How much rigid foam under a concrete slab do we really need?
I should have noted that this is in northern central Massachusetts.
In my son’s house we are rehabbing he needs to pour a new concrete floor in the kitchen addition. It is currenly loose sand as we have taken out the poorly constructed slab that was there.
We plan to add 2 inches of graded base to pack on top of the sand and then install 2 inch thick XPS rigid foam on top of the packed gravel, then a 4 inch slab (vapor barrier too).
Steve spoke with a cement guy yesterday who said, (and I quote) “You better put in 4 inches of insulation or you will be bleeped”, and he didn’t say “bleeped”.
Steve did not question him on this point, so I thought I would ask you guys as to why 4 inches would be necessary. There is no water issues in this room, I was hoping to avoid removing another 2 inches of sand to make room for this additional insulation.
Thanks in advance for any and all advise you may have!