Imagine an ice cream parlor that offers only one flavor of ice cream, one chosen by scientists after studying what the ‘average’ person wants. We would not expect such a business to succeed, and the idea seems absurd. Yet a similar logic has been employed for decades in designing buildings and establishing standards for thermal comfort in buildings. Buildings are designed to provide interior temperatures for a predicted ‘average’ occupant, and great effort is expended in design, construction and operation to assure those temperatures are uniform everywhere in the space. The closer a building interior adheres to this target temperature, the better it was thought to be.

However, recent examinations of ASHRAE comfort databases show that narrow temperature ranges do not provide comfort as predicted. An analysis of occupant surveys show that even so-called ‘ideal’ conditions leave more than 20 percent of the occupants dissatisfied. Furthermore, restricting temperatures to narrow bands is highly energy-intensive. From these and other studies we now know that a building can only provide 100 percent thermal satisfaction when it provides its occupants with effective ‘adaptive’ opportunities, that is the ability for people to control their immediate environment.

These insights led a group of CBE staff, industry partners and others to develop a revision to ASHRAE’s Standard 55 (thermal conditions for human occupancy) that acknowledges the variability in human comfort requirements. This work involved the synthesis of research findings, creative collaboration to draft a solution, and advocacy to move the proposal forward. In early 2020, ASHRAE adopted the revision to the standard with a five-level classification scheme that encourages providing control to occupants. Researchers from CBE, with co-authors and industry partners Taylor Engineering, and TRC, recently published a paper describing this evolution (from which elements of this post are adapted).

While the term ‘adaptive opportunities’ may not be familiar, it applies to many building features and policies that are quite familiar: operable windows and shading systems, open choice of workstations, ceiling fans, personal thermal devices that heat and cool occupants directly, flexible workplace dress codes, and of course thermostats. These vary in terms of effectiveness at providing comfort and also in how easily they can be incorporated into a building, but combinations of convenient and effective adaptive opportunities translate into better indoor comfort for more people.

The new comfort control classification scheme rates the degree of control at five levels, based on the number of options available to individuals. Options include more granular control of ceiling fans and thermostats, personal comfort systems (PCS) and other means. Their eligibility in the standard is determined by what we call the comfort corrective power, the difference in degrees between the ambient temperature and the temperature perceived by a person using a PCS device at its full capability. The classification scheme with examples is summarized in the table below.

Thermal Environmental Control Classification Levels (from ASHRAE Standard 55-2017)


Comfort control classification level Control measure(s) for environmental factors required to achieve level Informative examples meeting comfort control classification levels
1 Each occupant is provided two or more control measures for their personal environment
  • Private office with a ceiling fan and an occupant-adjustable thermostat
  • Shared office with a desktop fan and foot warmer for each occupant
2 Each occupant is provided one control measure for their personal environment
  • Private office with an occupant-adjustable thermostat
  • Shared office with a desktop fan for each occupant
3 The room or thermal zone provides multi-occupant control of at least two control measures in their shared environment.
  • Shared office with an occupant adjustable thermostat and ceiling fan control
4 The room or thermal zone provides multi-occupant control of one control measure in their shared environment.
  • Shared office with an occupant adjustable thermostat
5 No occupant control of any environmental factors
  • Shared or private office with an unadjustable thermostat or no thermostat


While a combination of ambient and personal systems may seem to be a stretch beyond the conventional concept of a building, many of its control measures are familiar, for example thermostats (though they must be granular and accessible) and operable windows, the latter providing a mix of physical and psychological effects. The ceiling fan is an intermediate PCS step, and we propose that this should ultimately become better integrated with HVAC systems for more efficient and individually-responsive environments. CBE has been investigating how to integrate ceiling fans with mechanical systems, both by sequencing the use of fans and mechanical cooling, and by physically integrating the airflows from diffusers and fans.

Since personal comfort systems are currently not considered part of the building’s design or installed equipment, they are included in Standard 55’s Section 6 compliance documentation as a comfort classification system, with an intended audience of third-party green-building rating systems. CBE researchers and others have published numerous papers describing the performance of the compliant types of PCS devices, including the use of heated/cooled office chairs in an actual office building, the use of low-power footwarmers in cool conditions, and studies on combinations of devices.

The new classification approach was created to positively impact building rating systems, such as LEED and WELL, and to provide a resource for building operators. Basing this scheme on the availability of controls to individuals is a promising new approach that we hope will encourage designs and operational practices providing both improved comfort and energy-efficiency. Just as no one flavor of ice cream will suit everyone (though we can’t fathom why not everyone wants Bourbon Praline Pecan) we believe that acknowledging and designing for the variability among individuals will lead to happier and more satisfied building users.


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