ArticlePDF Available


In recent years, many studies have been carried out on green roof systems because of the multiple benefits that they offer. The energy savings within the building is one of the benefits that have been studied more than the other benefits. The results obtained show that energy saving is achieved by decreasing cooling during summer and reducing heating during winter. This paper examines the current literature related to energy savings in buildings due to green roof systems, phenomenological models that describe the heat transfer process, and the experimental analysis of green roofs as well as highlights some of the factors with more impact. The results of this work show that it is necessary to analyze the heat transfer processes in green roofs at different length scales to evaluate the energy savings in buildings.
J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013 105
Building Energy Savings with a Green Roof
S. Quezada-García,1 R. Vázquez-Rodríguez,2 J. J. Ambriz-García,2 and
G. Espinosa-Paredes,2,*
1Department of Energy Systems, National Autonomous University of Mexico, Mexico
2Area of Energy Resources Engineering, Metropolitan Autonomous University -
Iztapalapa, Mexico
Received October 05, 2012; Accepted October 24, 2012
Abstract: In recent years, many studies have been carried out on green roof systems
because of the multiple benefi ts that they offer. The energy savings within
the building is one of the benefi ts that have been studied more than the
other benefi ts. The results obtained show that energy saving is achieved by
decreasing cooling during summer and reducing heating during winter.
This paper examines the current literature related to energy savings in
buildings due to green roof systems, phenomenological models that describe
the heat transfer process, and the experimental analysis of green roofs as
well as highlights some of the factors with more impact. The results of this
work show that it is necessary to analyze the heat transfer processes in green
roofs at different length scales to evaluate the energy savings in buildings.
Keywords: Energy saving, experimental analysis, green roof, heat transfer, phenomeno-
logical models
1 Introduction
In recent years, many studies have been carried out on green roofs systems (GRSs)
due to the multiple benefi ts that they offer, such as Increase in urban forest area [1],
air purifi cation [2,3], reducing runoff [1,4,5], reduction in the urban heat island effect
[6–10], life extension of the roof [11–13], and energy saving in the building [14–18].
To take advantage of the benefi ts of green roofs, the governments in some of
the highly urbanized societies such as Japan, Singapore, Germany, and Belgium
have offered incentives to encourage or even impose their use [10]. Mexico City
has laws that encourage the use of green roofs. The Green School Project, imple-
mented in South Korea, undertook the installation of GRSs in existing elementary
and middle school facilities [18].
*Corresponding author:
DOI: 10.7569/JSEE.2012.629506
S. Quezada-García et al.: Building Energy Savings with a Green Roof
106 J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013
DOI: 10.7569/JSEE.2012.629506
However, sometimes incentives alone are not suffi cient for project develop-
ment. The economic aspects need to be taken into account for decision making.
Therefore, some studies have assessed the fi nancial viability of GRSs and have
found that the net present value of GRSs is lower than that of conventional roofs
mainly because of energy savings [12,19].
To know the potential energy savings in a building, it is necessary to know the
heat transfer through the roof before and after installing the GRS. Therefore, it
is desirable to have mathematical models that accurately describe the heat fl ow
through the various biotic and abiotic layers that form the GRS to predict energy
exchange and determine the temperature inside the building.
This paper examines the current literature related to energy savings in build-
ings due to GRSs, phenomenological models that describe the heat transfer pro-
cess, and the experimental analysis of green roofs as well as highlights some of the
factors with more impact.
2 Building Energy Savings
The roof is a component of the building envelope that can provide advanced solu-
tions for energy savings. On average, the roof of a building receives twice as much
solar radiation than the walls [20]. GRSs can reduce the magnitude of heat fl ux
through one of the building envelopes as a result of the insulation provided by the
growing medium and provide additional benefi ts such as evapotranspiration that
can cool the roof under the sun [16]. Therefore, GRSs have the potential to reduce
energy consumption for heating and cooling [14,15].
Teemusk and Mander analyzed GRSs in an extreme climate such as in Estonia
and found that during summer the GRS with a substrate layer of 100 mm depth
signifi cantly reduced temperature fl uctuations compared to a conventional ceil-
ing [21]. During fall and spring, the substrate layer protects the roof from rapid
cooling and freezing, and also provides effective thermal insulation during
winter. Using dynamic simulations Zinzi and Agnoli analyzed GRSs and found
that they can improve the energy effi ciency of buildings. The authors concluded
that the water content in the roof signifi cantly impacts the energy savings in the
building; that is, the lack of water has a negative impact on reducing building
energy savings by cooling [17]. Hong et al. demonstrated, using numerical simu-
lations, the energy saving effect due to the introduction of GRSs in elementary
schools [18].
