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Using Ventilated Envelopes to Improve the ThermalPerformance of Buildings in Hot-Humid Climate
The thesis titled “Using ventilated envelopes to improve the thermal performance of buildings in
Hot-Humid Climate” prepared by Miassar Bakri has been submitted in partial fulfillment of
requirements for a master’s degree at the University of Arizona and is deposited in the University
Library to be made available to borrowers under rules of the Library.
Brief quotations from this thesis are allowable without special permission, provided that an
accurate acknowledgement of the source is made. Requests for permission for extended quotation
from or reproduction of this manuscript in whole or in part may be granted by the head of the
major department or the Dean of the Graduate College when in his or her judgment the proposed
use of the material is in the interests of scholarship. In all other instances, however, permission
must be obtained from the author.
SIGNED: Miassar Bakri
APPROVAL BY THESIS DIRECTOR
Ray Barnes
This thesis has been approved on the date shown below:
Defense date
Thesis Director Ray Barnes December 14th, 2015
Professor of Architecture
2 December 14, 2015
Using ventilated envelopes to improve the
thermal performance of buildings in Hot-
Humid Climate
By
Miassar Bakri
Supervisors:
Ray Barnes Nader Chalfoun Colby Moeller
3 December 14, 2015
Table of content
Subject Page
number
Introduction 5
Background information of Jeddah, Saudi Arabia 6
Problem statement 15
Hypothesis 16
Experiment description 17
Anticipating the data 18
The significance of the experiment 19
Understanding thermal mass 20
Building envelope characteristics in hot humid climate 30
Understanding ventilated envelopes 37
Case studies 66
The experiment assembly 72
Pictures from the test location 79
The data acquisition instruments 83
The results 86
Discussion 99
Conclusion 99
References 100
4 December 14, 2015
Abstract:
Many attempts have been made to design buildings that reduce the heat gain inside the building. In hot-humid region, architects deal with many forces of nature. These forces might be Rain, Humidity, and solar heat gain. In order to deal with the excessive heat gain, builders have used thermal mass materials to mitigate the heat gain inside the building. Hot arid region was known to be the best climate region to apply thermal mass due to the diurnal temperature swing. However, there are some architects who agree that thermal mass materials could be used in hot-humid climate. This thesis project suggests using ventilated envelope that incorporates thermal mass in the design of the ventilated envelope. The result of the experiment shows that using ventilated envelopes with thermal mass would allow the heat gained in the cladding and in the thermal mass to be released to the air cavity and; therefore, releasing the heat from the building to the exterior atmosphere. The ventilated facade could be improved by adding thermal insulation and by using reflective materials on the cladding
5 December 14, 2015
Chapter 1: Introduction:
The building sector consumes an extravagant amount of energy. According to the department
of energy’s website “In the United States, the buildings sector accounted for about 41% of primary energy
consumption in 2010, 44% more than the transportation sector and 36% more than the industrial sector”
(DOE, 2015). Heating and cooling loads have the most profound impact on the energy consumption in
most buildings. In the past buildings were not using as much energy as they are using today. Nowadays,
seldom would someone find a building that does not rely on energy. There are many aspects that led to
the typical consumption of modern day buildings. One of these reasons is the transition from the old type
buildings to the new modern style buildings. Old buildings have no electricity running through them to
power anything inside the house. Most old building occupants relied on what is called active usage, the
engagement of the occupants with the building in order to adapt the building to the users’ needs. This will
include activities like, for example, opening the window to allow for natural ventilation, wetting the floor
to reduce the ambient air temperature by evaporative cooling, and, in some places, sleeping on the rooftop
of the house to take advantage of the cool breeze of the night. These activities were exercised inside the
building naturally. People at that time were engaging with the building and the climate around them in a
very effective way according to their intrinsic feelings of what might lead them to feel comfortable.
In addition to the active usage of the building, vernacular buildings have utilized the passive
strategies fully. Passive strategies are the methods and implementation of environmental strategies that
would use the forces of nature to create an acceptable microclimate inside the building space. Some of
these passive strategies are thermal mass and natural ventilation. Thermal mass is essential especially in
hot-aired regions where there is a wide range of diurnal temperature swing. The most appealing thing
about thermal mass is the simplicity and the availability of the material. It is so simple that even the
poorest person in the community could build his/her own house by backing the molded mud in the sun
and then stack them to construct the walls. Thermal mass has proven to be one of the best passive
strategies that architects could apply to their building due to the affordability and ease of construction.
Architects could save energy and choose a natural way of constructing buildings.
Each place has its own microclimate and hot humid is no exception. Many describe the
hot-humid regions as challenging. There are many reasons to believe so. Some of these reasons are high
levels of humidity, and, in many tropical regions, heavy rain. All these elements make the hot-humid
region very difficult to deal with. Some of the most renown methods that have been used in hot-humid
climate are cross ventilation and reflective light surface. High levels of humidity could be as discomfort
as high air temperature. Natural ventilation is utilized to mitigate the effect of high humidity levels. Many
other strategies are employed to condition the building as passively as possible without relying on
mechanical systems to condition the occupied space. What is very important is to have the right
environmental strategy at the right location. Combining two or more strategies would not be necessarily
better, but if they were chosen carefully, they will make strong contribution to energy savings.
6 December 14, 2015
1.2 An overview of the region of Jeddah, Saudi Arabia:The location:
This thesis project is concentrated on the study of ventilated envelope in hot-humid region. The
representative city of this project will be the city Jeddah which is located in Saudi Arabia. Jeddah is a
coastal city located at the west side of the Kingdom of Saudi Arabia with the latitude of 29.21 north, and
39.7 east (Jeddah Municipality, 2015). It is considered as the second largest city in Saudi Arabia. It has a
very long history dating back centuries even before the beginning of the Islamic era. As most cities in the
middle east, Jeddah was surrounded by barricades that surround the perimeter of the city. After the
beginning of the establishment of the third Saudi Kingdom, the city has undergone several changes at the
building scale and the urban scale. The city has been transformed into one of the main cities in the
Kingdom of Saudi Arabia. The city has an international airport, seaport, and many governmental
buildings and embassies that are not located in most cities in the country.
Figure 1 the location of Jeddah in the Kingdom Saudi Arabia (CDM Smith.com, 2015)
7 December 14, 2015
1.2.2 The climate:
According to the municipality of Jeddah, the climate is very hot during the summer with an
average of 40° C and high humidity due to the rise of sea temperature. In the winter, the temperature is
moderate with low levels of humidity due to the impact of moderate winds that blows through the city
that is associated with high air pressure (Jeddah Municipality, 2015).
