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We can get frost below 3 feet, typically we bury our waterlines at 4 foot. We have isolated the heat sink from the cold outside soil with sytrofoam. It goes all the way around the rock to a depth of 6 feet. The foam is 4 inches thick and is inside the ground poles that holds up the arches.

That is excellent to know! That should work.

Just this morning someone offered a 7' x 15' hoophouse on BuyNothing. I wonder....

Are you in an area that freezes in the winter? We haven't had our greenhouse long, but we did have a single layer of plastic on the side walls for several months. Doubling up all the plastic layers made a significant increase in our over night greenhouse temps.

Yes, we freeze alright, down to -25F most winters. Ground freezes 4' down.

That's cold! Then for sure a single double layer of greenhouse plastic would serve your needs better than a single layer found on moat hoop houses.