ASHRAE Standard 55-2020: Thermal Environmental Conditions for Human Occupancy
About this book
ASHRAE Standard 55, "Thermal Environmental Conditions for Human Occupancy," is the primary American national standard defining the indoor thermal environment conditions that are acceptable to a majority of building occupants. Published by ASHRAE and continuously updated, the standard provides the technical framework for specifying and evaluating thermal comfort in occupied spaces, drawing on decades of fundamental research into human physiological and psychological responses to the thermal environment. The standard's intellectual foundation rests principally on the work of Danish engineer Povl Ole Fanger, whose 1970 book established the Predicted Mean Vote (PMV) and Predicted Percentage Dissatisfied (PPD) indices as quantitative measures of thermal acceptability.
These indices, which relate to the six fundamental parameters governing thermal comfort — metabolic rate, clothing insulation, air temperature, radiant temperature, air velocity, and humidity — form the core of the standard's "Analytical Comfort Zone Method." In its current form, ASHRAE 55 specifies acceptable ranges for operative temperature and humidity that correspond to less than 20 percent predicted dissatisfaction among building occupants. The 2020 edition includes two primary methods for determining acceptable thermal conditions: the graphical (psychrometric chart) method for quick assessment of steady-state conditions, and the PMV/PPD analytical method for more detailed evaluation including the effects of elevated air speed and asymmetric thermal radiation. A significant development in recent editions is the formalization of the "Adaptive Model" for naturally ventilated and mixed-mode buildings.
This model, based on field studies showing that occupants in buildings with operable windows develop different thermal expectations from those in air-conditioned buildings, allows wider ranges of acceptable temperatures when occupants have behavioral control over their environment. The adaptive model is grounded in research showing that outdoor climate, clothing choice, activity level, and psychological expectations all influence what people find comfortable. The standard is extensively referenced in green building certification programs including LEED, WELL, and BREEAM, all of which include thermal comfort requirements derived from or cross-referencing ASHRAE 55.
It is also referenced in building codes and design guidelines worldwide. ISO Standard 7730 covers the same subject matter and is largely harmonized with ASHRAE 55, making the PMV/PPD framework a truly international standard for thermal comfort design. Sources: ASHRAE (ashrae.org); ASHRAE Standard 55-2020 official publication; ASHRAE Thermal Comfort Tool documentation; Indoor Air journal.