Choosing a Garage Foundation Type in Maine on Heavy Clay Soil

A homeowner in coastal Maine is planning a two-story building with a garage below and living space above, on a site with heavy clay soil. He's weighing foundation options for the garage portion, since the ceiling above it will be air sealed and insulated separately to protect the living space. The main question is whether to build a frost wall (a foundation wall that extends below the frost line) or an insulated slab, and how to handle insulation placement, thickness, and whether radiant heating makes sense given the clay soil.

He also raises a technical question about the actual purpose of slab insulation in a garage that's only heated part of the time. He plans to keep the garage around 45 degrees with a heat pump in winter, but wonders what would happen if it were left unheated for a long stretch, like a year. Specifically, he wants to know whether the insulation is meant to prevent frost heave (ground swelling from freezing moisture) or freeze-thaw movement in the soil, and how insulation changes soil behavior compared to a bare slab. He's also asking how much insulation is actually needed for Maine's climate, whether insulating just the perimeter is enough, or whether "frost wings" extending out beyond the slab edge are necessary.

This is a builder's forum question, not a program or product announcement. There's no rebate, deadline, or new technology involved. It's useful mainly for homeowners in cold, clay-heavy regions thinking through similar foundation choices for a garage or outbuilding.