ARC 810 – AIR
FLOW THROUGH
BUILDINGS IN
NIGERIA.
OLAOGUNODEOLUWATOSIN A.
ARC/05/5633
AUGUST, 2011
ABSTRACT
This report describes techniques for observing airflow through buildings. We believe that studies
of air motion will most likely be undertaken in search of information about some “larger”
building quality or performance issue. For example, an investigator may be evaluating thermal
comfort levels for occupants of a building, estimating heat transfer through the envelope of a
building, or determining whether the air quality within a building is satisfactory. In each of these
instances, tests for air movement will be conducted to provide information that will be used to
answer “larger” questions. Although the study exercises described in this report are presented
as if they were going to be carried out independently, we expect that the exercises will indeed be
used to determine such "larger" issues.
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OBSERVING AIR FLOW IN BUILDINGS
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 be.
Alternatively, when interior air is cool and flows around an occupant, then s/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
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.
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CAUSES OF 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 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.
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.)
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
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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) 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 duct-enclosed 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 rates.
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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.)
(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 lower-height
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
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.)
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Finally, designers may intentionally rely on unpredictable and possibly difficult-tocontrol 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. 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, 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 those openings in the envelope be used judiciously. But it is difficult
to predict with certainty the amount of infiltrating air that will pass through building envelope.
Further, regulating air infiltration may require special attentions not only by the designer, but
also by the builder, maintenance person, and occupant.
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.
A convective draft forming off of a cold window surface, suppose that you occupy an
older house, probably one built before the introduction of the various energy conservation codes
that now regulate the compositions of new building envelopes. Windows in the older house are
likely to be glazed with single-pane assemblies.
So, on a winter day if the ambient outside air temperature is 0 C (32 °F) and the inside
(room) air temperature is 20C (68 °F), then the interior surface temperature of an unheated glass
pane will be about 5 C (40 °F). Because a temperature gradient would then exist between the
room air and the interior surface of the glass light, convective heat exchange would occur in the
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vicinity of this window surface. As the air layer adjacent to this inside surface is cooled by its
proximity with the glazing pane, the air layer would flow down and inward from the building
envelope across the floor of the room.
A more precise explanation of the heat transfer mechanisms present for this scenario
would state that the transfer from the air molecules in contact with the window surface occurs
conductively. As these molecules lose heat, the volume of space they occupy becomes smaller.
Then, because this localized volume is denser than the surrounding air, the denser volume will
move downward as this denser volume loses buoyancy and will be replaced by warmer air. This
overturning of air volumes resulting from temperature differences is known as convection.
Heating with a finned-tube radiator
The composition of a finned-tube radiator includes a copper tube with many steel fins
joined to the tube at right angles to the principal axis of the tube. Then, this finned-tube is
mounted inside a sheet metal enclosure which has a vented opening along the top of the
enclosure. This assembly functions with the passage of hot water (at about 60-82 C [140-180
°F]) through the tube. Heat from the water is conducted to the copper tubing and, thence, to the
fins. The warmed fins then promote convective movement of air in the vicinity of the finned-tube
assembly. Note that this assembly functions most usefully as a convector and less so as a radiator
(i.e., a more appropriate appellation for this assembly is finned-tube convector.)
Sitting under an air-admitting diffuser
In many internal-load-dominant buildings a principal function of the mechanical (HVAC)
system is the dilution of the heat generated by internal heat production sources. Often in such
buildings this heat dilution is accomplished by supplying relatively cool air (at temperatures of
about 10-15 C [50-60 °F] to the overheated spaces. For these buildings conventional design
practice calls for this cool air to be supplied through diffusers located in the ceiling (or, less
often, in the upper walls.) The entering air leaves the diffusers at velocities of 2.5 m/s [500 fpm]
or greater. If occupants sit directly under these diffusers and the air is projected downward
(rather than being spread outward), the occupants will likely be chilled by the cold air and will be
made thermally uncomfortable.
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Encouraging the natural ventilation of interiors of buildings subjected to warm-humid
climates
In regions where external air temperatures and relative humidities are frequently above
accepted thermal comfort ranges (e.g., temperatures above 27 C [ca. 80 °F] and humidities
greater than 60%), a traditional design strategy employs building forms that are single-banked
(i.e., floor plans that minimize or eliminate the use of floor-to-ceiling partitions [see Figure 6].)
Additionally, this strategy incorporates large, operable fenestration assemblies, which when left
open will allow external air to flow across the building. Thus, any gentle breezes can be admitted
to the building interior and pass over and around building occupants providing them with
beneficial convective (and evaporative) cooling. Another component of this strategy involves
raising the primary living areas above the existing ground plan. The premise of this last
component is that ambient air flow will be freer and less inhibited by natural ground forms and
built structures.