The energy saving depends on many factors such as the composition of the
growing medium, depth, moisture content, type of plants, irrigation, and local cli-
mate [17,20–22]. Plants may reduce the amount of heat stored in a building through
an increase in the amount of radiation refl ected, the shade provided by the canopy,
and evapotranspiration. Plants with green leaves have better temperature reduc-
tion effects than plants with purple leaves [20]. It has been found that increasing
the depth of the substrate layer up to 20 cm results in an increase in the thermal
S. Quezada-García et al.: Building Energy Savings with a Green Roof
J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013 107
DOI: 10.7569/JSEE.2012.629506
transmittance coeffi cient (U), but for depths of the substrate layer greater than 20
cm the U value decreases [23]. Experimentally, it has been found that solar radia-
tion accounts for 99.1% of the total heat gain in green roofs. Evapotranspiration
from plants was of the total heat loss in green roof 58.4% is due to the evapo-
transpiration, net long-wave radiative exchange correspond to 30.9%, and the net
photosynthesis of plants is around 9.5% [24].
Increasing the depth of the growing medium and supplying water for irriga-
tion allow the use of plants with higher biomass and greater leaf area index, which
results in higher evapotranspiration rates [22].
GRSs are passive systems for energy saving and are based on four fundamental
mechanisms: (i) interception of solar radiation by the canopy of the vegetation; (ii)
thermal insulation provided by the vegetation and the substrate; (iii) evaporative
cooling that occurs by evapotranspiration from the plants and the substrate; and
(iv) effect of wind on the building [25].
3 Mathematical models
Some authors have calculated heat transfer through GRSs in terms of a global heat
transfer coeffi cient to evaluate the thermal performance of GRSs in real scale and
dynamic conditions [23]. These studies are generally conducted on green roofs
that are already installed and are valid only for the roof in question.
To predict energy savings, it is necessary to know the heat fl ux before and after
installing a GRS. Some authors have developed mathematical models for the anal-
ysis of energy fl ow through GRSs [26–30].
The energy balance in the energy exchanges between the plants–soil system and
the environment, which is illustrated in Figure 1, is given by equation (1) [24]. In
this equation, the thermal effects of plant metabolism such as respiration and tran-
spiration and the thermal effects of microorganisms in the soil are not considered;
similarly, conditions such as precipitation and dew are not included. Moreover, it
is assumed that the green roof is large enough to assume horizontal homogeneity
and apply one-dimensional (vertical) analysis.
Ambient air
qrp qsp
qcv qem qtp System boundary
Figure 1 Energy exchange between a green roof and its environment [24].
S. Quezada-García et al.: Building Energy Savings with a Green Roof
108 J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013
DOI: 10.7569/JSEE.2012.629506
sr lr cv em tp ep sp ss tf ps rp
+ + + + + + + + + + = 0qqq q qq q qqq q , (1)
where sr
q is the heat gain from solar radiation, lr
q is the heat gain from long-wave
radiation, cv
q is the heat transferred by convection, em
q is the heat loss by emis-
sion, tp
q is the heat loss by transpiration, ep
q is the heat loss by evaporation, sp
q is
the heat storage by plants, ss
q is the heat storage by soil, tf
q is the heat transferred
into a room, ps
q is the solar energy converted by photosynthesis, and rp
q is the
heat generation by respiration.
Jim and Tsang presented a mathematical model by considering that conduction
is the main factor affecting soil temperature [31]. A typical heat conduction equa-
tion, equation (2), can be used to estimate the i-layer temperature:
dT T
dt C z z , (2)
where i
T is the i-layer temperature and ri, i
C, and i
k are the mass density, spe-
cifi c heat, and thermal conductivity of the i-layer, respectively.
The dominant form of heat dissipation in GRSs is evapotranspiration; therefore,
it is important to take into account the mass balance [15,16,24,26,27,32]. The mass
balance for the substrate layer is given by equation (3) [31]:
dW W
dt z z
, (3)
where g
W is the volumetric water content that the soil can hold and g
D is the dif-
fusion coeffi cient of water in soil.
A basic model was proposed for calculation of the convective heat transfer,
equation (4), which is a modifi ed version of Newton’s cooling law [33]. Plants
have a symmetric leaf structure that can be assumed to have equal two-sided heat
transfer rates for both forced and free convection conditions. Therefore, h values
can be doubled for a two-sided leaf:
, (4)
where a
is the air temperature, l
T is the leaf temperature, h is the convection
coeffi cient, and H is the sensible heat fl ux.
