Use foam board underneath fountain footer?
I am installing this fountain in PA:
I don’t want to dig 36 inches to the frostline.
Can I dig 2 feet, put tamped 3/4 clean stone, place XPS foamboard and then pour concrete?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Can you? Yes, but I don't advise it.
Once you've dug a 24 inch hole, it's only another 12 inches to turn it into a 36 inch hole. Dig down 3 feet. You won't regret it.
If you hit a very big rock -- what we often call "ledge rock" in Vermont -- you can modify the target depth of 36 inches (you can turn the target into "as deep as I can go") and then use rigid foam.
The problem with the rigid foam idea is that frost moves sideways as well as down. With a wide enough piece of rigid foam that is thick enough, you'd probably be OK with your 24-inch plan. But why risk it?
What about foam board on the sides?
5 sides of foam board including the bottom.
Even if you go to 36 inches