The energy and comfort performance of buildings using underfloor air distribution (UFAD) has been of interest, with some contention, in the building industry for many years. It is not often that an opportunity to address that question directly appears. This project represented such an opportunity to compare and contrast two similarly designed buildings in the same climate and co-located near one another, both occupied by California state employees, one with a UFAD system (B225) and the other with an overhead (OH) variable air volume (VAV) system (B172). At the outset there was hope that we could settle the question definitively due to the highly instrumented systems in these buildings that afforded an opportunity to use measured data for the comparison. This report contains a detailed description of the measured data and simulated analyses used to compare the energy performance of UFAD vs. OH systems, and a summary of the post occupancy evaluations (POE) used to study and compare the occupant satisfaction and indoor environmental quality (IEQ) of both buildings. In addition, we report on field measurements conducted in B225 to investigate two key performance issues with UFAD systems: (1) room air stratification and (2) temperature gain in underfloor plenums. The key findings from the study are listed here: • The measured energy performance data indicates improved efficiency for the UFAD system (B225) vs. the OH system (B172), as annual cooling energy is 31% higher and total annual fan energy is 50% higher for B172 compared to B225. • To account for all major design and operating differences between the buildings, we developed an alternative analysis method based on estimating the impact on B172 energy performance as if it was configured and equipped with central system equipment similar to B225; aka “apples to apples” comparison. When this “apples to apples” comparison method was applied, the total annual HVAC energy use (including cooling, heating, and fans) for OH (B172) is 20% higher than UFAD (B225) and total annual whole building energy use for B172 is 8% higher than B225. • Based on the calculated Energy Star ratings, both B225 and B172 demonstrate excellent energy performance overall. B225 showed a very high site Energy Star rating of 98 and the Energy Star rating for the B171-174 complex (B172 could not be calculated separately) was 91, both well above the 75 required to receive the Energy Star label. • The final POE surveys conducted during October 2007 in the two buildings found that the satisfaction ratings were generally positive and very nearly the same for most of the categories. An important lesson learned from the repeated surveys in B225 between 2003 and 2007 was the value of continuous commissioning of a building’s HVAC system. Efforts by building operations staff and the research team led to an improved understanding of the unique features of the UFAD system, and as a result, greater occupant satisfaction with the quality of the indoor environment in B225.