The mathematical models to describe the heat fl ow through green roofs have
been validated by some authors through experimentation.
4 Experiments in the literature
To validate the mathematical models, some authors have used simulation pro-
grams while others have resorted to experimentation to verify the accuracy of a
mathematical model [23,26,34]. Jim and Tsang validated their model with empiri-
cal results from three experimental plots [31]. Jim and He performed a similar
S. Quezada-García et al.: Building Energy Savings with a Green Roof
J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013 109
DOI: 10.7569/JSEE.2012.629506
experiment and measured variables such as solar radiation and microclimatic and
soil conditions; four solar radiation components were recorded: incoming and out-
going shortwave and incoming and outgoing longwave; soil moisture and tem-
perature; meteorological factors including the relative humidity of air, air tem-
perature, dew point temperature, wind speed, wind direction, and rainfall [35].
The quantitative evaluation of the heat transfer through a GRS requires precise
knowledge of the thermal properties of the growing medium. Previous experi-
ments have measured the thermal conductivity, the heat capacity, and the thermal
diffusivity of different soil samples [36,37].
5 Discussion
It has been shown that GRSs offer many benefi ts; however, using current math-
ematical models and experimentation we are still far from adequately describ-
ing the heat fl ux through these systems. The transfer of energy through GRSs
has been studied until now at the macroscopic level , that is, global energy bal-
ances where the length scale is of the order of magnitude of the green roof.
This type of approach has limitations as it does not go deep into the problem of
understanding the mechanisms of heat transfer in the energy transfer processes
in green roofs, especially near the boundaries between adjacent layers that con-
stitute the GRS.
Of the different layers that make up a GRS are porous media in which important
phenomena for the heat and mass transfer processes take place, but the porosity
is not taken into account in many existing mathematical models. Another limita-
tion of macroscopic description is that it is not possible to analyze the interaction
between the leaves of the plants and the environment.
A more complete study of heat transfer through GRSs can be done by using dif-
ferent length scales, that is, by studying the phenomenon at a large and small scale
(Figure 2) [38,39]. The small-scale approach has the advantage that all the factors
involved in the interfacial heat fl ux in the GRS are taken into account and the lay-
ers are considered to be multiphase systems.
Regarding experimental work, measurements to validate the mathematical
models and calculate the energy savings have been done to get experimental plots
for green roofs that are already installed, that is, these have been made in outdoor
experiments with multiple factors that affect the heat transfer. In some cases, the
models are only valid for the particular case of the study. Therefore, it is important
to carry out controlled experiments in a laboratory [40].
6 Conclusions
GRSs offer many benefi ts, one of which is the energy savings in buildings by
reducing the need for cooling in summer and heating in winter. However, many
of the studies that quantify the energy savings are only valid for the building in
S. Quezada-García et al.: Building Energy Savings with a Green Roof
110 J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013
DOI: 10.7569/JSEE.2012.629506
To predict the energy saving, it is necessary to know the heat transfer processes
in the roof. This requires mathematical models based on the physical processes
that describe the net heat fl ux both in conventional roofs and in green roofs. At
present, although mathematical models have been proposed to describe the heat
ow through GRSs, it is desirable to have more precise models that can be applied
in any situation (e.g., Figure 2). Solar radiation, type of plants, the composition and
depth of the growing medium, moisture content, leaf area index, irrigation, and
local climate are some of the factors involved in the heat fl ow that must be taken
into account in the mathematical models.
To describe the phenomenon in each stage, it is more convenient to carry out
controlled experiments. Therefore, to get a deeper understanding of the mecha-
nism of heat transfer processes in GRSs, it is necessary to develop more rigorous
experiments and models.
1. J. C. Berndtsson, L. Bengtsson, and K. Jinnob, Runoff water quality from intensive and
extensive vegetated roofs. Ecol. Eng. 35, 369–380 (2009).
2. J. Yang, Q. Yu, and P. Gong, Quantifying air pollution removal by green roofs in Chicago.
Atmos. Environ. 42, 7266–7273 (2008).
3. J.-f. Li, O. W. H. Wai, Y. S. Li, J.-m. Zhan, Y. A. Ho, J. Li, and E. Lam, Effect of green roof
on ambient CO2 concentration. Build. Environ. 45, 2644–2651 (2010).
