What's the right approach for insulating the slab & footings underneath a masonry fireplace/heater?

Question

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:

1. Uninsulated fireplace footing - thermal bridge right into the ground, but in the middle of the slab. How significant is that?
2. Insulate the fireplace footing - do building codes generally allow that?
3. 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.

Answer 1

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

Answer 2

If this will be used as a serious heating appliance, (I'm inferring from "masonry heater") not merely decoration (as many fireplaces are), I'd go with uninsulated footing, especially if the stemwall is insulated - whatever thermal bridging takes place will be to the "bubble" of dirt inside the stemwall, which can play into your thermal mass.

http://www.greenbuildingadvisor.com/blogs/dept/musings/foam-under-footings suggests both that it should be entirely feasible to place foam under, and that it may require negotiations with the LAHJ for code/inspection purposes.

Answer 3

While foam insulation is capable of taking a large pressure, it also has the tendency to compact a bit over time. This would cause some settling of the slab and/or footing above it. Now, settling is not in itself necessarily a problem - but differential settling would be:

when rigid foam supports a load, it can suffer from “creep” or deflection. Over 50 years, the foam can shrink by 10%... The real problem isn’t settling, it is differential settlement...

Therefore I believe you must insulate under both the slab and footing, or under neither.

That said, you have another option, which is to not use a footing at all. The function of a footing in this case, as I understand it, is not actually to transfer load to the ground - if you put the heater right on the slab itself, the same thing would happen (as opposed to, say, putting it on a wooden framed floor). Instead, the function of the footer is to prevent the slab from cracking under the weight.

Now, an 8000 lb weight over a 3x6 area is 444 psf. I believe this is well within the load range of what a normal 4" slab can handle. for comparison, a 2000 lb automobile sits on a total tire contact area of perhaps 1 sqft. That's 2000 psf, obviously way more pressure (and also more concentrated) but no one puts footings under their garage floors for that.

(You might elect to add some extra rebar under that area and extending outwards from it, since that is cheap and easy to do. In fact even pouring the slab 2" thicker uniformly would not cost that much more and could give added peace of mind.)

Hope that helps. (We're planning a masonry heater too.)