Passive building design opening approach

ANNALS of Faculty Engineering Hunedoara – INTERNATIONAL JOURNAL OF ENGINEERING
Tome XXI [2023] | Fascicule 1 [February]
79 | F a s c i c u l e 1
ISSN 1584 – 2665 (printed version); ISSN 2601 – 2332 (online); ISSN-L 1584 – 2665
1.
Fatima M ELAIAB, 1.
Gada AlMAGRE
PASSIVE BUILDING DESIGN: OPENINGS APPROACH FOR NATURAL VENTILATION
TO CONTROL INDOORS ENVIRONMENT
1.
Collegeof Art& Architecture, OmarAl–Mukhtar University, LIBYA
Abstract: This studywas inspired byacontemporarytrend inarchitecture, whichis theinterestof passiveand lowenergyapplications inbuildings.Wherea
natural cross ventilation can be a promising passive solutionfor summer thermal comfort in buildings especially in hotcountry like Libya. It takes advantage
of the night temperature of the air to cool the walls at the buildings, although this technique is well–known in hot climate, its use in new buildings requires
being able to predict the quantity of heat that can be dissipated. There is indeed a lack of experimental and research data either to build design rules for
engineers or to validate numerical code dedicated to the design. This paper concentrates on the area of cross ventilation and Highlights the opportunity of
using this passivelowenergycooling method hereinLibya.
Keywords: Cross ventilation, windowstypes, opining orientation, nocturnal cooling
1. INTRODUCTION
Tow– sided or cross ventilation occurs when air enters the room or buildings from one or more opening on
one side and room air leaves through one or more opening on another side of the room or buildings, the flow
of air in this case is mainly due to wind pressure, and buoyancy pressure becomes important only if there is a
significate difference in height between the inflow and outflow openings.
As well as, ventilation is the process by which fresh air is introduced and ventilated air is removed from an
occupied space. The primary aim of ventilation is to preserve the qualities of air. Sometimes, ventilation may
also be used to lower the temperature inside an occupied area. Without ventilation, a building’s occupants will
first be troubled by odors and other possible contaminants and heat. Humidity will rise, thus enhancing
moisture hazards (e.g. mold growth and condensation). Oxygen will not be missed until much later. The
purpose of ventilation is to eliminate airborne contaminants, which are generated both by human activity and
by the building itself. Therefore, designers should not only care about the design of the window aesthetically,
but they should also care about reducing the thermal transmission of the window through the load to achieve
the thermal comfort required for the users of the space.
Natural ventilation also, is the process of supplying and removing air by means of purpose–provided aperture
(such as openable windows, ventilators and shafts) and the natural forces of wind and temperature–difference
pressures. Natural ventilation may be divided into two categories:
▓ Controlled natural ventilation is intentional displacement of air through specified openings such as
windows, doors, and ventilations by using natural forces (usually by pressures from wind and/or indoor–
outdoor temperature differences). It is usually controlled to some extent by the occupant.
▓ Infiltration is the uncontrolled random flow of air through unintentional openings driven by wind,
temperature–difference pressures and/or appliance–induced pressures across the building envelope. In
contrast to controlled natural ventilation, infiltration cannot be so controlled and is less desirable than other
ventilation strategies, but it is a main source of ventilation in envelope–dominated buildings.
2. CONTROLLING THE AIR MOVEMENT THROUGH THE OPINING ZONES
The movement of air inside the space is controlled through ventilation holes, to achieve several main functions,
namely:
 Health Ventilation
Replacing clean air with unpleasant air, i.e. providing the building with the oxygen necessary for breathing to
prevent the increase in carbon dioxide, as well as getting rid of unpleasant and harmful odors and fumes. The
rate of renewal of the air of the space occupied by a person varies according to his job, in the living room, for
example, the air needs to be renewed from 1 to 1.5 times per hour, while in the kitchen, where odors and high
carbon dioxide levels increase, this rate increases to 4 or 5 times per hour. (Al–Zafaranim,2000)
 Thermal Comfort Ventilation
Cooling the human body when needed by controlling the air speed and its movement, because with the
increase in air speed, the rate of heat transfer from the body to the surrounding environment increases, as well

80 | F a s c i c u l e 1
as the increase in the evaporation capacity of the air, that is, the amount of water vapor or moisture absorbed
by the air, and then the cooling effect caused by sweat evaporation increases on the skin. (Al–Zafaranim,2000)
 Structural Cooling Ventilation
Cooling the origin, as the outside air entering through the opening’s mixes with the internal air, and heat is
transferred between them according to the difference between their two temperatures. Experiments have
proven that the cooling effect caused by ventilation inside buildings increases with the decrease in the
thickness of the external walls and their darkening in color, and it decreases with the increase in the thickness
of the wall and its resistance to heat penetration, because the air temperature in this case increases its
dependence on the temperature of the internal surfaces. (Al–Zafaranim, 2000).
