Frost-proof floating foundation for addition, tied to stem walls for Zone 5 – GreenBuildingAdvisor

We’re having a 40×40 garage/workshop with a bonus room above designed to attach to our cape, here in CT. I nearly had a heart attack after getting our first quote for the foundation! The excavators quoted the typical 42″ x 10″ stem wall on top of a 20″ x 10″ footing, with foam insulation on the first two feet, and a 4″ 4000 psi slab with fiber mesh. 

Is there any reason why we shouldn’t do a frost-proof floating slab? I was planning on having 2″ of foam under the entire slab anyway, since the shop will be heated. That would really reduce the amount of excavating and concrete needed. Trade-off would be adding 135′ worth of 4′ wide wing insulation around the perimeter. Would it be pinned to the existing stem-wall with rebar where the new slab meets the existing house?

Any concerns about mixing both styles (obviously I couldn’t put wing insulation where it meets the existing stem wall)? I figured both would keep from moving in the frost, so no harm done?

Thanks for any advice!