CO2 Payoff of Extensive Green Roofs with Different Vegetation Species

Abstract

Green roofs are considered effective in the reduction of atmospheric CO2 because of their ability to reduce energy consumption of buildings and sequester carbon in plants and substrates. However, green roof system components (substrate, water proofing membrane, etc.) may cause CO2 emissions during their life cycle. Therefore, to assess the CO2-payoff for extensive green roofs, we calculated CO2 payback time it takes their CO2 sequestration and reduction to offset the CO2 emitted during its production process and maintenance practices. The amount of CO2 emitted during the production of a modular green roof system was found to be 25.2 kg-CO2·m⁻². The annual CO2 emission from the maintenance of green roofs was 0.33 kg-CO2·m⁻²·yr⁻¹. Annual CO2 sequestration by three grass species with irrigation treatment was about 2.5 kg-CO2·m⁻²·yr⁻¹, which was higher than that of Sedum aizoon. In the hypothetical green roofs, annual CO2 reduction due to saved energy was between 1.703 and 1.889 kg-CO2·m⁻²·yr⁻¹. From these results, we concluded that the CO2 payback time of the extensive green roofs was between 5.8 and 15.9 years, which indicates that extensive green roofs contribute to CO2 reduction within their lifespan.

Full text

sustainability
Article
CO2Payoff of Extensive Green Roofs with Different
Vegetation Species
Takanori Kuronuma 1, Hitoshi Watanabe 1,*, Tatsuaki Ishihara 2, Daitoku Kou 2,
Kazunari Toushima 2, Masaya Ando 1and Satoshi Shindo 1
1Center for Environment, Health and Field Sciences, Chiba University, 6-2-1 Kashiwa-no-ha, Kashiwa,
Chiba 277-0882, Japan; t.kuronuma@chiba-u.jp (T.K.); m-ando@chiba-u.jp (M.A.);
shindo@faculty.chiba-u.jp (S.S.)
2Kyodo KY-Tec Corporation, 1-15-1, Ebisu-minami, Shibuya-ku, Tokyo 150-0022, Japan;
ishihara@ky-tec.co.jp (T.I.); kou@ky-tec.co.jp (D.K.); toushima@ky-tec.co.jp (K.T.)
*Correspondence: hwatanabe@faculty.chiba-u.jp; Tel.: +81-47-137-8106
Received: 6 June 2018; Accepted: 27 June 2018; Published: 30 June 2018


Abstract:
Green roofs are considered effective in the reduction of atmospheric CO
2
because of their
ability to reduce energy consumption of buildings and sequester carbon in plants and substrates.
However, green roof system components (substrate, water proofing membrane, etc.) may cause
CO
2
emissions during their life cycle. Therefore, to assess the CO
2
-payoff for extensive green roofs,
we calculated CO
2
payback time it takes their CO
2
sequestration and reduction to offset the CO
2
emitted during its production process and maintenance practices. The amount of CO
2
emitted during
the production of a modular green roof system was found to be 25.2 kg-CO
2·
m
−2
. The annual CO
2
emission from the maintenance of green roofs was 0.33 kg-CO
2·
m
−2·
yr
−1
. Annual CO
2
sequestration
by three grass species with irrigation treatment was about 2.5 kg-CO
2·
m
−2·
yr
−1
, which was higher
than that of Sedum aizoon. In the hypothetical green roofs, annual CO
2
reduction due to saved energy
was between 1.703 and 1.889 kg-CO
2·
m
−2·
yr
−1
. From these results, we concluded that the CO
2
payback time of the extensive green roofs was between 5.8 and 15.9 years, which indicates that
extensive green roofs contribute to CO2reduction within their lifespan.
Keywords:
building energy savings; CO
2
emission; CO
2
payback time; CO
2
sequestration; extensive
green roof; inventory analysis
1. Introduction
Global warming caused by the increase of greenhouse gases in the atmosphere has been
identified as one of the most important environmental issues currently faced by human civilization.
Carbon dioxide (CO
2
) is known to be a primary anthropogenic greenhouse gas. In urban areas,
according to the Intergovernmental Panel on Climate Change [
1
], climate change is projected to
increase environmental risks for people, assets, economies and ecosystems, including risks from heat
stress, storms and extreme precipitation, inland and coastal flooding, landslides, air pollution, drought,
water scarcity, sea level rise and storm surges. Increasing urban greenspaces is one proposed method
of mitigating these problems through urban planning [2–6].
Greenspace in urban areas reduces atmospheric CO
2
through sequestration, shading and
evapotranspiration. Shading and evapotranspiration reduce atmospheric CO
2
indirectly by reducing
the necessity for air conditioning [
7
–
11
], which decreases CO
2
emissions from electric power generation.
CO
2
sequestration is the direct removal of CO
2
from the atmosphere through photosynthesis and
the fixation of carbon in plant litter and root exudates. The capacity of urban greenspaces (urban
Sustainability 2018,10, 2256; doi:10.3390/su10072256 www.mdpi.com/journal/sustainability

Sustainability 2018,10, 2256 2 of 12
forests, parks, trees, etc.) to sequester CO
2
in plants and soils has already been quantified [
12
–
16
],
demonstrating that an ecosystem can serve as a carbon sink over a sufficient time period.
