Building ventilation involves introducing outdoor air into indoor spaces to provide healthy air and remove pollutants. There are three main methods: natural ventilation which relies on wind and buoyancy forces, mechanical ventilation which uses fans, and hybrid/mixed-mode ventilation which combines natural and mechanical methods. Natural ventilation works through cross ventilation driven by wind pressure differences or stack ventilation using warmer air rising, while mechanical ventilation is needed when natural forces are insufficient. Hybrid ventilation uses mechanical fans to supplement natural ventilation when flow rates are too low.

1. Building Ventilation
Architectural Climatology

2. Ventilation
⚫Ventilation moves outdoor air into a building or a
room, and distributes the air within the building
or room.
⚫The general purpose of ventilation in building
is to provide healthy air for breathing by
both diluting the pollutants originating in the buildi
ng and removing the pollutants from it.

3. Ventilation is necessary in buildings to remove
‘stale’ air and replace it with ‘fresh’ air:
⚫Helping to moderate internal temperatures.
⚫Replenishing oxygen.
⚫Reducing the accumulation of moisture, odors,
bacteria, dust, carbon dioxide, smoke and other
contaminants that can build up during occupied
periods.
⚫Creating air movement which improves the comfort
of occupants.

4. Building ventilation has three basic elements:
⚫ Ventilation rate —
the amount of outdoor air that is provided into the space,
and the quality of the outdoor air.
⚫ Airflow direction —
the overall airflow direction in a building, which should be
from clean zones to dirty zones; and
⚫ Air distribution or airflow pattern —
the external air should be delivered to each part of the
space in an efficient manner and the airborne pollutants g
enerated in each part of the space
should also be removed in an efficient manner.
There are three methods that may be used to ventilate a
building: natural, mechanical and hybrid

5. What is natural ventilation?
⚫Natural forces (e.g. winds and thermal buoyancy force d
ue to indoor and outdoor air density
differences) drive outdoor air through purpose
built, building envelope openings.
⚫ Purposebuilt openings include windows, doors,
solar chimneys, wind towers and trickle ventilators.
⚫ This natural ventilation of buildings depends on
climate, building design and human behavior.

6. Natural ventilation is generally
categorized as:
⚫Wind-driven (or wind-induced) cross ventilation,
where pressure differences between one side of the
building and the other draw air in on the high
pressure side and draw it out on the low pressure
side.
⚫Buoyancy-driven stack ventilation (the stack effect),
where cooler air enters the building at low level, is
heated by occupants, equipment, heating
systems and so on, becomes less dense and so
more buoyant and rises through the building to be
ventilated to the outside at the top.

7. Cross ventilation
⚫Cross ventilation occurs where there are pressure
differences between one side of a building and
the other. Typically this is a wind-driven effect in
which air is drawn into the building on the high
pressure windward side and is drawn out of the
building on the low pressure leeward side. Wind
can also drive single-sided ventilation and
vertical ventilation.
⚫Cross ventilation is suitable for buildings up to
approximately 12 to 15m in depth (five times the
floor to ceiling height, or 2.5 times the floor to
ceiling height if openings can only be provided on
one side).

8. Continued…
⚫Beyond this, providing sufficient fresh air creates
draughts close to openings, and additional design
elements such as internal courtyards are
necessary, or the inclusion of elements such as
atrium that combine cross ventilation and stack
effects.
⚫A disadvantage of cross ventilation is that it tends
to be least effective on hot still days, when it is
needed most.

9. Stack ventilation
⚫As air gets warmer it becomes less dense and so
more buoyant. This means that warm air has a
tendency to rise.
⚫This effect can be used to naturally ventilate
buildings. Cooler outside air is drawn into
buildings at a lower level, it is warmed by sources
of heat within the building (such as
people, equipment, heating and solar gain), and
then rises through the building to vent out at a
higher level. A positive pressure area is created at
the top of a building and negative pressure area
at the bottom.
⚫This process can take place without mechanical
assistance, simply by introducing openings at the

10. Continued…
⚫The effectiveness of stack ventilation is
influenced by; the effective area of openings, the
height of the stack, the temperature difference
between the bottom and the top of the stack and
pressure differences outside the building.
Where ventilation is needed high up in the
building, this can require the addition
of ventilation stacks that achieve the height
necessary to create a pressure difference
between the inlets and outlets. See Stack effect
for more information.

11. What is mechanical ventilation?
⚫A building ventilation system that uses powered fans
or blowers to provide fresh air to rooms when the
natural forces of air pressure and gravity are not
enough to circulate air through a building.
⚫Mechanical ventilation is used to control indoor air
quality, excess humidity, odors, and contaminants
can often be controlled via dilution or replacement
with outside air.
⚫However, in humid climates specialized ventilation
systems can remove excess moisture from the air.

12. where mechanical ventilation is
needed?
⚫ The building is too deep to ventilate from the
perimeter.
⚫ Local air quality is poor, for example if a building is
next to a busy road.
⚫ Local noise levels mean that windows cannot be
opened.
⚫ The local urban structure is very dense and shelters
the building from the wind.
⚫ Air cooling or air conditioning systems mean that
windows cannot be opened.
⚫ Privacy or security requirements prevent windows
being opened.
⚫ Internal partitions block air paths.
⚫ The creation of draughts adjacent to openings

13. What is hybrid or mixed-mode
ventilation?
⚫ Hybrid (mixed-mode) ventilation relies on natural
driving forces to provide the desired (design) flow
rate. It uses mechanical ventilation when the natural
ventilation flow rate is too low.
⚫ When natural ventilation alone is not suitable, exhaust
fans (with adequate pre-testing and planning) can be
installed to increase ventilation rates in rooms
housing patients with airborne infection. However, this
simple type of hybrid (mixed-mode) ventilation needs
to be used with care.
⚫ The fans should be installed where room air can be
exhausted directly to the outdoor environment through
either a wall or the roof. The size and number of
exhaust fans depends on the targeted ventilation rate,
and must be measured and tested before use.