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external pressure (in the absence of wind). Above the neutral plane, the internal air
pressure will be positive and air will flow out of any intermediate level apertures
created (Figure 8.6). Below the neutral plane the internal air pressure will be negative
and external air will be drawn into the space through any intermediate level apertures.

Buoyancy-driven ventilation has several significant benefits:
 Does not rely on wind: can take place on still, hot summer days when it
is most needed.
 Stable air flow (compared to wind)
 Greater control in choosing areas of air intake
 Sustainable method

Limitations of buoyancy-driven ventilation:
 Lower magnitude compared to wind ventilation on the windiest days
 Relies on temperature differences (inside/outside)
 Design restrictions (height, location of apertures) and may incur extra
costs (ventilator stacks, taller spaces)
 The quality of air it introduces in buildings may be polluted for example
due to proximity to an urban or industrial area (although this can also be a factor in
wind-driven ventilation)

Natural ventilation in buildings can rely mostly on wind pressure differences in
windy conditions, but buoyancy effects can
a) augment this type of ventilation and
b) ensure air flow rates during still days.
Buoyancy-driven ventilation can be implemented in ways that air inflow in the
building does not rely solely on wind direction. In this respect, it may provide
improved air quality in some types of polluted environments such as cities. For
example, air can be drawn through the backside or courtyards of buildings avoiding
the direct pollution and noise of the street facade. Wind can augment the buoyancy
effect, but can also reduce its effect depending on its speed, direction and the design
of air inlets and outlets. Therefore, prevailing winds must be taken into account when
designing for stack effect ventilation.

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