Figure(1). Structural Cooling Ventilation.
www.mirathlibya.blogspot.com.7/7/ 2021
Figure(2) Cooling Ventilation
www.mirathlibya.blogspot.com.7/7/ 2021
The assessment of ventilation inside the space depends on two main components,
▓ The first is that it is easy to achieve, as ventilation must meet the rates necessary to achieve its hygienic
function.
▓ The second is the extent of achieving thermal comfort for users within the space by achieving appropriate
air velocities inside the space. It is considered a variable element according to the type of activity within the
space.
3. DESIGN FOR NATURAL VENTILATION
The design of controlled natural ventilation systems requires identification of the prevailing wind direction, the
strategic orientations and positions of openings on the building envelope. These openings include windows,
doors, roof ventilators, skylights, vent shafts, and so forth.
 Ventilation rates
When designing a ventilation system, the ventilation rates are required to determine the sizes of fans, openings,
and air ducts. The methods that can be used to determine the ventilation rates include:
(a) Maximum allowable concentration of contaminants
A decay equation can be used to describe the steady–state conditions of contaminant concentrations and
ventilation rate, like this:
Ci = Co + F / Q (1)
where Ci = maximum allowable concentration of contaminants;
Co = concentration of contaminants in outdoor air
F = rate of generation of contaminants inside the occupied space (l/s)
Q = ventilation rate (l/s)
(b) Heat generation
The ventilation rate required to remove heat from an occupied space is given by:
(2)
where H = heat generation inside the space (W)
Q = ventilation rate (l/s)
cp= specific heat capacity of air (J/kg. K)
ρ = density of air (kg/m3
)
Ti = indoor air temperature (K)
To= outdoor air temperature (K)
(c) Air change rates
Most related professional institutes and authorities have set up recommended ventilation rates, expressed in
air change per hour, for various situations. The ventilation rate is related to the air change rate by the following
equation:

81 | F a s c i c u l e 1
Q =
V∗ACH
3600
*1000 (3)
where: Q = ventilation rate (l/s)
V = concentration of contaminants in outdoor air
ACH = air change per hour
Table 1 gives some recommended air change rates for typical spaces. Table
2 provides some examples of outdoor air requirements for ventilation.
Table2. Outdoorairrequirements for ventilation
Application
Estimated maximum occupancy
(persons per100 m2
floorarea)
Outdoor air requirements
(l/s/person)
Offices
– office space 7 10
– conferenceroom 50 10
Education
– classroom 50 8
– auditorium 150 8
– library 20 8
Hospitals
– patientrooms 10 13
– operating rooms 20 15
Note: Datasource: ASHRAEStandard 62–1989, Ventilationfor AcceptableIndoor Air Quality.
 Flow caused by wind
Major factors affecting ventilation wind forces include:
▓ average wind speed;
▓ prevailing wind direction;
▓ seasonal and daily variation in wind speed and direction;
▓ local obstructing objects, such as nearby buildings and trees;
▓ position and characteristics of openings through which air flows; and
▓ distribution of surface pressure coefficients for the wind.
Natural ventilation systems are often designed for wind speeds of half the average seasonal velocity because
from climatic analysis there are very few places where wind speed falls below half the average velocity for many
hours in a year.
