The document discusses the importance of natural ventilation for thermal comfort in buildings, emphasizing three main functions: supply of fresh air, convective cooling, and physiological cooling. It highlights various design strategies and principles for effective natural ventilation, such as optimizing orientation, opening sizes, and cross-ventilation techniques, and notes the impact of external features on airflow. Additionally, it touches on the use of empirical rules and models to predict air flow patterns and maximize indoor air movement.

1. M E A N S O F
T H E R M A L C O M F O R T :
NATURAL VENTILATION

2. FUNCTIONS OF VENTILATION
Natural ventilation and air movement could-be considered under the
heading of 'structural controls’ as it does not rely on any form of
energy supply or mechanical installation, but due to its importance for
human comfort, it deserves a separate section.
It has three distinctly different functions:
1 . SUPPLY OF FRESH AIR
2 . CONVECTIVE COOLING
3. PHYSIOLOGICAL COOLING
There is a radical difference in the form of provisions for 1 and 2 and
for 3: therefore, the first two functions will be considered as
'ventilation' but the last function is considered separately as
'air movement'.

3. The requirements of fresh air supply are governed by the
type of occupancy, number and activity of the occupants and
by the nature of any processes carried out in the space .
Requirements may be stipulated by building regulations and
advisory codes in terms of m 3/h person, or in number of
air changes per hour, but these are only applicable to
mechanical installations. Nevertheless, they can be taken as
useful guides for natural ventilation.
The aim of all these rules is to ensure ventilation, but the
rigid application of such rules may often be inadequate To
ensure a satisfactory performance the principles involved
must be clearly understood.
SUPPLY OF FRESH AIR

4. For natural ventilation usually certain limited solutions are prescribed
and not the expected performance.
•The provision of 'permanent ventilators',
i.e. of openings which
cannot be closed, may be compulsory.
•These may be grilles or 'air bricks‘
built into a wall, or may be incorporated
with windows.
•The size of openable windows may be stipulated in relation to the
floor area or the volume of the room.

5. The exchange of indoor air with
fresh out-door air can provide
cooling, if the latter is at a lower
temperature than the indoor air.
The moving air acts as a heat
carrying medium.
CONVECTIVE COOLING

6. •Ventilation, i.e. both the supply of fresh air and convective
cooling, involves the movement of air at a relatively slow
rate. The motive force can be either thermal or dynamic
(wind).
•The stack effect relies on thermal forces, set up by density
difference (caused by temperature differences) between the
indoor and out-door air.
•It can occur through an open window (when the air is
still): the warmer and lighter indoor air will flow out at the
top and the cooler, denser outdoor air will Flow in at the
bottom.
•The principle is the same as in Wind generation.
Provision for ventilation: STACK EFFECT

7. Special provision can be made for it in the form of VENTILATING SHAFTS. The higher
the shaft, the larger the cross-sectional area and the greater the temperature difference: the
greater the motive force therefore, the more air will be moved.Such shafts are often used
for the ventilation of internal, windowless rooms (bathrooms and toilets) in Europe.
Provision for ventilation: STACK EFFECT

8. VENTILATION SHAFT
FOR TOILETS &
BEDROOM
VENTILATION SHAFT
FOR TOILET S &
LIVING ROOM
VENTILATION SHAFT
FOR TOILET S &
LIVING ROOM

9. VENTILATION SHAFTS

10. Chimneys / atria with vents at top and bottom

11. Room organisation stragies that facilitate both Stack & Cross ventilation

12. WIND CATCHER

13. WIND CATCHER

14. WIND CATCHER

15. MODERN WIND CATCHERS

16. Provision for ventilation: STACK EFFECT
The motive force is the 'stack pressure' multiplied by the cross-
sectional area (force in Newtons– area in m²).
The stack pressure can be calculated from the equation:

17. This Graph gives
a quick guide
for establishing
the size of
ventilating shafts.
These systems
operate
satisfactorily
under winter
conditions when
the temperature
difference is
enough to generate
an adequate
air flow.
Provision for ventilation: STACK EFFECT

18. The movement of air past
the skin surface accelerates
heat dissipation in two
ways:
1. Increasing convective
heat loss
2. Accelerating evaporation
Cooling by air movement
is most needed where
there are no other forms of
heat dissipation available,
when the air is as warm as
the skin and the
surrounding surfaces are
also at a similar
temperature.
Physiological cooling

19. Thermal forces will
rarely be sufficient to
create appreciable air
movements. The only
'natural’ force that can
be relied on is the
dynamic effect of winds.
When the creation of
air movements indoors
is the aim, the designer
should try to capture as
much of the available
wind as possible.
Provision for air movement: WIND EFFECTS
Negative control – when the wind is too much – is easy, if windows
and openings can be shut.