Table 1 in this table the climate data for the city of Jeddah (Climatemp.com, 2015):
Climate
variables
January February March April May June July August September October November December
Figure 2 the average temperature, humidity, precipitation, sea temperature, days with frost, wet days, and Day length. (Climatemp.com, 2015)
9 December 14, 2015
1.2.3 The psychrometric chart:
Figure 3 the temperature and humidity levels in Jeddah plotted on the Psychrometric chart (House energy Doctor, 1986)
10 December 14, 2015
1.2.4 The vernacular architecture in Jeddah:
According to an online journal written by Mohammad Arif Kamal (2014), our ancestors have had
a better understanding of the conditions of local environment and they have built their building
accordingly. Jeddah is one of those cities that have been established to deal with excessive heat and
humidity in the summer. The art of construction was a great collaboration of many experiences that
influenced the way these buildings in Jeddah were built. For example, the Mashrabi’ah (the protruded
perforated windows) were taken from the Egyptian style of architecture.
The tall lavish buildings that have been made for the wealthy merchants have lasted two to three
hundred years. The buildings in Jeddah have utilized amazing passive strategies that helped to achieve the
thermal comfort of people at that time. The buildings in the old city are clustered in a place known to the
natives as “Al-Balad.” Unlike many vernacular cities in the hot Middle Eastern region, the houses were
separated from each other to allow the air to pass by the buildings freely. This gives the indication of how
vigilant the natives were about their surrounding environment. The buildings were separated by streets
that vary in their size. The street with the least width (secondary street) ranges from (2-4) meters in width,
the second widest street (primary street) ranges from (4-10) meters, and the widest street (the main street)
ranges from (10-20) meters. The main street is on the north –south access so that it will block the sun and
shade the buildings (Mohammed Kamal, 2014).
There are some open spaces that will allow the sun to shine down on them. When these spaces
are heated, the air becomes less dense. Therefore, the much heavier cold air that is in the shade will be
driven to these hot spaces allowing the air movement to occur in the city streets (Mohammed Kamal,
2014).
Figure 4 a simple equation that shows the relationship between the Egyptian style and the style of Jeddah (Mohammad A. Kamal, 2014).
11 December 14, 2015
Figure 6 street pattern in Al-Balad region of Jeddah (Mohammed Kamal, 2014).
Figure 5 settlement pattern and layout of Al-Balad region (Mohammed Kamal, 2014)
12 December 14, 2015
1.2.5 House form:
The size of the house Is Dependent on the socio-economic status of the residents. The rich houses
range from 15-18 meters in height. The design of the house is dependent on the privacy, and the
maximization of the natural ventilation. The first floor is allotted to the male guest room; the other floors
are for the family bedrooms. The ventilation occurs at the extended windows “Mashrabiyya”, and the
vertical opening “Al-Menwar” that brings the fresh air to the inside of the house (Mohammed Kamal,
2014).
The structure of the building is similar in techniques like those in Egypt. The walls are made out
of thick coral rocks that are been brought from the reefs of the red sea. The stones have lasted for a very
long time, almost two hundred years in many cases. The floors are constructed by using heavy hardwood
that is imported from either India or Burma, and some times from Indonesia (Llewelly-Jones, 1995).
Finally, the gypsum has played an important role in the construction of the house. It had been used for
plastering the walls, and for a bonding agent for the walls.
Figure 7 ground floor plan, elevation, and section of a typical rich house in Jeddah (Mohammed Kamal, 2014)
13 December 14, 2015
Figure 9 ocean rocks and coral stone that are used for the load bearing walls (Mohammed Kamal, 2014)
Figure 8 the timber roofing system of a typical house (Mohammed Kamal, 2014)
14 December 14, 2015
Figure 10 picture of the protruded-perforated windows (Mohammed Kamal, 2014)
Figure 11 picture of the protruded-perforated windows (Mohammed Kamal, 2014)
15 December 14, 2015
1.3.1 Problem statement:
Hot humid climate is one of the most challenging climates. Architects and building engineers
have to deal with heat, humidity, and, in some occasions, heavy rain. Using thermal mass would not be
very efficient. The reason is that in order for a material with good thermal mass criteria to work well, it
needs to be located in a location where there is wide diurnal temperature swing. This phenomenon does
not happen in hot-humid regions. The temperature in hot-humid region is mostly hot during the summer
and spring, then the temperature will be mild in winter with a little temperature rise in the fall. The
diurnal swing is not really profound with the exception of sometimes during the winter season. Only in
the winter when the temperature might drop to 59° F. When the temperature drops significantly, then
thermal mass would work as a good strategy. The essence of thermal mass is that it stores the heat during
the day and releases it during the night. Assuming that in hot-humid regions occupants would need the
heat during the winter night, and then thermal mass could be used. However, many building engineers
would argue that savings from thermal mass in hot-humid regions are very limited, and it is hard to rely
on it as an environmental system that does not work efficiently most of the time.
In order for a thermal mass material to function well, it should release most of the heat that has
been stored during the day in order to have the chance to store heat in the next day. If the temperature in
hot-humid climate does not drop to a certain level, the material would not release that heat during the
night. The reason why there is no temperature swing is that the humidity in the air is preventing heat from
being released to sky. According to Meteorologist Jeff Haby high moisture content in the air cause
“greenhouse gas” effect that will trap the longwave radiation from being emitted to the atmosphere
(2015). For this reason, thermal mass would not work well in hot humid climate. In this thesis, the focus
is to optimize the use of thermal mass in hot-humid region by placing the thermal mass material in a place
where it is shaded from the sun.
The benefits of using a ventilated envelope (Fig 12) are many. The first benefit is the creation of
an interstitial area where the thermal mass could release the stored heat due to the low air temperature
inside the cavity. The reason why the temperature inside the air cavity is low is because the air cavity is
protected from the sun rays during the day. The protection from sun rays during the day will eliminate the
conductive heat transfer through the air cavity. The second benefit is the use of natural ventilation inside
the occupant space and air cavity. Air ventilation is key in this concept. Heat is transferred by three main
methods (conduction, convection, and radiation). Heat loss by convection is one of the three main ways of
heat transfer. The air that passes through the air cavity could bolster the process of heat loss from the
thermal mass material by convection, then the absorbed hot air will carry the heat by natural buoyancy to
the outlet slit and then after that to the atmosphere. This method of using ventilated walls or roof is not
new. Many architected in cold climate have used the ventilated roof to eliminate ice dam effect. Many
other practices have used the air cavity in masonry walls since the mid-twentieth century. Ventilated
envelopes might contribute in the mitigation of heat transfer from the outside of the building to the
occupant space by utilizing the effect of both thermal mass and natural ventilation.
16 December 14, 2015
1.3.2 The hypothesis:
The strategy that is going to be used is a ventilated envelope (Fig 12) that consists of a basic
envelope which will serve as the base envelope for the building. After that, there will be another envelope
that will cover the base envelope with an air gap cavity between the base envelope and the other
envelope. The hypothesis behind this experiment is to prove that the building envelope thermal
performance will improve by using air ventilation within the interstitial space of the envelope itself. The
ventilated envelope is assumed to improve the thermal performance of the building envelope by
eliminating the conductive heat transfer of the outer skin of the ventilated envelope. Secondly, the air that
passes through the air gap will help in the removal of the heat from the inner envelope and exhausts it
outside the building. In addition to the double envelope system, the inner envelope is proposed to be
made out of thick thermal mass material that will absorb the heat from the inside of the occupant space
and then it will be released through the air cavity. Furthermore, the design includes a tilted roof which
will allow for the hot air to be removed from the occupant space by creeping below the tilted ceiling. This
tilt position of the roof and ceiling will direct the hot air to the ceiling’s opening. The air that is coming
from the outside through the air cavity will induce this process, allowing a difference in air pressure to
speed up the exhaustion of hot air from inside the occupant space.