4. J. Mentens, D. Raes, and M. Hermy, Green roofs as a tool for solving the rainwater
runoff problem in the urbanized 21st century? Landscape Urban Plan. 77, 217–226 (2006).
Green layer
Substrate layer
Figure 2 A green roof system at different scales: (a) green layer and substrate layer at a very
large scale; (b) green layer at a large scale; (c) substrate layer at a small scale.
S. Quezada-García et al.: Building Energy Savings with a Green Roof
J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013 111
DOI: 10.7569/JSEE.2012.629506
5. V. Stovin, G. Vesuviano, and H. Kasmin, The hydrological performance of a green roof
test bed under UK climatic conditions. J. Hydrol. 414–415, 148–161 (2012).
6. Q. Weng and S. Yang, Managing the adverse thermal effects of urban development in a
densely populated Chinese city. J. Environ. Manage. 70, 145–156 (2004).
7. N. H. Wong and Y. Chen, Study of green areas and urban heat island in a tropical city.
Habitat Int. 29, 547–558 (2005).
8. C. Y. Lin, F. Chen, J. C. Huang, W. C. Chen, Y. A. Liou, W. N. Chen, and S. C. Liu, Urban
heat island effect and its impact on boundary layer development and land–sea circula-
tion over northern Taiwan. Atmos. Environ. 42, 5635–5649 (2008).
9. H. Takebayashi and M. Moriyama, Surface heat budget on green roof and high refl ec-
tion roof for mitigation of urban heat island. Build. Environ. 42, 2971–2979 (2007).
10. N. H. Wong, Y. Chen, C. L. Ong, and A. Sia, Investigation of thermal benefi ts of rooftop
garden in the tropical environment. Build. Environ. 38, 261–270 (2003).
11. S. Saiz, C. Kennedy, B. Bass, and K. Pressnail, Comparative life cycle assessment of
standard and green roofs. Environ. Sci. Technol. 40, 4312–4316 (2006).
12. C. Clark, P. Adriaens, and B. Talbot, Green roof valuation: A probabilistic economic
analysis of environmental benefi ts. Environ. Sci. Technol. 42, 2155–2161 (2008).
13. L. Kosareo and R. Ries, Comparative environmental life cycle assessment of green roofs.
Build. Environ. 42, 2606–2613 (2007).
14. H. F. Castleton, V. Stovin, S. B. M. Beck, and J. B. Davison, Green roofs: Building energy
savings and the potential for retrofi t. Energ. Buildings 42, 1582–1591 (2010).
15. T. G. Theodosiou, Summer period analysis of the performance of a planted roof as a
passive cooling technique. Energ. Buildings 35, 909–917 (2003).
16. S. Onmura, M. Matsumoto, and S. Hokoi, Study on evaporative cooling effect of roof
lawn gardens. Energ. Buildings 33, 653–666 (2001).
17. M. Zinzi and S. Agnoli, Cool and green roofs. An energy and comfort comparison
between passive cooling and mitigation urban heat island techniques for residen-
tial buildings in the Mediterranean region. Energ. Buildings (2011) doi:10.1016/j.
18. T. Hong, J. Kim, and C. Koo, LCC and LCCO2 analysis of green roofs in elementary
schools with energy saving measures. Energ. Buildings 45, 229–239 (2012).
19. T. Carter and A. Keeler, Life-cycle cost–benefi t analysis of extensive vegetated roof sys-
tems. J. Environ. Manage. 87, 350–363 (2008).
20. T. C. Liu, G. S. Shyu, W. T. Fang, S. Y. Liu, and B. Y. Cheng, Drought tolerance and ther-
mal effect measurements for plants suitable for extensive green roof planting in humid
subtropical climates. Energ. Buildings 47, 180–188 (2012).
21. A. Teemusk and U. Mander, Greenroof potential to reduce temperature fl uctuations of a
roof membrane: A case study from Estonia. Build. Environ. 44, 643–650 (2009).
22. K. L. Getter, D. B. Rowe, J. A. Andresen, and I. S. Wichman, Seasonal heat fl ux properties of
an extensive green roof in a Midwestern U.S. climate. Energ. Buildings 43, 3548–3557 (2011).
23. G. Kotsiris, A. Androutsopoulos, E. Polychroni, and P. A. Nektarios, Dynamic U-value
estimation and energy simulation for green roofs. Energ. Buildings 45, 240–249 (2012).
24. C. Feng, Q. Meng, and Y. Zhang, Theoretical and experimental analysis of the energy
balance of extensive green roofs. Energ. Buildings 42, 959–965 (2010).
