Building insulation materials consist of all types of products and materials to prevent the passage of heat. Insulation reduces unwanted heat loss or gain, while decreasing energy demands of heating or cooling systems. Building insulation refers to materials that slow heat loss, including cellulose, fiberglass, rock wool, polystyrene, urethane foam, and vermiculite. These materials can be used in various techniques to address heat transfer - specifically conduction, radiation and convection materials, and soil. Building insulation materials are often evaluated by its R-value, a measure of thermal resistance. However, R-value does not account for the quality of construction of a building, or local environmental factors. Typically, there are two types of building insulation: structural and non-structural. Structural building insulation refers to rigid panel insulation, structured panels, or straw bale. Rigid panel building insulation is usually foam-based plastic, but foam-straw composites are also available. This type of building insulation has integral strength, thus reducing the need for structural lumber, air leaks, and assembly time. Structured insulation panels (SIPs) consist of a sandwich of two panels of structural board with a foam insulating layer in between. The boards are often oriented strand board (OSB), and the foam is polyurethane foam, expanded polystyrene foam (EPS), or extruded polystyrene foam (XPS). Straw bale is another type of structural building insulation. Straw bale is a bundle of straw, which has been tightly bound with wire or twine. This type of building insulation is commonly finished with plaster, which provides excellent thermal mass, compressive ductile and compressive strength, and fire and thermal resistance. Non-structural building insulation is also available as batts, blankets, loose-fill, and spray foams. Batt is a common type of building insulation, although it often allows more air transfer than other types. Batts are cut from blanket insulation. Both batt and blanket building insulation consist of flexible fibers, often fiberglass; however, mineral wool, plastic fibers, and natural fibers may also be used. Batt and blanket insulation is available with out without facings, which acts as an air barrier. Building insulation materials are also available in loose-fill or spray. Loose-fill insulation consists of small fiber, foam, or other types of particles. The most common materials used are cellulose, fiberglass, and mineral wool. Loose-fill building insulation can conform to any space without disrupting structures. Spray-on building insulation is made of polyurethane foam, and is pushed through pressurized spray nozzles. As it reaches the surface, it expands for form a continuous layer of insulation – even covering cracks that may have gone unseen. Other types of building insulation may also be available.