The fireplace will be located in the middle of the concrete slab (away from the foundation stem walls) so it will have its own footing.
My impression is that footings should rest on soil & gravel and the slab should "float" on rigid foam. I've done a fair bit of homework on foundation and basement insulation in general, however when it comes to fireplaces and insulation the focus seems to be primarily on chimneys and doors (where I suppose it's too late to think about footings in most cases).
The options I've considered + diagrams of the footing cross-section:
- Uninsulated fireplace footing - thermal bridge right into the ground, but in the middle of the slab. How significant is that?
- Insulate the fireplace footing - do building codes generally allow that?
- Uninsulated footing with insulation between slab and footing.
RE: 40 PSI foam, 8000 lbs on a 3' x 6' footing works out to just over 3 PSI additonal load.
By the way this is in Zone 4 marine climate - gets cold but frost doesn't penetrate below the topsoil.