Our current root cellar was built as an experiment in the fall of 2019 and proved to be an excellent way to store food long-term in our climate which routinely dips to -40 in the winter. In essence our root cellar is a buried 8'x14' insulated aluminum box taken off an old refrigerator truck which then had several traditional root cellar modifications made to it. Total actual cost for this project was less than $500 CAD. I posted the details of this build with lots of construction pictures and a long list of learnings on another permies thread - "refrigerator that doesn't use power" (
https://permies.com/t/40/132501/Refrigerator-doesn-power-build#1393637).
We are now in the process of constructing another 8'x14' root cellar based on one major lesson learned from our first root cellar: as we age, we don't want to have to venture outside in the winter to access our veggies. So our new root cellar will be accessible directly from the basement of our new addition.
All of the block walls of both the addition and root cellar are being constructed of Nexcem (an NON-styrofoam ICF block which is fireproof; vermin, termite and insect proof; does not support fungus growth; highly energy efficient [up to R28]; constructed of 80% recycled material including clean softwood aggregates, does not contain or emit any toxic elements; and is lightweight and yet very durable to build with). Yet despite all these desirable traits, Nexcem still may have to be modified for a root cellar application as it is also hygroscopic. This vapor regulator trait typically keeps indoor relative humidity levels at healthy/comfortable levels (not exceeding 70%) - which is highly desirable for living quarters but not ideal for a root cellar. Thus we may find that we will need to seal the blocks inside the root cellar to prevent them from vapor regulating. We plan to use 1/2 the addition's basement (which will have a cement floor) as a dry cool storage, as <70% humidity is great for onions, garlic, and canned goods.