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PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
Low Cost Green Roofs for Cooling:
Experimental series in a hot and dry climate
PABLO LA ROCHE1
1 California State Polytechnic University, Pomona, USA
ABSTRACT: Green roofs have many benefits over conventional roofs, they reduce storm water runoff, the heat island
effect in cities, and energy requirements for cooling; all of this while sequestering some CO2 from the atmosphere. But
because of their expense, the building industry has yet to fully embrace their large scale implementation. Over the course
of three summers several test structures with green roofs have been built and tested at the Lyle Center for Regenerative
Studies in Cal Poly Pomona University to determine their cooling potential combined with night ventilation. Results
indicate that the test cell with the uninsulated green roof consistently performs better than the test cells with the insulated
green roof and the conventional white insulated roof. This indicates that in mild climates with warm summers, green roofs
can be thermally coupled with the interior of the space to improve the performance of night cooling systems, reducing
energy consumption and greenhouse gas emissions.
Keywords: night ventilation, passive cooling, green roofs, experimental series, low cost sustainable housing
INTRODUCTION
A living or green roof is a roof that is substantially
covered with vegetation. Such a strategy has been proven
to have positive effects on buildings by reducing the
stress on the roof surface, improving thermal comfort
inside the building, reducing noise transmission into the
building, reducing the urban heat island effect by
reducing “hot” surfaces facing the sky, reducing storm
water runoff, re-oxygenating the air and removing
airborne toxins, recycling nutrients and providing habitat
for living organisms, all of this while creating peaceful
environments.
The positive thermal effects of green roofs are
usually described by the reduction of the external surface
temperature due to the effect of vegetation, and the
reduction of the thermal transmittance of the assembly,
mostly due to the effects of insulation, usually placed
between the sustaining material and the interior space of
the building. A green roof without this added insulation
has a low thermal resistance, but does have thermal mass.
Apart from providing protection against overheating, a
green roof can also provide some cooling through the
evaporative process in the plants [1]. Their vegetative
matter absorbs solar radiation through the biological
processes of photosynthesis, respiration, transpiration,
and evaporation. However, the solar radiation that
bypasses these processes can seep into the building
envelope [2].
Studies have demonstrated that a well planned and
managed green roof –with insulation- acts as a high
quality insulation device in the summer [3]. But little has
been done to take advantage of the mass of the green roof
as a heat sink in temperate or hot climates. By reducing
daily thermal fluctuations on the outer surface of the roof
and increasing thermal capacity in contact with the
indoors, green roofs can contribute to the cooling of
spaces if the mass of the soil is cooled. There has been
some research in this direction that indicates potential to
reduce the cooling loads inside buildings [4] [5].
Nocturnal ventilative cooling occurs when an
insulated high-mass building is ventilated with cool
outdoor air so that its structural mass is cooled by
convection from the inside, bypassing the thermal
resistance of the envelope. During the daytime, if there is
a sufficient amount of cooled mass and it is adequately
insulated from the outdoors, it will act as a heat sink,
absorbing the heat penetrating into and generated inside
the building, reducing the rate of indoor temperature rise.
This ventilation system can be either fan forced or
natural through windows that are opened and closed at
appropriate times. During overheated periods the
ventilation system (windows or fans) must be closed to
avoid heat gains by convection. Nocturnal ventilative
cooling is a well known strategy that has been used for
many years, mostly in warm and dry climates [6]. The
main parameters that determine the efficiency of night-
ventilation can be classified in three broad groups:
climatic parameters, building parameters and technical
parameters of the technique [7]. This paper discusses
changes in the performance due to changes in building
parameters.
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
During the course of three summers several green
roofs have been built and tested at the Lyle Center for
Regenerative Studies in California State Polytechnic
University Pomona. Cal Poly Pomona is located in a hot
and dry climate with mild winters about 30 miles east of
Los Angeles in southern California (Fig 1).
