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Passive cooling with a hybrid green roof for extreme climates
Pablo La Roche
a,
⇑
, Dongwoo Jason Yeom
b
, Arianne Ponce
c
a
California State Polytechnic University Pomona, CallisonRTKL, CA, USA
b
Department of Architecture, Lawrence Technological University, MI, USA
c
CallisonRTKL, CA, USA
article info
Article history:
Received 30 January 2020
Revised 14 June 2020
Accepted 15 June 2020
Available online 20 June 2020
Keywords:
Passive cooling
Evaporative cooling
Green roof
Thermal comfort
Low carbon
Radiant cooling
abstract
In a warming planet, overheating is becoming an increasingly important issue at all latitudes and loca-
tions, even in areas which traditionally did not have this problem. Passive cooling systems improve ther-
mal comfort in a hot climate using a fraction of the energy used by conventional mechanical systems. This
paper investigates the performance of a novel passive cooling system, a hybrid green roof and verifies its
cooling potential, through test beds, in a hot climate in Southern California. The hybrid green roof com-
bines evaporative cooling with a radiant system integrated in the green roof to improve its cooling per-
formance. The concept is then implemented in projects under development in hotclimates in the United
States, Middle East and Asia. In these projects additional passive strategies are sometimes combined with
the hybrid green roof to substantially improve outdoor and indoor thermal comfort in an extreme climate
with minimal use of mechanical cooling.
Ó2020 Elsevier B.V. All rights reserved.
1. Introduction
A passive cooling system reduces indoor temperature by trans-
ferring heat from a building to various natural heat sinks [1]. They
are typically classified according to the heat sinks that they use to
store energy [1]: ambient air (sensible or latent), the upper atmo-
sphere, water, and under surface soil. Because of how they collect,
store and distribute energy, passive cooling systems provide ther-
mal comfort using a fraction of the energy used by conventional
mechanical systems, achieving thermal comfort with lower capital
and operating costs. Because of their simple design, they can also
be built at lower costs, using local labor and resources, generating
income that stays in the community. Considering the application of
mechanical air conditioning systems has increased in the last few
decades, passive cooling design is now more important to reduce
the cooling energy consumption and increase thermal comfort
[2,3].
Passive cooling systems and design strategies have been studied
and verified as an effective method for different built environ-
ments, such as existing buildings, pre-fabricated buildings, and at
the urban context [4–6], as well as various parts of the building.
According to the Environmental Protection Agency (EPA) evapo-
transpiration, alone or in combination with shading, can help
reduce peak summer temperatures by 1–5 °C. When combined in
canopies they can lower local temperatures by an additional 2 °C
and reduce peak loads in the summer and overall energy consump-
tion. Kurn et al. [7] estimated that near-surface air temperatures
over vegetated areas are 1–2 °C lower than air temperatures in
cities.
Considering the building envelope, Prieto et al. studied passive
cooling and climate-responsive façade design and suggested the
limit of the strategies through the literature review and simulation
analysis [8]. The cooling performance of the interstitial Venetian
blinds was verified as an effective method by the simulation and
physical experiments [9] and Ji et al. suggested a wall-mounted
ventilation system for night cooling under hot summer condition
and verified that it has significant performance [10]. Also, PCM
(phase change material) was investigated as a passive cooling
method in the wall assembly and the performance was verified
experimentally [11].
A roof receives solar radiation directly, which makes it an
important component to reduce cooling energy consumption. Cool
roofs have been studied and verified as an effective passive cooling
strategy for diverse conditions [12] and a green roof was studied
from many perspectives, such as energy saving potential in build-
ings, reduction of GHG emissions, etc [13–15]. A green roof reduces
the indoor temperature variation and energy consumption in both
warm and cold climates [16]. These papers explained that a green
roof decreases the cooling energy consumption by 2–48% and roof
surface temperature, based on its design and area coverage
https://doi.org/10.1016/j.enbuild.2020.110243
0378-7788/Ó2020 Elsevier B.V. All rights reserved.
