Building insulation materials is used depends on a wide variety of factors. Some insulation materials have health risks, some so significant the materials are no longer allowed to be used but remain in use in some older buildings such as asbestos fibers and urea. Factors affecting the type and amount of insulation to use in a building include: Considerations regarding building and climate: Often a combination of materials is used to achieve an optimum solution and there are products which combine different types of insulation into a single form. Spray foam is a type of insulation that is sprayed in place through a
Building insulation materials (short of a vacuum), and carbon aerogel absorbs infrared radiation (i.e., heat from sun rays) while still allowing daylight to enter. The combination of silica and carbon aerogel gives the best insulating properties of any known material, approximately twice the insulative protection of the next best insulative material, closed-cell foam. The use of highly compressed straw bales as insulation, though uncommon, is gaining popularity in experimental building projects for the high R-value and low cost of a thick wall made of straw. "Research by Joe McCabe at the Univ. of Arizona found R-value for both wheat and rice bales was