20. As no satisfactory and complete theory is available, air flow patterns
can only be predicted on the basis of empirical rules derived from
measurements in actual buildings or in wind tunnel studies.
Such empirical rules can give a useful guide to the designer but in
critical cases it is advisable to prepare a model of the design and test it
on a ‘Wind Simulator’.
Wind simulators may be of
1. The Open-jet Type
or
2. The Wind Tunnel Type .
Air flow through buildings

21. OPEN JET WIND SIMULATOR
This type is in use with the
Architectural Association
School of Architecture
which has been developed
with the cooperation of the
Department of Fluid
Mechanics, University of
Liverpool.

22. WIND TUNNEL WIND SIMULATOR
This type is best
represented by an
economical model
developed by the Building
Research Station which is
described in BRS Current
Paper 69/1968.

23. On the basis of such experimental observations the following factors
can be isolated which affect the indoor air flow (both patterns and
velocities):
1. ORIENTATION
2. EXTERNAL FEATURES
3. CROSS-VENTILATION
4. POSITION OF OPENINGS
5. SIZE OF OPENINGS
6. CONTROLS OF OPENINGS
Each of these will be examined in the following paragraphs.
Air flow through buildings

24. ORIENTATION
•The greatest pressure on the windward side of a building is
generated when the elevation is at right angles to the wind
direction, so it seems to be obvious that the greatest indoor
air velocity will be achieved in this case.
•A wind incidence of 45° would reduce the pressure by 50%.
•Thus the designer must ascertain the prevailing wind
direction from wind frequency charts of wind roses and must
orientate his building in such a way that the largest openings
are facing the wind direction.
•It has, however, been found by Givoni that a wind incidence
at 45° would increase the average indoor air velocity and
would provide a better distribution of indoor air movement.

25. ORIENTATION
Effect of wind direction and inlet opening size on air velocity distribution

26. ORIENTATION
Figure a shows the outline of air flow at 90° and Figure b at 45°, to a building
square in plan. In the second case a greater velocity is created along the windward
faces, therefore the wind shadow will be much broader, the negative pressure (the
suction effect) will be increased and an increased indoor air flow will result.
The size of outlet
opening was not
varied in his
experiments: it was
fixed at the
maximum possible
so that the suction
forces had full
effect. It is justified
to postulate that
with smaller outlet
openings this effect
would be reduced,
if not reversed.

27. ORIENTATION
If often happens, that the optimum solar orientation and the optimum orientation
for wind do not coincide. In equatorial regions a north-south orientation would be
preferable for sun exclusion but most often the wind is predominantly easterly.
The usefulness of the above findings is obvious for such a situation – it may resolve
the contradictory requirements
Massing & Orientation for Cooling
Massing and orientation are important design factors to consider for passive
cooling, specifically, natural ventilation. As a general rule, thin tall buildings will
encourage natural ventilation and utilize prevailing winds, cross ventilation, and
stack effect.
Massing Strategies for Passive Cooling
Thinner buildings increase the ratio of surface area to volume. This will make
utilizing natural ventilation for passive cooling easy. Conversely, a deep floor plan
will make natural ventilation difficult-especially getting air into the core of the
building and may require mechanical ventilation.
Tall buildings also increase the effectiveness of natural ventilation, because wind
speeds are faster at greater heights. This improves not only cross ventilation but
also stack effect ventilation.

28. ORIENTATION
While thin and tall buildings can improve the effectiveness of natural ventilation to
cool buildings, they also increase the exposed area for heat transfer through the
building envelope. When planning urban centers, specifically in heating dominated
climates, having the buildings gradually increase in height will minimize high speed
winds at the pedestrian level which can influence thermal comfort. The height
difference between neighboring buildings should not exceed 100%.

