This standard applies to spaces intended for human occupancy within single-family houses and multi-family structures of three stories or fewer above grade@ including manufactured and modular houses. This standard does not apply to transient housing such as hotels@ motels@ nursing homes@ dormitories@ or jails. This standard considers chemical@ physical@ and biological contaminants that can affect air quality. Thermal comfort requirements are not included in this standard (see ANSI/ASHRAE Standard 55-2004@ Thermal Environmental Conditions for Human Occupancy). While acceptable IAQ is the goal of this standard@ it will not necessarily be achieved even if all requirements are met a. because of the diversity of sources and contaminants in indoor air and the range of susceptibility in the population; b. because of the many other factors that may affect occupant perception and acceptance of IAQ@ such as air temperature@ humidity@ noise@ lighting@ and psychological stress; c. if the ambient air is unacceptable@ and this air is brought into the building without first being cleaned (cleaning of ambient outdoor air is not required by this standard); d. if the system(s) are not operated and maintained as designed; or e. when high-polluting events occur. This standard does not address unvented combustion space heaters. PURPOSE This standard defines the roles of and minimum requirements for mechanical and natural ventilation systems and the building envelope intended to provide acceptable indoor air quality (IAQ) in low-rise residential buildings.