Design Criteria Fire Ratings Building codes are quite specific in the degree of fire protection required in various areas of buildings. This protection is derived from the fire resistance of walls, partitions, and floors and is measured according to the Standard Methods of Fire Tests of Building Construction and Materials, ASTM E-119. Tests are conducted at Underwriters Laboratory, the National Bureau of Standards, or other recognized laboratories. The ratings indicate a component’s ability to withstand controlled fire intensities for a time equal to or in excess of the rating. They are expressed in whole hour increments up to four hours, which is usually the maximum rating required by building codes. The fire rating required for a building component becomes the function of design as well as the appropriate building code. Based upon actual experience, masonry walls have proven superior to other wall types when subjected to fire. Their excellent stability and load carrying capacity make them outstanding wall values. See Fire Rating for current codes. Sound Transmission Class Although considerable attention is given to a wall’s ability to absorb or reflect sound, we find a greater concern for the amount of sound passage through walls. We identify the rating of the ability to resist passage as the Sound Transmission Class. Briefly, the STC is a single number rating derived from the measured sound transmission loss of 16 standard frequencies through a wall. The value is expressed in decibels. Heat Loss And Heat Gain ASHRAE Standard 90-90.2 "Energy Conservation in New Building Design" recommends that in addition to the criteria set forth in the standard, a proposed design should consider thermal mass. In Section 4.4.3, Cooling Criteria, consideration is given to the mass of a wall when the formula given for compliance with the standard is applied. However, no provisions are given for consideration of mass when checking for compliance of a wall or roof system in heating applications. The "M" factor is a simplified correction factor that is intended to be used when checking walls and roofs for compliance with the requirements of 90-90.2 in heating applications.