29. EXTERNAL FEATURES
Wind shadows created by obstructions upwind, should be avoided in positioning the
building on the site and in positioning the opening in the building.
Building structures can redirect prevailing winds to
cross-ventilation
•External features of the building
itself can strongly influence the
pressure build-up.
•For example, if the air flow is at
45◦ to an elevation, a Wing Wall at
the downwind end or a projecting
wing of an L-shaped building can
more than double the positive
pressure created.
•A similar funneling effect can be
created by upward projecting eaves.
Any extension of the elevational
area facing the wind will increase
the pressure build-up.

30. EXTERNAL FEATURES
If a gap between two buildings is closed by a solid wall, a similar effect will be
produced. The air velocity between free-standing trunk of trees with large crowns
can be increased quite substantially due to similar reasons
The opposite of the above means will produce a reduction of pressures: if a wing
wall or the projecting wing of an L-shaped building is upwind from the oepning
considered, the pressure is reduced or even a negative pressure may be created in
front of the window
Wing Walls
Wing walls project outward next to a window, so that even a slight
breeze against the wall creates a high pressure zone on one side and
low on the other. The pressure differential draws outdoor air in
through one open window and out the adjacent one. Wing walls are
especially effective on sites with low outdoor air velocity and variable
wind directions.

31. Wing Walls

32. CROSS VENTILATION
When placing ventilation openings, inlets and outlets are placed to optimize the
path air follows through the building. Windows or vents placed on opposite sides
of the building give natural breezes a pathway through the structure. This is called
cross-ventilation. Cross-ventilation is generally the most effective form of wind
ventilation.

33. CROSS VENTILATION
Different amounts of ventilation and air mixing with different windows open
It is generally best not to place openings exactly across from each other in a space.
While this does give effective ventilation, it can cause some parts of the room to be
well-cooled and ventilated while other parts are not. Placing openings across from,
but not directly opposite, each other causes the room's air to mix, better distributing
the cooling and fresh air. Also, cross ventilation can be increased by having larger
openings on the leeward faces of the building that the windward faces and placing
inlets at higher pressure zones and outlets at lower pressure zones.

34. CROSS VENTILATION
Placing inlets low in the room and outlets high in the room can cool spaces more
effectively, because they leverage the natural convection of air. Cooler air sinks lower,
while hot air rises; therefore, locating the opening down low helps push cooler air
through the space, while locating the exhaust up high helps pull warmer air out of the
space. This strategy is covered more on the stack ventilation.

35. CROSS VENTILATION
The following figure in the absence of an outlet opening or with a
full partition there can be no effective air movement through a
building even in a case of strong winds. With a windward opening
and no outlet, a pressure similar to that in front of the building will
be built up indoors, which can make conditions even worse,
increasing discomfort. In some cases oscillating pressure changes,
known as 'buffeting' can also occur. The latter may also be
produced by an opening on the leeward side only, with no inlet.
Lack of Cross-Ventilation

36. CROSS VENTILATION
Air flow loses much of its kinetic energy each time it is diverted
around or over an obstacle. Several right-angle bends, such as internal
walls or furniture within a room can effectively stop a low velocity air
flow . Where internal partitions are unavoidable, some air flow can be
ensured if partition screens are used, clear of the floor and the ceiling.
Effect of opening positions

37. POSITION OF OPENINGS
Pressure build-up at inlet
To be effective, the air movement must be directed at the body surface. In building
terms this means that air movement must be ensured through the space mostly
used by the occupants: through the 'living zone' (up to 2 m high).
As Figure shows, if the opening at the inlet side is at a high level,
regardless of the outlet opening position, the air flow will take place
near the ceiling and not in the living zone.

38. POSITION OF OPENINGS
Air flow in a two storey building
The relative magnitude of pressure build-up in front of the solid areas of the
elevation (which in turn depends on the size and position of openings) will, in fact,
govern the direction of the indoor air stream and this will be independent of the
outlet opening position. The figure below shows that a larger solid surface creates a
larger pressure build-up and this pushes the air stream in an opposite direction,
both in plan and in section.
As a result of this, in a
two storey building the
air flow on the ground
floor may be
satisfactory but on the
upper floor it may be
directed against the
ceiling.
One possibilities
remedy is an increased
roof parapet wall.