Figure 1: Climate Zone 9, location of the tests
This paper analyzes the cooling potential of green
roofs combined with night ventilation by looking at the
internal temperature of experimental test cells and in a
full size residential building.
EXPERIMENTAL SYSTEM
Three test cells with a dimension of 1.2 x 1.2 x 1.2
meters were built using 2 by 4 inch stud wall
construction with drywall on the inside, plywood on the
outside and batt insulation in between for a U value of
0.12 W /m2 K. Exterior is white and the three cells have
0.61 m by 0.61 m (2’ x 2’) single glazed windows facing
south that were replaced by double glazed windows in
2007 the last series (Fig. 2). All of the cells have 3.8 cm
thick concrete pavers as the slab.
Figure 2: The Test Cells
The roof is the only difference between the cells.
The first cell has a code compliant insulated roof, with a
U value of 0.055 W /m2 K, painted white, the second cell
has an insulated green roof, and the third one an
uninsulated green roof (Fig 3). The growth medium in
the uninsulated green roof is thermally coupled with the
interior via a metal plate, while in the the other green
roof there are 10 cm of matt insulation between the space
and the soil. Night ventilation is provided with a fan and
all of the cells are equipped with dimmers and timers to
adjust the ventilation rate and start/end times.
The green roofs were of the extensive type. Two
types were built. The first one, in 2005, was covered with
Saint Augustine grass above a layer of soil 7.5 cms thick
with 2.5 cm of gravel and a plastic liner underneath.
Drainage tubes were spread through the gravel with
perforations that capture the excess water and drain it
outside the building. The plastic liner is spread above a
metal plate, supported by wooden joists. The metal plate
assures thermal coupling between the mass and the space
underneath (Fig 3). The green roof in the cells was
substituted by another one in 2007, designed so that it
could be built at a large scale with minimal technology,
low cost materials, and little maintenance [8]. “Rice
sack” tubular bags developed by the Cal Earth Institute,
were cut and filled with a growth medium containing
50% native soil and 50% perlite. These bags were placed
above an impermeable layer of plastic, which was placed
on top of the roof decking as a moisture barrier. Slits
were cut into the topside of the bag where sedums and
succulents were planted, which have little need for water,
maintenance, or soil depth. The bags were
photodegradable to sunlight, thus over time the surface of
the bags disappeared creating a soil strata evenly
distributed over the roof. This same system was also used
in an affordable low cost house prototype for Tijuana,
Mexico, that was tested in the Fall of 2008.
Figure 3: Cells with insulated white roof and green roof and
uninsulated green roof
EXPERIMENTAL RESULTS
Series were performed over the course of several years
and some of them are presented in this section.
Surface Temperature Surface temperatures in all
cells were higher than the ambient temperature and the
surface temperature of the white roof was significantly
higher than the surface temperature of the green roof. In
a series recorded in 2008 with succulent plants, the
surface temperature under the plants averaged 11.2 C less
than the surface temperature in the white painted roof
(Fig. 4). If the roof was painted a darker cooler the
temperature difference would be even more significant.
Surface and ground temperatures in these cells are
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
studied in more detail in another paper in this conference
titled “Green Roofs: Beneficial in a Semi-Arid Climate”
by M Figueroa.
Surface Temperatures : Aug 18 - Sep 4, 2008
0
10
20
30
40
50
60
70
1 100 199 298 397 496 595 694 793 892 991 1090 1189 1288 1387 1486 1585 1684 1783 1882 1981 2080 2179 2278 2377
Time
Temperature (C)
Outside Temperature Green Roof Surface Temperature White Roof Surface Temperature
Figure 4: Comparison of Surface Temperatures Fall 2008.
Control and Validation Series Several series were
performed in 2005 and 2008 to ensure that had similar
thermal performance. Figure 5 shows results of a series
in 2008 comparing both green roof cells with the same
level of insulation. Both curves are very close together
and there is no need for a correction factor. The
insulation was then taken off again in one of the cells to
continue comparing the performance of insulated and
uninsulated green roofs.
