Assuming everything goes to plan, we will be pouring two shallow frost-protected concrete slabs next month. One for our grindbygg workshop, and the other for a small timber frame cottage. We've chosen this style of foundation because our land is very ledgy—it's extremely unlikely that we would be able to dig deep enough for a typical frost wall foundation or rubble trench without hitting bedrock. Another option, a pier foundation, has been ruled out due to general incompatibility with radiant heating.
In order to protect our shallow foundations from frost heave (video below), we will be pouring them over a large platform of insulation. In addition to the primary goal of preventing the subsoil from freezing, this barrier will also inhibit thermal bridging between our concrete slabs and the ground. Given that we plan to install in-floor radiant heating, this is of crucial importance—without the insulation we'd be flooding any warmth we generate directly into a heat sink of truly massive proportions: the earth itself.
There are very few proven materials that can provide the insulation we need for this purpose. The industry standard is rigid polystyrene foam (video below). Some alternatives I am aware of include perlite, foam glass, strawbale-concrete sandwiches, and mineral wool. Aside from being largely untested, most of them involve complex installations and/or difficult material sourcing.
Given the permanence and importance of this particular aspect of our construction, as well as our lack of building experience, we've been very conservative in our selection for below-grade insulation. We will be using an EPS foam which has been manufactured and shipped from a company in Buffalo, New York. It should be arriving tomorrow!