⇑
Corresponding author at: California State Polytechnic University Pomona, 4105
W. University Drive, Pomona, CA 91768, USA
E-mail address: pmlaroche@cpp.edu (P. La Roche).
Energy & Buildings 224 (2020) 110243
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[17–19]. Additionally, plant varieties, depth of growing medium,
and density of the plants were also identified as significant factors
correlated to the cooling potential of the indoor thermal environ-
ment [20–23]. Comparative studies on the cooling potentials of
green roofs and cool roofs were conducted and showed contrasting
results [24,25].
Many recent studies have also developed the ideas of the hybrid
passive cooling system, combining a conventional passive system
with other passive or active systems, and tried to verify its cooling
performance and applicable conditions. The potential to improve
the cooling performance of the green roof with night ventilation
was simulated and monitored for verification [26,27] and a green
roof with smart ventilation control systems was developed and
its significant potentials were verified [28]. A roof pond was devel-
oped which employed an elevated shading structure for natural
ventilation and was tested to verify its passive cooling perfor-
mance[29], and conventional Trombe walls and a new design with
a solar chimney and evaporative system was simulated its cooling
performance verified [30]. Also, hybrid wind-tower with evapora-
tive cooling, heat pipe, and wing wall were studied via simulations
and field tests and the significant results were reported [31–34].
Many researchers have also tried to determine the optimum
passive cooling systems and performance for specific regions and
climates. Chen et al. [35] focused on natural ventilation and esti-
mated its potentials in 1854 locations around the world. He
reported that South-central Mexico, Ethiopian highland and South-
west China as well as Mediterranean, California, Western Australia,
Portugal, and Central Chile, had relatively higher potential than
other locations, and he also found a few locations for night-time
ventilation. Other researchers also studied different passive cool-
ing strategies in terms of the performance and efficiency in Asia,
such as a green roof in Shanghai, China [36], effective natural
ventilation in Malaysia [37], and optimum design parameters in
India [38], as well as passive design strategies for building retrofit
in Taiwan [39] and the passive design performance analysis of ver-
nacular housing in Vietnam [40]. Additionally, considering regions
where the climate is relatively hot in summer, various studies were
conducted with different passive cooling systems, such as counties
in the Middle East [41–43], South Europe [44,45], central America
[46,47], and Africa [48].
The applicability of a passive cooling system is affected by mul-
tiple climate variables and not all systems can be used in all cli-
mates. To apply passive design and systems in practice, a passive
performance optimization framework was suggested to improve
the performance of daylight, solar control, and natural ventilation
in the early design strategies [49],... and a design optimization
process in early design stages, considering passive design varia-
tions, was developed utilizing multi-objective optimization strat-
egy based on the non-dominated sorting genetic algorithm
(NSGA-II) [50]. However, further studies and practices are required
that adapt passive cooling design or systems, specifically for
extreme climates. Additional studies can verify the performance
of hybrid passive cooling systems for extreme climates and actual
design cases can validate how the system was implemented in
practice.
In this context, the purpose of this paper is to investigate the
performance of the proposed passive cooling system and its poten-
tials in practice for a hot and dry environment. In this study, a few
passive cooling systems were developed and verified by the exper-
iment and field application, and examples in practice were dis-
cussed to demonstrate how some of these ideas can be applied
effectively in a hot and dry climate.
2. Methodology
2.1. Hybrid green roof test cells in southern California
The cooling potential of different green roof configurations has
been evaluated over several years using test cells, located outdoors
and exposed to the weather at the Lyle Center for Regenerative
Studies at Cal Poly Pomona University, 30 km east of Los Angeles,
in California (Fig. 1).