18
20
22
24
26
28
30
32
34
36
38
1 60 119 178 237 296 355 414 473 532 591 650 709 768 827 886 945 1004 1063 1122 1181 1240 1299 1358 1417
Time (10 minute intervals)
Temperature (C)
Green Roof 1 Green Roof 2
Figure 5: Comparison of Insulated Green Roof Cells.
Indoor Temperature with Different Window
Dimensions The climatic parameters that determine the
effectiveness of nocturnal ventilative cooling are the
minimum air temperature, which determines the lowest
temperature achievable inside the building; the daily
temperature swing, which determines the potential for
lowering the indoor maximum below the outdoor
maximum; and the water vapor pressure level, which
determines the upper temperature limit of indoor comfort
with still air or with air movement [9]. During the
measurement period, night temperatures and relative
humidity were sufficiently low for nocturnal ventilative
cooling and all three cells have a timer that sets the fan
set to operate from 9 PM to 5 AM at 25 air changes per
hour.
The main building parameters that affect the
effectiveness of nocturnal ventilative cooling are the
insulation level, the amount of thermal mass and the
amount of glazing. Three series were performed in 2005
with the same amount of thermal mass and insulation.
The window dimension was “modified” by covering with
insulation, affecting direct solar radiation and
conduction. Three series were performed with different
window dimensions: 0.37 m2 window, 0.165 m2 window,
and no window.
Series 1: 100% window In this series initiated
September 7, 2005, the window area is 0.37 m2, or 25%
of the floor area. Temperatures in all three cells are
always higher than outdoor, especially during the
daytime, due to the solar gain through the windows (Fig.
6). The values of the maximum temperature in the
control cell are an average of 10.4 °C above the outdoor
temperatures in the insulated green roof they are 7.6 °C
above the outdoor temperatures, and in the uninsulated
green roof they are 4.5 °C above the outdoor
temperature.
10
15
20
25
30
35
40
1 6 11 16 21 26 31 36 41 46 51 56 61 66 71 76 81 86 91 96 101 106 111 116 121 126 131 136 141
Time (Hours)
Temperature (C)
outdoors control green roof insulated green roof no insulation
Figure 6: Series 1: 100% window, glazing to floor ratio 25%
Series 2: 50% Window In this series initiated
September 7, 2005, the window is 0.185 m2, or 12.5% of
the floor area. During the day, the values of the
maximum temperatures in the control cell are an average
of 4.8 °C above the outdoor temperature, in the insulated
green roof they are 3.2°C above the outdoor
temperatures, and in the uninsulated green roof they are
1.2 °C above the outdoor temperature. Because the
glazing to floor ratio is smaller, the difference between
the maximum temperatures inside all cells and the
outdoor maximum temperature is less than in the
previous series (Fig. 7).
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
10
15
20
25
30
35
40
1 7 13 19 25 31 37 43 49 55 61 67 73 79 85 91 97 103 109 115 121 127 133 139 145 151 157 163 169
Time (hours)
Temperature (C)
outdoors control green roof insulated green roof no insulation
Figure 7: Series 2: 50% window, glazing to floor ratio 12.5%
Series 3: No window In this series initiated
September 7, 2005, there is no window. During the day,
the values of the maximum temperatures in the insulated
control cell are an average of 0.7 °C below the outdoor
temperature, in the insulated green roof they are 2.6 °C
below the outdoor temperature, and in the uninsulated
green roof they are 3.6 °C below the outdoor maximum
temperature.
10
15
20
25
30
35
40
1 10 19 28 37 46 55 64 73 82 91 100 109 118 127 136 145 154 163 172 181 190 199 208 217 226 235 244 253 262
Time (Hours)
Temperature (C)
outdoors control green roof insulate d green roof no insulation
Figure 8: Series 3, No window.
Tests with different plants, shading and windows.
