A conventional root cellar, which essentially is a room buried in a hillside or underground, meets only a handful of criteria.
Insulation: The earth is a wonderful insulator. A typical old-world root cellar was literally buried on all sides except the entrance, utilizing the natural insulating properties of the soil.
Ventilation: It is important that your make-shift root cellar can breathe. Warm stale air needs to float out of the top of your chamber as fresh cooler air makes its way through the bottom. This is important to keep humidity levels under control in order to prevent the rot of moisture-sensitive vegetables such as squashes.
Darkness: Light accelerates the decomposition of fruits and veggies. An ideal means of storage incorporates complete darkness.
Source : http://www.realfarmacy.com/storing-the-food-you-grew/