Greetings, 

I live in mid Michigan.  Zone 5 (but lives like 6).  I have opted to do a radiant slab on grade in a small addition.  Existing home is 94 years old.  Fairly well tuned up envelope. 

I insulated 4’ down on the inside of the perimeter walls prior to fill.  Currently have vapor barrier and 2” of XPS installed over sand bed but am somewhat stuck on the perimeter.  I am also somewhat limited by the foundation design on continuing the 2″ foam.

Any thoughts or recommendations on the minimum thickness of foam (or other material) needed to create a thermal break?

Thanks in advance!