This one-room addition has no foundation. There is insulated in walls (likely R11) and ceiling (likely R19 – certainly no more), but not in the floor. As you can see, outdoor air can simply pass through the open space under the floor. When the flat EPDM roof needs to be replaced in 5-10 years, we will install exterior foam board outside the roof deck. Heating is primarily warmed air from rest of house or propane fireplace. Cooling is AC from rest of house.
I’m considering digging 2′ out under the floor, and sealing the underside with foam. What approach would you take?