The climate is hot and dry with an average high temperature of
31.5 °C in August and an average low of 5.3 °C in January. All test cells
are similar in size (1.35 m. 1.35 m. 1.35 m.) with a window facing
south. The only differences between test cells are the roof assembly
which is the variable tested. The wall of the test cells is 178 mm
thick, with drywall on the inside, 50.8 mm 101.6 mm studs with
glass wool insulation, OSB board, XPS insulation board, and plywood
on the outside. The floor of the cell is an OSB board and XPS
insulation board, and the walls were painted white to reduce the
heat gains. The U-value of the wall assembly is 0.308 W/m
2
K and
the U-value of the floor assembly is 0.299 W/m
2
K(Table 1). A
double-glazed window 610 mm (W) 610 mm (H) was installed
Fig. 1. Hybrid green roof test cells. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
Table 1
Wall and floor assembly of the test cell with U-value.
Wall Floor
# Material mm W/mK U-Value (W/m
2
K) # Material mm W/mK U-Value (W/m
2
K)
1 Drywall 10 0.180 0.308 1 OSB 11 0.130 0.299
2 Glass Wool 89 0.044 2 Vapor Barrier 0.15 –
3 OSB 11 0.130 3 XPS with Stud 51 0.043
4 Vapor Barrier 0.5 – 4 Plywood 5 0.130
5 XPS 51 0.043
6 Air Space 13 0.079
7 Plywood 5 0.130
2P. La Roche et al. / Energy & Buildings 224 (2020) 110243
in the south wall and was tested with and without shade. An exhaust
fan was installed for the night ventilation.
Data loggers by Onset computer were used for data collection
(Model: U12-012, UX 120-006 M, TMC6-HD). These sensors were
installed in multiple positions inside and outside of the test cells
to monitor dry bulb temperature and relative humidity. Every sen-
sor was connected to the data logger, and the sensors were
installed following DIN EN 60751 regulation. The specifications of
sensors are indicated in Table 2.
Several green roofs are compared to the radiant-evaporative
roof: an insulated green roof, an uninsulated green roof, and a gen-
erally insulated non-green roof. The insulated green roof has rigid
insulation underneath the planting material. The U-value of the
insulated green roof was 0.282 W/m
2
K. In the uninsulated green
roof, the planting material is thermally coupled with the interior
of the space via a metal plate under the green roof. Combined with
night ventilation it cools in two ways: during the night, the cell is
ventilated and cooled with outside air and during the daytime the
vegetation shades the roof, reducing solar gains while the growth
medium acts as a heat sink.
The radiant-evaporative green roof configuration consists of a
radiant system with a water pipe embedded in the soil and
exposed to space below, and an evaporative system with a sprin-
kler system. The radiant system consists of a closed-loop pipe with
a total length of 33 m. The upper portion is embedded in the soil of
the green roof and the lower portion is exposed at the ceiling of the
cell (Fig. 2). A pump (Max flow rate: 8 L/m) circulates the water
inside the pipe and is operated by a digital timer, turning on or
off according to selected schedules. The evaporative component
is achieved by a sprinkler system that irrigates above the ground
and reduces the air and soil temperature by evaporative cooled
water and air. The cooled soil then lowers the water temperature
of the radiant pipe embedded in it, which then absorbs heat from
the space below as it moves from the ceiling, through the ground,
and to the surface. The heat is thus transferred from the space to
the ground and dissipated to the exterior by evaporation above.
The water tank for the sprinkler was placed within 1 m from the
test cell and located outdoor with 2.5 cm insulation to minimize
the impact of outdoor temperature fluctuation. There was no
mechanical heating or cooling system for the water tank. Accord-
ing to Yeom and La Roche [3], the most effective schedule to oper-
ate the flow of water through the embedded pipes is continuously
moving the water, and the most effective schedule for the use of
the sprinklers was to turn on when exterior humidity was lowest,
typically around mid-day. We are currently testing with good
results a 24-hour schedule with 5 min of irrigation every 55 min.
