Rather than try to insulate and heat the large barn, I’ve decided to expand the attached shed (see pic).  This will be used to restore/repair cars and I’d like it to be VERY energy efficient.

Both the shed and the barn have concrete floors but as far as I can tell, neither have any kind of foundation below the slabs.  The shed (about 8 x 15′) concrete is cracked near the back/center but is not currently heaved around that crack.  The shed needs to be widened about 3-4 feet and lengthened about the same, so I’ll need to pour more concrete.  What I’d like advice about is whether to consider  a FPSF for the new pour, or would I be better off digging down to the frost line (30″)?  If the major portion of  this floor is in essence, ‘floating’, will attaching concrete with footings cause problems?  Also, as I understand it, I’ll need to drill horizontal holes in the old slab, for rebar, which will extend into the new concrete.  How far apart can/should these be? 

Any thoughts/suggestions greatly appreciated.

Thanks.

Thon