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Concrete floor overlay using rigid foam to save weight

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I have an existing elevated concrete floor slab that needs elevated 9 inches. Because the floor is only rated to 100 psf LL, I cannot add 9 inches of concrete. Therefore, I am going to use 6 inches of rigid foam insulation with a 3 inch reinforced concrete slab poured over the insulation (The architect & owner understand this will drop the allowable LL and are ok with this). I have see this done several times before, although I have never been involved in the design of such a system.


My question is this. Dow has rigid insulation rated for 25 psi compressive strength, where compressive strength is defined at 10% deflection under load. However, they also sell a material called high load, which has a compressive strength of 40/60/100 psi (there are 3 varieties) at 5% deflection under load. Dow technical services cannot provide me with any information regarding what the allowable load should be when used under a concrete floor slab. Has anyone here ever dealt with this issue?


It seems the 25 psi material should work as this provides a FS of about 18 under DL+LL (100 psf), even using the 5% deflection criteria (i.e. compressive stress of 12.5 psi rather than 25 psi).



Eric Green, PE