However, urban areas are covered mainly by impervious surfaces (e.g., streets, parking lots and
buildings), which makes it difficult to plant trees and increase urban greenspace. Accordingly, a green
roof, which replaces an impervious surface with greenspace, is a key solution to this problem. The total
area of green roofs in Japan actually increased about 29-fold between 2000 (135,222 m
2
) and 2013
(3,875,716 m
2
). Green roofs also contribute to atmospheric CO
2
reduction through their reduction
energy consumption of buildings and sequestration of carbon in plants and substrates. The energy
saving potential of green roofs has been widely investigated [
17
–
23
] and Sailor and Bass [
24
] developed
a web tool—the Green Roof Energy Calculator—to readily estimate the annual energy savings for a
building with a green roof. By contrast, although Getter et al. [
25
] and Whittinghill et al. [
26
] measured
the capacity of green roofs to sequester carbon in plants and substrates, efforts to quantify carbon
sequestration in green roofs have been limited.
A green roof can be classified as extensive or intensive. The extensive type is characterized
by a shallow substrate (<20 cm deep) and requires little maintenance. In contrast, the substrate
depth of an intensive green roofs is greater than 20 cm and can support the growth of woody plants.
However, an intensive green roof requires careful maintenance and is costly. Because of these reasons,
the extensive green roof is currently more common. In particular, modular extensive green roof systems
are used exclusively in Japan. They are designed for ease of installation and alteration and generally
consist of vegetation mats, substrate, substrate containers, water reservoir trays, water proofing
membrane, edge dividers and an irrigation system.
These green roof system components have potential environmental impacts throughout their life
cycles (raw material extraction, manufacture, distribution, use, repair and maintenance and disposal
or recycling). Several studies have used the life cycle assessment (LCA) methodology to determine
the environmental impacts of green roofs [
27
–
29
]. These studies compared the environmental load
and benefits of green roofs and assessed their overall environmental impacts. In particular, a study by
Bianchini and Hewage [
30
] indicated that the annual air pollution (NO
2
, SO
2
and O
3
) reduction from
a green roof will offset the emissions associated with its production after 13 to 32 years. The study
calculated the amount of air pollution created by the production of the polymers of a typical green
roof system and compared their results with its pollution removal capacity [
31
]. However, there have
been few studies on the CO
2
payoff of modular green roofs and the carbon balance of a modular green
roof system—that is, whether it acts as a sink or a source—is therefore open to debate.
In studies estimating the emissions reduction potential of power plants utilizing renewable energy,
CO
2
payoff is often defined as the CO
2
payback time [
32
–
34
]. This index is calculated as the ratio of
the CO
2
emissions from the production of each power plant to the annual CO
2
reduction resulting
from the generation of electricity from renewables.
In this study, therefore, we used LCA methodology to calculate the CO
2
emissions from the
production process and maintenance practices of a modular green roof and investigated the annual
CO
2
sequestration by several green roof plants (Figure 1). In addition, we estimated the amount of
energy saved of buildings with green roofs using the Green Roof Energy Calculator. We used these
parameters to assess the CO
2
-payoff for modular green roofs by calculating their CO
2
payback time.
We defined the CO
2
payback time of a green roof system as the time it takes total CO
2
reduction by the
system to offset the CO2emitted during its production and maintenance.

Sustainability 2018,10, 2256 3 of 12
Sustainability 2018, 10, x FOR PEER REVIEW 3 of 12
Figure 1. A schematic drawing of the investigation scope in this study to estimate the CO2 payback
time of a modular green roof.
2. Materials and Methods
In order to estimate CO2 emissions and energy savings from a modular green roof, we set a
hypothetical average for green roofs in Japan to a greening area of 200 m2 and a substrate depth of 5
cm. We conducted partial LCA for the hypothetical green roofs. The functional unit studied was 1 m2
of the modular green roof with a service lifetime of 45 years on a flat concrete roof. We therefore
converted our results for CO2 emissions and reductions into values per m2.
2.1. CO2 Emission from a Modular Green Roof
2.1.1. Definition of Goal and Scope
In this section, we calculated the CO2 emitted during the production and maintenance of a
typical modular green roof.
We defined the system boundaries as the production processes for the system components
(substrate, substrate containers, water reservoir trays, water proofing membrane, edge dividers,
irrigation tubes, irrigation pipes and automatic watering device) and the maintenance practices
(irrigation and fertilization) of a modular green roof system. The life cycle system included the
extraction and refinement of raw materials and the consumption of natural resources. The production
process for vegetation mats in a farm and transportation of the components were not taken into
account because of a lack of relevant information.