During this time there would be more air cooling and more cooled
water to the substrate, reducing its temperature and increasing its
capacity to absorb heat from the interior. Some series were also
tested combining the radiant evaporative green roof with night
ventilation (Fig. 3).
Table 2
Specification of sensors.
Monitoring Range Accuracy
Dry Bulb Temperature 20°to 70 °C ± 0.35 °C from 0°to 50 °C
Wet Bulb Temperature 20°to 70 °C ±0.35 °C from 0°to 50 °C
Relative Humidity 5–95% RH ±2.5% from 10% to 90% RH
Fig. 2. Component of the radiant evaporative green roof. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web version of
this article.)
Fig. 3. Radiant evaporative green roof combined with night ventilation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web
version of this article.)
P. La Roche et al. / Energy & Buildings 224 (2020) 110243 3
A version with the pipes separated from the space by a plenum
and an insulated ceiling (Fig. 4), also with a sensor operated fan
that provides cool air when needed below. The radiator absorbs
heat from the interior of the cells which is dissipated through
the green roof and the evaporation. The U value of the radiant-
evaporative green roof was also 0.282 W/m
2
K. (See Fig. 4). The
activation temperature of the fan is set a 21 °C, transferring air
from the plenum, cooled by the radiant system, to the interior of
the space, thus cooling it. Summer results with this system were
not as favorable. The ceiling blocked some of the heat transfer from
the interior to the exterior and the performance was better when
the pipes were directly exposed to the space [51].
3. Results
3.1. Test Results: Hybrid green roof in Southern California
Results combining night ventilation with the radiant evapora-
tive system were best in hot and dry days with cooler nights. Dry
days provide more potential to cool by evaporation during the
day with the radiant/cooling system, while cooler nights provide
more potential for cooling with night ventilation and radiant cool-
ing at night. Fig. 5 shows a series in July and August of 2018. During
this period all test cells had windows shaded, night ventilation
from 9 pm to 6 am provided with a fan, and the irrigation system
operating from 1 pm to 1:30 pm. The embedded water pipes were
operating continuously 24 h.
Outdoor dry bulb temperatures were above 40 °C while inside
the test cell with the radiant/evaporative system temperatures
were 9–13 °C below exterior temperatures and several degrees
below the other test cells. The maximum average temperature in
the radiant evaporative green roof was 27.6 °C compared to
29.7 °C in the control cell with the insulated roof, 30.3 °C in the
uninsulated green roof, 29.7 °C in the insulated green roof and
36. 4 °C outside. The average minimum temperature outside dur-
ing the night was 19.9 °C.
The predictive equations were developed by utilizing the lin-
ear regression for the cell with the radiant evaporative system,
shaded, with night ventilation. Figs. 6–8 show the relationship
between the reduction of the maximum temperature (difference
between maximum outside and maximum inside) and the out-
door temperature swing in cells with different amounts of mass:
low, medium and high. Concrete bricks in the floor were used to
adjust this amount of mass and test different options: there are
no bricks in the low mass cell (0 kg/m
2
), 8 bricks in the medium
mass (20 kg/m
2
), and 16 in the high mass (40 kg/m
2
). Each point
in the figure contains one day’s data, comparing the difference
between the indoor and outdoor maximum temperature (y-
axis) with the outdoor diurnal temperature swing (x-axis). As
the swing increases, the difference between indoor and outdoor
maximums also increases. This trend line describes the equation
that predicts maximum indoor temperature as a function of the
outdoor maximum temperature and the outdoor temperature
swing.
In all three test cells and all series, better performance as indi-
cated by higher reduction in the maximum indoor temperature, is
achieved with higher temperature swings, typically also associated
with lower humidity. More variability in outdoor conditions pro-
vide more precision in the equations. Unfortunately, during the
data collection period in the series with low mass there was not
enough variation in outdoor conditions to generate an equation
with a good coefficient of determination.
