COOLING BY NATURAL VENTILATION
The Causes of Natural Ventilation. Natural ventilation in
buildings is produced by pressure differences between the inside and the
outside of the building. The magnitude of the pressure difference and the
resistance to flow across the openings in the envelope will determine the rate
of airflow through the openings. The two main forces producing pressure
differences are the wind force and the thermal force or stack effect.
The amount of pressure induced by thermal differences in a building
is directly proportional to the vertical height of the enclosed volume of
heated or cooled air. Tall room volumes will have strong stack effects, while
short room volumes will have little or no stack effects. For low-rise
buildings or in medium to high wind conditions, the stack effect may be
considered negligible in comparison to wind pressure forces. The stack effect
rarely creates enough air movement to cool the occupants directly, but it can
provide enough ventilation for fresh air and health requirements. In
high-rise buildings, the stack effect may cause strong air movement through
elevator shafts and stair towers, but the individual floors are usually
separated from other floors so that the stack effect within the floors will be
small. This handbook emphasizes the use of wind-induced ventilation.
The Cooling Process. Although there are many strategies for
naturally cooling a building, the primary ones are:
a) Convective Cooling--cooling of the occupants and/or of the
structural mass by air movement,
b) Radiant Cooling--heat in the building's structure is discharged
c) Evaporative Cooling--water is evaporated to cool the interior
air or building structure, and
d) Earth Cooling--soil is used as a heat sink and heat is
transferred by direct contact with the soil or through air or water pipes.
Natural ventilation, a form of convective cooling, has the
potential to cool the human body directly through convection and evaporation,
or indirectly by cooling the structure of the building surrounding the
occupants. The choice of cooling strategy is dependent on the climatic
factors, the type of building, and the indoor climate desired.
Bodily Cooling. Bodily cooling is effective during overheated
periods when the temperature and humidity of the air are above the still air
comfort range (refer to para. 2.3 for the definition of the comfort zone).
Bodily cooling is especially useful in hot-humid climates where high humidity
suppresses the range of daily temperature fluctuation making structural
cooling difficult to achieve.
When bodily cooling is desired, buildings should allow maximum
airflow across the occupied area and provide protection from the sun and rain.
Lightweight structures which respond quickly to lower night temperatures are