Here is a very rough cross section sketch of what is currently happening...

I'd look into extending the pink foam over to the form board - filling the cavity underneath it with concrete first. You might have to live with the rebar still forming a much less critical thermal bridge.

2 inch pink under as much as possible, and around perimeter before you back fill, Staple pex to foam, Done.
Dan

What jonr & Dan said.

An inch of pink isn't enough for under a radiant slab (it'll work, but it'll be lossy) but would be appropriate under a other non-heated slab in conditioned space in NoCal. If the rebar isn't already tied, slip in another layer of the 1". Once the concrete is poured it's not fixable.

BTW: The XPS itself is an adequate vapor barrier from a ground moisture transfer point of view, and with taped seams its pretty good even against radon & other gases.

You will find that many codes and experts specify poly. Go for < .01 perms.

And poly is cheap stuff...

Sub-slab insulation is specification is determined by heat load, which has to do with ground temperature, indoor design temperature, outdoor design temperature and design water temperature.

The one inch is adequate in many climes but perimeter insulation is the most critical and should not be omitted in any radiant floor heating application. A typical FPSF is insulated from the outer shallow foundation to the interior.

Thanks everyone -We will be doing perimeter insulation on the exterior of the foundation. Our under slab design requirements called for R4 so the 1" just covers that at R5.

A concern I do have is that we have designed some large areas of southwest facing concrete for a courtyard - it was designed to be a bit of a heat sink for passive solar heating - this wasn't extensively modeled or anything that I am aware of. We are only planning to run the insulation on the outer perimeter of the building - not on the inner courtyard area that will get lots of sun exposure - Based on our climate, and the number of sunny winter days we get, it looks like the heat gain would be good, and perimeter insulation would negate that in those areas...Does that sound about right?

Right now the perimeter insulation is really easy to handle on the back of the house, as walkways will be poured in those areas - we will just add the insulation, with a chamfered top, and pour the walkways. One side of the house will have dirt/decomposed granite backfilled to the foundation - so we are planning to have the foam extend above the finished grade and have the stucco weep screed above the top of the foam - we might need a bit of flashing...I will leave that to others to tell me what is best.

I"m not quite understanding the passive-solar courtyard configuration. Is the courtyard slab outside, with a slab continuous to the interior, or is it a glazed ventable sunroom or...??? Insulating the perimeter doesn't negate anything. Were you thinking the exterior slab would be conducting more heat into the building than it extracted at night or during cold weather?

On the S side the reflected solar gain off bright exterior concrete through the windows can be substantial if the windows are designed for solar gain. (The solar-mirror effect.) The re-radiated infra-red off the slab won't get through the windows, but it will raise the temp of the siding, lowering the conducted heat loss. Insulating under the outdoor slab would raise it's peak temp, for more IR radiation, but dialing the thermal mass to optimize it for seasonal gain isn't a back-of-napkin calc. (And there are probably too many variables for too many unknowns to hit it with any accuracy unless you do a lot of data-diggging and measuring.)

Posted By Dana1 on 14 Jun 2011 10:34 AM
I"m not quite understanding the passive-solar courtyard configuration. Is the courtyard slab outside, with a slab continuous to the interior, or is it a glazed ventable sunroom or...??? Insulating the perimeter doesn't negate anything. Were you thinking the exterior slab would be conducting more heat into the building than it extracted at night or during cold weather?

On the S side the reflected solar gain off bright exterior concrete through the windows can be substantial if the windows are designed for solar gain. (The solar-mirror effect.) The re-radiated infra-red off the slab won't get through the windows, but it will raise the temp of the siding, lowering the conducted heat loss. Insulating under the outdoor slab would raise it's peak temp, for more IR radiation, but dialing the thermal mass to optimize it for seasonal gain isn't a back-of-napkin calc. (And there are probably too many variables for too many unknowns to hit it with any accuracy unless you do a lot of data-diggging and measuring.)

The courtyard slab will be outside - it is not continuous to the interior as it will be a separate pour.  However, it will be poured directly up to the current interior slab & footings...So it could be insulated before hand if it was better to do so - The idea was to allow the mass of the exterior courtyard slab to help reduce conducted heat loss by it acting as a heat sink.  That is what the designer has told me anyway, and I don't know what his calculations were (if any).

Since our coldest days here are almost always sunny days...I am assuming (not a good thing) that having a large surface mass of concrete against a section of the house would be beneficial for storing some amount of heat.

Insulating the interior from the outdoor temperature swings will be more comfortable & reliable than any gains you got from being edge-butted against a massive unglazed solar collector (aka exterior slab.) Insulate the slab edge, don't make it conductive with the exterior slab.

If you had a vent-able sun room with an insulated slab for storing the heat at low temp you could manage the gains for the seasonal need for supplemental heat, but that's a whole 'nuther project.

Posted By Dana1 on 14 Jun 2011 02:35 PM
Insulating the interior from the outdoor temperature swings will be more comfortable & reliable than any gains you got from being edge-butted against a massive unglazed solar collector (aka exterior slab.) Insulate the slab edge, don't make it conductive with the exterior slab.

If you had a vent-able sun room with an insulated slab for storing the heat at low temp you could manage the gains for the seasonal need for supplemental heat, but that's a whole 'nuther project.

Thanks for the help!