This paper addresses the evolution of ASHRAE Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, with regard to the continued long-term increase in thermal performance levels required of commercial building envelope and fenestration systems. The trend of increasing thermal performance levels for building envelopes has had a significant impact on design and the materials, systems, and products available for use in exterior building envelopes. This is particularly true when considering buildings of size or simplicity that do not warrant thermal modeling of the entire building or envelope system to validate compliance with mandated energy performance levels. As a widely recognized, and most often code-mandated, standard, ASHRAE 90.1 significantly impacts the selection of materials, products, and systems available for compliance with required energy standards. As increases continue through the most recent edition of ASHRAE 90.1 and as U-values become more stringent for materials, products, and systems in specific envelope applications, identifying options for the design of buildings becomes more challenging. The demand for additional options also increases, which will hopefully spur greater interest in the development of new and improved envelope materials, systems, and products to meet the demands of aesthetics and the more stringent energy performance requirements. As requirements for energy performance increase, the number of buildings and envelope systems evaluated by thermal modeling to confirm compliance with new standards is also likely to increase. Compliance with the new ASHRAE 90.1 “prescriptive” requirements has been challenging and appears likely to become even more so. This paper includes consideration of a 20-year period (1999–2019) of evolving ASHRAE 90.1 standards and criteria for prescriptive building envelope thermal performance, and specific examples of the types of restraints currently being experienced by designers is included.