AIR FLOW AROUND BUILDINGS IN NIGERIA – TWO CASE
STUDIES
BY
ADEDEJI DAUD AKINKUNMI
ARC/05/5572
APPLIED CLIMATOLOGY
SUBMITTED TO
THE DEPARTMENT OF ARCHITECTURE, FEDERAL UNIVERSITY
OF TECHNOLOGY AKURE, ONDO STATE, NIGERIA.
LECTURER
PROF.OGUNSOTE
IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE
AWARD OF BACHELOR OF TECHNOLOGY (M-TECH) IN
ARCHITECTURE
SEPTEMBER, 2011
1
INTRODUCTION
The movement of air in a building affects the thermal comfort of occupants, influences
the rates of heat gain or loss through the building envelope, and determines whether good-quality
indoor air will be present. How a building designer lays out a building, chooses materials,
defines building details, and participates in the construction process will influence the nature and
magnitude of subsequent air movement in the building. Thermally-comfortable environments
depend on four environmental parameters and two personal parameters. The four environmental
parameters are the dry-bulb temperature of the air surrounding the occupant; the relative
humidity of the air; the (net) radiant exchange between the occupant and the surrounding
surfaces; and the rate of air movement around the occupant. Further, the two personal parameters
are the metabolic rate for the activity in which the occupant is engaged and the insulative value
for the occupant’s clothing ensemble. Thus, one determinant of whether a building occupant will
feel thermally comfortable is how rapidly or slowly air moves in the vicinity of the individual.
For example, on a day when the air in a building is warm and humid, air flow around an
occupant can improve how comfortable the individual will.
Alternatively, when interior air is cool and flows around an occupant, then she/he will
perceive the air motion as a draft and may be made uncomfortable by this motion. Second, air
movement can induce significant heat exchange through the building envelope. For instance, the
infiltration of cold air through the envelope can require that heat be furnished by some internal
source to maintain the inside air temperature at a comfortable level. Thus, to reduce heating
2
requirements for building operation, the infiltration of cold air should be curtailed. Conversely,
cool external
air will often be intentionally admitted into an internal space (by active air-handling systems) to
offset — or dilute — heat build-up that results from internal heat production sources. Supplying
good-quality air to building spaces is also a requisite for maintaining healthy and comfortable
conditions. The air within a building should be free of dust, dirt, allergens, and any potentially
toxic substances; it should be clean-smelling and free from possibly offensive odors; and its
humidity should be within acceptable ranges, so that stuffiness will not be experienced by
occupants. Further, this good-quality air should be distributed uniformly and at acceptable
Velocity rates throughout each occupied space.
FUNDAMENTALS
For air flow to occur, there must be both:

a pressure difference between two points, and

a continuous flow path or opening connecting the points.
Although the prerequisites are obvious and simple to state, in practical design applications it is
not always clear what the pressure differences are or how to assess the existence and nature of
flow paths.
In general, the approach taken to control air flow is to attempt to seal all openings at one plane in
the building enclosure. This primary plane of air tightness is called the air barrier system. The
3
word system is used since airflow control is not provided by a material, but by an assemblage of
materials which includes every joint, seam, and penetration.
The following sections will present forces driving flow, air barrier systems, a discussion of flow
within building enclosures, and air leakage tolerant enclosure designs
WHAT CAUSES AIR MOVEMENT IN BUILDINGS?
