing but including perimeter AND vetical insulation below frost line----- http://www.buildingscience.com/documents/insights/bsi-059-slab-happy/images/bsi059_figure_01_web.jpg-----

foam is NOT STRONG ENOUGH to support the concentrated loads transmitted through the wall's bottom plates

A heavy wood framed house with maximum snow load, 5.5" bottom plates and no interior supports = 15 psi
STYROFOAM™ HIGHLOAD 100 = 100 psi

Two independent foundations for a single wall - an expensive way to get nasty settlement problems.

"A heavy wood framed house with maximum snow load, 5.5" bottom plates and no interior supports = 15 psi
STYROFOAM™ HIGHLOAD 100 = 100 psi

Two independent foundations for a single wall - an expensive way to get nasty settlement problems."

The styro foam has creep (long term deformation) issues and is not recommended to sustain stress over 20 PSI, some say 33 PSI The load is concentrated at the studs, not spread, all that much by the bottom plate. I also disagree with your number, consider a 36 foot wide house with 70 #/sq ft snow load (northern lower peninsula Michigan code) and 2x6 framing just the snow load is 19 PSI & that's ignoring all structure weight and assuming complete spreading by the sheathing, both not at all realistic.

The problems of differential settlement must be 'designed around'. One way, as I've described above is to make the inner wall the primary load bearing member and place a 'crush strip' of foam under the outer wall.
Even if there was an appropriately strong foam, placing it on top of a continuous single slab leaves the problem of conduction through the slab.  Concrete is a really bad insulator!
The simple fact is: Thermal conduction in the foundation can dramatically add to heat loss & it makes absolutely NO sense to ignore that reality and pretend the wall r is what it's not.

Liebler-have you seen the product, Pro Clima Intello Plus vapor barrier that changes its perm rating in summer and winter, more permeable in summer to allow for drying and less permeable in the winter. Could my vapor/air barrier be placed on the inner side of the inner stud wall, with furring strips and drywall beyond that, rather than your suggestion of liquid vapor barrier on plywood/OSB?

"Liebler-have you seen the product, Pro Clima Intello Plus vapor barrier that changes its perm rating in summer and winter, more permeable in summer to allow for drying and less permeable in the winter. Could my vapor/air barrier be placed on the inner side of the inner stud wall, with furring strips and drywall beyond that, rather than your suggestion of liquid vapor barrier on plywood/OSB?"

Yes, I'm familiar with "smart vapor retarders" !  They do not work as you describe but their permeability changes with relative humidity being more permeable under higher humidity.  The fact is that both plywood and OSB BY THEMSELVES have similar characteristics with greater permeability in high humidity.   At least 1 layer of plywood or OSB is required in any wall for structural bracing.  We also need an air barrier so sealing up the plywood adds minimal cost.  I choose to make the sealing also a vapor barrier but it need not be.  You could use use, for instance, zip sheathing and tape with acceptable results.You would have a bit more, but probably acceptable,  water vapor diffusion into the outer insulation during winter high indoor humidity intervals.  Using the liquid vapor barrier material is  possibly 'overkill" but adds negligible expense for the largest 'safety" margin.  One of the advantages of locating the vapor control and air barrier on the outer face of the inner wall is that it is far less likely to be compromised by the occupants doing normal things like hanging pictures.

Myrtleboone,
http://www.buildingscience.com/documents/insights/bsi-059-slab-happy/images/bsi059_figure_01_web.jpg

Is very close, just add an inner stud wall spaced in from the one shown, on the slab inside the vertical insulation around the slab note the slab will be carrying the wall loads and may need to be strengthened somewhat.  Also What is called rigid insulation is the fiberboard sheathing.

"What's your opinion on a continuous plastic vapor barrier underneath the concrete but above the EPS that extends up into the inside stud wall and sealed to"

That's good but I'd add be sure to seal around all the penetrations (plumbing etc.)

Is very close, just add an inner stud wall spaced in from the one shown, on the slab inside the vertical insulation around the slab note the slab will be carrying the wall loads and may need to be strengthened somewhat.

So, one footing correct? Do you know how thick a footing typically is? If I have (2) 2x4 walls + 5 1/2" cavity (12 1/2"), is this how thick the footing should be? Thanks again for your insight.

liebler-should be rigid insulation also be placed on the outside of the footing?

Myrtleboone,
        The footing width really should be engineered for the actual house being supported and the soil it sits on  I'd guess between 18" and 2 foot. OTH the foundation wall needs to wide enough to transmit the building load through the foam that is between the foundation wall and the slab with out over stressing the foam. Using the highest strength foam (100PSI ultimate compressive strength) 20 PSI can be tolerated long term. This foam is what is holding up the slab and all the rest of the house.  How wide the foundation wall needs to be depends on how much vertical foam(or other insulation) is around the slab and how wide the concrete needs to be under the outer stud wall.  My guess is an 18" foundation wall will work with 6" perimeter foam and 5" concrete to support the outer wall ( this would give  a 7" strip of foam supporting the slab edge) .   I'd also add a 1"or 2"  layer of foam (ordinary low compressive strength) under the outer wall's sill plate as a "crush" gasket in the event the foam between the footing and slab does take a 'set".  

There is no need for vertical foam on the outside of the footing or foundation wall,  Vertical foam is needed  on the inside to "frost protect" the slab. 

You might consider using flat topped foundation walls,. Using, let's say, 18" foundation walls with the sub slab foam on the inner 1/3 or so and taller outer stud walls (taller by the thickness of the slab and the sub slab insulation).  You would need to build temporary forms or the slab's edges.  This would allow more effective insulation around the perimeter of the slab because it's a 2x4 wall instead of concrete.  You also could eliminate the foam around the slab & just make it the same as the rest of the wall insulation, making the outer edge of the slab line up with the outer edge of the inner walls. Just be sure you keep the wood far enough above the finished grade.  You are trading a simpler concrete foundation wall for more complex framing.  If you choose the flat topped foundation wall, I'd still recommend the crush gasket  under the outer stud wall.