Page 69 - 6848

wind pressure will vary considerably creating complex air flows and turbulence by
its interaction with elements of the natural environment (trees, hills) and urban
context (buildings, structures). Vernacular and traditional buildings in different
climatic regions rely heavily upon natural ventilation for maintaining thermal comfort

conditions in the enclosed spaces.
Design guidelines are offered in building regulations and other related
literature and include a variety of recommendations on many specific areas such as:
 Building location and orientation
 Building form and dimensions
 Indoor partitions and layout
 Window typologies, operation, location, and shapes
 Other aperture types (doors, chimneys)
 Construction methods and detailing (infiltration)
 External elements (walls, screens)
 Urban planning conditions

Some of the important limitations of wind driven ventilation:
 Unpredictability and difficulties in harnessing due to speed and direction
variations
 The quality of air it introduces in buildings may be polluted for example
due to proximity to an urban or industrial area
 May create a strong draught, discomfort.

Buoyancy driven ventilation arise due to differences in density of interior and
exterior air, which in large part arises from differences in temperature. When there is
a temperature difference between two adjoining volumes of air the warmer air will
have lower density and be more buoyant thus will rise above the cold air creating an
upward air stream.
Forced upflow buoyancy driven ventilation in a building takes place in a traditional
fireplace. Passive stack ventilators are common in most bathrooms and other type of spaces
without direct access to the outdoors.

In order for a building to be ventilated adequately via buoyancy driven
ventilation, the inside and outside temperatures must be different. When the interior
is warmer than the exterior, indoor air rises and escapes the building at higher
apertures. If there are lower apertures then colder, denser air from the exterior enters
the building through them, thereby creating upflow displacement ventilation.
However, if there are no lower apertures present, then both in- and out-flow will
occur through the high level opening. This is called mixing ventilation. This latter
strategy still results in fresh air reaching to low level, since although the incoming
cold air will mix with the interior air, it will always be more dense than the bulk
interior air and hence fall to the floor. Buoyancy-driven ventilation increases with
greater temperature difference, and increased height between the higher and lower
apertures in the case of displacement ventilation. When both high and low level
openings are present, the neutral plane in a building occurs at the location between
the high and low openings at which the internal pressure will be the same as the

4