Walk-in and reach-in freezers should have fully-gasketed doors to fully contain the cold air within, and walls should be adequately insulated. When available, select units that carry the Energy Star label for energy efficiency. Look for freezer equipment that does not use chlorofluorocarbon (CFC) based refrigerants which deplete the ozone. Freezers may be connected to back-up power supplies such as generators to preserve food during power outages.