To build a root cellar, also called a vegetable or fruit cellar, build a small insulated structure, or dig an insulated space out of the ground, and add a ventilated roof or cover. Fruit cellars can also be built into the ground, preferably in a sloped piece of land like a hill, to provide thermal insulation and easy access. Most fruit cellars are constructed underground to take advantage of the cooler temperatures to naturally preserve produce.
When building a fruit cellar in an existing structure like a basement, use existing insulated walls and floors to cut down on time and materials. Make sure all walls and ceiling are insulated. Also check that the walls and openings are sealed tightly to preserve humidity levels and the cool temperature.
If constructing a fruit cellar by digging a space out in the ground, dig deep enough that the warmer top soil temperature does not reach the produce. Flare the sides at the opening of the cellar to avoid the sides of the hole from caving in. Top the fruit cellar with some kind of insulated material like layers of straw or clay.
To keep the produce from spoiling, ventilate the fruit cellar without compromising the humidity levels or temperature. In more formal structures, use piping or valves to siphon out heat and circulate air. In ground fruit cellars, use breathable material like straw or create ventilation holes or slats on the roof of the structure.