Figure 12 Conceptual drawing of the Idea and how the air flows through the air cavity. The room is supplied by air coming through the window which is shown on the façade of the section, and then it is exhausted by the small opening on the top of the ceiling.
17 December 14, 2015
1.3.3 Experiment description:
In order to conduct the research, a testing model would have to be built in order to verify the
hypothesis. To test the thermal behavior of this model, a half-scale test chamber is recommended. The
half scale model size is more accurate then smaller size model. The test chamber will be four feet wide,
four feet long, and four feet high resembling a cubic shape. In addition, there will be an additional one
feet that is added on the top south side of the test chamber to create the tilted roof. The material that is
chosen for this test has to resemble the specification of a brick material. Concrete board has been chosen
to resemble the specification of the brick material, and it has the ability to act as a thermal mass. It will be
used for the inner envelope of the system. The outer envelope will be constructed of a simple plywood.
Due to weather condition in Tucson Arizona, the outer envelope has to be protected from the rain with a
waterproof paint. This paint will not alter the results since it is translucent. On the outer surface of the
outer envelope, a bottom slit will be made on both the south side and the north side of the test chamber.
These slits will allow the air to enter through them so that the ventilation process will take place. On the
top of the tilted roof, another slit will be made so that the air inside the air cavity could exit through it.
Figure 13 3-D view of the proposed model
18 December 14, 2015
1.3.4 Anticipating the data:
It is believed by the author that the temperature inside the air cavity will be lower than that of the
outer atmosphere ambient temperature.
The thermal mass will help in the absorption of heat from the interior space, and it will release the
heat to the air cavity during nighttime.
The overall temperature of the interior of the test chamber will be lower than the atmospheric
temperature.
19 December 14, 2015
1.3.5 The significance of this experiment:
The importance of this experiment stems from the fact that designing an effective passive
building envelope in hot humid climate is very difficult. Some of the basic strategies that are
implied in hot/ or warm humid climate will be explained in the following chapters. Most of these
strategies involve using a thin (low mass) layer of building envelope with another thin layer of
envelope that serves as a shading element for the lower layer.
This thesis is suggesting the use of a higher thermal mass in hot humid climate to utilize
the benefit of heat storage in internal surfaces of the building envelope. The most important
element in the experiment is to prove that thermal mass can function in hot-humid climate if an
area adjacent to the thermal mass is lower in temperature. The lower temperature created next to
the thermal mass element will allow the thermal mass to absorb more heat and then releases it to
the atmosphere. There are many benefits are gained if the system has proven to work properly.
The first benefit is the reduction of solar heat gain. With this design, the solar radiation
could drop significantly since the outer envelope shades the inner envelope from the sun. In
addition, the conductive heat transfer is being eliminated by creating the air gap that only allow
for the convective heat and radiative heat to pass through the air gap. Heat is one of the most
influential element that affects the performance of the building envelope. Therefore, it has to be
taken into consideration when designing buildings in hot arid or hot hot-humid climate.
The second benefit is that this system uses a passive strategy that relies on the forces of
wind and air movement to cool off the internal layer of thermal mass. If this experiment was
successful, it will allow architects to design a better productive buildings with very little reliance
on active systems to condition the spaces. Furthermore, this system will allow the occupants to
drag the heat from the room using the upper opening to the air cavity. This allows the room to
exhaust the hot air that is trapped inside the room and to be released to the outside. Not only that
the internal hot air is driven outside, it also allows the room to have better cross ventilation and
stack ventilation working in both horizontally and vertically.
Lastly is the energy savings. The main objective is to save as much energy as possible.
With the reduction in heat gain inside the building, Occupants would not need to consume an
excessive amount of energy to reach thermal comfort. Energy savings will reduce the operational
cost of the building, reduce CO₂ emissions, and saves the energy needed to fulfill the needs of
the future generation. This research experiment will contribute to the thousands of research
papers on energy consumption.
20 December 14, 2015
Chapter 2: Understanding thermal mass
There are many ways to use passive strategies that would reduce the heat gain inside the building,
and would reduce the energy consumption of the building. A material that has been used for millennials
which is thermal mass. Many builders in the past used thermal mass for its natural capability to store heat
for a long period of time. It allowed builders to reduce the heat gain inside the building and stabilize the
temperature swing inside the building. Thermal mass could be beneficial in many parts of the world
where hot climate exists. However, not all regions could benefit from the attributes of thermal mass
material. There are some places that would not benefit from thermal mass the same way as other regions.
Hot humid climate is one of the regions that thermal mass would not work very effectively.
Although that might be true; however, not all researchers agree that thermal mass is not beneficial
in hot humid climate. In this chapter, the general understanding of thermal mass will be described. The
chapter will encounter the basic characteristics of thermal mass, where it is best to be applied, and the
possibilities of using thermal mass in hot humid climate.
2.1 How thermal mass works:
According to (Martin Holladay, 2013) thermal mass is a solid or liquid material that can store
heat. He also mentioned that many objects are considered thermal mass. However, they are not designed
specifically to store significant quantities of thermal mass. What distinguishes a material as a thermal
mass is the specific heat of that material. The reason why specific heat is important in a material with high
thermal mass is the fact that the denser the material is, the more heat is needed to raise its temperature.
That is why heavy concrete materials are preferred in hot regions.
The mechanism in which thermal mass work is that a special material suitable to be used as a
thermal is chosen. In the past, adobe and stones are the most well-known sources of thermal mass.
Thermal mass works in a cycle which starts from the early morning when the wall is cool. When the sun
starts to rise, it heats up the wall. Because the wall is thick and it has a high specific heat, it will take time
before the heat reaches the other side of the wall. At noon around 1:00 P.M., the wall is still cool. After
sunset, the heat had already reached the other side of the wall. Due to the temperature difference between
the recently heated wall and the cooler atmosphere, the heat inside the wall will start to emit. The wall
should lose all its heat so that it could store more heat the next day (Martin Holladay, 2013).
