Above ground root cellar

So I am working on plans, so that I can be ready to build a root storage building in the spring.  On our land the water table is very near the surface so a true cellar (below ground) is not an option.  I'm thinking I will build a shed inside a shed with about 12 inches between the inner and outer wall.  Then insulate the space between.  If I berm the walls about halfway up combined with the good insulation I should need only minimal heat to keep the inside from freezing in the winter.

Any thoughts on what I should put between the inner and outer walls.  I'm trying to minimize cost, so I don't really want to purchase that much insulation.  I'm thinking perhaps, scrape styrofoam, or cardboard or shredded paper or wood chips.  The challenge with anything organic is keeping it dry so it won't rot.

Pictures to follow, once the snow melts in the spring.

You do understand that the goal is to put slightly more dirt on all sides than is your normal frost line?  So if you frost line is 2 1/2 feet then you want say at least 3 feet of dirt on all sides.  Insulation will help but an above ground cellar has exposure on 5 sides while a normal cellar is mostly below ground on all but one side giving better geothermal heating and less loss area for heat.  For a fully exposed cellar you will likely need way more insulation.  In this area good root cellars have at least 4 feet of dirt in every direction.

Without putting a giant hill in my yard, there is no way to have 4-5 ft of dirt on all sides of the cellar.  As a result I am opting for a well insulated shed which will need some supplementary heating in the winter.  I have not finalized my decision on wall thickness yet.  My neighbour has a similar building but with 4 ft thick wall filled with sawdust.  It works well, but the insulation is slowly rotting in the walls.

Vermiculite may be a way to go if you can get it for a good price.

"The challenge with anything organic is keeping it dry so it won't rot."

Permaculture -> stacking functions -> the problem is the solution

Here's my suggestion:

A: Two nesting Quonset huts on a slab, one slightly larger than the other, with a 3 foot gap in between.
B: make stairs and a walkway to the top, allowing access to the entire length.
C: leave gaps in the very top of the inner Quonset hut, pour leaves, pine straw, grass clippings, and any fluffy organic matter into the gap, filling up the 3 foot space between the two shells.
D: add worms and a little water to keep it slightly moist.
E: make small doors at the bottom of the inner shell, remove finished compost, worms, and worm castings as the leaves and organic matter decay over time.

3 feet of slightly damp leaves should have an R-value of at least 60 (US units). The inner surface should maintain a fairly consistent temperature, allowing the worms to survive the winter, and keeping your root crops cool in the summer.

B Beeson wrote:

Here's my suggestion:

A: Two nesting Quonset huts on a slab, one slightly larger than the other, with a 3 foot gap in between.

How about nesting inflatables painted with re-inforced concrete?

Hey, Tim! Were you able to make any progress on this project? I'd be interested to know how it's going.

Things happened in life so I haven't been able to make much progress.  Hopefully I can gather some more materials over the next few months and start the build in the spring.  I will be sure to post some updates once there is something to see.