Hi All I’m new here and working on a single story 500SF ADU in the side yard in Mississippi (Zone 3A). I have never done insulation under a concrete slab and a foam manufacturer in the area has a bounty of 1/2″ Type VIII (1lb/ft) EPS sheets available at discount. 

My question is basically can I stagger and stack 4 layers of 1/2″ foam to achieve the 2″ thickness I would like. From what I’ve read minor gaps/imperfections in the foam under a slab shouldn’t negatively effect things, so I am inclined to use the 1/2″ discounted foam.

Any input welcome. I will be using 4″ compacted gravel below foam and a vapor barrier above foam and below concrete.