One very important aspect about thermal must be noted in order to use it well which is the
location of usage. The important thing to know about thermal mass is that this wall system works best
when it is subjected to wide diurnal temperature swing. If the temperature is constant throughout the day,
then this system will not work as efficient as planned. That is why thermal mass works best in hot arid
regions. According to Alex Wilson, the editor of Environmental Building News, “Nearly all areas with
significant cooling loads can benefit from thermal mass in exterior walls.” In an article called, “Mass
Confusion,” Charles Wardell reported, “Greg Kallio, a professor of mechanical engineering at California
State University in Chico who specializes in heat transfer, recently … model[ed] ‘the whole gamut’ of
wall systems, from stick-built to SIPs to insulated concrete, using industry standard energy analysis
programs like EnergyPlus, as well as his own custom software. His conclusion? ‘The effectiveness of
Thermal mass has special features that make it not practical to implement everywhere. As
mention earlier in the previous section, places, where large diurnal temperature swing occurs, are the best
places to implement thermal mass material. A study done by (Randa Ghattas, Franz-Joseph Ulm, Alison
Ledwith; 2013) highlighted the areas where thermal mass could produce potential savings. They made
computer simulation on the same building with different locations in the United States. In their study,
they gave a percentage of where the most savings occur in the different Parts of the United States.
One of the parameters that are measured in this research article is the diffusivity of the materials.
According to (Randa Ghattas et al; 2013), diffusivity is the measurement of heat flow through the
material or the ratio of heat transmittance to heat storage. For a given thickness, diffusivity captures both
thermal mass of the wall and the heat flow through the wall due to the temperature difference between the
inside and the outside. This phenomenon occur due to the fact that for a given conductivity, the higher the
density of the material and the specific heat, the lower the diffusivity. i.e. if we have a material that is
very low in diffusivity, we could determine along with the specific heat that this material will be a great
source of thermal mass. This parameter is very important in the mapping of the benefits of thermal mass
in different parts of the country. Here are some of the materials in regards to the thermal mass properties
of each material
The results:
The authors have the same wall but they have tried to change the location of the wall to determine the
benefit of the walls with high values of specific heat and low diffusivity (Randa Ghattas et al; 2013). Here
are some the outlines from the simulation results:
1- At all conductivities, walls with higher specific heat and densities will contribute in the energy
savings of the building.
2- The climate is a key factor in determining the range of benefits from low diffusivity walls.
3- Reducing the conductivity of an equivalent wall is a key factor in reducing energy consumption.
4- Thermal mass benefit has the most impact when daily outdoor temperature variations are above
and below the balance point of a building. Hence, cold climates benefit most in the summer
season and hot climates in the winter season.
5- For walls with the same density and specific heat and different conductivities, there is less
thermal mass benefit at lower conductivities.
Table 2.1 thermal properties of typical building material (Randa Ghattas et al; 2013)
23 December 14, 2015
Mild, Marine Climate
Mild climates benefit most from the use of walls with low diffusivity. Annually, the potential
savings is in the range of 22% annually for a typical wall. In addition, a wall with a higher conductivity
and low diffusivity can be exchanged with a wall with low conductivity and high diffusivity as a tool to
achieve comparable energy consumption. Hence, there is a double benefit of using a wall with low
diffusivity at lower conductivities. Seasonally, there are greater energy savings in the summer (Randa
Ghattas et al; 2013).
Hot, Dry Climate
Thermal mass benefit is in the range of 4.9% annually for a typical wall in a hot, dry climate. In
addition, lowering conductivities is a key factor in reducing energy consumption. However, lower
conductivities reduce the impact of thermal mass benefit. Seasonally, there are greater energy savings in
the winter (Randa Ghattas et al; 2013).
Hot, Humid Climate
Hot, humid climates exhibit similar characteristics to a hot, dry climate, but the impact of thermal
mass is lower. Thermal mass benefit is in the range of 3.1% annually for a typical wall (Randa Ghattas et
al; 2013).
Cold Climate
The conductivity of an equivalent wall is the primary driving force in reducing energy
consumption in a cold climate. When considered annually, thermal mass benefits are minimal in a cold
climate. For a typical wall, the annual benefit is in the range of 1.5% annually. The amount of thermal
mass benefits are dependent on the seasons, with greater impact in the summer than winter (Randa
Ghattas et al; 2013).
Table 3 prioritizing strategies to improve the energy efficiency of a typical wall (Randa Ghattas et al; 2013)
24 December 14, 2015
Figure 15 percentage annual energy savings vs diffusivity for a typical wall (Randa Ghattas et al; 2013)
Figure 14 percentage summer energy savings vs diffusivity for a typical wall (Randa Ghattas et al; 2013)
25 December 14, 2015
2.3.1 Studies on the implementation of thermal mass in hot humid region:
Some studies have been conducted to assess the implementation of thermal mass in regions where
heat and humidity is a major aspect regarding these areas. On one hand, there are some studies which
shows that using thermal mass could reduce the energy consumption in hot humid climate claiming that
thermal mass would reduce the peak maximum temperature gain in the building. However, other studies
are not suggesting using thermal mass due to limited savings from thermal mass.
A research that has been conducted in the University of Texas by (Mina Akhavan, Sara
Motamedi; 2012) studied the effect of thermal mass on a hot humid climate like Austin. The goal of this
study was to estimate the effectiveness of thermal mass in hot-humid climate even if there is no
fluctuation of diurnal temperature as seen in hot-aired regions. There have been some studies that
simulated the effect of thermal mass in hot humid regions; however, most of them concluded that thermal
mass does save energy in hot humid climate.
In their study, they simulated two models that reside in Austin Texas. The first model was a low-
mass office building, and the second model was the same office building with a high-mass envelope.
They kept all the other parameters the same such as the lighting, the R-value of the exterior walls, and the
electric consumption. The typical office building consumption in Austin was 13 KWh/ft². However, the
baseline model has a consumption of 11 KWh/ft², because it complies with ASHRAE 90.1 standards
(Mina Akhavan, Sara Motamedi; 2012).
The following chart shows the comparison between the energy consumption of both models. The
cooling load on the high mass model consumes 1015.8 KWh less than the low-mass model. For heating,
the high-mass model consumes 370.31 KWh less than the low mass model.
Figure 16 percentage winter energy savings vs diffusivity for a typical wall (Randa Ghattas et al; 2013)
26 December 14, 2015
By analyzing the results from the savings. It turned out that thermal mass has a very low energy
savings. There are energy savings from using higher mass, but the results are not promising. In all four
orientations, the south wall seemed to have the highest savings among the other orientations (Mina
Akhavan, Sara Motamedi; 2012).
Figure 17 annual fan load (Mina Akhavan, Sara Motamedi; 2012)
Figure 18 annual cooling load (Mina Akhavan, Sara Motamedi; 2012)
Figure 20 annual heating load (Mina Akhavan, Sara Motamedi; 2012)
Figure 19 total cost (Mina Akhavan, Sara Motamedi; 2012)
27 December 14, 2015
2.4.1 Other studies that indicate the benefit of thermal mass in hot humid climate:
Some architects do think that buildings with high thermal mass in hot humid climate is effective.
