California - Concrete
We ended up with three slab-on-grade buildings: the main house, the guest house, and a utility shed which holds the well pump equipment & PV. Overall we went thru somewhere around 60 yards of concrete (about 24 yards for footings & stem walls), for about 2600SF of slab, including porches. It all has some flyash in it--around 20%.
Since this is a passive solar house, the floor slab is stained dark and left exposed. Between the two houses there is about 25 yards of concrete (that is slab minus footings: 38mBTU embodied energy). So what is the return in solar energy? Assuming only about 10 yards of the slab are actually exposed to the sun, that's ballpark 32k lbs at about .22 Btu/°F/lb and a guess of 5°F temperature change yields 35k Btu/day; which is just a bit lower than the predicted 50-60k Btu/day night time heat loss. Multiply this by a guesstimate of 120 days of sun during the heating season, and you get ballpark 5mBtu/year and a payback of 9 years. Needless to say, there is a sizeable margin of error in that number.
The two houses are fully insulated slabs done with short stem walls that sit on top of sizeable footings, while the pump house is a mono-pour with a thickened edge. The footings are typically 12"wx18"d, and the little stem wall is about 8-12" above grade and 5" wide. The insulation then runs under the slab and then up against the inside of the stem wall where the top of the insulation is hidden under the bottom plate of the wall. Its not the cheapest way to insulate a slab, but its the best at protecting the insulation from decay.
The underlayment for the slab will look strange to anyone outside of a hot-dry climate (see photo below). As usual, it starts with aggregate (in this case sand rather than coarse gravel) and a heavy vapor barrier on top of that (neither of which shows in the photo), and then the insulation on top of that. Rather than pouring directly on the insulation, a couple of inches of sand are put on top of the insulation, and that sand is thoroughly soaked just before the pout. While this would cause terrible moisture problems in much of the country in our dry climate it prevents the slab from curling and excessive cracking due to drying.1
We used 3/4" rigid polystyrene insulation (about R3). The deep ground temperature in central California is around 62°F, but since our site is generally colder than town, our ground temperature is likely colder also--maybe as low as 57F, which compared to most of the country is still quite warm. Given this mild ground temperature, R3 is probably fine at center slab. The perimeter, however, faces much greater temperature extremes--the average daily temperature in the winter is in the 45-48F range, so we should have put at least R10, and maybe even R20 along the edge-possibly only for the 5" vertical piece. The bottom plate is a 2x10, so with a 5" wide stem wall, the plate can still cover 4" of insulation.
In the below right photo, we're shoveling dirt on the concrete as its pumped into the footing. The dry dirt stops the concrete from blowing out the side. In spite of this, we still used somewhat more concrete than was called for because the soil was such that it was impossible to dig a rectangular 12"x18" ditch. Getting 18" deep wasn't a problem, but the grain of the sandstone was at a sharp angle to the ditch, so the ditch ended up very ragged, and often much bigger than the required 12" wide, which means it used more concrete. Obviously its more cost effective to use a few more yards of concrete than it is to form the footing., so of course this is what everyone does. It seems if there is opportunity to design a better system there.
The pump house slab is a great contrast to the house slab, both in simplicity and use of materials. The slab is 10x11 (ie under the 120SF size where a permit is required), and the shed itself will only be 7x10: the rest of it is a porch. There are no footings at all--just a thickened edge, and even then the edge isn't thickened that much. In addition to this not being engineered, it also isn't insulated, which greatly simplifies forming. Total concrete use was around a 1-1/4 yards.
We ended up putting #4 rebar on a 2x2 grid (just like the house), but only a single piece along the edge, and somewhat smaller hold-downs. The main difference is that the slab is thinner there is no huge footing, yet the building's load isn't that different from the guest house.
Notes
1: Not having experience, I can't really verify this, but talking to multiple concrete people, this is the standard in California. Problems only arise if you cover the slab in some impervious material, like vinyl tile of sheet vinyl before it's fully dry, since that traps the water between the vapor barrier below and the vinyl on top.