In 2008 the single glazed window was substituted by a
double glazed window, shaded with a 60 cm overhang
that provided complete shading during the summer noon
hours and partial shading during the morning and
afternoon hours. The Saint Augustine grass was
substituted in 2007 by succulent species that needed little
water. Results of tests in 2008 are consistent with
previous series and also indicate that the green roof with
no insulation performs better than the insulated green
roof and the insulated white roof (Fig 9).
0
10
20
30
40
50
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
Time (Hours)
Temperature (C)
outdoor control
insulated green roof uninsulated green roof
Figure 9: Two days in the 2008 series.
Temperature Difference Ratio The three series
performed in 2005 are compared with each other using
the Temperature Difference Ratio, TDR. This concept
was proposed by Givoni and used with good results to
compare passive cooling systems with different
configurations [11] TDR is calculated using:
TDR = (Tmaxout - Tmaxin) / (Tmaxout - Tminout) (1)
Where:
TDR: Temperature Difference Ratio
Tmaxout: Maximum temperature outside
Tmaxin: Maximum temperature inside
Tminout: Minimum temperature inside
The TDR concept normalises the capacity to reduce
the indoor maximum temperature, as a function of the
outdoor swing, permitting comparison of the different
series. In a well ventilated building the TDR can’t be
higher than 1.0 and the resulting fraction can be
expressed as a percentage. A higher value (closer to one),
indicates a better performance and a larger temperature
difference between outdoors (hot) and indoors (cool) and
more cooling. A negative value indicates that the average
maximum temperature inside is higher than outdoors.
TDR is calculated for the three series and averaged for
each one. The best TDR is in the uninsulated green roof,
which performs much better than the other cells (Fig 10).
The equations that predict TDR as a function of the floor
to window ratio (FWR) are:
For the control cell:
TDR = -4.352*FWR +0.0361 (2)
In equation (2) R2 = 0.99
For the green roof with insulation:
TDR = -3.9325*FWR + 0.194 (3)
In equation (3) R2 = 0.98
For the green roof without insulation:
TDR = 3.0588*FWR+0.2955 (4)
In equation (4) R2 = 0.98
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
y = -3.0588x + 0.2955
R2 = 0.9852
y = -3.9325x + 0.194
R2 = 0.9815
y = -4.352x + 0.0361
R2 = 0.9904
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35
% Window to Floor ratio
TDR
TDR control TDR green roof w insulation
TDR green roof no insulation Linear (TDR green roof no insulation)
Linear (TDR green roof w insulation) Linear (TDR control)
Figure 10: TDR as a function of the window to floor ratio
After TDR is calculated for a building using
equations (2), (3), and (4) it is possible to predict the
indoor maximum temperature using equation (1) and
solving for Tmaxin
T maxin = Tmaxout – [TDR * (Tmaxout - Tminout)]
Where outdoor maximum and minimum
temperatures, or daily temperature swing, must be
known. These equations would be valid in buildings with
slab on grade concrete floors with a thickness of 4 cm.
Tests in a real building The green roof was tested in
a real size building: the low cost housing prototype built
at the Lyle Center for Regenerative Studies in Cal Poly
Pomona for Tijuana, Mexico [11]. The walls are made of
papercrete a mixture of cement and paper, the floor slab
is concrete and the windows are single pane glass [12].
Figure 11: Green Roof of the Tijuana House (north view)
A low cost green roof similar to the one in the test
cells has been built on the larger west-facing roof. The
green roof was built with an impermeable layer directly
on top of the roof deck with the perimeter of the roof
boxed in to contain the growth medium. The roof is
supported by a series of trusses built from reclaimed
pallet wood. The same “Cal Earth” bag system that is
used in the test cells is used here, with the growth
medium, made of a mixture of native soil and vermiculite
to reduce weight. The bags are cut in two foot sections
that are easy to transport onto the roof, and placed on the
roof in rows perpendicular with the downward edge of
the roof. They also provide structure around the soil
while the roots establish themselves.
Three series were performed in this building, in the
late summer of 2008: continuous ventilation, night
ventilation and minimal ventilation with all the windows
closed. Results for the series with night ventilation, the
same strategy used in the test cells are presented (Fig.