Resolving each of these four situations and their relationships to air flow
Each of these four examples illustrates situations where performance testing is needed to
ascertain how well (or poorly) the form, composition, and operation of a building may support
the building occupants. The air flows resulting from the first and third examples will cause
building occupants to feel cold, producing uncomfortable conditions. For someone charged with
correcting occupant discomfort for these two conditions, an initial task would be to ascertain
why the occupants felt uncomfortable. After conjectures about possible causes of the discomforts
had been proposed, series of tests might then be conducted to verify the accuracy of the
inferences. If the tests confirmed the predicted causes, suitable corrective means could be
suggested and undertaken. Following the completion of these corrections, further tests could then
be performed to insure that the corrective means were adequate. Additionally, the occupants
could be interviewed to ascertain whether (or not) they felt more comfortable.
The second example describes a common means for warming the perimeter surfaces of
buildings. Heat passes from the finned-tube convector to window and other perimeter surfaces,
principally to lessen temperature differences between cold window surfaces and the clothing and
exposed skin of occupants. To demonstrate that a finned-tube convector provides adequate
heating for a window surface, window surface temperatures could be measured, both when the
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finned-tube convector is operating and when hot water passage through the finned-tube is halted
(and the enclosed water has cooled.) Also, for the period when the finned-tube convector is
operating, the velocity of air rising from the convector could be measured. Further, if the
temperature of the hot water could be established (using a surface-contact temperaturemeasuring device), then the observed air velocity could be linked to a specific water temperature.
Subsequently, if the temperature of the hot water flowing through the finned-tube could
be raised or lowered, then air velocity measurements for alternative water temperatures could be
performed. From several measurements of alternative water temperatures and resultant air
velocities, it may thus be possible to develop a relationship describing finned-tube water
temperatures and associated air velocities. Also, the temperatures of the warmed window surface
could likewise be measured and compared for the alternative water temperatures and the
velocities of the air rising from the convector. Thus, this second example offers an opportunity to
examine how a common heating device participates in the maintenance of occupant thermal
comfort. Further, this example considers how some operating parameters of the finned-tube
convector could influence air flow in the vicinity of the convector. By examining and relating
water temperatures, air flow rates, window surface temperatures, and perceptions of comfort by
occupants, it may be possible to detect optimum conditions for the operation of the finned tube
convector.
For example, using natural ventilation to cool a building space the effectiveness of this
conditioning strategy could be tested. If a designer is considering the use of such a conditioning
strategy in a new building, then one would need to determine how much air flow is needed to
produce comfortable conditions. Tests could be conducted on similar existing buildings (if any
were available) or on reduced scaled models. Any data gathered from these activities could then
inform design decisions. Later, after the completion of construction of the building (with its
various means for fostering air movement), the designer could examine the effectiveness of the
natural ventilation devices to determine how well the observed air flows matched predicted air
flows and to what extent occupant thermal comfort needs were satisfied. The results of such tests
and comparisons could provide useful information for future building projects which may
present conditions similar to those found for the completed building.
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OTHER SITUATIONS WHERE AIR FLOW PATTERNS MAY BE OBSERVED 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.
What parameters could be manipulated to alter performance? Where potential problem
areas exist, what solutions should be employed? Where benefits are possible, are there means for
obtaining yet-better performances? When considering the probable nature of air flow in a
building, the key steps during programming involve being able to anticipate problem areas and to
understand how beneficial situations can be exploited. An extremely useful complement to this
first key step is recognizing potential operating difficulties and knowing how to find solutions.
Means for identifying problems or benefits can include the use of mental or written checklists (as
derived from past experiences or from the perusal of technical literature), brain-storming among
team members, or guidance from consultants.
Once the design team has accumulated sufficient information from the programming
phase, the members can commence schematic designing. During this phase the principal activity
involves creating a building organization that responds to formal requirements, occupant needs,
and important physical behaviors (as identified during the programming phase.) Because of the
complexity of considering many potentially disparate bits of information, the schematic design
process will often involve several iterations. Further, any design solution established during this
phase should only be regarded as a first-approximation. But at least this solution furnishes a
starting point for the subsequent fine-tuning that is usually conducted in the later design phases.
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The more systematically and rationally the information-gathering has been accomplished during
programming and the more closely the schematic designer pays heed to such information, the
higher the probability that any schematic solution will satisfy the building criteria established
during programming.
After one (or more) schematic design solution(s) has (have) been produced, the design
team proceeds to the third phase, design development. During this phase the team seeks to
evaluate and choose from alternative schematic designs, if more than one design has been
established. The chosen schematic design is then analyzed, and its components are tested and
resolved. Where design aspects concerning air flow in buildings are found, these aspects can be
evaluated by a variety of means. First, published literature can be examined for relevant
information. Second, further advice from consultants can be sought and integrated with the
design proposal. Third, analysis based on either the employment of accepted analytic protocols
delineated in the technical handbooks and standards or computations obtained from software
packages can be undertaken. Fourth, physical models usually, constructed at reduced scale can
be assembled and tested. And, fifth, tests can be carried out on existing buildings which are
similar to (or different from) the proposed building, and the data from such tests can be applied
to the design proposal.
