This research is to assess effects of a partition on thermal comfort, indoor air quality (IAQ), energy consumption, and perception in an air-conditioned space via computational fluid dynamics (CFD) analysis. The variables of indoor air are numerically determined before/after installation/removal of a partition. Accordingly, predicted mean vote (PMV) of thermal comfort, carbon dioxide concentration, rate of energy consumption in making up air, and an overall perception index are proposed to quantify effects in a partitioned space. For a case study, a partition is used to tightly separate a study area from a rest area in a library during peak time. The CFD analysis is performed so that the mean differences between the measured and simulated variables at 14 locations are less than 5%. After partitioning in the CFD analysis, it is found that the average PMV value decreases to −1.4 in the rest area, and it remains at −0.7 in the study area where occupants perceive a slightly cool sensation. In the study area, the carbon dioxide concentration increases to 450–500 ppm, while the rate of energy consumption increases by 8.3%. From the overall perception index of 0.9, the occupants feel spacious in the partitioned areas. Therefore, installing the partition is encouraged with the recommendation that cooling supply can be reduced for energy savings. It is apparent that the proposed methodology yields quantitative indicators for decision making of installation/removal of partitions. The interior investigation of partitions in buildings can be performed before making real physical changes.