The equations are below:
Low Mass
T
maxint
¼T
maxext
ð0:37 T
swing
Þþ1:05
Medium Mass
T
maxint
¼T
maxext
ð0:61 T
swing
Þþ3:99 ðR2 ¼0:89Þ
High Mass:
T
maxint
¼T
maxext
ð0:75 T
swing
Þþ3:87 ðR2 ¼0:96Þ
Predictive equations can be generated from the data collected:
Low Mass T
maxint
¼T
maxext
ð0:37 T
swing
Þþ1:05
Medium Mass T
maxint
¼T
maxext
ð0:61 T
swing
Þþ3:99
High Mass T
maxint
¼T
maxext
ð0:75 T
swing
Þþ3:87
Fig. 4. Radiant-Evaporative Green Roof with plenum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this
article.)
4P. La Roche et al. / Energy & Buildings 224 (2020) 110243
where:
T
maxint
=Maximum Temperature Inside
T
maxext
=Maximum Temperature Outside
T
swing
=Outdoor Temperature Swing
These equations are valid for these test cells, with specific phys-
ical properties and amounts of thermal mass tested, and while they
do not predict temperature in buildings with different configura-
tions, they provide an indication of performance in larger spaces
with similar properties.
4. Discussion
4.1. Validation of the test results
Indoor and outdoor measurements were compared on the same
day and compared to the comfort zone to determine the effective-
Fig. 5. Comparative monitoring results: High Mass (July 19–24); Medium Mass (Aug 11–16); Low Mass (Aug 23–28).
Fig. 6. Comparison between reduction of Maximum DBT and Outdoor DBT Swing
Low Mass.
P. La Roche et al. / Energy & Buildings 224 (2020) 110243 5
ness of the strategies [51]. The blue area in the psychrometric dia-
gram in Fig. 9 describes the conditions under which this radiant
evaporative cooling system will be most effective. This area is sim-
ilar to the evaporative cooling area typically proposed in most psy-
chrometric diagrams, up to a dry bulb temperature of 42 °C, an
absolute humidity below 12 g/Kg and wet bulb temperatures
below 24 °C. Above 42 °C there will still be cooling but comfort will
be difficult to achieve.
Different types of green roofs are more effective for cooling in
some climates. However, in all types of green roofs, the following
two conditions improve performance:
a) More vegetation in the canopy layer improves the perfor-
mance by providing shade and some evaporative cooling.
b) All test cells with green roofs when combined with night
ventilation and shade provide more comfort inside build-
ings, with increased energy efficiency. Additional thermal
mass also increases their performance.
Based on the thermal comfort range, the indoor temperature of
each cell was distributed by cold, comfortable, and hot hours, using
a comfort band between 21.1 and 25.5 °C[52]. The number of cold,
Fig. 7. Comparison between reduction of Maximum DBT and Outdoor DBT Swing
Medium Mass.
Fig. 8. Comparison between reduction of Maximum DBT and Outdoor DBT Swing
High Mass.
Fig. 9. Climate Applicability of the Radiant Evaporative Cooled Roof.
6P. La Roche et al. / Energy & Buildings 224 (2020) 110243
comfortable, and hot hours allows to examine the temperature
pattern in the test-cells. Fig. 10 shows the result from the mid-
mass test cell as an example. The 24 rows represent hours of the
day and vertical columns show each experiment day. The number
inside the cell is the average indoor temperature of the test cell.
Table 3 is the summary of all three options (Low, mid, and high-
mass). It is clear that the number of comfortable hours in every
experiment cell were higher than that of the control cells, which
verifies that the developed system works effectively in any mass
condition. The high-mass test cell showed the most comfortable
hours and the low-mass had the least number of comfortable
conditions. Also, the difference between the experiment and con-
trol cell was the largest in the high-mass test cell, which demon-
strates that the high-mass option is the most effective. This
result verifies that the high-mass green roof design will reduce
the operation hours of the building’s mechanical system (13%)
which will contribute to reduce the building’s overall energy
consumption.
