Since heat transfer through an insulator happens by conduction or radiation, thermal materials are used to lessen the flow of heat in or out of steam pipes, buildings, refrigeration units, etc. Different materials give better insulation for certain applications and are available in a variety of forms and materials such as foams of polyurethane or polystyrene; loose fill insulators are of diatomaceous silica, perlite, calcium silicate, plastic beads, charcoal granules, vermiculite or charcoal granules; rigid insulator are PVC, balsa wood, cellular glass, corkboard, urea, polyurethane or polyethylene. Fiberglass, sprays of macerated paper or insulating concrete, aluminum foil, etc. are other types of thermal insulation.