Structural Slab
Because the supply air in the underfloor plenum is in direct contact with the concrete structural slab, a number of energy- and cost-saving strategies are possible with underfloor air distribution systems. The underfloor thermal mass also has the effect of providing a consistent cool air temperature reservoir (for cooling applications), making UFAD systems extremely stable in their operation.
 

Thermal storage strategies can be used in temperate climates (cool night air temperatures, low humidity), where ventilating the underfloor plenum with nighttime air (termed nighttime pre-cooling, or night ventilation) effectively cools the slab overnight. During the following day's cooling operation, higher supply air temperatures from the mechanical system can be delivered to the underfloor plenum, thereby reducing refrigeration loads for at least part of the day. This 24-hour thermal storage strategy benefits from lower off-peak utility rates, extends the hours of economizer operation, and enables potential downsizing of equipment.