25. G. Pérez, L. Rincón, A. Vila, J. M. González, and L. F. Cabeza, Green vertical systems
for buildings as passive systems for energy savings. Appl. Energ. 88, 4854–4859 (2011).
S. Quezada-García et al.: Building Energy Savings with a Green Roof
112 J. Sustainable Energy Eng., Vol. 1, No. 2, April 2013
DOI: 10.7569/JSEE.2012.629506
26. E. P. D. Barrio, Analysis of the green roofs cooling potential in buildings. Energ. Buildings
27, 179–193 (1998).
27. D. J. Sailor, A green roof model for building energy simulation programs. Energ.
Buildings 40, 1466–1478 (2008).
28. R. Kumar and S. C. Kaushik, Performance evaluation of green roof and shading for
thermal protection of buildings. Build. Environ. 40, 1505–1511 (2005).
29. S. W. Tsang and C. Y. Jim, Theoretical evaluation of thermal and energy performance of
tropical green roofs. Energy 36, 3590–3598 (2011).
30. Q. Meng and W. Hu, Roof cooling effect with humid porous medium. Energ. Buildings
37, 1–9 (2005).
31. C. Y. Jim and S. W. Tsang, Modeling the heat diffusion process in the abiotic layers of
green roofs. Energ. Buildings 43, 1341–1350 (2011).
32. N. H. Wong, S. F. Tay, R. Wong, C. L. Ong, and A. Sia, Life cycle cost analysis of rooftop
gardens in Singapore. Build. Environ. 38, 499–509 (2003).
33. T. Ayata, P. C. Tabares-Velasco, and J. Srebric, An investigation of sensible heat fl uxes at
a green roof in a laboratory setup. Build. Environ. 46, 1851–1861 (2011).
34. I. Jaffal, S. E. Ouldboukhitine, and R. Belarbi, A comprehensive study of the impact of
green roofs on building energy performance. Renew. Energ. 43, 157–164 (2012).
35. C. Y. Jim and H. He, Coupling heat fl ux dynamics with meteorological conditions in the
green roof ecosystem. Ecol. Eng. 36, 1052–1063 (2010).
36. D. J. Sailor and M. Hagos, An updated and expanded set of thermal property data for
green roof growing media. Energ. Buildings 43, 2298–2303 (2011).
37. G. Pérez, A. Vila, L. Rincón, C. Solé, and L.F. Cabeza, Use of rubber crumbs as drainage
layer in green roofs as potential energy improvement material. Appl. Energ. 97, 347–354
38. G. Espinosa-Paredes, Instantaneous equations for multiphase fl ow in porous media
without length-scale restrictions using a non-local averaging volume. Nucl. Eng. Des.
240, 1160–1185 (2010).
39. B. Wood, The role of scaling law in upscaling. Adv. Water Resour. 32, 723–735 (2009).
40. F. G. Arroyo-Cabañas, J. E. Aguillón-Martínez, J. J. Ambríz-García, and G. Canizal,
Electric energy saving potential by substitution of domestic refrigerators in Mexico.
Energ. Policy 37, 4737–4742 (2009). doi:10.1016/j.enpol.2009.06.032.
ResearchGate has not been able to resolve any citations for this publication.
As the area of urban forests rapidly decrease in size, there is growing interest in green roofs as the only alternative to urban forests. This study aimed to evaluate economic and environmental effects of functional improvement in elementary school facilities by applying various improvement scenarios based on green roof systems (GRSs) with the combination of energy-saving measures (ESMs). A total of 16 possible improvement scenarios from the combination of GRSs and ESMs were developed, and energy modeling (Energy Plus ver. 6.0), based on the (i) characteristics of building, (ii) regional climate, and (iii) season, was performed. Using the energy modeling result, the amount of the CO2 emission reduction by energy savings and the CO2 absorption by GRSs’ plants was calculated, and a life cycle cost analysis was conducted with the consideration of the life cycle CO2 (LCCO2). The results of this study can be used (i) to introduce the most appropriate ESMs for the specific facility when applying GRSs, (ii) to decide which location is proper to implement GRSs considering characteristics of regional climate, and (iii) to select energy- and cost-efficient elementary school when applying GRSs.
