The control of room-air stratification is critical to the design and operation of successful underfloor-air-distribution (UFAD) systems, representing an oftentimes complex balancing act: Increasing stratification by reducing airflow or mixing for a given space heat load saves energy, while decreasing stratification by boosting airflow or mixing for a given space heat load improves occupant comfort.

The Center for the Built Environment (CBE) has conducted full-scale laboratory experiments to study the impact of stratification in interior and perimeter office spaces. Because the findings cover topics that are of considerable interest to the industry at this time, the following interim guidelines are offered. Each of the topics outlined in this article require more research for specific guidelines to be offered. For a more complete overview of UFAD design requirements, see “Underfloor Air Distribution (UFAD) Design Guide” and the Underfloor Air Technology Web site (www.cbe.berkeley.edu/underfloorair).

The use of an underfloor plenum to deliver conditioned air directly into an occupied zone of a building is a key feature distinguishing underfloor-air-distribution (UFAD) systems from conventional ducted overhead systems. Current research (www.cbe.berkeley.edu/underfloorair) and design practices are focusing on two plenum-related topics that can have significant impacts on the successful operation of UFAD systems:

- Air leakage from pressurized plenums.

- Underfloor-plenum-supply-air heat gain (thermal decay)

This article will present updated design and operating guidelines addressing both of these topics.