2.1.2. Inventory Analysis
We collected data from companies and experts about the components and maintenance practices
of a typical modular green roof system. We calculated the CO2 emission factors for each component
and maintenance practice using the MiLCA LCA software developed by the Japan Environmental
Management Association for Industry. We used inter-industrial relation analysis to calculate the CO2
emission factors of the irrigation tube and automatic watering device. This analytical method is a
form of economic analysis based on the interdependencies between economic and environmental
sectors. It enables us to estimate the environmental impacts of products throughout their costs. For
all of the other components and maintenance practices, we used a bottom-up approach for building
the inventory. We calculated the amount of CO2 emitted by each component or maintenance practice
by multiplying its CO2 emission factor by the quantity of that component used in the hypothetical
green roof.
We set the dimensions of a typical green roof at 12.5 m by 16 m (200 m2) and we calculated the
use of each green roof system component in this size. A schematic drawing of a typical modular green
roof system (with the irrigation pipes and automatic watering device excluded) is shown in Figure 2.
The main raw materials used to produce each component and the quantity of each component used
in the hypothetical green roof, are shown in Table 1.
Figure 1.
A schematic drawing of the investigation scope in this study to estimate the CO
2
payback
time of a modular green roof.
2. Materials and Methods
In order to estimate CO
2
emissions and energy savings from a modular green roof, we set a
hypothetical average for green roofs in Japan to a greening area of 200 m
2
and a substrate depth of
5 cm. We conducted partial LCA for the hypothetical green roofs. The functional unit studied was
1 m
2
of the modular green roof with a service lifetime of 45 years on a flat concrete roof. We therefore
converted our results for CO2emissions and reductions into values per m2.
2.1. CO2Emission from a Modular Green Roof
2.1.1. Definition of Goal and Scope
In this section, we calculated the CO
2
emitted during the production and maintenance of a typical
modular green roof.
We defined the system boundaries as the production processes for the system components
(substrate, substrate containers, water reservoir trays, water proofing membrane, edge dividers,
irrigation tubes, irrigation pipes and automatic watering device) and the maintenance practices
(irrigation and fertilization) of a modular green roof system. The life cycle system included the
extraction and refinement of raw materials and the consumption of natural resources. The production
process for vegetation mats in a farm and transportation of the components were not taken into account
because of a lack of relevant information.
2.1.2. Inventory Analysis
We collected data from companies and experts about the components and maintenance practices
of a typical modular green roof system. We calculated the CO
2
emission factors for each component
and maintenance practice using the MiLCA LCA software developed by the Japan Environmental
Management Association for Industry. We used inter-industrial relation analysis to calculate the CO
2
emission factors of the irrigation tube and automatic watering device. This analytical method is a form
of economic analysis based on the interdependencies between economic and environmental sectors.
It enables us to estimate the environmental impacts of products throughout their costs. For all of
the other components and maintenance practices, we used a bottom-up approach for building the
inventory. We calculated the amount of CO
2
emitted by each component or maintenance practice
by multiplying its CO
2
emission factor by the quantity of that component used in the hypothetical
green roof.
We set the dimensions of a typical green roof at 12.5 m by 16 m (200 m
2
) and we calculated the
use of each green roof system component in this size. A schematic drawing of a typical modular green
roof system (with the irrigation pipes and automatic watering device excluded) is shown in Figure 2.
The main raw materials used to produce each component and the quantity of each component used in
the hypothetical green roof, are shown in Table 1.

Sustainability 2018,10, 2256 4 of 12
Sustainability 2018, 10, x FOR PEER REVIEW 4 of 12
Figure 2. A schematic drawing of a typical modular green roof system that was calculated the CO2
emissions from the production process in this experiment (with the irrigation pipes and automatic
watering device excluded).
Table 1. The main raw material and the quantity of each component used for the hypothetical green
roof in production and maintenance practice.
System Components and Maintenance
Main Raw Material
Required (for 200 m2)
Substrate
Perlite
1572 kg
Substrate container
Polypropylene
800 kg
Water reservoir tray
Polyvinyl chloride
174 kg
Water proofing membrane
Polyvinyl chloride
55 kg
Edge divider
Aluminum
75 kg
Irrigation pipe
Polyvinyl chloride
5 kg
Irrigation tube
Special polyethylene
208 m (142$)
Automatic watering device
‐
1 machine (231$)
Irrigation
Water
161.6 t･year−1
Fertilizer
Compound fertilizer
8 Kg･year−1
The substrate contains more than 50% perlite, along with compost and zeolite. We calculated the
CO2 emission factors of each substrate component using MiLCA and weighed their individual
content ratios (kg･kg−1). We used the results to calculate a CO2 emission factor for the substrate.
The substrate containers were made of polypropylene, with numerous small holes for drainage.
The substrate containers were 50 cm long by 50 cm wide and 6.5 cm deep. Vegetation mats were
planted in the containers after filling with substrate to a depth of 5 cm. Each container was connected
to adjoining containers to prevent wind uplift.
Drainage from the containers was collected in the water reservoir trays, which were 50 cm long
by 50 cm wide by 1.5 cm deep and made of polyvinyl chloride. Each reservoir tray was also connected
to adjoining trays.
The irrigation pipes connected the automatic watering device to the irrigation tubes, which were
aligned under every second substrate container (13 lines × 16 m). The irrigation pipe needed to be
aligned with the edge of the green roof, so we set the length of the irrigation pipe to 15 m. Irrigation
pipe consists mainly of polyvinyl chloride.
