Insulation in buildings is an important factor to achieving thermal comfort for its occupants. Insulation reduces unwanted heat loss or gain and can decrease the energy demands of heating and cooling systems. In a narrow sense, insulation can just refer to the insulation materials employed to slow heat loss, such as cellulose, glass wool, rock wool, polystyrene, urethane foam, vermiculite, perlite, wood fiber, plant fiber (cannabis, flax, cotton, cork, etc.), recycled cotton denim, plant straw, animal fiber (sheep's wool), cement, and earth or soil, reflective insulation (also known as a radiant barrier) heat transfer - conduction, radiation and convection materials. Many of the materials in this list deal with heat conduction and convection by the simple expedient of trapping large amounts of air (or other gas) in a way that results in a material that employs the low thermal conductivity of small pockets of gas, rather than the much higher conductivity of typical solids. (A similar gas-trapping principle is used in animal hair, down feathers, and in air-containing insulating fabrics).