Choosing a Garage Foundation Type in Maine on Heavy Clay Soil – GreenBuildingAdvisor

I’m planning a two-story garage with living space above in coastal Maine. The site has heavy clay soil, and I’m trying to determine the best foundation approach for a cold climate. I will air seal and insulate the ceiling joists of the garage to keep heat in the apartment above, so this question is primarily about a garage foundation.

What foundation types would you consider for this situation, and what factors would drive the decision between options like an insulated slab or frost wall? Radiant heating or not? Insulation placement and thickness?

I’m especially interested in lessons learned from building on heavy clay soils in Maine – what has worked well, what has caused problems, and what you would do differently.

Bonus question: As an engineer, I’m skeptical of the role of insulation under and around a garage slab. Insulation reduces heat flow, so I’m curious about the purpose of insulating an unheated or intermittently heated garage.

I do intend to keep the garage at about 45 degrees in the winter with a heat pump, but what if I winterize the building and leave it unheated for an extended period (for example, a year)? What failure mode is the insulation intended to prevent? If the answer is frost heave or freeze/thaw movement, how does the insulation change the soil behavior compared to an uninsulated slab?

If insulation is recommended, how much is actually needed for a Maine climate? Is perimeter insulation alone sufficient, or are frost wings extending beyond the slab perimeter necessary? I’m trying to understand where the point of diminishing returns is for a garage foundation.