The bases for air movement, both inside and outside of buildings, are temperature and
pressure differences. When temperature differences exist between adjacent volumes of air, there
will be accompanying air density differences between these volumes. Or, when an air
temperature difference exists between a building surface and the air adjacent to this surface (see
Figure 1), the density of the air close to the surface will be different than the ambient air. Where
less-dense and more-dense volumes are present, the lighter air will rise and the heavier air will
sink, causing air flow. Another example of air flow occurs when air in the atmosphere of the
Earth moves (as wind) to a building surface. This air movement exerts a pressure on the building
surface. This wind-induced pressure will be incrementally-greater than the ambient atmospheric
pressure. If there are openings on the windward side of the building, the pressure difference
(between the wind and the building interior) will cause outside air to pass in through the
openings, producing air flow within the building. Note that wind flow across the surface of the
Earth is the product of temperature differences between adjacent regions of the atmosphere:
indeed, both atmospheric pressure systems that exist across large-scale areas on the surface of
the Earth and localized winds which flow against a building surface result from temperature
differences. But, pressure differences can also initiate air flow without a temperature difference
occurring between adjacent air volumes. For instance, a fan in an air-handling (ventilation)
4
system will propel air through ducts and out of diffusers. Additionally, occupants can cause air
movement as they move around a building. The presence of localized temperature differences in
buildings and the accompanying air movements caused by these temperature differences can
generally be anticipated. But the rates with which such movements will occur and the patterns
that these movements exhibit are usually difficult to predict. So, determining the patterns and
estimating the rates of air motions resulting from temperature differences will commonly require
observations for each specific instance. Two important distinctions can be used to describe air
flow in buildings and to understand what incites this phenomenon: whether the air movement
results from active or passive means; and whether air flow is present because of intentional or
accidental interventions? For example, the fan-powered air flow (initiated by an air-handling
system) forces air through ducts and ejects the air into rooms. The air movement in the ducts is
driven by a fan which pressurizes the air. Thus, the pressure difference between the ductenclosed air and the room air enables the entering air to pass into and flow throughout the room.
Such air movement results from an active control system which is operated intentionally. Indeed,
this air handling system operates because an engineer has designed the system to supply specific
volumetric flow.
5
Figure 1: Placing your hand against the interior surface of a window pane will cause the exterior surface to become
warmer. This warmth will cause the adjacent air to become buoyant and to rise. (Drawing by Lisa Kirkendall.)
rates (described in liters per second or cubic feet per minute) moving at expected velocities
(meters per second or feet per minute) to various locations in the building. As a second example,
a “stack effect” occurs when there is a temperature difference between volumes of air in a room:
when the air at lower heights in a room is warmer than the air at upper heights, then the lowerheight air will be less dense and will rise (and be replaced by more dense air.) If the temperature
difference between the lower and upper heights is produced by the passage of solar energy into
the room through windows (at the lower height), then this “stack effect” behavior is a passive
phenomenon and may be an accidental result (i.e., the building designer had not foreseen that
entering solar energy would overheat the lower volume of the room.) But, if the presence of a
temperature difference between the lower and upper heights of a room is anticipated by the
building designer (perhaps heat is generated during food preparation), then the designer can
incorporate operation strategies to offset the discomforting heat gain. Thus, the temperature
difference can promote a “stack effect” and induce the movement of the warm air away from the
6
occupants (see Figure 2). In this latter instance, inclusion of lower and upper-wall venting can
enhance air flow in the building providing an intentional and passive remedy. Additionally, a
slowly-revolving ceiling fan can further enhance the “stack effect” movement of the air thereby
offering an intentional, active treatment for offsetting the heat gain. Finally, designers may
intentionally rely on unpredictable and possibly difficult-to-control means to promote air
movement in buildings. One example of this practice is using infiltration to provide adequate
fresh air for the occupants of a building.
Figure 2: Air flow is promoted by a "stack effect", as warm air rises in the house and exits at the roofline vents.
Exterior air is drawn in through the floor vents (and, also, through the jalousies in the walls.) The fan aids the
ascension of the war med air. (Drawing by David Hudacek.)
Air infiltration is generally caused by pressure differences between the external air and the
internal air. These pressure differences can result from wind-loading — wind flow onto the
exterior surface of a building — or from a “stack effect” (e.g., where warm air in a space rises,
7
sucking in outside air through the building envelope.) In either of these instances, the primary
pathways for air admission are through cracks in the building envelope or through opened
operable parts in the envelope (e.g., doors and windows.) Because anticipating (and then
providing controls for) all of the likely pathways through which infiltration will occur is virtually
impossible, some infiltration of external air is unavoidable. Thus, a designer seeking to insure
that some fresh air will be present for building occupants can rely on it arriving by infiltration.
To control the rate of infiltration through a building envelope, it is essential that envelope
components fit tightly and that openings in the envelope be used judiciously. But it is difficult to
predict with certainty the amount of infiltrating air that will pass through a building envelope.
Further, regulating air infiltration may require special attentions not only by the designer, but
also by the builder, maintenance person, and occupant.