Throughout these evaluations the goal of the design team should be to seek information
which can support the design proposal or lead to the successful modification of the proposal.
Ultimately, the purpose of this design development phase is to fine-tune the design proposal, to
arrive at a specific overall building form, to decide on exact dimensions for the building and its
components, and to identify appropriate materials and assemblies. After such information and
definitions have been established, then the design team can undertake the preparation of the
construction documents. But, for the characterization and/or regulation of airflow in buildings,
their roles appear less central.
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
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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 become
more buoyant (less dense) and will rise upward through a building. As in 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? The term air exchange generally refers
to the passage of some volume of air between an individual room or a whole building and the
building exterior. This term most often is used to describe the volumetric flow rate in units such
as liters per second (l/s) or cubic feet per minute (CFM) achieved by a mechanical (HVAC)
system or some component of the system. Thereby, air exchange can indicate at what rate
Ventilation air is provided to a space by the mechanical system or how rapidly the system
exchanges “used” air with “fresh” air. But the term is also used to characterize the infiltration
rate for an overall building or, even, for individual rooms. Regardless of whether the air
exchange is produced by the operation of a mechanical system or by infiltration action, such
exchange may be defined by answering questions similar to those stated in the previous
paragraph. At what rates does air exchange happen?
What environmental, building, and operational parameters affect exchange rates? What
corrective means are available for altering observed exchange rates? And by how much can
exchange rates be reduced by undertaking corrective means?
A CATALOG OF TECHNIQUES FOR ASSESSING AIR MOVEMENT IN BUILDINGS
Sometimes, the best way to recognize the motion of air within a building is to rely on one
(or another) of our five senses. For example, we have suggested using chalk dust to make visible
the air movement present in the incoming rays of sunlight. We have also mentioned how the
occupant of a room can become aware of the scent of cologne or perfume worn by someone
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newly entering that room. Thus, the sense of smell provides the means for observing air motion.
We can discern air flow by our sense of feeling, either by wetting a finger and holding it up into
what we anticipate to be a moving stream of air or we may feel air moving across our bare skin,
especially, when the air is much cooler than the skin. Hearing can provide evidence of air
movement when we are outside of a building and we observe the whoosh of windblown air.
Alternatively, for fan-driven air within a building, we can often hear fan noise borne by the
moving air. Also, we can sometimes discern air movement as fan-blown air passes over the
blades of a ceiling diffuser causing them to vibrate. In some instances, two senses functioning
together can make us aware of air movement. For example, we can observe air movement in the
presence of burning incense. We see smoke rising from the smoldering incense stick, and we
smell the aroma of the combusted incense. A number of other simple techniques can be
employed to observe air movement in a space. A well-balanced mobile can offer an attractive,
pleasing demonstration of air motion. Or we might use a child’s pinwheel or a helium-filled
balloon to detect moving air streams. We can watch the flame of a candle to see if the flame
moves and flickers, particularly when a door between this space and an adjacent one is opened or
closed. We also could bring some dry ice into a room and observe the motion of the vapor as the
solid sublimes. And, if the air stream moves at a rapid enough pace, usually-stationary, loose
papers lying on the horizontal surface of a table (or desk) will be blown about.
In conclusion, we recommend using a powder “gun” to inject finely-divided particles into
the air. If this air is moving, then its motion can be readily seen as the powder particles disperse.
Essentially, propelling the particles into air to see if the air is moving is an action similar to
beating a chalk eraser against some hard surface. In both instances the finely-divided particles
appear buoyant and are carried by the moving air stream. If you wish to accentuate the visibility
of these particles, you can project a laser beam through the cloud of particles. Indeed, a rubycolored laser beam passing across a moving dust cloud can make a very engaging display.
Another method for indicating air flow is to use a helium bubble machine. With this device small
buoyant droplets filled with helium gas can be injected into moving air.
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REFERENCES
Brooks, 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 Rasmessen, J. “Measured Thermal Resistance
of Frame Walls with Defects in the Installation of Minerals Fibre Insulation”, J. of Thermal
Insulation and Building Envelopes, Vol. 16, April 1993, Pp. 318-319.
Diamond, R.C., H.E. Feustel, and D.J.Dickerhoff. (1996). “Ventilation and Infiltration in High
Rise Apartment Buildings,” Lawrence Berkeley Laboratory Report, LBL-38103, Berkeley,
California.
Feustel, H.E. and R.C. Diamond (1996). “Diagnostics and Measurements of Infiltration and
Ventilation Systems in Three High-Rise Apartment Buildings,” in the Proceedings of the 1996
ACEEE Summer Study, vol. 1, American Council for an Energy-Efficient Economy,
Washington DC, 1996.
Kelly, Mark E., McQuail, John E., and O'Brien, Robert. (1992). “Case Study of Ventilation
Improvements in a Multifamily Building," in the Proceedings of the 1992 ACEEE Summer
Study, vol. 2, American Council for an Energy-Efficient Economy, Washington DC, 1992.
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