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!

Liebler-A couple more questions for you. How far away horizontally do you run the foam from the foundation perimeter?

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.

I'm a very visual person; does the inner vertical foam board travel along the inner footing wall all the way to the building surface, thus separating the footing from the slab? Anyway that you could sketch it for me and scan as a jpeg? That way I can describe it better to my foundation guy. It's assumed the wall is what you had described a couple days ago. (double 2x4 wall with 5 1/2" cavity). Thanks again.

Myrtleboone,

"I'm a very visual person; does the inner vertical foam board travel along the inner footing wall all the way to the building surface, thus separating the footing from the slab? Anyway that you could sketch it for me and scan as a jpeg? That way I can describe it better to my foundation guy. It's assumed the wall is what you had described a couple days ago. (double 2x4 wall with 5 1/2" cavity). Thanks again."

The simple answer to you question is NO!
In explaining I'm going to assume you'll adopt the "flat topped" foundation wall.
The vertical foam is on the insides of the 'foundation walls" and extends ,vertically, from the top of footings under the foundation walls to the top of the foundation walls. The sub slab foam sits on the vertical foam as well as the foundation wall. The foundation walls are thick, let's say 18". Once the foundation walls and the vertical foam is in place the area inside the foam is prepared for the slab including,fill, drainage material and placing the sub slab foam which extends OVER the foundation wall. At least the foam over the foundation wall needs to be 100 PSI compressive strength. Temporary forms for the edges of the slab are constructed on top of the foundation walls, these forms sit on say 1/2" of  the sub slab foam and are positioned so that the edge of the slab will be under the outer face of the inner stud wall's sheathing. Once the slab is cured the outer stud walls (taller) are framed (with pressure treated bottom plates) on the slab,temporary braces installed on their outer face and tilted into place on the foundation wall over the crush strips, temporarily braced and secured to the foundation wall, all without sheathing. Once the outer walls are up, the inner walls are framed, sheathed,treated with liquid vapor barrier, then tilted up sitting on the poly that is wrapped around the edge of the slab and a generous bead of acoustic sealant, the bottom plates are secured to slab, bracing installed etc. Since there is 5 1/2" of insulation between the stud walls and 3 1/2" in the studs of the outer wall and 3/4" of fiberboard and 1/4" of DC14 This puts about r36 around the edge of the slab for a lot less $ than using a 'stepped' foundation wall and 7" of vertical foam around the slab. Send an email to me at [email protected] & I'll send you scans of sketches

Just for grins I repeated the whole wall r value calculation with r36 around the 4" slab under a 9 foot r51 wall, the whole wall r is reduced to r 50.5.

".Liebler-A couple more questions for you. How far away horizontally do you run the foam from the foundation perimeter? "
There is NO foam outside the foundation walls!

Here is a less expensive alternative, foam under either bottom plate is optional. It's just a standard frost protected slab + slab edge insulation (which could be covered with stucco).

I would probably prefer the vertical outer foam to stop at the base of the outer wall. Thanks.

I do like this wall assembly attached here. It's based on Swedish building practices that creates a simple, efficient wall that requires less labor to erect versus a double stud wall and doesn't run into weight support issues. Granted, this particular assembly doesn't achieve my required r-value but could create 3.5" cavities on each side of the 2x8 stud (3.5" + 7.25" + 3.5" = r-60 not taking into account the u-value). The only thing I'm not sure of is how to maintain the space for rock wool insulation on each side of the cavity without placing a stud (ie. how/what do I attach the exterior cladding, WRB wrap and interior sheetrock without compressing those spaces)? If I chose to include an interior "wall" against the 2x8 main stud, I guess it could be termed a hybrid double wall, but with no cavity and structural support issues to contend with. The 2x8 and 2x4 studs could be staggered so as not to promote thermal conductance through the entire wall but their sill/top plates would be pushed against each other (proper insulating/sealing of sill plates obviously required).

http://www.lamidesign.com/blog/imgs/best_2x8s.jpg

"The 2x8 and 2x4 studs could be staggered so as not to promote thermal conductance through the entire wall but their sill/top plates would be pushed against each other (proper insulating/sealing of sill plates obviously required). "

In a "proper" 2 dimensional thermal analysis, staggering the studs, in a double wall, has NO effect at all.  Rigorous 3 dimensional analysis and testing show a benefit  (gain in actual r value over the aligned case, predicted by the 2 dimensional analysis) of approximately 1%.  Staggered studs simply have no, economically justifiable, benefit in thermal performance.

"that requires less labor to erect versus a double stud wall"

This is an assumption , that I believe is false, constructing the inside "service cavity" is as much work as building another wall!
In addition, compared to a simple double stud design,  such a wall uses more lumber and more expensive insulation materials to achieve equivalent whole wall r value.