R value of styrofoam insulation VS dirt

[Icandoit]

Hey all,

I would like to pose the following scenario. I am putting a cistern system in my house because my well has failed. I am burrying 2 ROTH MODEL 1500RMT tanks. I will tie both tanks together and run a a single line to my existing water line from my well and tie into it using a curb stop. I will also install a curb stop on my well line so I can choose which system to use. I don’t want my well water contaminating my cistern for various reasons.
I live in central BC, so frost is an issue. The tanks can not have any more than 36” of soil on top of them. I want to use a combination of bedding sand and sheets of styrofoam over top of the tanks to prevent the frost.
I researched R values of soil and the general consensus is R=0.125 to 1.
Styrofoam has different values.
Which foam should I use in dirt?
How much do I need to prevent frost from getting to my tanks?

NOTE: my neighbors tanks are only half buried and then covered with about 1 foot of soil. He has never had a frozen tank or water line but I am skeptical and want to do it right.

Thanks

[Shannon]

On the farm growing up in Saskatchewan we had a 8000 Imp. gal. concrete cistern 8' in the ground and sticking above ground by about 16". I don't remember it ever freezing up. The top usually got a little ice but down were the suction hose was never froze. I don't think it should be an issue.

[kenora]

Both the white EPS and blue XPS foam have an R value of about 4.5 per inch.... they are both OK to bury since they are often used along the side of foundations or under slabs...the only foam NOT to use is isocryocynate (spelling is wrong but its called iso)..its absorbs water

edit.... white EPS is waaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaay cheaper