As part of the desire to minimize the overall energy usage of the house we developed a passive-solar design that situated the house with a southern orientation, maximized south-facing glazing and minimized northern, and incorporated a thermal mass for storing thermal energy gained from the sun over the course of the day. The most effective way to do this (and to avoid the potential problems of a wet basement) would be to build a slab-on-grade. Since we intended to use the floor slab as a thermal mass for storing heat gained from the sun it also made sense to incorporate radiant in-floor heat and use the slab as the primary heat source of the home.
We were also interested in minimizing the overall amount of concrete used in the home, another reason for avoiding a full basement. Even though the Dragon Cement plant is less than 10 miles away from the site we still had concerns about the high embodied energy in concrete. This article addresses the carbon-load and energy intensity of producing and transporting concrete. Although the amount of concrete we saved by not building a full basement is insignificant in global terms it was still important to us to minimize the carbon footprint of this home.
For the sake of simplicity we initially considered several monolithic slab techniques, but since the slab was to be the primary heat source for the home it made sense to isolate the slab from the foundation wall. The problem with a typical monolithic slab is getting enough insulation around the outside of the slab to keep the heat you pump into it in a radiant application from escaping to the cold outside during the winter. The detail below shows a wall construction with an R-value of somewhere between R-21 and R-30 depending on the insulating materials and thicknesses used, but around the foundation there isn't even R-10. Since the framed wall needs to sit on top of the foundation it's nearly impossible to get adequate insulation in place if your monolithic slab is also your heat source.
Additionally, because the slab is continuous with the wall in this detail the wall becomes part of the thermal mass being heated by the radiant floor system. While we want to have a substantial thermal mass it's possible to have too much mass to heat efficiently, and in this type of detail there is a high surface area relative to the mass which would lead to greater potential for heat loss to the earth below (not to mention the fact that this particular detail doesn't illustrate any insulation below the slab).
The detail we ended up going with is a modification of the one below, a more traditional foundation wall with separate footing, foundation wall, and most importantly a thermal break between the floor slab and the foundation wall. While both of these details illustrated have the same amount of insulation between the slab and the outside air, the placement of that insulation makes all the difference.
The soil on site was very sandy which made for quick excavation for the foundation wall. The photo below shows the footing form-work with a step down towards the front of the house where grade slopes away.
Mom and Dad enjoying their new foundation:
The completed foundation walls with partial backfill and the beginning of insulating to the interior side of the wall.