embed pex in slab or pourover?

baltik

In my new addition I have 8" of structural floor depth to play with. The engineer specd 6" slab leaving me 2".
So essentially i can do:
2" insulation then 6" slab with embedded pex
OR
No insulation, 6" slab, thermal break then 2" gypcrete.

I am leaning towards option 2 since the area I live in is cool but mild (never below freezing) and I like the idea of having a responsive system where I can turn down a few degrees at night. However gas is also very expensive where I live, so my primary concern is overall efficiency. What's the better setup for my use case and are there other factors I'm forgetting?

Hot_water_fan

That thermal break is supposed to stop all heat loss? How?

Option 1 makes more sense. If you want responsive, neither fit the bill.

hot_rod

Add 2" fill, compact, 2" insulation and 4" slab, tube suspended 2" below top.

Really no need for 6" unless you plan on working on heavy equipment in there

Spray foam is becoming the norm around here for underslab, gets you a perfect vapor barrier from Radon concerns, etc.

baltik

I need the 6” for structural reinforcement of existing foundation….

hot_rod

somehow, somewhere you want a 2” minimum insulation layer. Anything less will just increase the downward heat loss and operating costs over the life of the system. That 6” slab will be a large, conductive heat sink. At some point as that mass warms it would be hard to control over heating potential.

Any or the radiant simulation programs would show the loss and related costs associated with insulation thickness options.