Greetings,

Soon we'll be pouring the slab for a house, about 1200 sqft.  The location is in Southern Oklahoma and as per the data I found we'll put down a 6mil barrier then 1" of "250" foam, renforcement wire, and 1/2 PEX tied to it.   The question comes about the fact the slab is considered being done as a monolithic slab, thus thicker at the edges about (12 inches) then tapered to 4 inches for the most of the floor.  Thus, foam will run up to the edge but, edge will be bare.  My thinking is after the pour, I'd come back in with 2 inch 150 foam and do the edge, then cover with either cement board or something similar.  For the most part this site is basically sold rock, but there is one corner that had some fill.  There they will  auger a 12 inch hole down to rock  (about 12-16 inches deep) and pour it full. Thus those spots won't have foam I don't think. 

Does this sound reasonable? Is this pratical vs. pouring a foundion, then a (very) short stem wall, then insluating inside that and pouring a slab? Needless to say the later is a lot more costly given the 2 extra steps.

Just looking for confirmation I'm on the right path or ideas to correct course before its cut in "stone".[script removed]ajccmc

The best way is the stem wall, you may save some $ as the thickened edge is a large amount of concrete that now can be replaced with compacted fill.
You then will make a cup of foam and pour your concrete in to the cup.
If you are on bed rock you may not need wire , in that case you can staple to the foam.
perimeter foundations have advantages. You can come back after the roof is up and pour in the dry area, a considerable amount less lost heat with out the thickened edge.
I am clearly bias.
But the o'l thickened edge is 1 stop shoppin. should you go this way you will be well served with exterior perimeter foam and your cement board as mentioned.
Good Days,
Dan

If your beams are sitting on bedrock, I'm not sure that exterior foam is going to be much help. You might need to rethink the slab or radiant. ICF makes a great stem wall for radiant. When you pour a slab inside it and over foam, it is indeed sitting in a foam cup as Dan says. But I'm guessing your foundation costs will considerably higher.

Thanks for the replies. I did a quick estimate using ICF 1 high to create the stem wall , about $850. If buy 2x12 is only $350 plus foam 160 of foam to line it... and I could re-purpose the 2Xs afterwards. I'm at stand still at the moment with the concrete guy... he is busy... so I may sit my own stem wall forms and bring him in later to fiinish the floor. Given this sits on bedrock, do you think I need to pour a footer under the stem walls if I make them 8-10 inches wide?

No footers necessary if you are building on bedrock. For the corner that doesn't reach, dig a step (steps) so the icf sits on bedrock continuously.Cutting the foam to fit isn't difficult. Needless to say, check with your building inspector first. Two cautions: typically you'd need a pumper truck (extra expense) to fill the icf although one course could be done with wheelbarrows/skid steer. ICF tends to lift at corners as you are filling it. This effect is stronger on short walls so don't let anyone tell you it's only one course.

Posted By toddm on 22 Nov 2012 08:58 PM
  ICF tends to lift at corners as you are filling it. This effect is stronger on short walls so don't let anyone tell you it's only one course.

Good ICF blocks have locking tabs that lock each ICF. How would the corners lift in that case?