The water proofing membrane was also made of polyvinyl chloride and was 0.3 mm thick. This
membrane was the final layer of the modular green roof system and served as water proofing and a
root barrier. It was intended to protect the building from penetration by water and roots.
The edge dividers were made of aluminum and used for sealing the edge of the green roof (57
m). The edge dividers play a crucial role in locking the green roof components in place and
preventing wind uplift.
According to the inter-industry relations analysis implemented in MiLCA, the prices of the
irrigation tubes and automatic watering device for the hypothetical green roof were $142 and $231,
respectively, at an exchange rate of 110 yen to the U.S. dollar.
The use of water and fertilizer during maintenance of the hypothetical green roof are shown in
Table 1. According to an interview with a relevant company, a green roof is irrigated 101 times
Figure 2.
A schematic drawing of a typical modular green roof system that was calculated the CO
2
emissions from the production process in this experiment (with the irrigation pipes and automatic
watering device excluded).
Table 1.
The main raw material and the quantity of each component used for the hypothetical green
roof in production and maintenance practice.
System Components and Maintenance Main Raw Material Required (for 200 m2)
Substrate Perlite 1572 kg
Substrate container Polypropylene 800 kg
Water reservoir tray Polyvinyl chloride 174 kg
Water proofing membrane Polyvinyl chloride 55 kg
Edge divider Aluminum 75 kg
Irrigation pipe Polyvinyl chloride 5 kg
Irrigation tube Special polyethylene 208 m (142 $)
Automatic watering device - 1 machine (231 $)
Irrigation Water 161.6 t·year−1
Fertilizer Compound fertilizer 8 Kg·year−1
The substrate contains more than 50% perlite, along with compost and zeolite. We calculated the
CO
2
emission factors of each substrate component using MiLCA and weighed their individual content
ratios (kg·kg−1). We used the results to calculate a CO2emission factor for the substrate.
The substrate containers were made of polypropylene, with numerous small holes for drainage.
The substrate containers were 50 cm long by 50 cm wide and 6.5 cm deep. Vegetation mats were
planted in the containers after filling with substrate to a depth of 5 cm. Each container was connected
to adjoining containers to prevent wind uplift.
Drainage from the containers was collected in the water reservoir trays, which were 50 cm long
by 50 cm wide by 1.5 cm deep and made of polyvinyl chloride. Each reservoir tray was also connected
to adjoining trays.
The irrigation pipes connected the automatic watering device to the irrigation tubes, which were
aligned under every second substrate container (13 lines
×
16 m). The irrigation pipe needed to be
aligned with the edge of the green roof, so we set the length of the irrigation pipe to 15 m. Irrigation pipe
consists mainly of polyvinyl chloride.
The water proofing membrane was also made of polyvinyl chloride and was 0.3 mm thick.
This membrane was the final layer of the modular green roof system and served as water proofing and
a root barrier. It was intended to protect the building from penetration by water and roots.
The edge dividers were made of aluminum and used for sealing the edge of the green roof (57 m).
The edge dividers play a crucial role in locking the green roof components in place and preventing
wind uplift.
According to the inter-industry relations analysis implemented in MiLCA, the prices of the
irrigation tubes and automatic watering device for the hypothetical green roof were $142 and $231,
respectively, at an exchange rate of 110 yen to the U.S. dollar.

Sustainability 2018,10, 2256 5 of 12
The use of water and fertilizer during maintenance of the hypothetical green roof are shown in
Table 1. According to an interview with a relevant company, a green roof is irrigated 101 times annually,
with 8 L
·
m
−2
of water used for each irrigation. Fertilizer is applied twice annually, with 20 g
·
m
−2
used
each time.
2.2. CO2Sequestration by Several Green Roof Plants
In order to quantify CO
2
sequestration in green roofs, we investigated the annual CO
2
sequestration
by three grass species—Cynodon dactylon Pers., Festuca arundinacea Schreb. and Zoysia matrella L.
“Himekourai-shiba”—and a flowering plant, Sedum aizoon L. Grasses and Sedum species are the most
common green roof vegetation in Japan.
This experiment was conducted at the Center for Environment, Health and Field Sciences at
Chiba University over one year. All plants in this experiment were propagated as cuttings in plug flats
(128 cells
·
tray
−1
) filled with seedling propagation soil (Metro Mix; Sun Gro Horticulture, Agawam,
USA). After approximately one month, the plugs were planted in 0.2 L polyethylene pots (44 cm
2
)
filled to a depth of 5 cm with commercial artificial soil for green roofs (the same as the substrate
mentioned above) and grown in a greenhouse for two months. They were then placed on the rooftop
and acclimated for three weeks. For a more accurate simulation in this experiment, we should have
used the same modular green roof system. However, we used polyethylene pots and irrigated by hand
sowing because we had to test a number of experimental plants using a limited roof area.
At the start of the experiment, on October 20, 2014, we sampled all species from a total of 15 pots
over a period of about 10 days. Green roofs composed of grasses are generally fitted with irrigation
systems to prevent drought stress. In contrast, irrigation systems are less common in Sedum green roofs
because Sedum uses the CAM photosynthetic pathway and is thus better adapted to drought conditions.
