But for the average builder, how deep you go is limited because of expenses. You can also borrow cool from the air. Often the night s air temperature will be cooler than the air in your cellar. And finally, you should do what you can to prevent heat from having access to your cellar. Even if kept cool, in a low humidity environment, your vegetables will soften and shrivel up.
Most vegetables require high humidities.
A typical underground root cellar will generally maintain a high humidity all by itself if it has an earth or dirt floor. This is because the vegetables in your cellar give off gasses that often are conducive to either spoilage or sprouting. For example, apples naturally give off ethylene gas which makes potatoes sprout prematurely. The outlet must always be at the highest level in the cellar with the outlet tube flush with the inner wall. The inlet should come into the cellar at the bottom.