The following equation shows the air flow rate through ventilation inlet opening forced by wind:
(4)
where Q = air flow rate (m3
/s)
A = free area of inlet openings (m2
)
v = wind velocity (m/s)
Cv = effectiveness of the openings (assumed to be 0.5 to 0.6 for perpendicular winds and 0.25 to 0.36 for
diagonal winds)
 Flow caused by thermal forces
If the building's internal resistance is not significant, the flow caused by stack effect may be estimated by:
(5)
where Q = air flow rate (m3
/s)
K = discharge coefficient for the opening (usually assumed to be 0.65)
A = free area of inlet openings (m2
)
h= height from lower opening (mid–point) to neutral pressure level (m)
Ti = indoor air temperature (K)
To = outdoor air temperature (K)
 Guidelines for natural ventilation
The following guidelines are important for planning and designing natural ventilation systems in buildings:
Table1. Recommended air change rates
Space
Air changerates
per hour
Garage 6
Kitchen 20 – 60
Bathrooms 6

82 | F a s c i c u l e 1
▓ a natural ventilation system should be effective regardless of wind direction and there must be adequate
ventilation even when the wind does not blow from the prevailing direction;
▓ inlet and outlet openings should not be obstructed by nearby objects;
▓ windows should be located in opposing pressure zones since this usually will increase ventilation rate;
▓ a certain vertical distance should be kept between openings for temperature to produce stack effect;
▓ openings at the same level and near the ceiling should be avoided since much of the air flow may bypass
the occupied zone;
▓ architectural elements like wingwalls, parapets and overhangs may be used to promote air flow into the
building;
▓ topography, landscaping, and surrounding buildings should be used to redirect airflow and give maximum
exposure to breezes;
▓ in hot, humid climates, air velocities should be maximized in the occupied zones for bodily cooling;
▓ to admit wind air flow, the long façade of the building and the door and window openings should be
oriented with respect to the prevailing wind direction;
▓ if possible, window openings should be accessible to and operable by occupants;
▓ vertical shafts and open staircases may be used to increase and generate stack effect;
▓ openings in the vicinity of the neutral pressure level may be reduced since they are less effective for
thermally induced ventilation;
▓ if inlet and outlet openings are of nearly equal areas, a balanced and greater ventilation can be obtained.
 Air movement inside the space and position of windows in the horizontal plan
To obtain ventilation inside the space, several factors must be present in the external openings, the most
important of these factors are the following (Al–Zafaranim,2000):
▓ Availability of at least one air inlet and at least one air outlet for a single space, or defining a field for wind
movement that helps direct the air into the space.
▓ The pressure difference between the internal and external voids should be large in a way that helps to draw
air and move it within the void.
▓ Putting the wind–receiving window in the direction of the preferred wind.
It is possible to summarize the effect of these factors as shown in Table (3), which shows several different cases
of the external openings and the possibilities of their different patterns and the prevailing form of the wind
direction inside the void at the level of the horizontal projection (Source: Al–Esawy, 2003):
Table3. Therelationship betweenthelocationof windows inthehorizontal positionand thedirection
of wind movementwithinthespace. (Al–Esawy, 2003)
Placementof windows and wind direction
Elevation
Wind direction
single window blank
Ventilation inside the space is somewhat weak, And not
enough for the entire void
Two opposite windows have the same width, and the
direction of the wind is perpendicular to them
The air flows directly from these openings to the opposite
opening, forming an air stream that causes a kind of
inconvenience to the users of the space, and the ventilation
is not homogeneous in the space
Two opposite windows of the same width, and the
direction of the wind is tilted on them
Most of the air volume passes and moves through the space
of the room and increases its flow at the corners, thus
achieving a more homogeneous ventilation inside the space
Two opposite windows
(the width of the entrance is smaller) and the direction of
the wind is perpendicular or inclined to them
Air flows inside the space, whether at a slope or
perpendicular to the outer opening, and the highest wind
speed inside the space is at the smaller opening, whether
air enters or exits from it.

83 | F a s c i c u l e 1
Two opposite windows
(The width of the entrance is greater), and the direction of
the wind is perpendicular or inclined to them
Air flows inside the space, whether at a slope or
perpendicular to the outer opening, and the highest wind
speed inside the space is at the smaller opening, whether
air enters or exits from it.
Two adjacent windows, and the direction of the wind is
perpendicular to the entrance
Homogeneous ventilation can be obtained within the space.
Two adjacent windows and the direction of the wind is
tilted on the entrance towards the other window
Air passes from the entrance window to the exit window
without achieving homogeneous ventilation of the void,
especially at the other corners
Two adjacent windows, and the direction of the wind is
tilted against the entrance
The direction of the other window
Homogeneous ventilation can be obtained within the space.