12). The maximum temperature inside the house is an
average of 6.1 C below the maximum average
temperature outdoors. The daily temperature swing
averages about 21.3 C and the reduction in maximum
temperature is about 29% of the average swing.
Night Ventilation Tijuana House
0
5
10
15
20
25
30
35
40
45
1 16 31 46 61 76 91 106 121 136 151 166 181 196 211 226 241 256 271 286 301 316 331 346 361 376
Time
Temperature (C)
outside center space
Figure 12: Green Roof of the Tijuana House
Equation (4) for night ventilated spaces with
uninsulated green roofs and concrete slabs was tested.
Maximum and minimum outdoor values were used. The
south window to floor glazing ratio of the main living
space where the main data logger sensor that is placed is
5% and TDR is 0.14256 applying equation (4). Predicted
values using equations (1) and (4) are compared with
measured values and a good correlation is observed
between the predicted values and measured values on
cloudy days (Table 1 & Fig 13). Because the house was
not built to test these equations there are several variables
that could account for the difference in performance
between cloudy and sunny days: a) In the building there
are additional windows that did not exist in the test cells,
including east facing clerestory windows, b) the volume
to envelope ratio of the space is much larger, and c) the
U value and thermal lag of the walls is much different.
Table 1: Indoor Predicted and Measured Temperatures
SUNNY CLOUDY CLOUDY CLOUDY SUNNY SUNNY
Predicted Tmaxin= 34.8 28.5 25.7 25.1 30.7 35.9
Measured Tmaxin= 28.3 27.9 26.3 25.2 27.1 28.1
numerical difference 6.5 0.6 -0.6 -0.1 3.6 7.8
difference ratio 22.9% 2.2% -2.2% -0.2% 13.4% 27.9%
PLEA2009 - 26th Conference on Passive and Low Energy Architecture, Quebec City, Canada, 22-24 June 2009
Figure 13: Calculated and Predicted Indoor Temperatures in
the Green Roof of the Tijuana House
CONCLUSION
An uninsulated green roof combined with night
ventilation can help cool a space in two ways: the canopy
layer reduces the effect of solar gains by reducing the
sol-air temperature, and the growth medium acts as a
heat sink.
Results of tests over several years with different
types of plants, windows, and shading systems,
consistently indicate that in this climate, the uninsulated
green roof performs better than the insulated green roof
and the insulated white roof. This indicates that in a
warm or mild climate with cool nights it is possible to
combine a green roof with night ventilation, coupling the
soil layer with the interior of the building. The vegetation
in the canopy layer improves the performance of the
system by blocking solar gains.
Figure 14: Applicability of uninsulated green roofs with night
ventilation.
Simple equations are derived from the experimental
work that permit to calculate internal maximum
temperatures as a function of outdoor maximum
temperature, daily swing, and glazing to floor ratio. One
of these equations is tested in a real size building with
acceptable results on cloudy days. More series should be
performed under different climates and with different
types of buildings to determine the effect of volume,
thermal lag, thermal capacity and other building
variables. Until these series are performed an
applicability range similar to that indicated for thermal
mass and night ventilation in Givoni and Milne’s chart is
proposed (Fig 14). In climates with moderate heating
needs, where heat loss by conduction is not a critical
issue, the uninsulated green roof could probably also be
combined with a passive solar heating system (direct or
indirect). This will be the subject of future research.
Green roofs, when combined with night ventilation
can lead to more comfortable conditions inside buildings,
with increased energy efficiency. This should give them
added value increasing their applicability.
ACKNOWLEDGEMENTS. This project was partially
supported by a 2005 RSCA grant from Cal Poly Pomona.
I am also grateful to the Cal Poly Pomona students that
helped in the construction and monitoring of the test cells
and house, especially Jonah Swick, Rael Berkowitz,
Charles Campanella, Nick Klank. Travel support has
been provided by Energy Design Resources, which is
funded by the California Public Utilities Commission
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