Image courtesy of author.
By Daniel Overbey
According to the National Oceanic and Atmospheric Administration (NOAA), the wet-bulb globe temperature (WBGT) is a measure of the heat stress from an outdoor environment on the human body in direct sunlight. As such, WBGT takes into account:
- Dry-bulb temperature
- Relative humidity
- Wind velocity
- Sun angle
- Cloud coverage
WBGT is derived from a combination of temperature measurements from three different thermometers:
- Black globe to account for the solar factors of sun angle and cloud cover
- Dry-bulb to account for the ambient air temperature (sensible heat)
- Wet-bulb to account for humidity (latent heat)
Although both WBGT and Heat Index take into consideration dry-bulb temperature and relative humidity, Heat Index is calculated specifically for outdoor shaded areas and does not consider conductive heat transfer from wind speed or the heat stress added by direct sun exposure.
For outdoor activities with sun exposure, WBGT can serve as a good metric by which to monitor thermal safety from heat stress. The U.S. military agencies, the Occupational Safety and Health Administration (OSHA), and various nations use the WBGT as a basic guide to managing a person's workload in outdoor spaces with direct sunlight.
WBGT applied as a passive survivability metric
The major limitation of WBGT is that it is best suited to gauge heat stress in outdoor environments with direct sun exposure rather than indoor environments. However, LEED pilot credit IPpc100: Passive Survivability and Back-up Power During Disruptions defines an option through which building project teams may use the Center for the Built Environment (CBE) Thermal Comfort Tool’s psychrometric chart, demonstrate that indoor conditions (dry-bulb air temperature and humidity) never breach the overheating and under-heating thresholds defined by the pilot credit. These thresholds allow teams to assess the Heat Index and/or WBGT during the peak summer and winter analysis periods.
The pilot credit defines a WBGT summertime overheating threshold and a minimum wintertime dry-bulb temperature threshold for various building types as follows:
For Commercial buildings or buildings without overnight occupancy: No hours during the summertime analysis period permitted to exceed above either the 88°F (31°C) WBGT overheating threshold indicated on the Psychrometric chart in Figure P3-1.
For Residential buildings or building with overnight occupancy (such as lodging): No hours during the summertime analysis period permitted to exceed above either the 82.5°F (28°C) WBGT overheating threshold indicated on the Psychrometric chart in Figure P3-1.
Heating (Winter) : No hours during the wintertime analysis period permitted to fall below 50°F (10°C) (residential or non-residential buildings) dry-bulb air temperature on the psychrometric chart in Figure 3-1.
For hospitals and nursing homes: It is assumed that they will have sufficient backup power to enable them to meet their legal obligations—at no point during the year are they permitted to exceed above 81°F (27°C) on the Heat Index or fall below 71°F (21.7°C) on the Heat Index. This is a backup power requirement and is separate from this passive survivability requirement.
The figure below is an adaptation of the referenced "Psychrometric chart in Figure P3-1" from the Center for the Built Environment (CBE).
Psychrometric chart exhibiting thermal safety thresholds (using WBGT and Heat Index) in relation to the psychrometric chart. Adapted from: Hoyt Tyler, Schiavon Stefano, Piccioli Alberto Moon Dustin, and Steinfeld Kyle, 2013, Psychrometric Chart in Figure P3-1. CBE Thermal Comfort Tool. Center for the Built Environment, University of California Berkeley, http://cbe.berkeley.edu/comforttool/. Figure adapted by Daniel Overbey.
