The Thermal Chimney
This pattern is shaped by
Problem
When a building needs to move air without wind, the occupant faces a choice: run a fan, which costs energy and noise, or sit in stillness, which costs comfort. But air wants to move when temperatures differ — warm air rises, cool air falls. A building that captures this force can breathe on its own. The tension: how to create reliable airflow without depending on breezes that may not come, and without consuming energy to manufacture what physics offers for free.
Evidence and Discussion
The principle is ancient. Roman hypocausts moved hot air upward through wall cavities. Persian wind towers combined stack effect with evaporative cooling. The physics is straightforward: warm air is less dense than cool air, so it rises. A vertical shaft, heated by the sun or by the building's internal gains, creates a column of rising air. As this air exits at the top, it draws replacement air in at the bottom. The taller the shaft, the greater the pressure difference; the warmer the shaft relative to inlet air, the faster the flow.
The Eastgate Centre in Harare, Zimbabwe, designed by architect Mick Pearce and engineer Ove Arup, opened in 1996 as the most-cited example of this approach. The building uses no conventional air conditioning. Instead, heavy masonry floors and walls absorb heat during the day, while a system of vertical shafts — thermal chimneys — draws air through the building. Cool night air flushes the thermal mass; during the day, the mass absorbs occupant and equipment heat while the chimneys continue to ventilate. Published accounts claim energy consumption 35% lower than comparable Harare buildings, though primary measurement data remains difficult to locate in peer-reviewed literature. What is documented: the building has operated without conventional HVAC for nearly three decades in a climate where afternoon temperatures regularly exceed 30°C.
The BedZED development in London (2002), designed by Bill Dunster Architects with Arup, uses colorful wind cowls atop each unit — hybrid devices that combine wind-catching with stack-effect ventilation. When wind is present, the cowls scoop it into the building; when air is still, thermal buoyancy in the shafts continues to draw stale air out. Monitoring by the BioRegional Development Group found that the passive ventilation system provided adequate air quality in most conditions, though some residents installed supplementary mechanical ventilation for summer comfort — a reminder that passive systems work within limits.
Alexander's original Pattern 162, "North Face," addressed passive cooling through orientation and shading but did not elaborate the mechanics of stack ventilation. This pattern fills that gap: the thermal chimney is the engine that makes natural ventilation work on windless days.
Therefore
where a building requires summer cooling, include at least one thermal chimney — a vertical shaft, glazed or dark-surfaced on its sun-facing side, rising from the occupied zone to an outlet above the roofline. Size the shaft to provide at least 0.3 square meters of cross-sectional area per 50 square meters of floor served. Locate cool-air inlets low on the shaded side of the building, sized to match or exceed the chimney outlet area. The test: on a still day with outdoor temperature at 28°C and the chimney sun-heated, hold a tissue at the inlet opening — it should draw inward with visible movement. If the air does not flow, the chimney is too short, the inlet too small, or the path too restricted.