Based on the developed equations, the maximum indoor tem-
perature for all three conditions (High, medium, and low mass)
were calculated to validate the projected data by the actual data
and the projected data. In Fig. 11, the monitored line shows the col-
lected data from the experiments and the estimated line indicates
the calculated data by applying the equations with the maximum
outdoor temperature and outdoor temperature swing. The t-test
proved that the equations for the high and medium mass are sig-
nificantly accurate (p > 0.05). However, the projected data from
the low mass equation was not statistically accurate (p < 0.05),
thus additional monitoring experiments are required to improve
and validate the equation.
In hot and humid days, the vegetated canopy will provide
shade, and insulation will reduce exterior heat gains, but increased
Fig. 10. The average indoor temperature distribution matrix: Medium-mass.
Table 3
The summary of the average indoor temperature distribution matrix.
Low-Mass Mid-Mass High-Mass
Cold Comf Hot Cold Comf Hot Cold Comf Hot
Qty Exp 56 42 46 16 69 59 0 82 62
Ctrl 61 33 50 25 61 58 16 62 66
% Exp 38.9 29.2 31.9 11.1 47.9 41 0 56.9 43.1
Ctrl 42.4 22.9 34.7 17.4 42.4 40.3 11.1 43.1 45.8
Fig. 11. Comparative validation of the indoor Max. temperature: Monitoring vs Estimated.
P. La Roche et al. / Energy & Buildings 224 (2020) 110243 7
humidity and reduced daily swing will reduce the effect of evapo-
rative cooling and night ventilation on the cooling performance of
the green roof.
4.2. Integrating passive strategies for outdoor comfort in a hot and dry
climate
It is possible to implement multiple passive strategies to
improve outdoor comfort. For example, in the middle east, shade,
air movement, evaporative cooling, thermal mass with embedded
pipes and green surfaces are proposed to improve outdoor comfort
in several projects. None of these are effective by themselves to
achieve thermal comfort during all outdoor conditions of the year.
However, together, they can improve conditions during all the year
improving occupant experience and thermal comfort in the court-
yards (Fig. 12).
Shade is needed most of the year and in this climate provides
two important benefits, it blocks solar radiation to the human body
which would increase discomfort on warm days and reduces solar
gains on exterior surfaces which would otherwise be absorbing
solar radiation, storing it and then re-radiating it as long wave radi-
ation towards the courtyard, negatively affecting thermal comfort.
Solar studies have been done to propose shade where it will be
more effective. Shade can be retractable or operable and can be
adjusted as appropriate, opening at night to provide additional
cooling of the surfaces to the night sky. During days with high out-
door temperatures, shade by itself is not enough to achieve ther-
mal comfort because it does not reduce air temperature and
other strategies must be implemented.
Air movement can improve comfort when the air temperature
is below 32 °C so that the body feels several degrees cooler. How-
ever, with higher outdoor temperatures, air movement is not help-
ful because it will feel warm as it touches the skin. Fans provide air
movement where it is most needed in the dining area and an exte-
rior solar chimney provides an opportunity for warm air to exit the
courtyard. Air movement is helpful during most of the year to
increase comfort but it some cases must be previously cooled
through evaporative cooling before it passes through the body.
As already discussed, evaporative cooling is an effective strat-
egy to reduce air temperature through evaporation of water when
the air is hot and dry. In this process, the sensible heat in the air is
exchanged for the latent heat of water droplets or wetted surfaces
and the air temperature is reduced with a gain in humidity. This
process is adiabatic, which means that no energy is gained or lost.
