Boost Happiness and Save Energy with Adaptive Thermal Comfort

The most common complaint facility managers hear from building occupants is that their office space is too cold. That would seem an easy enough problem to solve, except that the number two complaint is that it’s too hot.

Different people, it turns out, are comfortable under different conditions, and keeping everyone comfortable at the same time is an elusive goal. Offices are considered thermally successful if just 80% of their occupants are reasonably comfortable at any given time. This is the goal laid out by ASHRAE in the industry’s gold standard of comfort—Standard 55, Thermal Environmental Conditions for Human Occupancy.

Humans have been creating spaces to provide thermal comfort for eons. The long-established components of comfort include air temperature, mean radiant temperature, humidity, and air velocity within the space, along with the personal factors of clothing insulation and activity level. But our understanding of what makes a space comfortable is still evolving, and these components, we’re discovering, represent only part of the puzzle of thermal comfort.

A collection of less tangible components also plays a role. These include the amount of control occupants have over the thermal conditions of their environments and their expectations stemming from the weather, the season, and even their own habits and culture. We expect outdoor temperatures to be colder during the winter, for example, and we actually shift our range of indoor comfort slightly to accommodate the corresponding shift outside. In the thermal comfort world, elements related to our expectations and level of control are grouped under the term “adaptability.” Adaptability plays a significant role in determining the comfort of building occupants. ASHRAE Standard 55 focuses primarily on the “analytical” model, which sets relatively narrow acceptable temperature ranges based on the six basic thermal comfort factors. But even Standard 55 recognizes adaptability; since 2004, it has included an expanded comfort zone for optional use in naturally ventilated spaces.

The adoption of a more adaptive comfort model is significant for the world of green building. If we can achieve occupant comfort with greater swings in temperature or humidity, then less energy is needed to condition the air.

The acceptance of an adaptive model could also result in buildings that keep more people comfortable more of the time—which might increase productivity, keep people healthier, and have other benefits. This report takes a look at how our evolving understanding of comfort could shift our expectations for buildings, encourage the design of naturally ventilated and mixed-mode buildings, and increase occupant comfort and productivity, all while reducing operating costs and saving energy.