with no air temperature rise, elevated diffuser air temperatures can lead to higher supply airflow rate and increased fan and chiller energy consumption. The same study found that air temperature rise in summer is higher than in winter and it also depends on the climate. The ground floor with a slab on grade has less temperature rise compared to middle and top floors, and an increase of the supply air temperature causes a decrease in the temperature rise. The temperature rise is not significantly affected by the perimeter zone orientation, the internal heat gain and the window-to-wall ratio. Supply plenum air temperature rise, thus, has implications on the energy saving potential of UFAD systems and their ability to meet cooling requirements with supply temperatures above those of conventional overhead systems. Current research suggests that both energy and thermal performance can be improved in UFAD systems by ducting air to perimeter zones where loads tend to be the greatest. Critics suggest however that such underfloor ducting reduces the benefit of having a low-pressure plenum space, as well as adding design and installation complications when fitting ducts between floor tile pedestals. Air leakage in UFAD plenums Leakage in UFAD supply plenums can be a major cause for inefficiency in a UFAD system. There are two types of leakage—leakage into the space and leakage into pathways that bypass the space. The first category of leakage does not result in an energy penalty because air is getting to the zone it is intended to cool. The second category of leakage increases fan energy in order to maintain a constant plenum pressure, resulting in increased energy use. Careful consideration needs to be paid in the construction phase of UFAD systems to ensure a well-sealed plenum. UFAD and energy The energy assessment of UFAD systems has not been extensive, but some studies indicates potential energy savings due the lower pressure drop and lower air flow rate. Typical plenum pressures are (0.1 inch of water column) or less. UFAD is particularly suitable for buildings with high height ceilings, where the energy saving effect is more pronounced due to thermal stratification. Because UFAD is accomplished by supplying air through a raised floor using different types of distribution configurations and outlets, the key issue for efficient performance of the system is to ensure thermal stratification. The inefficient operation of the UFAD system virtually deteriorated the potential savings presumed from