What kind of foam will work under the liner????

[MarkC]

I used 1/2" carpet padding under a liner in my old above ground pool. The blue foam board may dent and not bounce back like a carpet pad like foam would. Try denting a small piece and see if it returns to normal. It would still probably be OK with the denting but just might end up collecting dirt in the depressions.

[scootchu]

I used 1/2" carpet padding under a liner in my old above ground pool. .

Was that directly on the ground/sand? Won't it rot or just become nasty when damp?