39. SIZE OF OPENINGS
•Window or louver size can affect both the amount of air and its
speed.
•For an adequate amount of air, one rule of thumb states that the
area of operable windows or louvers should be 20% or more of the
floor area, with the area of inlet openings roughly matching the area
of outlets.
•However, to increase cooling effectiveness, a smaller inlet can be
paired with a larger outlet opening.
•With this configuration, inlet air can have a higher velocity.
•Because the same amount of air must pass through both the bigger
and smaller openings in the same period of time, it must pass
through the smaller opening more quickly.

40. SIZE OF OPENINGS
•Air flows from areas of high pressure to low pressure.
•Air can be steered by producing localized areas of high or low
pressure.
•Anything that changes the air's path will impede its flow, causing
slightly higher air pressure on the windward side of the building and
a negative pressure on the leeward side.
•To equalize this pressure, outside air will enter any windward
openings and be drawn out of leeward openings.
•Because of pressure differences at different altitudes, this
impedance to airflow is significantly higher if the air is forced to
move upward or downward to navigate a barrier without any
corresponding increase or decrease in temperature.

41. With a given elevational area – a given total wind force (pressure x area) – the largest air
velocity will be obtained through a small inlet opening with a large outlet.
This is partly due to the total force acting on a small area, forcing air through the opening
at a high pressure and partly due to the ‘Venturi Effect’: in the broadening funnel (the
imaginary funnel connecting the small inlet to the large outlet) the sideways expansion of
the air jet further accelerates the particles. Such an arrangement may be useful if the air
stream is to be directed (as it were focused) at a given part of the room.
SIZE OF OPENINGS

42. When the inlet opening is large, the air velocity through it will be less, but the total
rate of air flow (volume of air passing in unit time) will be higher. When the wind
direction is not constant, or when air flow through the whole space is required, a
large inlet opening will be preferable.
The best arrangements is full wall openings on both sides, with adjustable sashes or
closing devices which can assist in channeling the air flow in the required direction,
following the change of wind.
SIZE OF OPENINGS
Venturi Effect
The Venturi Effect is the
reduction in fluid pressure that
results when a fluid flows
through a constricted section
of pipe. The Venturi effect
is named after Giovanni
Battista Venturi (1746–
1822), an Italian physicist.
The pressure in the first measuring tube (1) is
higher than at the second (2), and the fluid speed
at "1" is lower than at "2", because the cross-
sectional area at "1" is greater than at "2".

43. SIZE OF OPENINGS
Venturi Effect
The Venturi Effect is a
phenomenon of the flow of fluids.
Fluids in this case are all gases &
liquids. The experience of this
effect happens in many places in
our world. You may have
experienced this dynamic effect
when trying to open a door on a
windy day that does not want to
open, or when walking through a
windy urban canyon or narrow
passage. The phenomenon of
high wind areas and difficult doors
is created by Venturi effect. The
Venturi Effect is created by a
fluids natural tendency to equalize
pressure across two or more
zones.

44. Venturi Effect
The Venturi Effect is utilized in
buildings for natural ventilation.
Passive cooling is a method of
cooling a building’s exterior or
interior surfaces. The purposeful
creation of positive and negative air
pressure zones can create an
increased air flow through a
building or across a surface
creating a cooling effect. This
cooling of surfaces helps to reduce
the amount of conductive energy
in a material that can in turn
remove cool air from the interior
of a building. A building’s position
and orientation in relation to
predominate wind direction can
create predictable zones for
positive & negative air pressure.

45. Venturi Effect

46. CONTROLS OF OPENINGS
Sashes, canopies, louvres
and other elements
controlling the openings,
also influence the indoor air
flow pattern.
Sashes can divert the air
flow upwards. Only a
casement or reversible pivot
sash will channel it
downwards into the living
zone .
Effects of Sashes

47. CONTROLS OF OPENINGS
Canopies can eliminate the
effect of pressure build-up
above the window, thus the
pressure below the window will
direct the air flow upwards. A
gap left between the building
face and the canopy would
ensure a downward pressure,
thus a flow directed into the
living zone
Effects of Canopies

48. CONTROLS OF OPENINGS
Louvres and shading
devices may also present a
problem. The position of
blades in a slightly upward
position would still channel
the flow into the living zone
(up to 20° upwards from the
horizontal) .
Effects of Louvers

49. CONTROLS OF OPENINGS
Fly screens or mosquito nets are an absolute necessity not only in
malaria infested areas, but also if any kind of lamp is used indoors
at night.
Without it thousands of insects would gather around the lamp.
Such screens and nets can substantially reduce the air flow.
A cotton net can give a reduction of 70% in air velocity.
A smooth nylon net is better, with a reduction factor of only
pproximately 35%.
The reduction is
Greater with higher
wind velocities
and is also increased
with the angle of
Incidence,as shown
by the findings
of Koenigsberger et al.

