How much rigid foam under a concrete slab do we really need?
I should have noted that this is in northern central Massachusetts.
tom
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!
Best regards,
Tom
Replies
Very dependent on your geographical location and climate.
Some places you dont want ANY foam (mostly AC needed). Other location no amount is enough and you should buld on piers (e.g permafrost areas).
etc..
Hi Tom,
R-10 is considered a good target for sub slab insulation for your climate. That can be achieved with 2" of foam. 4" is overkill for your location. However the foundation walls should be insulated to R-20 (4" of foam). Best of luck.