If you aren't going to have radiant in-floor heating and you have slab on grade construction and passive solar design.... ...how much insulation under the floor and around the perimeter (assume polished concrete floor so no padding or other insulation on the surface) for Zone 4C? Is there a point at which the insulation is sufficient that the floor should be at the same temp as internal room temp even without in-floor heating? TIA.

Is there a point at which the insulation is sufficient that the floor should be at the same temp as internal room temp even without in-floor heating?

Ignoring the solar gain - no, you can only get close. What is it worth to have the floor slightly above room temp vs slightly below - that's a good question.

And I've read that you need MORE sub-slab insulation if you use a radiant infloor system, which I guess makes sense, but just adds to the cost of that.

I suppose I could handle a floor that was slightly below room temp in terms of touch (some throw rugs and good wool slippers would likely suffice), but wonder how comfortable that is on a body (thinking about how heat radiating from below is more comfortable that heated air blowing around....so is cold radiating from below less comfortable?).

About to start my slab on passive build, skipping underslab, but using 2" 3lb foam under footings or edges, 2" wing insulation 4' all around.

If you look at thermal imaging this makes a lot of sense.

search "wing insulation"

A quick Google suggests that wing insulation has to do with frost-protective measures in cold climates. That isn't a concern where I live. Nevertheless, is it just horizontal sub-slab insulation around the perimeter? I'll have to check local codes, etc to see what the usual practice is here. I'm striving for "better than code", however, and thinking that an extra 2" of foam couldn't hurt, right?

Not really, it is not just frost areas

your choice

Been doing more reading. Found a few tables that suggest in my zone (4C) one should use 2" of insulation underneath the slab (to get to R-10). And you can use Roxul products instead of foam (maybe not under the footings, but definitely under the slab). I will have to price this, but I figure if it doesn't cost too much it sure can't hurt. If it is costly, I'll have to research further...

Would love to hear from any body in my climate zone who has slab on grade....