Use foam board underneath fountain footer?

ps24eva | Posted in Green Building Techniques on May 22, 2017 11:53am

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?

Answer

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Martin Holladay | May 23, 2017 05:30am | #1
John,
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?
ps24eva | May 23, 2017 04:08pm | #2
What about foam board on the sides?

5 sides of foam board including the bottom.
ps24eva | May 24, 2017 04:08pm | #3
Even if you go to 36 inches

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