Climate zone: Canadian Zone B. Would be a US zone 5B. 3850 HDD and dry.

I am building a shop with a heated slab. The slab will be insulated at the perimeter (via ICF foundation/stem walls) and underneath to try to thermally isolate the slab from ground and exterior.

The slab has two exterior transitions: a person door and a car door.

I will deal with the thermal bridging at the person door by cutting down the ICF only to grade level. The slab will be poured to be level with the top of the ICF, and then a wide door sill used to protect the top of the exposed styrofoam. (Looking for a thermally efficient door sill…)

However, I am concerned with the thermal bridge that happens at the car door opening – every detail I’ve seen has the ICF cut down to allow a continuous 4″ (or more) slab to bridge the exterior and interior. While this is obviously sensible for preserving the integrity of the concrete for vehicular traffic, it is equally insensible from a energy efficiency point of view. The lack of thermal break effectively turns the exterior part of the slab into a large heat radiator.

I am taking care to try to build the shop with energy efficiency in mind, and this sticks out as the most obvious source of heat loss at this point. Any suggestions on how to deal with this transition in an energy efficient manner yet still support some vehicular traffic??

If it matters, the car door will see only occasional use, maybe opened once a week. Perhaps less.

Thanks for any input,
Mark