Larry Speck was visiting an old building that was built during the roman ruling in Turkey and says, “I
became interested in using high thermal mass as an alternative while traveling in Turkey with my son
Sloan, eight years ago. He and I visited remote Roman ruins on the south coast and the interior, where the
sites are in raw states and are not much frequented by tourists. The summer climate in Turkey is very hot
and humid, not unlike Texas. But it was strikingly comfortable inside the stone ruins with their high
thermal mass.” (L. Speck, 2012). He also noted the same effect in Ping Yao in china where he lived in a
home that has high thermal mass material. The bed was made out of stone and according to him even
when the temperature was 100° F outside, it was remarkably cold from the inside. Convinced by the
results, he decided to build a simple single story office building in Austin Texas for a firm called “Wiss,
Janney, Elstner Associates (WJE).” During the final stages of the construction, he and the rest of the
construction crew noticed that even without using the air condition, it was surprisingly cold inside the
building (L. Speck, 2012).
There is another study, which takes place in Ghana. The study was focusing on the effect of
thermal mass, and night ventilation on the buildings in the city Kumasi. The climate condition in Kumasi,
the second largest city in Ghana, is considered hot and humid. The office building’s design adopted the
international style which focus on the use of large parts of glazing. The use of glazing has caused some
problems in the interior temperature of the office building which raises the indoor temperature due to the
admittance of sun radiation into the building (S. Amos-Abanyie, F.O.Akuffo, V. Kutin-Sanwu; in 2013).
The typical building material that is used in Ghana is sandcrete. This material is known to be low
in thermal mass and according to the author of this article, it does not benefit from the night relatively low
temperature. In addition, it does not take advantage of the use of night ventilation due to the fact that the
windows are always closed in office buildings during the night (S. Amos-Abanyie, et al; in 2013).
The research was carried out by computer simulation and by the experimental use of test
chambers that were built for the research.
The temperature difference ratio TDR = (𝑇max out –𝑇max in) / (𝑇max out –𝑇min out)
28 December 14, 2015
This equation was proposed by Givoni which was described by the authors as “…been used with
good results to compare passive cooling systems with different configurations” (S. Amos-Abanyie et al;
2013). The numerator calculates the difference between the indoor maximum temperature and the outdoor
maximum temperature, while the denominator calculates the temperature swing of the outdoor
temperature. The higher the number, which is the temperature difference, the better in delaying the peak
energy demand for the indoor space.
The results of this study have shown that using materials with heavier thermal mass would delay
the peak indoor maximum temperature. This reduction in the peak maximum temperature would reduce
the energy consumption of HVAC systems. This table will illustrate the time lag of each material (S.
Amos-Abanyie, et al; in 2013).
Figure 21 schematic drawing of the experiment setup (S. Amos-Abanyie et al; 2013)
Table 4 description of models (S. Amos-Abanyie et al; 2013)
29 December 14, 2015
In this study, the nighttime ventilation was analyzed and it was noticed that among the nine-test
model that they had, the one with the most thermal mass, air change per hour (ACH), and no windows has
the best performance. The researchers have noticed that increasing the ACH from 10 to 20 or even 30 did
not produce significant results. However, the researchers do confirm that night ventilation in the baked
bricks and the concrete model is beneficial when the windows are open to remove the heat that is
observed by the thermal mass. In addition, it allows the material to become a “heatsink” so that it could
absorb heat from the following day. Another finding in the study is that in Kumasi the wall temperature
difference between the low mass and high mass material is negligible. According to (S. Amos-Abanyie et
al; 2013) this might be the result of the limited diurnal temperature difference in the region.
Table 5 outdoor and indoor air temperature of model 9 and the controlled model (S. Amos-Abanyie et al; 2013)
Table 7 indoor air temperature models with varied windows pattern (S. Amos-Abanyie et al; 2013)
Table 6 maximum temperature differences, temperature difference ratios, and percentage of overheated hours (S. Amos-Abanyie et al; 2013)
30 December 14, 2015
Chapter 3: The basic design principles for hot humid climate region:
Building in tropical region is very challenging. The temperature is high most of the year that
ranges from 30° – 35° C almost all the year. The wind is almost non-existent due to the stability of the
temperature; also, the high levels of humidity make it very hard for wind to pass through the tropical
regions. The sun radiation in the tropical region is very high. Many designers recommend using large
amount of shading instrument on the building to protect the building from the direct solar radiation and
from the diffused solar radiation (Paul Gut, Fislisbach, Dieter Ackerknecht, Zollikon; 1993).
One of the benefit of living in the tropical region is the abundance of vegetation throughout the
land areas. The presence of vegetation will reduce the ambient temperature because of the
evapotranspiration. Two things must be taken into consideration when using vegetation for evaporative
cooling. The first is the exacerbated amount of humidity that might be generated from vegetation.
Secondly, the vegetation cover should not restrict the movement of air to the building. According to (Paul
Gut, et al; 1993), the use of thermal mass is highly not recommended. Instead, the use of low mass with
highly reflective surfaces and / or the use of double structure is much more appropriate in hot-humid
climate. Like in many other locations with hot-humid region, the use of natural ventilation is the key.
3.1.1 The main point:
Tropical location with maximum natural ventilation and shading.
Orient the building so that lowest amount of solar radiation would impact the building.
Scattered pattern of building.
Avoid locating the building in places of hazard such floods and hurricanes.
3.1.2 Sun orientation:
If the building would to be located on a mountain, then east and west slope better to be avoid for
they have the most impact of sun radiation than the other orientation (Paul Gut, et al; 1993).
3.1.3 Wind orientation:
The windward direction should be on main façade of the building. The use of vegetation could
assist in the orienting the wind toward the building (Paul Gut, et al; 1993).
31 December 14, 2015
Buildings should not block each other from receiving wind.
Therefore, a scattered layout of neighborhood is preferred. The
pavement should not be left without shading. If the pavements are
not shaded, the air that passes by the pavement will heat up by
convection and then it will hit the building envelope causing the
building to heat up even more (Paul Gut, et al; 1993).
3.1.4 Building design considerations:
The building rooms should face south and north so that the
cross ventilation movement could take place. To avoid the solar
radiation from the east and from the west, the majority façade of the
building should be oriented to face the south elevation. It is hard to
fulfill the need for sun protection and providing the best orientation
for the building. That is why some designers advice to rotate the
building so that it could use the benefit of both air ventilation and sun
protection. As a rule of thumb, in low-rise building when the sun
radiation would not hit the building too often, orientation should be
according to the wind direction. When a high-rise building is
considered, protection from sun radiation should be the decisive
factor (Paul Gut, et al; 1993).
32 December 14, 2015
3.1.5 Room arrangements:
Some architects suggest that the arrangement of rooms should be dependent depends on their
function. Since the thermal load is related to the orientation, rooms on the east side are warm in the
morning. However, they will cool down in the afternoon as soon as the sun moves toward the south
elevation. Rooms on the west side are cooler in the morning and heat up in the afternoon. Rooms facing
north and south remain relatively cool if provided with adequate shading. Thus, the rooms can be
arranged according to their functions and according to the time of the day, they are in use (Paul Gut, et al;
1993).
3.1.6 Room arrangement according to climatic preferences:
It may not always be possible to arrange all the main rooms in an ideal manner. In this case,
special care must be taken for the disadvantaged rooms (Paul Gut, et al; 1993).