50. Exclusion of rain is not a difficult task and making provision for air
movement does not create any particular difficulties, but the two
together and simultaneously is by no means easy. Opening of
windows during periods of wind-driven rain would admit rain and
spray; while closing the windows would create intolerable conditions
indoors. The conventional tilted louvre blades are unsatisfactory on
two counts:
1 strong wind will drive the rain in, even upwards through
the louvres
2 the air movement will be directed upwards from the
living zone
Verandahs and large roof overhangs are perhaps the best traditional
methods of protection.
Air Movement & Rain

51. Koenigsberger, Millar and Costopoulos have carried out some experimental work,
testing four types of louvres . Only type 'M' was found to be capable of keeping out
water at wind velocities up to 4 m/s and at the same time ensuring a horizontal air
flow into the building. The air velocity reduction varies between 25 and 50%.
Air Movement & Rain

52. When the architect’s task is the design of more than one building, a
cluster of buildings or a whole settlement, especially in a warm
climate, in deciding the layout, provision for air movement must be
one of the most important considerations. After a careful analysis of
site climatic conditions a design hypothesis may be produced on the
basis of general information derived from experimental findings,
such as those described below. A positive confirmation (or rejection)
of this hypothesis can only be provided by model studies in a wind
simulator. If the construction of adjustable or variable layout models
is feasible, alternative arrangements can be tested and the optimum
can be selected
Air Flow Around Buildings
Air stream separation at the face of buildings

53. Air Flow Around Buildings
Air stream separation at the face of buildings
The effect of tall blocks in mixed developments has been examined in experiments conducted
by the Building Research Station at Garston. Figure shows how the air stream separates on the
face of a tall block, part of it moving up and over the roof part of it down, to form a large
vortex leading to a very high pressure build-up. An increased velocity is found at ground level
at the sides of the tall block. This could serve a useful purpose in hot climates, although if the
tall block is not fully closed but is permeable to wind, these effects may be reduced.

54. If a low building is located in the wind shadow of a Tall block , the increase in height
of the obstructing block will increase the air flow Through the low building in a
direction opposite to that of the wind. The lower (return-) wing of a Large vortex
would pass through the building.
Air Flow Around Buildings
Reverse flow behind a tall block

55. a if in a rural setting in open country, single storey buildings are placed in rows in a
grid-iron pattern, stagnant air zones leeward from the first row will overlap the second
row (Figure 83). A spacing of six times the building height is necessary to 129 ensure
adequate air movement for the second row. Thus the 'five times height' rule for
spacing is not quite satisfactory
Air Flow Around Buildings
Air flow: grid-iron lay-out

56. b in a similar setting, if the buildings are staggered in a checker-board
pattern, the flow field is much more uniform, stagnant air zones are
almost eliminated.
Air Flow Around Buildings
Air flow: checkerboard lay-out

57. HUMIDITY CONTROL
Dehumidification is only possible by mechanical means, without
this, in warm-humid climates, some relief can be provided by air
movement. In hot-dry climates humidification of the air may be
necessary, which can be associated with evaporative cooling. In these
climates the building is normally closed to preserve the cooler air
retained within the structure of high thermal capacity, also to exclude
sand and dust carried by winds. However, some form of air supply to
the building interior is necessary. All these functions: Controlled air
supply , Filtering out sand and dust, Evaporative cooling &
Humidification are served by a device used in some parts of Egypt –
the Wind Scoop.
The following figure illustrates an example of this. The large
intake opening captures air movement above the roofs in densely built
up areas. The water seeping through the porous pottery jars evaporates,
some drips down onto the charcoal placed on a grating, through which
the air is filtered. The cooled air assists the downward movement – a
reversed stack effect

58. This device is very useful for ventilation (the above four functions), but
it cannot be expected to create an air movement strong enough for
physiological cooling.
HUMIDITY CONTROL
WIND SCOOP