Figure 3: Forces Driving Air Flow through Building Enclosures
8
Low-slope roofs tend to have mostly negative (uplift) pressures, especially on the leading edge
(Figure 3). Roofs with slopes above about 25 degrees experience positive pressures on the
windward face, and suctions on the leeward.
Figure 2: Wind Pressure Effects on Representative Buildings
EXAMPLES WHERE AIR FLOW IN BUILDING SPACES CAN AFFECT
OCCUPANT COMFORT
There are numerous conditions in our present-day buildings, where air movement affects
not only the functioning of building spaces, but also how occupants feel in these spaces. For
instance, large, cold window surfaces in buildings will promote air movement (by convective
drafts), if not suitably treated. Secondly, air will rise from a finned-tube radiator assembly
(through which hot water is pumped), and these finned-tube radiators can be employed to warm
otherwise-cold glazing surfaces. Thirdly, air-handling systems supply cool air to offset heat
generated by people, electrically powered lights and equipment. And, fourthly, in regions
subjected to warm, humid climates, various building forms and organizations can beneficially be
employed to enhance occupant thermal comfort.
OTHER SITUATIONS WHERE AIR FLOW PATTERNS MAY BE OBSERVED
9
AND STUDIED
Evidence that air flow occurs in buildings can be found in a number of additional
examples. For instance, if you look across rays of sunlight as they pass inward from a closed
window, you will often see motes of dust as they move randomly in the air crossed by the
sunlight. If you wished to accentuate the dust movement, try slapping a chalk eraser on a surface
near the streaming sunlight and watch the great increase in the dust presence. Another example
of air movement can be observed by your sense of smell: when someone walks by you wearing a
strong cologne or perfume or enters a room in which you have been sitting for some period of
time, often you will immediately be able to detect the introduction of this new scent. A third
example of discernible air movement can be seen when air flows around and across a hanging
mobile: the air motion will cause the mobile to twirl in space. Additional related examples where
air movement causes the motion of usually-stationary objects include the rotation of a toy
pinwheel and papers blown off a desktop when a window is opened.
INFLUENCES ON BUILDING DESIGN DECISION-MAKING
The fundamental reason for seeking information about air flow in buildings is to gain
insights which can be used to improve building performance. Thus, information can be sought to
upgrade the operation of an existing building. Or, alternatively, a search could be conducted to
gather information which might insure the satisfactory functioning of some future building. The
building design process traditionally includes the following tasks: programming, schematic
designing, design development, preparation of construction documents, and construction
supervision. Two potential further tasks, which might be taken on by a design team, are
10
Commissioning and post-occupancy evaluation (including testing of both the physical
performance of the building and its suitability for the occupants.) Building performance criteria,
such as information about air flow patterns, should be identified and worked with during the first
three phases of the traditional design process (i.e., programming, schematic designing, and
design development.) So, considering air flow, let us examine how (and what) information about
this performance attribute can be utilized in these three design-process phases. During the
programming phase the design team should identify, as systematically as possible, how the
operation of the future building can be affected by air flow characteristics. It should examine
how air flow can be influenced by how the building is sited, how the building is arranged (or laid
out), what possible envelope compositions might be used, and what internal organizational
features may be employed (e.g., partitions, furnishings, finishes, and so forth.) Climatic (and
microclimatic) information about a proposed building site should be examined to determine wind
directions, velocities, and frequencies of occurrence. Vegetation types and locations,
topographical forms, and nearby buildings should be noted. Then, data about wind flow,
vegetation presence, ground forms, and existing buildings should be considered to see if wind
shielding can occur and might reduce infiltration heat loss. Alternatively, if natural ventilation
would be an advantageous feature for building operation, what properties should the building
plan display? Of what assemblies should the envelope be composed to facilitate air flow through
it? Or how should likely internal partitions and furnishings be placed, so that ventilating air can
readily move through the building? Further, what rates of air flow would be desirable for
insuring thermal comfort for the occupants? During this programming phase the design team
needs to identify possible situations for which air flow would degrade or enhance the operation
of the building.