In all middle eastern projects a climate analysis was done to
determine if there was sufficient wet bulb depression for evapora-
tive cooling. During the summer, from noontime to 6 pm, the tem-
perature is typically between 35 and 44 °C with a relative humidity
between 25% and 60%. Even though the temperature is high there
is potential for evaporative cooling, especially when the relative
humidity is closer to 30%. The WBT depression is above 10 °C from
March to October indicating potential for cooling during this sea-
son. The highest potential for cooling is in May with 14 °C. From
June to September it is possible to cool the air to 24 °C and it is
possible to cool up to 13 °C below the outdoor temperature. One
scenario is shown in Fig. 13. It is possible to significantly reduce
the air temperature, from 38 °Cto27°C, following the wet bulb
temperature line, greatly improving comfort.
Evaporative cooling can be implemented through different
mechanisms: misters cool the air above the dining areas while
water features at the ground and lower levels provide additional
evaporating cooling, also enhancing the qualities of the space with
aesthetics and sound. Evaporative cooling is most effective during
the daytime when relative humidity is lower, and temperatures are
Fig. 12. Integration of passive cooling systems in the proposed middle eastern courtyard.
8P. La Roche et al. / Energy & Buildings 224 (2020) 110243
higher, and during the shoulder seasons when transitioning to the
hot muggy season. Water bodies at the ground level also keeps the
ground from overheating and re radiating energy to the exterior.
Thermal mass has cool radiant pipes embedded in it. When the
water features are activated, the water is cooled by evaporation
and then this water cools the slabs from inside. The embedded
pipes are thermally coupled with the water features, acting as a
heat exchanger, transmitting the energy from the slab to the water
and then to the air. This cool slab will provide radiant cooling and
improve comfort to the people sitting above. This strategy is espe-
cially effective during the late afternoon and dinner when evapora-
tive cooling is not as effective. Care must be taken to keep the slab
in the shade and below the dew point temperature to avoid
condensation.
Green surfaces have a double objective, they protect and shade
the walls, especially the west wall so that it does not overheat, and
they also provide some evaporative cooling. The courtyard floor
includes water and green areas that also reduce solar gains. Green
on the solar chimneys keeps the walls from overheating and can be
integrated with misters to improve comfort.
4.3. Design options generated from the hybrid green roof tested
The experiments in this study evaluated the performance of the
cooling systems and the architectural design can be different if the
heat flow paths are maintained. The concepts tested in the cells can
take multiple forms, more appropriate to the architectural concept
being developed. An example is the Xylem, that implements sev-
eral principles of the radiant/evaporative test cell system on an
outdoor space [53]. The Xylem proposes a green roof system that
can also be implemented in a hot and humid climate using the
strategies tested at the Lyle Center for Regenerative Studies and
described below.
The goal of the Xylem is to improve outdoor thermal comfort
while mitigating the heat island effect. Four cooling strategies are
integrated: a large canopy for shading, natural ventilation, a vege-
tated roof for thermal mass, and water circulation for radiant cool-
ing (Fig. 14). The form of the Xylem maximizes shade by its large
diameter at the top while its curvature and slender stem provides
for air flow at the bottom. As in the radiant evaporative cooling
green roof previously tested, liquid-filled tubes embedded in the
soil of the planting material are cooled to a lower temperature than
the air temperature. In this case the water in the pipes does not go
to the ceiling of a space but instead the water in these tubes cools
panels at the occupant level and in the ground around the center of
the pod, cooling the person by radiation from the ground or the
panels (Fig. 14). An air space between the earth and the panels also
helps to insulate the earth while providing an opportunity to also
cool the air as it flows from the top to the ground providing cool
air at the lower level. Photovoltaic panels integrated at the top of
the Xylem provide electricity to the pumps that circulate the
water.
As cities replace vegetation with paved surfaces, ‘‘islands” of
higher temperatures increase energy consumption in buildings,
emissions of GHG, and pollutants while compromising human
health. As in the green roofs that have been tested at the Lyle Cen-
ter, the Xylem incorporates multiple strategies to reduce the heat
island effect; the vegetation above the Xylem shades its surface
Fig. 13. Potential evaporative cooling effect.