3.1.7 Bedrooms:
Bedrooms can be adequately located on the east side, where it is coolest in the evening. Good
cross-ventilation is especially important for these rooms because, at rest, the human body is more
sensitive to climate. On the other hand, stores and other auxiliary spaces can be located on the west side
(Paul Gut, et al; 1993).
3.1.8 Kitchen:
Provided the kitchen is mainly used during morning and midday hours, it can be located on the
west side as well (Paul Gut, et al; 1993).
Figure 22 the arrangement of room as suggested by (Paul Gut, et al; 1993).
33 December 14, 2015
3.1.9 Main room:
The main rooms which are in use most times of the day, such as living rooms, should not be
located on the east or west side (Paul Gut, et al; 1993).
3.1.10 Rooms with internal heat load:
Rooms where internal heat occurs, such as kitchens, might be detached from the main building,
although they can be connected by a common roof (Paul Gut, et al; 1993).
3.1.11 Building components:
The main points
• Heat storage and time lag should be minimal.
• Thermal insulation is not effective except on surfaces exposed to direct radiation.
• Materials should be permeable to air.
• Reflectivity and emissivity are important.
According to (Paul Gut, et al; 1993), the main three strategies in hot humid climate. First, avoid
heat storage as much as possible to take advantage of the night cooling. Second, use natural ventilation to
take advantage of cooling the building by perspiration. Third, avoid the direct and indirect solar radiation.
However, in some cases, high thermal mass with a time lag of five hours would prove to be beneficially
in some cases. This might happen in early morning when the temperature of the wall surface is below the
ambient air temperature. This is also dependent on the diurnal temperature difference. There is one thing
that must be bore in mind which is the condensation of the cold surfaces during the early morning hours.
Due to high level of humidity, condensation could occur very often in humid climates (Paul Gut, et al;
1993).
According to (Paul Gut, et al; 1993), the use of thermal insulation has little effect in improving
the thermodynamic features of the building envelope. The reason is because of the use of natural
ventilation which make the inside temperature almost equal to the outside temperature. Insulating the roof
however, could help in reducing the heat transfer of solar radiation through the roof. The use of reflective
color on the roof and walls could improve the thermal performance of the building envelope.
Direct contact with the ground does not necessarily help, because if the ground is shaded from the
sun, the temperature of the shaded part of the ground will be equal to the ambient temperature. Therefore,
Paul Gut suggests that the ground floor be lifted to cool of underneath the floor by ventilation (Paul Gut, et
al; 1993).
34 December 14, 2015
3.1.12 Using double ventilated roof:
The use of double skin roof has proven to be very effective. It will perform better if the lower
skin is insulated with at least a 1.5 W/m² U-value insulation. The surface temperature of the inner ceiling
should not be higher than 4° C than the ambient temperature (Paul Gut, et al; 1993).
A book written by (Patti Stouter; 2008) give very important guidelines into the basic designs
principles of hot humid climate. In places where people live the traditional life style, buildings are
designed to meet the need of the people. They use simple treatment to adjust the building to their need.
For example, the kitchen and washing room are kept separate from the main building to reduce the source
of moisture. In addition, they are allocated in a way to remove the moisture by the breeze.
3.2.1 1- Ventilation:
The house interior should be ventilated for removing the heat from the inside of the building.
Ventilation could also reduce moisture levels by having the air dehumidified so that mold would not
grow. The speed of wind could be increased with the use of wing walls (Patti Stouter; 2008)
Figure 23 layer suggested by (Paul Gut, et al; 1993) to improve the thermal performance 1) reflective metal membrane; 2) air gap; 3) wood cover; 5) thermal insulation; 6) interior finish.
35 December 14, 2015
3.2.2 2- Shading:
The building in hot humid climate should be
shaded from the sun. The orientation could help in
reducing the heat gain. East-west orientation is the
most beneficial orientation. The east and west
façades produce harmful effect on the building. They
could increase the building’s temperature during the
early morning and during the afternoon. Therefore,
the use of windows at these orientations should be
kept at its minimum. In addition to orientation, the
color of the building can reduce the heat gain of the
building significantly (Patti Stouter; 2008)
.
3.2.3 3- Planting:
According to (Patti Stouter; 2008)
heavy walls should not trap air inside a
pavement area that might increase the air temperature by convection. Plants could reduce the
temperature of the air by the effect of evapotranspiration. In addition, they could funnel the wind
in order to increase the air speed within the building. Some of the benefits of using plant is that
they reduce the ambient air temperature. The temperature in the city could be higher than places
near green places. In hot humid climate wet land are used to trap the rain underneath them. With
the growth of the city, floods may occur because there is no place for the rain to go. These
damped lands are also beneficial in reducing the ambient air temperature. It happens when air get
in close contact with the damped soil therefore reducing the air temperature above it.
3.2.4 4- Insulation:
Two ways to deal with overheated periods in hot humid climate. The first way is to vent
the roof so that the hot air will raise and exists the space from the upper vents. The second way is
to insulate the roof. According to (Patti Stouter; 2008) however, thermal insulation should not
soak up the humidity.
Figure 24 the use of wing walls to drag the wind inside the house (Patti Stouter; 2008)
Figure 25 this figure illustrate the best orientation of building in hot humid climate (Patti Stouter; 2008)
36 December 14, 2015
3.2.5 Light weight construction:
Building in the hot humid regions are not the same as in hot dry regions. The buildings are light
and do not trap the heat so that it might reradiate into the inside of the house. Indigenous people in many
tropical regions have used trees bamboos and other materials instead of concrete because they are much
cooler than concrete. However, they may not use them some times because insects might eat them.
Therefore, many of them use concrete on the first floor and bamboo on the second floor.
Some materials are used as insulators. An example of these materials is reed-thatched roof.
These materials are natural and do not transfer heat but are flammable and must be treated to create a
much safer roof. Another inexpensive material is earth materials. This material is available everywhere
and is much better insulator than concrete or brick. There are many ways to build walls with earth
materials like Cob walls, compressed earth block (CEBs). However, these materials should not be
exposed to the rain and therefore a good roof protection and strong dry foundation is needed. In (table 8) a
list of materials and a comparison between them and concrete in terms of insulation and heat storage.
Table 8 some of examples of materials and the amount of heat and insulation each materials could stand compared to concrete (Patti Stouter; 2008)
37 December 14, 2015
Chapter 4: Understanding Ventilated Envelope:
Vented cavities are not new to the building industry sector. The idea of having an air gap between
two layers of wall assemblies have been in use for decades. It was the method commonly adopted in
building brick buildings especially in the 1940s (department of labor – Australia, 1946) at first it was not
considered as ventilated façade due to the fact that the bricks would only vent the walls so that moisture
would not accumulate. These walls had no openings from the top and bottom to vent the wall. However,
there are some blocks that are pierced to vent the walls. In this chapter, the focus is on the performance
and the applicability of vented façade in different part of the world. Researches have studied the effect of
ventilated façade on the energy performance of the building during the cooling season and the heating
season. This chapter also discusses the difference between sealed cavity facades and open joint cavity
façades. These studies will help in the understanding of the thermal behavior of these types of wall
system.
