and must be enclosed in insulated glazing or an Insulated Glass Unit (IGU) to maintain their properties over time. Specially designed coatings may be applied to one or more surfaces of insulated glass. One type of coating (low-e coatings) reduces the emission of radiant infrared energy, thus tending to keep the heat on the side of the glass where it originated while letting visible light pass. This results in glazing with better control of energy - heat originating from indoors in winter remains inside (the warm side), while heat during summer does not emit from the exterior, keeping it cooler inside. Glass can be made with differing thermal emissivities, but this is not used for windows. Certain properties such as the iron content may be controlled, changing the thermal emissivity properties of glass. This "naturally" low thermal emissivity is found in some formulations of borosilicate or Pyrex. Naturally, low-e glass does not have the property of reflecting near infrared (NIR)/thermal radiation; instead, this type of glass has higher NIR transmission, leading to undesirable heat loss (or gain) in a building with that type of window. Criticism of low-E windows It has been suggested that the high reflectivity of low-E windows can contribute to a concentration of solar radiation which can potentially cause damage to their surroundings; damage to the sidings of homes and to automobiles has been reported not only in news stories, but may cause legal issues as well. Low-e windows may also block radio frequency signals. Buildings without distributed antenna systems may then suffer degraded cell phone reception. Reflective thermal insulation Reflective thermal insulation is typically fabricated from aluminum foil with a variety of core materials such as low-density polyethylene foam, polyethylene bubbles, fiberglass, or similar materials. Each core material presents its own set of benefits and drawbacks based on its ability to provide a thermal break, deaden sound, absorb moisture, and resist combustion during a fire. When aluminum foil is used as the facing material, reflective thermal insulation can stop 97% of radiant heat transfer. Recently, some reflective thermal insulation manufacturers have switched to a metalized polyethylene facing. The long-term efficiency and durability of such facings are still undetermined. Reflective thermal insulation can be installed in a variety of applications and locations including residential, agricultural, commercial, aerospace, and industrial structures. Some common installations include house wraps, duct wraps, pipe wraps, under radiant floors, inside wall cavities, roof