Assessing Heat Stress for Passive Survivability

With increasing evidence of a shifting climate, many building project teams are confronting the compounded threat of increasingly severe heat waves and the vulnerability of our country's electrical grid. Under conditions of severe heat and discontinuity of utility-provided electricity, passive survivability becomes a critical consideration. If you are unfamiliar with this term, please see this piece by Alex Wilson, president of the Resilient Design Institute. The concept of passive survivability is akin to how you would treat your refrigerator during such conditions - only applied to your home or business: keep the enclosure buttoned up to minimize heat exchange as you buy time awaiting power restoration.

Assessing heat stress

In order to assess the passive survivability of a structure, a project team will need to simulate how a building will perform during extended periods of severe temperatures and no power supply. As heat will pose one of the greatest chronic human health and safety threats as the climate shifts, we will focus on heat stress here. How long might your building thermally "sail" before indoor temperatures becomes a human health and safety risk? This is an essential question of passive survivability. Yet, the building design and construction community is still wrestling with how to most appropriately and effectively gauge heat stress in indoor environments specifically. 

There's a LEED pilot credit for passive survivability

LEED Pilot Credit 100, Passive Survivability and Back-up Power During Disruptions, features an option to provide for passive survivability / thermal safety. The credit allows three paths: 

Path 1 requires that a project team assess the psychrometry of the indoor conditions using either the specified heat index (HI) or wet bulb global temperature (WBGT) threshold values and requirements.

Path 2 requires that teams demonstrate that the de-energized project will stay within livable Standard Effective Temperature (SET) thresholds for a certain period of time.

Path 3 allows credit compliance through Passive House Institute U.S. (PHIUS) or Passive House Institute certification and effective natural ventilation.

In accordance with the LEED pilot credit, the building design and construction industry is lending credence to HI and WBGT as valid proxies to assess heat stress in indoor spaces. However, both metrics have notable shortcomings in such application.

Heat index (HI)

The heat index, sometimes referred to as the apparent temperature, is used to communicate what the ambient air temperature is considered to feel like to the human body when relative humidity is taken into consideration. In other words, the heat index is a reference temperature that considers both the sensible and latent heat of the air. It also typically presumes strenuous activity by the individual. 

As such, HI offers a gauge of heat stress when relative humidity is taken into effect. When the body overheats, it perspires to cool itself off. If the perspiration is not able to evaporate because the air has too much moisture in it, the body cannot regulate its temperature. Therefore, when humidity is high, the rate of evaporation diminishes and heat stress increases.

The HI is widely used in the United States (by comparison, the humidex is commonly used in Canada). However, HI comes with a few caveats when it comes to its application to assess heat stress:

• HI assumes the person is in a shaded area.
• HI does not take into consideration mean radiant temperature (MRT).
• HI does not take air velocity into consideration (i.e., it assumes a dead air space).

Web bulb globe temperature (WBGT)

The Occupational Safety and Health Administration (OSHA) uses HI as an indicator to assess heat stress; although, the organization is quick to point out that HI may not measure worksite heat as accurately as wet bulb globe temperature due to the additional factors that it considers. According to the National Oceanic and Atmospheric Administration (NOAA), WBGT measures heat stress in direct sunlight; as such, it takes into account: temperature, humidity, wind speed, sun angle, and cloud cover (solar radiation). For workers in outdoor environments exposed to direct sunlight, WBGT may serve as a more suitable gauge of heat stress compared to heat index. For interior environments, it is up for debate. 

We may need a new metric; but for now, heat index might be our best shot

The validated, open-source building energy modeling tool, EnergyPlus, has recently had multiple resilience metrics added, including HI, humidex, and SET. However, given the inadequacies of HI and WBGT (which is also in EnergyPlus), the industry might be in need of a psychrometric assessment metric more suitable for gauging passive survivability. In the meantime, choose wisely and be mindful of the caveats with any heat stress assessment metrics utilized.