You want the temperature of your root cellar to stay near freezing. Depending on where you live, that might not be a problem during winter months. But, a couple of sunny days might bring the temperature up as winter turns to spring. To avoid too much heat, borrow colder temperatures from the ground. Earth, even two feet down, has a remarkably stable temperature. The farther down you go, the more stable it is. You must go down a full ten feet before complete temperature stability is reached, and for the average builder, depth is limited because of the expense of excavating. You can also borrow cool temperatures from the air. Often the nighttime air temperature will be cooler than the air in your cellar, so open a vent to take advantage of the cooler air. You should also think about the location of your root cellar in regard to passive solar heat. Build your root cellar in a place that is shady throughout the day, on the north side of your property, and use insulation to keep out the heat.

Your second most important consideration is humidity. Even if kept cool, vegetables will soften and shrivel up in a low-humidity environment. 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.

Because the vegetables in your cellar give off gasses that often are conducive to either spoilage or sprouting, you need to plan for good air circulation. Have an inlet vent and an outlet vent.