Loved this post, lots of good info!
We have a root cellar but it was put in after the foundation was poured. So I don't have the humidity needed. When we add on, I am making sure we have a REAL root cellar.
I can't dig one here as the water table is normally at 4', this year around 2.5 - 3'. And it's just too cold for a straw bale one here. I tried it one year and everything froze.
We did the proper building of the straw bales with 3 layers of stucco on the inside and outside and the roof is a standard roof with 6 inch deep fiberglass insulation in it and plywood on the underside. Our straw bale root cellar is attached to our house and we can open the door to it to let some of the wood heat in. We also put a little heater in it, on a thermostat to ensure things didn't freeze. We have temperatures of 40 below Celsius.
We didn't make anything as proper as that, and it was freestanding in the yard. You do get a good bit colder than us. Mostly we get to -25F (-31C).
I'll be looking forward to a proper root cellar when we add on...