Tiered Clerestory Natural Air Stack Convection Drafting in Balinese Wantilans

In the equatorial maritime climate of Bali, temperatures remain consistently high throughout the year, hovering between 28°C and 34°C, accompanied by relative humidity levels that routinely exceed 80% to 90%. In such an environment, the primary challenge of architectural comfort is not heating or mechanical air tightness, but the continuous dissipation of heat and moisture. When relative humidity is exceptionally high, the human body cannot cool itself effectively through radiant heat loss; its sole remaining physiological defense is the evaporation of perspiration from the skin.

Evaporative cooling on the human body, however, stalls in stagnant air because a boundary layer of moisture-saturated air forms immediately over the skin. To restore comfort, architecture must induce continuous, uninterrupted air movement across the body at velocities between 0.5 and 1.5 meters per second. Long before the invention of electric fans or mechanical HVAC compressors, the master builders (undagi) of Bali solved this physiological challenge through the brilliant thermodynamic engineering of the wantilan.

The core thermodynamic engine of the wantilan is the stack effect, governed by the physical principle of thermal buoyancy. In any high-volume architectural space, warm air expands, becomes less dense than the surrounding cooler air, and rises naturally toward the highest point in the structure according to Archimedes’ principle. In a poorly designed building with an unventilated roof, this rising heat accumulates beneath the ceiling, creating an intense radiant heat pocket that reradiates downward onto the occupants below.

In a wantilan, the multi-tiered tumpang roof turns this natural phenomenon into an active exhaust pump. By separating the monumental roof into overlapping tiers with an open, unglazed vertical clerestory gap of 800 to 1200 millimeters, the undagi creates a continuous natural chimney. Solar radiation striking the black sugar-palm fiber thatch warms the air in the upper apex cone to temperatures several degrees higher than the ambient air below. This superheated air rises aggressively and exhausts out through the clerestory openings into the passing wind stream. As this hot air is continually expelled from the top of the pavilion, it generates a continuous low-pressure suction at floor level, drawing fresh, cool ambient air into the pavilion from all four open perimeter directions in a perpetual convective siphon.

The effectiveness of the thermal stack exhaust is entirely dependent on the complete absence of exterior walls. In a wantilan, there are no solid brick facades, glass partitions, or airtight doors. The entire perimeter is 100% open to the tropical atmosphere, supported solely by the rhythmic colonnade of slender carved teak posts. This absolute porosity eliminates aerodynamic resistance, allowing ambient ground-level cross-breezes—whether diurnal sea breezes from the Indian Ocean or mountain breezes descending from the volcanic caldera of Mount Batur—to sweep unimpeded through the pavilion.

To optimize this cross-flow, the pavilion is constructed atop an elevated volcanic andesite stone podium known as the batur. In fluid dynamics, air movement immediately adjacent to the ground is significantly retarded by friction against grass, shrubs, and topographical irregularities (the planetary boundary layer). By elevating the pavilion floor 600 to 1000 millimeters above the surrounding ground level, the living plane is lifted above this stagnant friction zone, intercepting higher-velocity laminar wind currents. The raised platform also features broad, deep overhanging thatched eaves that extend 1.5 to 2.0 meters past the column line, completely shading the floor terrace from direct solar radiation so the stone platform remains pleasantly cool under bare feet.

A critical engineering paradox of the open clerestory is rain protection. In Bali’s tropical monsoon season, precipitation does not fall solely as gentle vertical showers; it frequently arrives as violent, torrential squalls accompanied by driving horizontal wind gusts. Without precise geometric detailing, rain would blow straight through the open clerestory gap, drenching the central pavilion floor and ruining sacred gamelan instruments.

Traditional Balinese carpentry overcomes this challenge through sophisticated overlapping eave geometry. The eaves of the upper pyramidal roof tier are extended downward and outward so that they project at least 500 to 600 millimeters past the outer plane of the clerestory opening, creating an overlapping horizontal shadow baffle. As driving rain strikes the upper roof, it is thrown clear of the clerestory opening by the steep 50-degree pitch. Within the clerestory throat itself, builders often mount angled, fixed timber louvers (jalusi) pitched at 45 degrees. These louvers create a tortuous pathway that causes driving rain droplets to lose momentum and drain downward onto the lower roof, while offering negligible resistance to rising warm air currents, ensuring that the wantilan maintains continuous, non-stop natural ventilation and supreme thermal comfort even in the midst of a tropical monsoon storm.