How does cross-ventilation differ from stack ventilation, and when is each most effective in tropical design?

• A. Cross-ventilation uses parallel openings on opposite walls for direct air flow; stack relies on vertical air movement through openings at different heights. Cross-ventilation is effective in steady wind conditions; stack is effective when hot air accumulates at the ceiling and doesn't rely on external wind.
• B. Cross-ventilation relies on ceiling fans; stack relies on windows.
• C. Cross-ventilation works only at night; stack works only during day.
• D. Cross-ventilation relies on fans; stack relies on windows.

Answer: (A)

Ventilation in tropical design relies on two different driving forces: wind pressure versus buoyancy from temperature differences. Cross-ventilation uses openings on opposite walls to create a direct air path; when the wind pushes air in from one side and out the other, a breeze moves through the space. It works best when there is a steady prevailing wind and the openings are aligned and unobstructed, so you get a continuous cross-breeze that cools occupants.

Stack ventilation, or buoyancy-driven ventilation, depends on warm air rising. As indoor air heats, it becomes lighter and exits through openings at higher levels, drawing cooler outdoor air in through lower openings. This process doesn’t rely on external wind, so it’s most effective when hot air tends to accumulate near the ceiling or in spaces with tall volumes and well-placed vertical openings.

In tropical design, you’d choose cross-ventilation for days with reliable breezes to maximize immediate cooling, and use stack ventilation when wind is weak or you need to remove heat that collects at higher levels. Often, buildings blend both strategies to maintain comfort across varying conditions.