'relatively constant temperature' is relative. if your outside temps have large seasonal differences, your temps inside your cellar will have swings seasonally as well. basically, you can put crops in the passive cellar in fall, once the cellar has cooled off. it will stay cold through the winter and into the spring, but it will definitely warm up by may/june. so a passive cellar works well for storing crops from november to april, not year round. to help cushion the temp swings, we put down a 4" layer of blueboard insulation over the top of the
concrete structure before backfilling 2 feet of soil. the walls are not insulated, just deeply buried underground. we used a large commercial refrigerator door to minimize heat transfer through the one part of the cellar that has contact with the outside air.
the crops you mentioned are mostly suited to cellar storage, but not squash. a cellar in winter would be too cold and damp for squash to store. they are better in an above ground structure that is warmer and drier. onions will do much better hung in mesh sacks from the ceiling than in bins, so it is good to have metal hooks in your ceiling for hanging.
you might calculate carefully the amount of volume you actually need for storage. a full cellar will be more stable with temperature compared to a large cellar with a lot of empty air volume. when you run the calculations, you may be suprised by how much root crops fit into how small of a volume. and cellar construction is not cheap with all the concrete and rebar, not to mention earth moving costs.
I didnt run any mathematical calculations for the air tempering. mostly just went off of
local country wisdom, for what that's worth. if your pipe runs are too long, you wont move
enough air to ventilate. our climate is fairly severe, with winters averaging around 25 degrees F, and summer average around 75 degrees F. the cellar buffers this, but not like a thermostatically controlled refrigeration room would. the air exchange is very slow, you cannot feel the air moving generally at the point where the pipes enter the cellar.
for the vent pipes, I used corrugated pipe with drainage slits, so any condensation drains into the soil. no problems there with mildew. make sure you screen the pipes for rodents, that would be a more likely problem.
consider the cleanability of the materials you use in construction. we chose all concrete walls and ceiling, coated with many layers of lime. easy to wipe down and disenfect, and the lime is strongly anti-fungal and anti-bacterial.
part of our floor is concrete slab, easy to clean for cheese and ferments. part of our floor is gravel, good for drainage and humidification. if we
water down the gravel, it keeps the ambient air humidity levels very nice for the vegetables.
hope those suggestions are helpful. like I said, we really love our cellar, and I think every farm
should have one.
good luck!
.png)
2