11
FUNDAMENTAL CONDITIONS INVOLVING AIR FLOW IN BUILDINGS
There are basically three circumstances for which air flow in buildings merit study. First,
air movement is generally present within building spaces and results from any one of several
different mechanisms (or causes.) Second, air leakage through the building envelope commonly
exists and occurs either by air passing from the exterior into the interior (as infiltration) or by air
passing from the interior out to the exterior (as exfiltration.) And, third, air exchange takes place
from one space to another and most often occurs through the operation of a heating, ventilating,
and/or air-conditioning system (i.e., note that air exchange also happens between buildings and
the external environment.) Several causes of air movement in building spaces have previously
been identified. The most pervasive of these causes are temperature differences between building
surfaces and air volumes or between adjacent air volumes, effects induced by mechanical
systems, and actions carried out by occupants while performing normal living and working
functions. Also, air motion will result from intentional or accidental forces, and the motion can
be caused by either active or passive devices.
The leakage of air through the building envelope is caused by pressure differences
between the air volumes on the exterior and interior sides of an envelope. Such pressure
differences can be caused by wind-loading: the incident wind blows on a building elevation,
exerting a pressure level that is incrementally greater than the ambient pressure experienced
inside the building. At the opposite side of the building (from the incident wind), a leeward
situation exists with the exterior pressure on this opposite side being slightly less than the
ambient pressure condition present within the building. In addition to pressure differences caused
by wind-loading, pressure differences can also be the result of stack effects. For stack effects, air
warmed by heat sources — for example, solar radiation, people, or lighting systems — will
12
become more buoyant (less dense) and will rise upward through a building. As these stack
effects develop, pressure differences form across the envelope, thus promoting air leakage
through the envelope.
For assessing air leakage through a building envelope, several important questions can be
asked. Along what paths across the envelope does such leakage occur? At what rates does
leakage happen? What environmental, building, and operational parameters affect leakage rates?
Can leakage rates be established for individual paths or can the rate of leakage only be
determined in an overall manner (i.e., for the whole building volume?) And, finally, what
corrective means are available for altering observed leakage rates, and by how much can leakage
rates be reduced by undertaking these corrective means?
Case study 1: showing the use of high level and low level fenestration on Representative Buildings which allows for
the inlet of cold air via the low level window and outlet of warm via the dormar
13
CONCLUSIONS
Air flow control is important for several reasons: to control moisture damage, reduce energy
losses, and to ensure occupant comfort and health. Airflow across the building enclosure is
driven by wind pressures, stack effect, and mechanical air handling equipment like fans and
furnaces. A continuous, strong, stiff, durable and air impermeable air barrier system is required
between the exterior and conditions space to control airflow driven by these forces. Air barrier
systems should be clearly shown and labeled on all drawings, with continuity demonstrated at all
penetrations, transitions, and intersections. In addition, enclosure assemblies and buildings
should be vertically and horizontally compartmentalized, may require secondary planes of air
tightness (such as those provided by house wraps and sealed rigid sheathing) and may need
appropriately air impermeable insulations or insulated sheathing. It must be noted that increased
air tightness must be matched by an appropriate ventilation system to dilute pollutants, provide
14
fresh air, and control cold weather humidity levels. Good airflow control through and within the
building enclosure will bring many benefits: reduce moisture damage, energy savings, and
increased health and comfort. However, while airflow usually causes wetting in enclosures, it
also can be a powerful drying mechanism. Therefore, enclosures with increased air flow control
demand greater attention to other sources of drying (diffusion is the only practical mechanism
available) and the reduction or elimination of other sources of wetting (built-in, rain and
diffusion).
References
Brook, M.S., "Rationalizing Wall Performance Criteria", Proc. Sixth Conference on Building
Science &Technology, Toronto, March 5-6, 1992, pp.145-161.
Brown, W.C., Bomberg, M.T., Ullet, J.M. and Rasmussen, J. "Measured Thermal Resistance of
Frame Walls with Defects in the Installation of Mineral Fibre Insulation", J. of Thermal
Insulation and Building Envelopes, Vol 16, April 1993, pp. 318-339.
Controlling Stack Pressure in High-Rise Buildings by Compartmenting the Building. Research
Report for CMHC, March, 1996.
15
Garden,G.K., Control of Air Leakage is Important. Canadian Building Digest 72, National
Research Council of Canada, Ottawa, 1965.
16