The S. aizoon pots were therefore assigned randomly to irrigation and non-irrigation treatments after
the first sampling. Plants in the irrigation treatment group were watered once a week from January to
March, once every two days from April to June, every day from July to September and once every two
days from October to December. The non-irrigation treatment group was never irrigated. The three
grass species received only the irrigation treatment, in keeping with general cultivation practice.
All treatments received about 20 g
·
m
−2
(0.1 g
·
pot
−1
) of controlled-release fertilizer (8N-8P-8K) on
June 13 and August 13, 2015. As the end of the experiment, on 20 October 2015, we harvested 15 pots,
including all species and treatments.
All plants were dried at 70
◦
C for 72 h and then divided into plant and substrate matter. Plant and
substrate carbon concentrations were measured using an organic elemental analyzer (2400 Series II
CHNS/O System; PerkinElmer, Waltham, MA, USA). Carbon content was quantified by multiplying
carbon concentration by the dry weight. Annual CO
2
sequestration was calculated by subtracting the
total carbon content in October 2014 from the total carbon content in October 2015.
In order to calculate the leaf area index (LAI) of the four species in summer, 15 pots, including all
species and treatments, were sampled on August 20, 2015 and divided into leaves and non-leaf parts.
Leaves were scanned (LP-A500; EPSON, Nagano, Japan) and image analysis software was used to
measure the leaf area (ImageJ [
35
]). The LAI was calculated as leaf area per 44 cm
2
(the area of each
polyethylene pot).
Data were analyzed using IBM SPSS Statistics version 22.0 (IBM Japan, Tokyo, Japan). Differences in
mean values were assessed with a Student's t-test.
2.3. Estimation of the Energy Savings Amount
We used the Green Roof Energy Calculator to estimate the energy saved by a building covered with
the hypothetical green roof (200 m
2
greening area of rooftop, 5 cm substrate depth). This web tool was
developed from the Energyplus-based green roof model [
24
] and requests the following information:
state and city, surface area of the roof, building type (old or new, office or apartment), substrate depth
(limited to 5 cm < depth < 30 cm), leaf area index (limited to 0.5 < LAI < 5), irrigation flag (yes or

Sustainability 2018,10, 2256 6 of 12
no), percentage of roof covered by the green roof system and roofing type for the non-green roof area
[black (albedo: 0.15) or white (albedo: 0.65)]. Because this tool requires LAI values, we ran separate
simulations for the C. dactylon,F. arundinacea,Z. matrella and S. aizoon green roofs, as was done in the
CO
2
sequestration experiment. The growing media characteristics for all green roof simulations were
set as follows: thermal conductivity 0.35 W
·
mK
−1
, density 1100 kg
·
m
−3
, specific heat 1200 J
·
kgK
−1
,
saturation volumetric moisture 0.3, residual volumetric moisture 0.01, initial volumetric moisture 0.1.
See the article for further details of GREC [24].
The location was entered as Houston, Texas, because the meteorological conditions in that city
resemble those in Tokyo, Japan. The percentage of roof covered by the green roof system was set at
50%, which set the surface area of the roof at 400 m
2
. In Japan, about 90% of green roofs are located
on new building, so the building type was set as “new office”. In order to embed the LAI values in
summer into this model, the results in August 2015 was added. The irrigation flag was set as “yes” for
all simulations, expect for that of the non-irrigated S. aizoon roof. The roofing type for the non-green
roof area was set as “black”.
The reduction in electricity and gas consumption was estimated in kWh and converted to CO
2
emissions reduction (kg-CO
2
) using the CO
2
emission factor (0.505 kg-CO
2·
kWh
−1
) obtained from the
Ministry of the Environment in Japan.
2.4. CO2Payback Time of Modular Green Roofs
In non-irrigated green roof systems (the non-irrigation treatment), CO
2
emitted during the
production of the irrigation system (irrigation tubes, irrigation pipes and automatic watering device)
and associated with the annual water supply was excluded from the calculation of the total CO
2
emissions from the modular green roof. The amount of annual CO
2
sequestration by the green
roof plants may decrease with their age because carbon in plants and soils eventually reaches a
carbon equilibrium, with sequestration offset by decomposition [
36
,
37
]. Therefore, we calculated the
CO
2
payback time based on two different scenarios. In Scenario 1, we hypothesized that the CO
2
sequestration by green roof plants occurs only during the first year after construction. CO
2
payback
time for this scenario was defined as
CO2payback time = (CO2 e-p - CO2 r-s) / (CO2 r-e - CO2 e-m) (1)
In Scenario 2, we hypothesized that the same amount of CO
2
sequestration by the green roof
plants occurs every year. CO2payback time for this scenario was defined as
CO2payback time = CO2 e-p / (CO2 r-s + CO2 r-e - CO2 e-m) (2)
where CO
2 e-p
is the amount of CO
2
emitted during the production of the modular green roof system,
CO
2 r-s
is the annual CO
2
reduction owing to CO
2
sequestration, CO
2 r-e
is the annual CO
2
reduction
owing to energy savings and CO
2 e-m
is the annual CO
2
emission from maintenance of the hypothetical
green roof.