P. La Roche et al. / Energy & Buildings 224 (2020) 110243 9
while evaporation from these vegetated surfaces also cools the air
by using heat from the air to evaporate water; the Xylem itself
shades the ground below reducing heat absorption (Fig. 15).
Variations of the Xylem can be developed to increase cooling by
radiation, evaporation, or convection. Fig. 16 shows a version
developed and integrated in the Master Plan of California State
San Bernardino University’s Palm Desert campus, developed at Cal-
lisonRTKL. The concept shows a Wind Capture Area (WCA) in
which air is captured, misted, and cooled when conditions are
appropriate for evaporative cooling. Some shade is also provided.
There is also a Personal Comfort Area (PCA) in which thermal com-
fort is improved at the occupant level through evaporative and
radiant cooling.
The air that is cooled by evaporation as it descends through the
tower and cools people at the lower level. The tower needs a cer-
tain height (at least 4 m) to cool the air and the water as it falls.
Two sizes of drops could be used, small drops will evaporate and
cool the air while larger drops will partially evaporate and fall as
cool water below. This cooled water then cools a heat exchanger
that cools the fluid that circulates to the radiant surfaces and pro-
vides comfort by radiation. These surfaces must be shaded for best
performance and vegetation provides shade to reduce the effect of
solar radiation at the upper portion of the cooled area.
5. Conclusion
It is possible to continue developing and improving the perfor-
mance of passive cooling systems integrating them with other
building components such as roofs, green roofs, walls, and win-
dows as long as the materials and design respond to required heat
flow paths and climate. By integrating these systems into building
components, the building components can do more, and value is
added to them.
This study developed a new design of the green roof with a radi-
ant/evaporative cooling system and verified its significant cooling
performance under various thermal mass conditions, and also pro-
vided a method to develop an equation that can be applied to pre-
dict indoor temperatures. This type of green roof performs better
Fig. 14. Radiant Evaporative Green Roof. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
10 P. La Roche et al. / Energy & Buildings 224 (2020) 110243
on hot and dry climates and when combined with night ventilation
in climates with daily temperature swings above 12 °C. This green
roof had additional value compared to a traditional green roof that
already has added value.
Additionally, diverse system alternatives should be developed
and analyzed to increase the accuracy of the results. Different sys-
tem options, such as diameters and length of the radiant pipe, sur-
face area and volume of thermal mass, night ventilation, and water
circulation schedule, as well as green roof options (depth, height,
exposed soil area to the atmosphere, wind-protection, parapets,
etc), will increase the accuracy of the equation as well as a diver-
sity of possible systems. Also, the analysis of additional environ-
Fig. 15. Shade effect by the Xylem.
Fig. 16. Design modification of the Xylem.
P. La Roche et al. / Energy & Buildings 224 (2020) 110243 11
mental factors, such as heat flow, relative humidity, air velocity,
will be beneficial to understand the dynamics of indoor environ-
mental factors and behavior of the systems.
Lastly, it is also important to integrate Passive Cooling Systems
in the design of interior and exterior spaces, so they are not simply
accessories attached to the project. They should be integral to the
architectural design intent which is what each of these projects
tries to demonstrate. For example, the form and construction of
the xylem responds to the flow of energy from the green roof
towards the people below to achieve comfort; and the integrated
strategies in the courtyard provide an opportunity to cool through
the thoughtful integration of these strategies adapted to daily and
seasonal cycles.
CRediT authorship contribution statement
Pablo La Roche: Conceptualization, Methodology, Validation,
Investigation, Resources, Data curation, Writing - original draft,
Writing - review & editing, Supervision. Dongwoo Jason Yeom:
Conceptualization, Investigation, Data curation, Validation, Formal
analysis, Writing - review & editing. Arianne Ponce: Visualization.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgement
The authors want to express gratitude to the John T. Lyle Center
for Regenerative Studies at Cal Poly Pomona for its support with
the facilities.
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