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

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IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF BACHELOR OF TECHNOLOGY (M-TECH) IN

ARCHITECTURE

SEPTEMBER, 2011

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

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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.

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. 

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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

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

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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.

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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 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

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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,

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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

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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

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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

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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.

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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

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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

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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

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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.

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Garden,G.K.,  Control of Air Leakage is Important.  Canadian Building Digest 72, National

Research Council of Canada, Ottawa, 1965.

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