3. Results and Discussion
3.1. CO2Emissions from a Modular Green Roof
The CO
2
emission factors and total CO
2
emissions from each component are shown in Table 2.
The aluminum edge divider had the highest CO
2
emission factor out of all of the components analyzed
using a bottom-up approach. The water reservoir tray, water proofing membrane and irrigation pipe
had similar CO
2
emission factors because they are all made from the same raw material (Tables 1
and 2).
Aluminum is a lightweight construction material and is therefore suitable for use in green roofs,
whose weights are subject to architectural constraints. However, Bribián et al. [
38
] found that primary

Sustainability 2018,10, 2256 7 of 12
energy demand and global warming potential are higher for aluminum production than for the
production of several other common building products. Using materials other than aluminum for edge
dividers may therefore decrease the environmental load, including CO
2
emissions, associated with the
production of green roof systems.
The amount of CO
2
emitted during the production of substrate was higher than that from any
of the other components (Table 2). This was due mainly to the relatively large quantify of substrate
used in green roof systems (Table 1). The automatic watering device exhibited the second lowest CO
2
emissions out of all of the components. However, this result is the amount of CO
2
emitted during the
production of one machine and, unlike the results for the other components, is independent of the area
of the green roof. It is therefore clear that the environmental load per m
2
of the automatic watering
device increases as the green roof area decreases.
The CO
2
emission factors and total CO
2
emissions for each maintenance practice are shown in
Table 3. The total annual CO
2
emissions from maintenance practices were 0.33 kg-CO
2·
m
−2·
yr
−1
.
The CO
2
emissions from the maintenance of a non-irrigated green roof were generated by fertilizer
alone, at 0.04 kg-CO2·m−2·yr−1.
Table 2.
CO
2
emission factors and CO
2
emissions for each modular green roof system component in
this experiment.
System Components CO2Emission Factor CO2Emission CO2Emission
(kg-CO2·200 m−2) (kg-CO2·m−2)
Substrate 1.15 kg-CO2·kg−11809 9.04
Substrate container 1.89 kg-CO2·kg−11512 7.56
Water reservoir tray 3.70 kg-CO2·kg−1644 3.22
Water proofing membrane 3.29 kg-CO2·kg−1182 0.91
Edge divider 10.26 kg-CO2·kg−1769 3.85
Irrigation pipe 3.56 kg-CO2·kg−116 0.08
Irrigation tube 0.55 kg-CO2·$−178 0.39
Automatic watering device 0.13 kg-CO2·$−130 0.15
Total – 5040 25.2
Table 3.
CO
2
emission factors and annual CO
2
emissions for each maintenance practice in the
hypothetical modular green roof.
Maintenances CO2Emission Factor CO2Emission CO2Emission
(kg-CO2·200 m−2·yr−1) (kg-CO2·m−2·yr−1)
Water 0.36 kg-CO2·t−158.8 0.29
Compound fertilizer 0.90 kg-CO2·kg−17.2 0.04
Total – 66.0 0.33
Total CO
2
emissions from the production of a modular green roof were 25.2 kg-CO
2·
m
−2
(Table 2).
This result is similar to the results of previous research quantifying the amount of CO
2
emitted
during the manufacturing phase of layered green roofs [
27
]. For a green roof without an irrigation
system (i.e., a non-irrigated green roof), the total CO
2
emissions from production process were
24.6 kg-CO2·m−2.
3.2. CO2Sequestration by Several Green Roof Plant
The plant and substrate dry weight, carbon concentration and carbon content of the different
plant species, as recorded on 20 October 2014 and 20 October 2015, are shown in Table 4. For all species
and treatments, plant dry weight was significantly higher at the end of the experiment than at the start.
There were no significant differences in plant carbon content between October 2014 and October 2015,

Sustainability 2018,10, 2256 8 of 12
except for that of S. aizoon with irrigation treatment. F. arundinacea was the only species for which
substrate dry weight at the end of the experiment was significantly different from that at the beginning.
The substrate carbon concentration of all species and treatments increased during the experimental
period and C. dactylon was the only species for which it did not increase significantly. In addition,
the plant and substrate carbon contents for all species and treatments increased significantly during
the experimental period. These results suggest that some carbon from the plant litter and root exudate
was fixed in the substrates.
Table 4.
Mean values (n = 15) for plant and substrate dry weight, carbon concentration, carbon content
and total annual carbon sequestration by four green roof plants (C. dactylon,F. arundinacea,Z. matrella
and S. aizoon).
