Designing a building such that its interiors may allow for passive survivability and the ability for occupants to shelter in place during an extreme temperature event can be assessed using the standard effective temperature (SET) metric. 

What is standard effective temperature (SET)?

Developed by A.P. Gagge and accepted by ASHRAE in 1986, ASHRAE Standard 55 defines SET as follows:

The temperature of an imaginary environment at 50 percent relative humidity, less than 20 feet per minute (fpm) average air speed, and mean radiant temperature equals average 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.

Basically, SET is a comfort indicator that considers indoor dry-bulb temperature, relative humidity, mean surface radiant temperature, and air velocity, as well as the activity level and clothing of occupants in the space. 

Defining "livable conditions" using SET

LEED pilot credit IPpc100: Passive Survivability and Back-up Power During Disruptions allows project teams to prove that a building will maintain passive survivability during an extreme temperature event by maintaining livable (not necessarily comfortable) conditions indoors if the building loses all mechanical functionality for a week. 

In accordance with industry guidance, the pilot credit defines "livable conditions" as maintaining an SET between 54 degrees Fahrenheit and 86F. 

Allowing SET degree-hours beyond the passive survivability thresholds

This might sound simple enough: design a building such that critical indoor spaces maintain livable conditions during an extreme hot week and extreme cold week.

However, even buildings constructed to today's most up-to-date energy codes may struggle to stay within the SET range of 54F to 86F. If the SET does breach either threshold, it does not necessarily mean that the building would not achieve suitable passive survivability. As such, the LEED pilot credit adopts the industry's established parameters to define when the cumulative SET values beyond the 54F or 86F threshold (expressed as SET degree-hours) over a seven-day period indicate inhabitable conditions:

Cooling (residential): Not to exceed 216F SET-hours above 86F SET

Cooling (non-residential): Not to exceed 432F SET-hours above 86F SET

Heating (all buildings): Not to exceed 216F SET-hours below 54F SET 

Passive survivability is easier said than done

Achieving passive survivability will take rigor and intentionality by project teams. The data I have seen to date indicates that the majority of buildings in most climate zones would struggle to meet the SET compliance path for the passive survivability pilot credit.

Our current energy codes will not get us there in most cases. Consider the following example: A single-family residential structure designed to meet the 2021 International Energy Conservation Code (IECC) located in a hot-humid climate similar to New Orleans, Louisiana. 

During the extreme hot week, outdoor temperatures routinely surpassed 100F. The SET inside the house was pulled above 86F threshold for several consecutive days. The SET degree-hours beyond the 86F threshold surpassed 900 (Figure 1). 

The project did not fare well during the extreme cold week either. The house drifted below the 54F threshold by over 1,100 degree-hours (Figure 2).

We need more industry support if design teams are really going to dig-in on using SET to assess passive survivability 

From various accounts, I perceive three issues holding the building design and construction industry back from being able to design for passive survivability with greater rigor and effect using SET:

1. We need more information resources. The SET metric is not new, but most of us in the industry have never before been prompted to consider SET and cumulative SET degree-hours are esoteric. I hope this column helps readers start to visualize how SET can be used to assess passive survivability. I wrote this because I could not find simplified, clear information to help me understand what the SET compliance path in the LEED pilot credit could look like so that I could explain it to others.

2. We need accessible (and graphic) tools. Most of our early-stage, user-friendly, building energy modeling tools do not exhibit modules to examine SET degree-hours. EnergyPlus calculates and reports SET as a time-step report variable, but we need to migrate this functionality to platforms with accessible, visually-engaging interfaces.

3. We need updated energy codes. Data suggests that the gulf between the vast majority of jurisdictional energy code requirements and what would be required to achieve passive survivability using SET is vast. Updated energy codes will close the gap and make passive survivability achievable for more projects.