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.
Best Answer
Ecnerwal is right that the if the footing is uninsulated, some of the heat will escape into the dirt under the house, where it will sort of be stored due to (I hope) your insulated slab perimeter walls keeping the heat in. However, unless that space is fully insulated on all sides (e.g. under the dirt on the same plane as the footer), there will be heat escaping into the surrounding soil. However, if this dirt is going to be used to store heat (which is fine), then you shouldn't insulate under the floor slab, either; if the dirt below the slab is effectively being brought into the mass envelope, and it doesn't make sense to thermally block off the finish flooring from it. In this case, you would want to insulate the slab perimeter, under the footings, and under the dirt that the slab sits on.
If, as your plan indicates, you're going to stick with insulating right under the rest of the slab, you should insulate under the masonry heater footing as well. Foam will be fine, assuming you can find someone willing to sign off on it--especially for a very heavy masonry heater that is probably unfamiliar enough to most building inspectors. An option that would be more conventionally-acceptable to these typically very conservative folks might be some sort of higher-strength insulation material, such as foamglas, AAC, or perlite concrete. Or even just bags of perlite, with the slab poured right on top of it. For an example of this, see https://perlite.org/library-perlite-info/insulation-perlite/Perlite-underslab-insulation.pdf