After reading quickly through the posts, I should give my two bits:
1. Root cellars take advantage of the average temperature of the region you are in.
2. When it is cold out, the relatively warm air of the root cellar will rise and vent out the higher vent.
3. When it is warm out, the relatively cool air of the root cellar will sink and draw INTO the higher vent, and out the lower vent.
4. When the outside temperature is the same as the root cellar, there will be no natural air movement. Thus, during stagnant periods, a computer or other fan would be beneficial.
Anywhere warm air joins cold surfaces, condensation will develop, such as on cold floors, ceiling, or walls.
Vapor barriers on the wrong side (cold side) of a surface can condensate moisture on the back side (warm side) of the barrier, rotting out wood.
Exposed floor joists conduct warmth from overhead living quarters past (between joist insulation) into basement root cellars, warming them. These exposed joists need to be heavily insulated as well as the spaces between them.
Moisture coming out of concrete can be considerable, and largely controlled by using sealers. I'd be surprised if basement root cellars are really practical if kept at high humidities. But maybe experiment with filling the entire joist cavities with insulation, and then bridging the bottom of the joists with at least 4" of foam insulation. The dew point would theoretically be somewhere in the foam. Once the cellar is at a cool temperature, dew should no longer collect on the cool surfaces except near the incoming or outgoing warm vent.
Seal all penetrations and even small cracks up into the heated building above to prevent any cool air wanting to be drawn up into the house through the chimney effect of the living quarters themselves. All of this can be complicated and tricky! Kevin Allred, Tenakee Springs, Alaska
