A single number index, called the exterior wall rating, or EWR, has been developed for rating the sound transmission loss of A‐weighted sound levels for exterior facades. This index is closely patterned after, but distinctly different from, the STC single number index. The latter provides a reliable rating for the sound transmission loss of interior partitions for typical interior sounds, but is not suitable for assessing the sound transmission loss of exterior walls to major sources of outdoor noise. However, by applying principles similar to those upon which the STC rating was developed, a practical index was constructed which can reliably rate exterior walls in terms of their ability to attenuate A‐weighted sound levels. EWR originally evolved out of a systematic analysis of 1/3‐octave‐band sound transmission loss data for over 225 wall constructions and 33 different window constructions with four different relative areas from 0%–20% of the wall areas. From this analysis of over 22 500 combinations of structural assemblies, EWR was found to be a substantially more accurate predictor of attenuation of A‐weighted sound levels than other candidate indices. These included a modified STC, SIL, and average TL values for 16 1/3‐octave bands. EWR includes a necessary correction for the different spectra of aircraft and highway noise, and thus provides an efficient method for the design or analysis of sound insulation treatment of buildings to attenuate noise from these major outdoor noise sources. One such application is considered in paper HH4 by David Brown [J. Acoust. Soc. Am. Suppl. 180, S67 (1986)]. The basic methods for calculating EWR values from standard TL data are defined to encourage sound‐transmission‐loss‐testing‐laboratory operators and users to apply this practical method for rating sound transmission loss of exterior walls.