text: Two regimes of stack effect can exist in buildings: normal and reverse. Normal stack effect occurs in buildings which are maintained at a higher temperature than the outdoor environment. Warm air within the building has a low density (or high specific volume) and exhibits a greater buoyancy force. Consequently, it rises from lower levels to upper levels through penetrations between floors. This presents a situation where floors underneath the neutral axis of the building have a net negative pressure, whereas floors above the neutral axis have a net positive pressure. The net negative pressure on lower floors can induce outdoor air to infiltrate the building through doors, windows, or ductwork without backdraft dampers. Warm air will attempt to exfiltrate the building envelope through floors above the neutral axis. 
  Mechanical refrigeration equipment provides sensible and latent cooling during summer months. This reduces the dry-bulb temperature of the air within the building relative to the outdoor ambient air. It also decreases the specific volume of the air contained within the building, thereby reducing the buoyancy force. Consequently, cool air will travel vertically down the building through elevator shafts, stairwells, and unsealed utility penetrations (i.e., hydronics, electric and water risers). Once the conditioned air reaches the bottom floors underneath the neutral axis, it exfiltrates the building envelopes through unsealed openings such as through dampers, curtainwall, etc. The exfiltrating air on floors underneath the neutral axis will induce outdoor air to infiltrate the building envelope through unsealed openings.