# Post-Occupancy Evaluation in High-Rise Apartent Buildings in Vietnam
## 3 Results and Discussion
### Impact of OB on the Room Ventilation  
The CO\({}_{2}\) concentrations in the measured bedrooms and living rooms under various room states and seasons are presented as box plots in Figure 11. When unoccupied, the CO\({}_{2}\) levels in these rooms are similar, with a median value of about 470 ppm. Some elevated CO\({}_{2}\) concentration values are recorded shortly after the occupants leave, likely due to the time lag in air exchange.  
During occupied periods without AC usage, most measured CO\({}_{2}\) concentrations remain below 1000 ppm, indicating adequate ventilation. The CO\({}_{2}\) concentrations in the occupied bedrooms during the non-cooling season are slightly higher than those recorded during the cooling season. This can be attributed to the occupants' ventilation habits: windows tend to be closed when outdoor temperatures are too low, thus reducing natural ventilation.  
A significant increase in CO\({}_{2}\) concentrations is observed when ACs are used, particularly in bedrooms, where the median value reaches 1100 ppm, with maximum con  
Figure 11: Comparison of CO\({}_{2}\) concentrations under various room states during different seasons.  
centrations exceeding 2500 ppm. This is primarily due to occupants closing windows to minimize cooling energy loss and, in some cases, closing interior doors when using AC in bedrooms. Since split-type ACs do not bring in outdoor air, ventilation is significantly reduced, leading to higher CO\({}_{2}\) concentrations in enclosed spaces.  
The significant variation in CO\({}_{2}\) measurements can be attributed to differences in ventilation habits and AC usage intensities. To further analyze the impact of occupant behavior on the indoor environment, temperature and CO\({}_{2}\) data from the bedrooms of two representative apartments (AP 25 and AP 49) are visualized as heat maps with a high temporal resolution (15 min intervals), as shown in Figure 12. Among all the apartments investigated, AP 25 exhibits relatively high AC usage, whereas the occupants of AP 49 never use AC.  
During the cooling season, the bedroom in AP 49 maintains relatively high temperatures due to the absence of AC usage. In contrast, AP 25 shows significant temperature drops at certain times, indicating AC usage. The occupants of AP 25 typically operate the AC between 20:00 and 6:00 the following day, during which CO\({}_{2}\) concentrations also increase noticeably. This suggests that windows and interior doors are kept closed to minimize cooling energy loss. Conversely, the bedroom in AP 49 remains well-ventilated throughout the cooling season. Given that AC use can negatively impact indoor air quality and pose health risks, appropriate measures are needed to ensure adequate ventilation while saving cooling energy.  
During the non-cooling season, temperature patterns remain relatively stable, with no significant fluctuations, indicating the absence of space heating. However, inadequate ventilation frequently occurs in AP 25, and elevated CO\({}_{2}\) concentrations are observed in AP 49, particularly in January and February. As mentioned earlier, occupants tend to close windows to reduce heat loss in cold weather. Additionally, in both apartments, bedroom doors are commonly kept closed during sleeping hours, further restricting outdoor air  
Figure 12: Comparison of room temperatures and CO\({}_{2}\) concentrations in the bedrooms of representative apartments.  
supply and resulting in inadequate ventilation. Since tolerance to cold conditions varies among individuals, the frequency of closing windows in winter differs between apartments, explaining the variation in the inadequate ventilation between AP 49 and AP 25 during the non-cooling season.