Standard Effective Temperature (SET) and Thermal Comfort
Recently, the U.S. Green Building Council adopted three new pilot credits on resilient design for use by LEED project teams for innovation credit.
One pilot credit in particular, Passive Survivability and Functionality During Emergencies, endeavors to ensure that buildings will maintain reasonable (i.e., "survivable") functionality, including access to potable water, in the event of an extended power outage or loss of heating fuel. Power outages are frequently one of the primary impacts of natural disasters and there are growing concerns about terrorist actions targeting energy infrastructure.
This Passive Survivability pilot credit includes three options, two of which are required to earn a LEED point. A detailed description of all three Resilient Design pilot credits can be found on the Resilient Design Institute's website at resilientdesign.org.
Although the Passive Survivability pilot credit language is fairly self-explanatory, one of the three options introduces a thermal comfort metric that will be unfamiliar to many design professionals - standard effective temperature (or SET). Option 1 of the pilot credit addresses thermal resilience and requires thermal modeling to demonstrate that a building's interior environment will maintain “livable temperatures” during a power outage that lasts seven days during the peak summertime and wintertime conditions of a typical year. The credit language goes on to carefully defined the parameters of "livable temperatures" with regard to SET.
So, What is SET?
ANSI/ASHRAE Standard 55-2010 defines SET as follows:
temperature, standard effective (SET): the temperature of an imaginary environment at 50 percent [relative humidity], less than 0.1 meters per second air speed, and [the mean radiant temperature equals the air temperature], in which the total heat loss from the skin of an imaginary occupant with an activity level of 1.0 met and a clothing level of 0.6 clo is the same as that from a person in the actual environment, with actual clothing and activity level.
SET is essentially the dry-bulb air temperature of a hypothetical environment at 50 percent relative humidity for occupants wearing clothing that would be standard for the given activity in the real environment. The SET assumes a standard environment in which both air and surface temperatures are the same and the air velocity is below 0.1 meters per second. SET also incorporates a two-node method to represent human physiological factors (such as skin temperature and skin wettedness).
In simpler terms, SET is a temperature metric that factors in relative humidity, mean radiant temperature, and air velocity, while also considering the anticipated activity rate and clothing levels. SET is essentially a comprehensive comfort index that endeavors to incorporate all six basic physical factors of thermal comfort as well as physiological considerations. (Please note that human thermal comfort is also influenced by psychological factors.)
Using SET to Define "Livable Temperatures" in Option 1 - Thermal Resilience
"Livable temperatures" in Option 1 are defined as a SET range between 54 degrees Fahrenheit and 86 degrees F. Deviations from this range are limited to a certain number of degree-days (or degree-hours) using SET resultants during peak winter and summer conditions.
It gets a bit complicated. For single-family and multifamily residential buildings during a one-week period occurring over the summertime peak, the building may not exceed 86 degrees F SET for more than 9 SET degrees F degree-days (216 degrees F SET hours). During the wintertime peak, the building may not drop below 54 degrees F SET for more than 9 SET degrees F degree-days throughout a one-week period.
For non-residential buildings, the wintertime criteria are the same as for residential, but in the summertime, a greater deviation above 86 degrees F SET is permitted: 18 degrees F SET degree-days (432 degrees F SET degree-hours) during a one-week period.
Essentially, a project team pursuing Option 1 needs to use thermal modeling to demonstrate that the building will maintain conditions within the "livable temperature" parameters. Such performance may be realized through a balanced combination of a highly insulated building envelope, high-performance glazings, optimized orientation, passive heating and cooling strategies, appropriate shading, and other energy conservation measures.
For more information about the LEED Pilot Credits on Resilient Design, please visit: www.resilientdesign.org/leed-pilot-credits-on-resilient-design-adopted.