 Air velocity in inner space:
To achieve the movement of the wind inside the space, the window does not have to be opposite to the
direction of the wind, but the wind can enter the void in the event that the direction of the window is parallel
to the direction of the wind, and if the movement of the wind inside the void in this case is few, and the designer
in this case must use the auxiliary plant elements in Directing the wind inside the space to get the required
wind movement inside it. (Givoni, Baruch, 1992)
 Orientation of windows with respect to winds
It is a common belief that for good ventilation in elongated buildings the main walls should be perpendicular
to the prevailing wind direction. With such an orientation the largest pressure differential is created between
the windward and the leeward walls. It is assumed that this orientation provides the best ventilation. In reality,
however, the situation is often different. Buildings that are exposed to oblique winds, with angles of 300
to 600
away from the normal, can provide better ventilation conditions in individual rooms and in the house as a
whole. When the wind is oblique to the building, a pressure gradient is created along the windward walls. If
two windows are provided in a given room along the windward walls, the upwind window is at a higher
pressure than the downwind one. Thus, air enters the room through the upwind window and leaves through
the downwind aperture. When the wind is perpendicular to the wall, the two openings are exposed to the
same pressure and this reduces the ventilation of the room. When the wind is oblique to the wall, it is possible
to greatly increase the pressure difference between the two windows by adding a single wing wall (a vertical
projection on one side of the window). If such a wing wall is placed downwind of the first window, high pressure
is created in front of it. A wing wall upwind of the second window creates a suction in front of it.
 Windows types and ways of opening
Different types of windows, when serving as inlets, produce different patterns of indoors airflow and provide
different options for controlling the direction and level of the flow.
▓ Double–hung windows, by their height, determine the vertical level of airflow but not its direction and
pattern. The maximum free opening is less than one–half of the total area of the sashes, a factor that limits
the effective ventilation rate.
▓ Horizontally sliding windows also provide less than half of the free–opening area. They allow less control
of the indoor flow pattern than double–hung windows because horizontal variations in the flow direction
are much greater than in the vertical plane as a result of changes in wind direction.
▓ Casement windows opened to the outside can serve as wing walls, creating an elevated pressure when
one sash, the downwind one, is opened, or creating a suction zone when the upwind sash is opened.
However, when both sashes are opened, they may provide a smaller airflow than when only the downwind
sash is open, because when both are open there is interference in the flow.
▓ Horizontal center pivot hung windows permit control of the vertical pattern of airflow (either upward or
downward) if the sashes can be made to open downward on the room side, 100
below the horizontal.

84 | F a s c i c u l e 1
Experiments by Givoni (1976) have demonstrated that by altering the angle to which the sash is opened it
is possible to modify and alter flow patterns and distribution of velocities throughout the indoor space. There
are many types of windows that can be used to obtain good ventilation of which as shown in the below
table.
Table4. Thetypes of windows and their effectontheventilationof thespace, (Givoni, 1976)
Air Flow
Ventilation
Control
Weather
Protection
NightVentilation
WindowType
WindowShape
v. good
middle
middle
middle
horizontal slide
good
good
good
middle
tilt and swivel
v. good to
average
middle
good
good
center pivot window
middle
Good
good
v. good
hinge at the bottom
good
middle
v. good
good
hinge on top
good
middle
middle
weak
side hinge
side – hung
casement
good
v. good
v. good
very good
upper fanlight and
outward opening
casement
v. good
good
middle
middle
vertical
double sash
 Barriers to the application of natural ventilation
A successful application of natural ventilation strategies is only possible when there are no problems in many
areas at various levels from the design stage to actual operating demands placed on the building users (Allard,
1998). These potential barriers include:
▓ Barriers during building operations
≡ safety concerns
≡ noise from outdoor
≡ dust and air pollution
≡ solar shading covering the openings
≡ draught prevention
≡ knowledge of the users about how to take the best advantage of natural ventilation
▓ Barriers during building design
≡ building and fire regulations
≡ need for acoustic protection

85 | F a s c i c u l e 1
≡ difficult to predict pattern of use
≡ devices for shading, privacy & daylighting may hamper the free flow of air
≡ problems with automatic controls in openings
≡ lack of suitable, reliable design tools
▓ Other barriers
≡ impact on architectural & envelope design
≡ fluctuation of the indoor conditions
≡ design a naturally ventilated building requires more work but could reduce mechanical system (design
fee on a fixed percentage of system's cost)
≡ increase risk for designers
≡ lack of suitable standards
 Design for natural ventilation
With the number of pleasant days provided during our fall, winter, and spring seasons we would use our air–
conditioners less if our buildings were designed for natural ventilation. There are a few basic principles of airflow,
based on the application of biology, meteorology, and engineering science to architecture in hot– regions. The
prevailing breezes, averaging 10 mph, during the warm seasons are from the east–southeast. Sea breezes winds
can reach speeds of 20 to 30 mph.
