Tar might dissolve the foam. Test it before you try a big area. They do make special adhesives for foam. If it's outside, you need to protect any foam above ground from sunlight- it degrades foam as well.

Thank you flgargoyle for this information!
Testing first is a good idea.

Finally got some time this year..used the adhesive for foam boards, put some used insulated tarp over to protect..was afraid these boards might get damaged or ripped off when back filling…

Have the names changed to protect the innocent?

really glad my root cellar is under my house. I can't even imagine trekking out to the cellar to get a bottle of peaches in the middle of an Edmonton winter. BRRRRR!

Lol..I’m an outdoor person and out every day.
Of course you can’t compare this to a house cold rom..this will have multiple advantages..overwintering all kinds of plant materials like cuttings/none hardy fruit trees / berry plants etc. drive right in with wheel barrow and store veggies / put carrots into sand …

Not sure how this last picture got in again and I don’t see of a editing future..

Some finishing pictures …

Very nice project.