Leveraging Thermal Comfort to Achieve High Performance
December 14, 2018
By Alissa Kingsley
User control may seem antithetical to high-performance buildings: imagine the amount of energy that might be wasted when building occupants are able to adjust the thermostat or open and close windows at will.
Occupants who have perceived control of temperature, however, tend to be more comfortable in their environment. Is it possible for architects to design buildings that are high performance and allow occupants thermal control?
Yes, but to achieve that goal, it is critical to understand a building’s use and the comfort of its occupants when designing for high performance.
The basics of thermal comfort
Thermal comfort is a subjective state. It is both psychological and physiological, and as such is one of the most complex but important aspects of building design.
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There are three types of heat transfer: conduction (transfer through direct contact with solid materials, like holding a hot cup of coffee), convection (transfer through liquids and gasses, like feeling colder when it’s windy) and radiation (transfer through electromagnetic waves, like feeling hot when close to a fire).
In addition, there are six factors that influence thermal comfort:
- Air temperature: the temperature of the air in the space. This is the factor most think of when considering comfort (it’s too hot, too cold or just right).
- Humidity: the moisture in the air. With higher humidity, perspiration evaporates less efficiently, resulting in a given air temperature feeling warmer.
- Mean radiant temperature: the perceived temperature in an environment, created by the average of the air temperature and radiant temperature of all facing surfaces. Mean radiant temperature explains why it can feel cold in the winter adjacent to a large window, even when the room is heated to a “comfortable” 72ºF air temperature. The cool window surface radiantly cools your body.
- Air speed: the rate of air movement. With higher air speeds (more air movement, within limits), we perspire more efficiently and increase convective heat loss, resulting in a given air temperature feeling cooler.
- Metabolic rate: the rate of transformation of calories into heat and mechanical work by metabolic activities within an organism. This is directly related to activity level in an environment. With lower levels of physical activity, warmer air temperatures are necessary for comfort, as well as the converse.
- Clothing insulation: the increased resistance to sensible heat transfer obtained from increasing the amount of clothing. Increasing the amount of clothing insulation decreases the air temperature that is comfortable.
Inefficiency + discomfort = business as usual
Most modern work spaces are heated and cooled to a uniform standard that addresses only air temperature and sometimes humidity, often resulting in excessively conditioned environments. Not only is this an inefficient use of energy, but it also leaves most building occupants dissatisfied. Building occupants can adjust their clothing level or use personal comfort devices to control their immediate space, but these are not holistic solutions.
When designing the Kendeda Building for Innovative and Sustainable Design for the Georgia Institute of Technology, a team of architects, engineers, and analysts looked for:
- alternative solutions to address thermal comfort
- a range of standards based on space function rather than a uniform standard throughout the building
Considering all six comfort factors
This interdisciplinary education building, which is located in the heart of Atlanta and slated for completion in 2019, is pursuing Living Building Challenge Certification, one of the most progressive green building certification programs. There are 20 required imperatives to Living Building Challenge Certification, including a requirement for net-positive energy.
To achieve this, the Kendeda Building will generate its own power through a large photovoltaic (PV) array sized to exceed the projected operational energy requirements. This is no easy task in Atlanta, where cooling and dehumidification loads are high due to both high sensible loads (high summer air temperatures) and high latent loads (accompanying high humidity). The energy budget was limited based on the amount of energy the project can generate on site, and cooling systems can be energy intensive.
For decades, building designers have primarily considered air temperature and humidity, which are the easiest thermal comfort factors to measure and therefore control, but the Kendeda Building team is considering all six factors of thermal comfort, rather than the traditional two. This comprehensive approach allows for a broader thermal comfort zone with higher-than-typical cooling setpoints.
No conventional AC in the Deep South?
The team decided to forgo traditional air conditioning and instead will utilize a radiant heating and cooling system coupled with ceiling fans to generate greater air velocity throughout the building. A dedicated outdoor air system (DOAS) will provide dehumidified ventilation air, controlling humidity in the building more efficiently than an all-air conventional system.
The building will also optimize passive systems to achieve thermal comfort, including external operable blinds in select locations and triple glazing throughout. The large PV array creates a canopy to shade the building, thus reducing mechanical interior cooling requirements.
Operable windows are a requirement of the Living Building Challenge and will allow end users to independently control their thermal comfort. The windows will be tied to a weather station, which will inform the building and its occupants if the climatic conditions are optimal for open windows. If they are, the building systems will open them, and when conditions shift, this system will close them. To balance the concerns of too much heat and humidity in the building against the occupants’ ability to control the systems, each operable window zone is equipped with a timed override to reverse the window position (open to closed and closed to open) that occupants can use to control their space. The same controls will automatically close windows when conditions are not favorable for natural ventilation, with overrides that deactivate a space’s radiant cooling if windows are opened.
Solutions by space type
As a multidisciplinary academic facility, the Kendeda Building will have a wide range of programmed spaces—classrooms, a maker space, offices and open collaboration spaces, to name a few—and their related activities happening simultaneously. Each of these spaces has different, tailored sets of heating and cooling setpoints to accommodate the planned activity level and the number of bodies in a given space.
The team is also taking Georgia Tech’s campus culture into account. The building will be primarily occupied by students who are not encumbered by professional dress codes, and in the summer, they will be dressed appropriately for the hot, humid outdoors. The building will offer students respite from the heat without making it so cold that they need to bring a sweater with them—a common occurrence in Atlanta’s conventionally air-conditioned buildings. This will be a cultural shift for those accustomed to traditionally cooled buildings in the Southeast. As more designers employ an adaptive approach to thermal comfort, building occupant expectations will evolve over time.
Lastly, during the year-long performance period prior to Living Building Challenge certification that will follow the building’s completion, Georgia Tech will measure the building’s performance and make changes to the setpoints based on post-occupancy surveys.
The Kendeda Building for Innovative and Sustainable Design is intended to show that enriched thermal environments can be achieved in a sustainable manner and still offer some control to occupants.
Thermal comfort must be considered for any building striving toward high performance, and when developing systems for controlling the indoor environment, the type of building and use, the climate, including seasons, and the culture of the place should all be taken into account.
Alissa Kingsley, R.A., is an associate at Lord Aeck Sargent, where her areas of expertise include disaster-response architecture and design-build services for underserved communities. Lord Aeck Sargent, a Katerra company, is an award-winning architecture, planning, landscape architecture, and interior design firm. For more information, visit the firm website at www.lordaecksargent.com.
Kendeda Building Project Team
Architects: Lord Aeck Sargent, The Miller Hull Partnership
Landscape Architect: Andropogon
Mechanical, Plumbing, and Electrical Engineer: PAE, Newcomb and Boyd
Structural Engineer: Uzun and Case
Civil Engineer: Long Engineering
Net-Positive Water Engineer: Biohabitats
Code Analysis: Jensen Hughes