There is no consensus among engineers as to whether the use of ventilated façade is beneficial or
not. Therefore, it is important to understand the benefits behind using ventilated façade and the
disadvantages from using ventilated façade. here are some of the benefits from using ventilated façade: 1)
Provide a capillary break form water penetration; 2) Reduce direct moisture bridge; 3) Allow the removal
of moisture that might have penetrated the cladding; 4) It can permit pressure equalizer (Mikael
Salonvarra, Achilles N. Karagiozis, Marcin Pazera, William Miller; 2007).
The ventilated cladding provides the opportunity for moisture removal through convective and
diffused air transportation.
4.1.1 The walls:
Numerous researches have been done to assess the performance of the cladding system that
contradicts each other. For example, a study at Belgium have determined that the ventilated cladding have
no effect on heat transmission within the air gap space. A study done by Guy and Stathopoulus (1982) has
shown that a reduction of 35% of cooling load was achieved when using the vented area that is 100% of
the cross sectional area of the cavity. In addition, they included that if the area is reduced the saving will
also be reduced. They have also demonstrated that by reducing the emissivity within the cavity from (0.90
- 0.40) with a simultaneous 25% reduction of the cavity size, a reduction of 50 % of cooling load was
achieved (Mikael Salonvarra, et al; 2007).
38 December 14, 2015
4.1.2 Roof:
Many techniques are used on the roof to increase the performance of the roof. The roof
construction has utilized ventilated roof to reduce the cooling load. In a study done by Beal and Chandra
(1995) who discovered that daytime heat flux reduction by 45% is obtained in a counter batten concreate
tile compared to direct-nailed shingles. Another study done by Miller (2006) decided to test the effect of
roof ventilation and the tile color of the roof and found that when they used darker color, the buoyancy
movement was faster than in light colors.
In addition, the effects of the air cavities extend beyond heat reduction. It could be used to dry out
the inner surfaces of the wall and block the moisture migration into the inner wall of the ventilated roof.
In order to improve the desiccation of the cavity, semi-permeable vapor barrier materials (0.4 -0.5 perm)
could be used to increase the desiccation of the cavity. The reason is that semi permeable vapor barrier
materials would hold moisture for too long. Thus allowing the air that passes through the cavity to carry
out the water from inside the cavity (Mikael Salonvarra, et al; 2007).
Figure 26 the assembly of counter batten (radiant Guards, 2015)
39 December 14, 2015
4.1.3 Comparing vented roof to unvented roofs:
Vented roof:
Some studies have investigated the effect of vented roofs and unvented roof as the online article
posted on the (Building envelope theory, 2015) which discusses the basic principles and benefits using ventilated roofs. In their article, they discuss the traditionally four main reasons for venting a roof:
Removing moisture from roof cavities, structural members, sheathing and insulation.
Controlling ice damming by keeping the roof cold.
Enhancing roofing material life span by reducing sheathing temperature.
Reducing cooling loads and increased occupant comfort during the cooling season.
In their study, they highlighted the reason for applying ventilated roofs in cold climate. The first
reason is to remove excessive moisture from the roof assembly. The desiccation of moisture will
occur if the air passes though the ventilated roof cavity without any obstructions. The second reason
for ventilating the roof is to reduce the ice dam effect. This phenomenon occurs when the heated air
inside the space rises up and gets in contact with the unvented roof. The heat from the room will melt
the snow that is located on the roof. However, when the snow melts it will leave room for the rest of
the snow on the roof to accumulate on top of the melted ice (Building envelope theory, 2015).
Moisture can enter a roof assembly in several ways:
Roof leaks (which may be caused by ice dams), flashing problems, roofing failure, and wind-driven rain and snow.
An air leak in the building envelope which transports water vapor into the roof cavity.
From inside the building via water vapor diffusing through the interior sheathing.
From inside the building through holes in the vapor retarders that allow water vapor suspended in the air to bypass the vapor retarder.
In a study done by Bill Rose of the University of Illinois stated in an unpublished report that,
“Researchers have compared the shingle temperature of both vented and un-vented roof systems. It has
been shown that ventilation has a great effect on the attic air temperature, but much less on shingle
temperature. The exterior surface of the shingle is practically unaffected by the presence or absence of
ventilation in the attic.” (Bill Rose, 2002). Some researchers have found that ventilated roofs do not
reduce the cooling load of the building. The National Bureau of Standards published an article “Summer
Attic and Whole House Ventilation” (Dutt and Harrji, 1979), in which the authors stated that “with
recommended levels of insulation, the attic air temperature had little effect on the cooling load.” They
also observed an increase in cooling costs because venting the roof caused pressure differences within the
building envelope. These pressure differences caused cool air from the interior of the house to escape into
the attic and out via the ventilation system.
40 December 14, 2015
4.2.1 The pattern of heat transfer through the vented wall system:
A research that has been done by (Griffith, 2006) highlights the method of heat transfer and types
of heat transfer through the ventilated wall system. This research represents a whole building numerical
model system that will analyze the use of ventilated envelopes. According to (Griffith, 2006) the wall is
made out of the outside layer that faces the atmosphere which is called the baffles, the air cavity layer,
outside plane and the underlying layer. The author assumes that the outer baffles are restricting the
transfer of convection and radiation heat transfer.
The test was run in Chicago Illinois at July 8. The result is comparing the dry bulb temperature of
the surfaces of the two configurations.
In the results, the baffles are heated much faster than the integrated wall system (which is a wall
system made of a typical from wall with solar panels attached to it), and cools of much faster than the
integrated wall system. The reason is that the baffles are made of highly conductive materials that absorb
the heat and emits it very fast. The result shows that the interior surface of the ventilated wall
Figure 27 the configuration of the wall assembly (Griffith, 2006)
Figure 28 exterior temperature results (Griffith, 2006)
41 December 14, 2015
configuration is cooler by 1.6° C. The integrated system is warmer during the night due to the fact that it
losses the heat in a slower rate than the ventilated wall system (Griffith, 2006).
According the author (Griffith, 2006) the underlying surface temperature is much higher than the
cavity air temperature, which indicates that the predominant source of heat transfer is radiation. It also
shows that the convection heat transfer is very low. This also tells that that ventilated-driven cooling is
limited inside the cavity.
Figure 29 Photovoltaic roof configuration, (a) with ventilation cavity, and (b) with integrated surface mounting (Griffith, 2006)
Figure 30 interior surface temperature results (Griffith, 2006)
42 December 14, 2015
4.2.2 using phase change materials with ventilated façade system:
A research done by (Alvaro de Gracia, Lidia Navarro, Albert Castell, Álvaro Ruiz-Pardo,
Servando Álvarez, Luisa F. Cabeza ; 2012) focuses on the usage of double ventilated façade system with
the use of phase change materials (PCM). The test has been done in Spain in the winter season. According
to some research on the topic of phase change materials, the benefit of using phase change materials is to
improve the performance of the building façade by storing the heat in the PCMs. However little research
has been done to prove the effectiveness of using PCMs with ventilated façade systems. Moreover, the
PCMs could provide a storage of cool air that is trapped inside the cavity when needed.