Species and Treatments
Dry Weight Carbon Concentration Carbon Content Total Annual Carbon
Sequestration
(g·pot−1)(%) (g-C·pot−1)
Oct-14 Oct-15 Oct-14 Oct-15 Oct-14 Oct-15 (g-C·m−2·yr−1)
C. dactylon irrigation plant 0.4 ±0.0z7.0 ±0.4* 39.2 ±0.3 40.7 ±0.4 0.1 ±0.0 2.9 ±0.2* 690
substrate
30.7 ±0.7 32.4 ±0.7 5.1 ±0.3 5.9 ±0.2 1.6 ±0.1 1.9 ±0.1*
F. arundinacea irrigation plant 0.4 ±0.0 7.2 ±0.3* 38.5 ±0.2 36.3 ±0.5 0.1 ±0.0 2.6 ±0.1* 751
substrate
30.0 ±0.3 33.6 ±0.6* 5.7 ±0.1 7.6 ±0.3* 1.7 ±0.0 2.5 ±0.1*
Z. matrella irrigation plant 0.6 ±0.0 6.6 ±0.4* 42.7 ±0.2 43.2 ±0.5 0.2 ±0.0 2.9 ±0.2* 671
substrate
30.8 ±1.3 31.8 ±2.5 6.2 ±0.2 7.0 ±0.2* 1.9 ±0.1 2.2 ±0.1*
S. aizoon
irrigation plant 0.6 ±0.0 3.9 ±0.2* 38.9 ±0.5 41.5 ±0.3* 0.2 ±0.0 1.6 ±0.1* 459
substrate
30.7 ±0.4 30.9 ±0.5 5.5 ±0.2 7.6 ±0.2* 1.7 ±0.0 2.3 ±0.0*
non plant 0.6 ±0.0 3.1 ±0.1* 38.9 ±0.5 38.5 ±0.2 0.2 ±0.0 1.2 ±0.0* 336
substrate
30.7 ±0.4 30.5 ±0.3 5.5 ±0.2 6.9 ±0.3* 1.7 ±0.0 2.1 ±0.0*
z
Represent means
±
SE. * represent significant differences between the results of October 2014 and October 2015
(Student’s t-test, P< 0.05).
The total annual carbon sequestration by F. arundinacea was higher than that of any other species
and was similar to those of the other grasses (Table 4). S. aizoon exhibited higher total annual carbon
sequestration with irrigation treatment than with non-irrigation treatment. The results for S. aizoon were
similar to results from previous research investigating Sedum green roofs [
25
]. With the assumption
that carbon sequestration resulted only from CO
2
uptake, total annual carbon sequestration was
converted to annual CO
2
sequestration. C. dactylon sequestered 2.530 kg-CO
2·
m
−2·
yr
−1
,F. arundinacea
sequestered 2.754 kg-CO
2·
m
−2·
yr
−1
,Z. matrella sequestered 2.459 kg-CO
2·
m
−2·
yr
−1
,S. aizoon with
irrigation treatment sequestered 1.684 kg-CO
2·
m
−2·
yr
−1
and S. aizoon with non-irrigation treatment
sequestered 1.232 kg-CO
2·
m
−2·
yr
−1
. This experiment thus supports the CO
2
sequestration capacity of
several green roof plants.
Kuronuma and Watanabe [
39
] suggested that competence for carbon sequestration of Sedum under
wet and increased nutrient conditions are equivalent to those of other green roof plants (Zoysia matrella
and Ophiopogon japonicus). However, it was disaccorded with the present results. This is probably
because the present experiment conditions keeping with general cultivation practice was nutrient-poor
for Sedum species. From this reason, it is suggested that carbon sequestration of Sedum aizoon was
lower than those of the grasses. For the carbon sequestration of green roofs, therefore, more suitable
design and maintenance practice of vegetation should be studied further.
The LAI of each specie and treatment, as recorded in August 2015, is shown in Table 5.S. aizoon
with irrigation treatment exhibited the highest LAI out of all of the species and treatments and
C. dactylon exhibited the lowest.
3.3. Estimation of the Energy Savings Amount
The annual energy savings from the hypothetical green roofs are shown in Table 5. Of the irrigated
green roofs (with irrigation treatment), the annual energy saving from the S. aizoon green roof was
the highest because the LAI of S. aizoon was higher than that of the three grass species. In contrast,
although S. aizoon with non-irrigation treatment exhibited the median LAI of all species and treatments,
its annual energy saving was the lowest because its irrigation flag was set as “no”. However, annual
energy savings did not differ much between species and treatments and annual CO
2
reductions due to

Sustainability 2018,10, 2256 9 of 12
energy savings were between 1.703 and 1.889 kg-CO
2·
m
−2·
yr
−1
(Table 5), being similar to those in
previous research [22].
Table 5.
