to the appropriate supply conditions so it can be delivered to the occupied zone. While overhead systems typically use ducts to distribute the air, UFAD systems use the underfloor plenum formed by installation of a raised floor. The plenum generally sits above the structural concrete slab, although lower heights are possible. Specially designed floor diffusers are used as the supply outlets. The most common UFAD configuration consists of a central air handling unit delivering air through a pressurized plenum and into the space through floor diffusers. Other approaches may incorporate fan powered terminal units at the outlets, underfloor ducts, desktop vents or connections to Personal Environmental Control Systems. UFAD air distribution and stratification Thermal stratification is the result of processes which layer the internal air in accordance with relative density. The resulting air stratum is a vertical gradient with high-density and cooler air below and low-density and warmer air above. Due to the naturally convective movement of air, stratification is used predominantly in cooling conditions. UFAD systems capitalize on the natural stratification that occurs when warm air rises due to thermal buoyancy. In a UFAD design, conditioned air stays in the lower, occupied part of the room, while heat sources such as occupants and equipment generate thermal plumes, which carry the warm air and heat source generated pollutants towards the ceiling where they are exhausted through the return air ducts. The temperature stratification created by the UFAD system has implication for space setpoints. Most of an occupant's body is in an area that is colder than the temperature at the thermostat height; therefore, current practice recommends raising thermostat setpoints compared to traditional overhead systems. The optimal ventilation strategy controls the supply outlets to limit the mixing of supply air with room air to just below the breathing height of the space. Above this height, stratified and more polluted air is allowed to occur. The air that the occupant breathes will have a lower concentration of contaminants compared to conventional uniformly mixed systems. The theoretical behavior of UFAD systems is based on the plume theory for DV systems. In comparison to classic displacement ventilation (DV) systems that deliver air at low velocities, typical UFAD systems deliver air through floor diffusers with higher supply air velocities. In addition to increasing the amount of mixing (and therefore potentially diminishing the ventilation performance compared to DV systems), these more powerful supply air conditions can