Figure 31 heat transfer coefficient results (Griffith, 2006)
43 December 14, 2015
The experiment is been done on two test chambers. One is made with a simple cubic shape room
with (2.4 * 2.4 * 5.1 m). The other was a chamber that has a ventilated system on the south wall of the
chamber. The dimension of the second chamber is the same as the first with the exception of the
ventilated system attached to it. Inside the ventilated chamber, there are 112 panels of PCM that are made
Q Rad iW –eL = radiative heat transfer between the inner wall and the external layer
Q Conv iW –gap = convective heat transfer between the inner wall and the air gap
Q Rad iW –room = radiative heat transfer between the inner wall and the room
Q Conv iW –room = convective heat transfer between the inner wall and the room
As been said earlier that the main difference between the sealed and open –joint cavity is that in the
sealed joint system the air inside the cavity will develop a convective loop inside the air gap. According
to (Cristina Sanjuan, et al; 2011) the convective heat transfer between the external layer and the air gap
has the same impact compared to the convective heat transfer between the inner wall and the air gap with
opposite signs. It is illustrated in the equation:
- Q Conv eL-gap = Q Conv iW –gap
The mean air temperature in the cavity will be somewhere between the ambient temperature of the
exterior outside air temperature and the inner surface temperatures of both side (Cristina Sanjuan, et al;
2011).
The difference between the sealed joint and open joint is very prominent in two conditions. The first
condition in the summer when the outside radiation and temperature is far higher than the room
temperature, so the heat transfer will be more intense especially the radiative heat transfer. In the situation
is reversed in the winter where the room temperature is higher than the outside air temperature. This will
let the temperature inside the room to transfer to the outside by a much faster rate. Thus, using open-joint
ventilated façade will be very beneficial in the summer, whereas the sealed-joint façade is better at the
winter time when the the heat could be trapped inside the air gap (Cristina Sanjuan, et al; 2011).
The results:
The results were obtained by the mathematical and numerical methods equations and simulation
methods. Two different results were generated from the simulation and calculation. The first was the fluid
dynamics of the air movement inside the air cavity. The second was the thermodynamics behavior that
occurred between the external layer, the air gap, and the internal wall (Cristina Sanjuan, et al; 2011).
62 December 14, 2015
The fluid dynamic behavior:
The fluid dynamics inside the open-joint slabs is effected by the solar radiation that hits the
external layer of the slabs. The air velocity increases as the air rises and then it reaches its peak around the
middle portion of the cavity because it escapes through the open joints. According to , the increase in air
velocity in the middle portion of the air cavity is explained by the fact that an intake of air is produced in
the middle part of the wall system (Cristina Sanjuan, et al; 2011).
In (figure 52) it shows the difference of air speed in the section profile between the open-joint
ventilated façade and the sealed-joint ventilated façade. It could be noticed that the air speed in the open-
joint façade has higher air velocity than the sealed-joint façade. In addition the ascension of air in the
open joint façade is in the whole width of air cavity and it does not form the convective loop that occurs
in the sealed-joint façade (Cristina Sanjuan, et al; 2011).
Figure 50 horizontal average velocity magnitude and z velocity component, at the middle vertical plane in the open-joint ventilated facade, at summer condition (24 C room, 30 C exterior, 400 W/m absorbed solar radiation)
63 December 14, 2015
Figure 51 air flow velocity at mid-height comparison between open-joint ventilated facade and sealed cavity facade
64 December 14, 2015
The thermal behavior analysis:
The results have shown that when the heat enters the open-joint system, it temperature is lower
than the two layer of the wall system. Thus, the cooler air will absorb the heat from both sides of the
cavity. The process could be reversed if the air entering the cavity was higher than the two layers. It will
losses the heat to the adjacent layers and exits the cavity with cooler air. In the sealed joint however, the
convection loop increase the convective heat transfer between the external layer and the inner wall.
(Figure 54) shows the temperature of air inside the cavity for both systems. Even though the numbers
might seem very for the sealed joint, it represent the heat that is been trapped inside the cavity due to the
solar radiation (Cristina Sanjuan, et al; 2011).
In (figure 55) it shows the heat flux that goes through the room from the inner wall. In the open-
joint case, the heat flux increases as the air ascend through the wall. However, in the sealed-joint façade,
the heat flux is almost uniformed through the height of the wall (Cristina Sanjuan, et al; 2011).
Figure 53 heat flux to room comparison
Figure 52 cavity temperature comparison
65 December 14, 2015
The evaluation of the energy consumption of open-joint ventilated façade compared to the
sealed joint ventilated façade:
The performance of open-joint ventilated façade is very prominent in the south façade at summer
times. It performs 26% better than the sealed joint faced in cooling load needed. However, in winter
times, the open joint façade system behaves very poorly. The heat loss during a typical winter is 50%
more than the sealed joint façade which make the open-joint ventilated faced performs better in the
summer and not so much in the winter because it main strategy is to loss heat and not restore heat.
Figure 54 energy performance of the two wall system
66 December 14, 2015
Chapter 5: Case study:
This article written by professor Chalfoun explains the benefit of using ventilated cavity to reduce
the heat gain on the building envelope. Some of the benefits that are addressed in this paper is the shading
effect on the inner layer of the ventilated cavity. The second benefit addressed in this paper is the
blackbody radiation. This research takes place in southern Arizona. This region is a hot arid region that is
hot during the summer times and with mild cold winters. In that region, building envelopes experience a
phenomenon known as the blackbody radiation, which is the process of releasing the stored heat during
the day and release it during nighttime. Third benefit of using ventilated cavity system is the loss of heat
from these building by convection (Nader Chalfoun, 2012).
Project description:
The project started in 2005. It is a joint effort between the University of Arizona college of
Architecture and Landscape Architecture through the Drachman institute, and the City of Tucson. The
project was aim to study the possibility of building five residential building and incorporating energy
efficient strategies into these buildings. Professor Chalfoun was designing the DDBC2 building that
included the ventilated cavity on the south side of the house (Nader Chalfoun, 2012).
The house is a three-bedroom home with 1,072 ft². Three of the five home were built with light-
gauge steel framing, concrete masonry unit (CMU), and mud adobe construction respectively. The subject
building is a wood frame house. The house has a long east west axis, which is the best orientation for
building in hot climate (Nader Chalfoun, 2012).
Figure 55 Barrio San Antonio site of the project in Tucson (Chalfoun, 2012)
67 December 14, 2015
The roof is design as a heat regulator and it consist of a ventilated space and with a tilt angel in
order to ease the heat out of the house. In addition, the south facing wall has an air cavity of 5’’.
Figure 56 floor plan of the DDBC2 wood farm house (Chalfoun, 2012)