And to benefit from this breeze the height of the window has a great impact on the ventilation of the space
and the movement of winds inside the space, and also affects the level at which the wind moves inside the
space. It is important to achieve ventilation at the level of the users of the space according to the activity they
perform inside the space, and these are different locations for the levels of ventilation entry and exit holes (Al–
Wakeel, 1985).
Considerations to be consider when design for natural ventilation:
▓ The placement and size of inlets and outlets can affect the flow of cooling breezes through a room.
Figure3a. Inletplaced lowcauses airflowto sweep thefloor
▓ Maximum airflow can be achieved when the inlet and outlet are of equal area and placed opposite each
other.
Figure3b. Louvers placed inadownward positionattheinletdiffuseairflow
▓ Higher velocities of air movement occur when the outlet is larger in area than the inlet.
Figure3c. Inletplaced highdirects flowupward resulting inlossof cooling effect.
▓ Low energy consumption can be achieved by combining this strategy with compact development, multi–
use spaces in buildings, designing a privacy gradient, and providing a rain and sun screen.
Figure3d. Inletplacedlowdirects flowdownward. Thelocationof theoutlethas no effectontheinternal flowpattern.

86 | F a s c i c u l e 1
▓ Use shading devices to protect from the sun and also to direct cooling breezes.
Figure3e. 1. Providing aslotbetweenacanopyor‘eye–brow’canincreasedownward pressureand resultinamorecomfortableair flowwithinaroom. 2.
Solid overhangs, or‘eye–brows’, directlyoverawindowcandirectairflowupward awayfrom theoccupied zoneof aroom.
▓ Use rain screens to improve thermal performance of the building envelope.
Figure3f. 1. Overhangs collectbreezesand enhanceairflowto interiorspaces.2. Louvered overhangs and sunshadesenhancedownward pressureof airflow
throughaspace.
4. CONCLUSIONS
It is important to conclude by emphasizing that the assessed natural ventilation potential is the potential of the
site. Once a site with a good potential is found, the designer’s task is to construct a building or to refurbish an
existing one in a way that makes the most out of this potential. In other words, both an appropriate site and an
appropriate building are necessary conditions if natural ventilation is to be applied. Moreover, in some cases,
natural ventilation cannot alone provide the required airflow rate. This could be the case when the ventilation
openings are too small for the wind and temperature conditions present on the site, or in rooms not directly
connected to the outdoor environment. Where or when the natural ventilation can no more be ensured by
either stack effect or wind, fans may be installed and switched on to ensure the necessary ventilation flow rate.
Such fans may be installed either on stack ducts, or in walls or windows. What is important is that, when the
fans are off, their openings are either tightly closed or part of the natural ventilation design.
References
[1] Allard, F., 1998. Natural VentilationinBuildings: A Design Handbook, James &James, London. [697.92 N2]
[2] ASHRAEStandard 62–1989, VentilationforAcceptableIndoorAir Quality.
[3] Al–Zafarani,Dr.AbbasMohamed,(2000AD),ClimaticDesignofArchitecturalStructures,Master'sThesis,DepartmentofArchitecture,CairoUniversity.
[4] Al–Esawy, m. Mohamed Abdel–Fattah Ahmed, March 2003, The effect of the design of the outer shell on heat gain and thermal comfort, Master's
thesis, Facultyof Engineering, Departmentof Architecture, Cairo University.
[5] CIBSE, 1997. Natural Ventilation in Non–domestic Buildings, CIBSE Applications Manual AM10: 1997, Chartered Institution of Building Services
Engineers (CIBSE), London. [LB697.92 N28]
[6] Cristian Ghiaus and Claude–Alain Roulet, Strategies for Natural Ventilation, Natural Ventilation in the Urban Environment. Assessment and Design,
Series EditorM. Santamouris, Firstpublished byEarthscanintheUKand USA in2005
[7] Evans, J. Martin, 1980, ''housing climateand comfort'', Architectural Press; NewYork: J. Wiley, London,UK.
[8] Givoni, Baruch, 1976,''ComfortClimateanalysis and building designguidelines'', Energyand building, NewYork, Vol18, 11–23, p12.
[9] Martin,A.J.,1996.ControlofNaturalVentilation,TechnicalNoteTN11/95,BuildingServicesResearchandInformationAssociation,Berkshire,England.
[LB697.92 M37]
[10] Watson& Labs, 1983,''Climatic Design'', McGrawHill,L.T.D., U.S.A., pp. 26.
[11] Woe, ShafakAl–Awsi, (1989 AD), Climateand Architecture of HotAreas, Cairo, World of Books.
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