Mean values (n = 15) for leaf area index (LAI), energy saved annually and annual CO
2
reduction due to saved energy of modular green roofs with different species (C. dactylon,F. arundinacea,
Z. matrella and S. aizoon) and treatments (irrigation and non-irrigation).
Species Treatments LAI Energy Saved Annually Annual CO2Reduction
(m2·m−2) (kWh·200 m−2·yr−1) (kg-CO2·m−2·yr−1)
C. dactylon irrigation 2.21 ±0.16 696.4 1.758
F. arundinacea irrigation 3.68 ±0.21 747.1 1.886
Z. matrella irrigation 2.45 ±0.17 713.5 1.802
S. aizoon irrigation 3.71 ±0.15 748.2 1.889
non 3.07 ±0.19 674.3 1.703
3.4. CO2Payback Time of Modular Green Roofs
The CO
2
payback time calculated from the Scenario 1, which hypothesized the CO
2
sequestration
by the green roof plants occurs only first year after construction, was in the range of 14.0 to 15.9 years
(Table 6). On the other hand, Scenario 2 was hypothesized that the same amount of annual CO
2
sequestration by the green roof plants occur every year. Thus, the CO
2
payback time calculated
from the Scenario 2 was in the rage of 5.8 to 8.5 years and shorter than that calculated from the
Scenario 1. For the S. aizoon green roofs, the amount of CO
2
emitted during the production of a
non-irrigated green roof was lower than that from an irrigated green roof. Therefore, the CO
2
payback
time calculated from Scenario 1 was longer for the irrigated green roof than for the non-irrigated green
roof. However, the CO
2
payback time calculated from Scenario 2 was longer for a non-irrigated green
roof than for an irrigated one because the increase through in CO
2
reduction from irrigation is greater
than the amount of CO
2
emitted by the addition of an irrigation system and annual water supply
(Tables 4and 6). However, these results indicate that there were not significant differences among the
CO
2
payback time of modular green roofs with different species and treatments in this experiment.
Our results for the hypothetical green roofs indicate that CO
2
reduction from the combination of CO
2
sequestration and energy savings by a green roof can offset the CO
2
emitted during its production
and maintenance after 5.8 to 15.9 years (Table 6). The lifespans of green roof components are generally
thought to be between 40 and 50 years [
28
–
30
,
40
], so it is clear that the lifetime CO
2
reduction of
modular green roofs offsets the CO
2
emitted during their production and maintenance. Accordingly,
our results suggest that modular green roofs are an effective way to reduce atmospheric CO
2
and
mitigate global warming. In addition, Tripanagnostopoulos et al. [
34
] clarified CO
2
payback time
of solar photovoltaic systems which is between 1.3 and 4.1 years. Thus, CO
2
payback time will be
one of the index which could compare the carbon balance of green roofs with that of rooftop solar
photovoltaic systems [41].
In this study, we focused on the production processes and maintenance practices for green roofs.
However, CO
2
sequestration in plants and substrates seems to be greatly influenced by their disposal
phase. In addition, the period for which green roofs serve as carbon sinks may be shorter than those
of other urban greenspaces (e.g., urban forests, parks and trees) because of the more frequent repair
and demolition of buildings. Therefore, the disposal phase also needs to be discussed to ensure the
sustainable design of green roofs although the CO
2
payback time calculated from energy savings
alone was shorter than the system component’s lifespan (17.6 years for C. dactylon, 16.2 years for
F. arundinacea, 17.1 years for Z. matrella, 16.2 years for S. aizoon with irrigation treatment and 14.7 years
for S. aizoon with non-irrigation treatment).
This study did not cover all green roof types (e.g., different green roof systems, system components,
vegetation plants, substrate depth and locations) that could influence the CO
2
payback time results.
Thus, the CO
2
emissions from a green roof and CO
2
reductions from secondly environmental benefits

Sustainability 2018,10, 2256 10 of 12
should be studied further. Our results will serve as a baseline for future research on the CO
2
payoff of
green roofs.
Table 6.
CO
2
payback time of modular green roofs with different species (C. dactylon,F. arundinacea,
Z. matrella and S. aizoon) and treatments (irrigation and non-irrigation).
Species Treatments CO2Payback Time (years)
Scenario 1 Scenario 2
C. dactylon irrigation 15.9 6.4
F. arundinacea irrigation 14.4 5.8
Z. matrella irrigation 15.5 6.4
S. aizoon irrigation 15.1 7.8
non 14.0 8.5
Means ±SE 11 ±4.3
Standard uncertainty 1.4
Note: Scenario 1 was hypothesized that the CO
2
sequestration by the green roof plants occurs only in the first year
following construction. Scenario 2 was hypothesized that the same amount of annual of CO
2
sequestration by the
green roof plants occurs every year.
4. Conclusions
This study quantified the CO
2
emitted during the production and maintenance of a hypothetical
modular green roof and estimated the CO
2
reduction from energy savings and CO
2
sequestration.
The results of the study show that CO
2
emissions are offset through CO
2
sequestration and energy
savings after 5.8 to 15.9 years, which indicates that extensive modular green roofs contribute to
atmospheric CO
2
reduction and global warming mitigation within their lifespan. In addition, this study
provided a method to assess the CO
2
payoff of green roofs. It is hoped that the findings presented
in this paper will contribute to the development and design of green roofs more suitable for CO
2
reduction and global warming mitigation.
Author Contributions:
All of the authors contributed to the work in the paper. T.K. designed the research and
wrote the paper. H.W., M.A. and S.S. provided advice and suggestions. I.T., D.K. and K.T. collected the data. H.W.
contributed to project supervision. All authors reviewed the manuscript.
Funding: This research was funded by [JSPS KAKENHI] grant numbers [15K07663].
Conflicts of Interest: The authors declare no conflict of interest.
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2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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