Today, green roofs are a building system which provides interesting benefits over traditional roof solutions. The most important advantages are the reduction of surface runoff in cities, improvement of the urban climate, biodiversity support, improvement of the durability of roofing materials, and, especially, energy savings. This paper has the aim of studying the performance of green roofs as a passive system for energy savings, within a wider objective of seeking constructive solutions suitable for sustainable and environmentally friendly architecture. This idea is tested at an experimental installation available at the University of Lleida, with several cubicles testing the energy performance of different construction solutions. This work raises the possibility of using recycled rubber from tires as a drainage layer in green roofs, substituting the porous stone materials currently used (such as expanded clay, expanded shale, pumice, and natural puzolana). This solution would reduce the consumption of these natural materials, which also require large amounts of energy in its transformation process to obtain their properties. Moreover it would provide a solution to the problem of waste rubber from the tires, known as rubber crumbs. Since the purpose of the drainage layer is the optimum balance between air and water in the green roof system, first the ability for draining of recycled rubber granules was studied and was compared with the offered by stone materials. The new solution using rubber crumbs is also studied to test if it would keep the same insulating properties that the green roof with stone materials presented in previous studies. Early results show that this extensive green roof system can be a good passive energy savings tool in Continental Mediterranean climate in summer, and that rubber crumbs can be an interesting substitute for stone materials used as drainage layer in this type of green roofs.
Vegetated (green) roofs alter the roof surface energy balance and hence affect both building energy consumption and the transport of heat into the environment. Quantitative evaluation of the energetics of green roof systems requires accurate knowledge of the moisture-dependent thermal properties of the growing media. To support this need for data and to supplement previously published data we conducted a laboratory study to measure thermal conductivity, volumetric heat capacity, and thermal diffusivity of 12 green roof soil samples of varying composition. The results indicate that thermal properties vary significantly as a function of growing media design. Growing media incorporating expanded slate as their aggregate had thermal conductivities that were two to three times those of media that used a porous silica-based aggregate. Media incorporating expanded clay as the aggregate had thermal conductivities roughly in the middle of these extremes. In general the thermal conductivity nearly tripled as the growing media moisture levels were increased from relatively dry to saturated. Also, it was found that compaction typical of green roof systems that have been installed for multiple seasons can increase thermal conductivity of moist soils by 30–40% over their uncompressed values.
Highlights ► New continuous 5-min hydrological performance data from a UK green roof test bed. ► 50.2% Overall retention, falling to 30% for significant events (>1 year return period). ► Regression analysis shows retention is not simply a function of ADWP. ► Performance is dependent upon substrate moisture fluxes; ET is critical. ► The roof has a finite retention capacity, in this case a maximum of 20 mm.
Green areas in cities have been considered as potential measure in mitigating the urban heat island (UHI) effect. In this paper, a mobile survey was conducted to explore both the severity of UHI effect and cooling impacts of green areas at macro-level in Singapore. Islandwide temperature distribution was mapped relying on data derived from the mobile survey. This study has indicated a strong correlation between the decrease of temperature and the appearance of large green areas in the city. Although there is no distinct borderline between ‘urban’ and ‘rural’ areas in Singapore, maximum temperature difference of 4.01°C was observed.
This paper analyzed the energy balance of extensive green roofs and presented a simple but practical energy balance model. Field experiment justified the validation and accuracy of this model. Experimental results demonstrated that within 24h of a typical summer day, when soil was rich in water content, solar radiation accounted for 99.1% of the total heat gain of a Sedum lineare green roof while convection made up 0.9%. Of all dissipated heat 58.4% was by the evapotranspiration of the plants–soil system, 30.9% by the net long-wave radiative exchange between the canopy and the atmosphere, and 9.5% by the net photosynthesis of plants. Only 1.2% was stored by plants and soil, or transferred into the room beneath.
A method of laying a layer of humid porous medium on roof to gain free cooling effect by passive water evaporation is proposed. Numeric model for simulating cooling effect is built with the help of experimental results of physical properties for humid porous medium, which shows advantage over analytical solutions because of the supposition of constant physical properties in the latter. Through the comparison between simulated and experimental results, the model is validated. And the method of evaporation cooling effect with humid porous medium on the roof is tested to be feasible.
The increase of peak and energy demand during the cooling season is becoming a crucial issue, as well as the intensification of the urban heat island effect. This trend is observed at several latitudes, including areas where overheating was unknown at building and urban levels. This phenomenon involves different issues: reduction of greenhouse gases, quality and comfort in outdoor and indoor environment, security of energy supply, public health. The building sector is directly involved in this change and adequate solutions can provide great benefit at energy and environmental levels. Roofs in particular are envelope components for which advanced solutions can provide significant energy savings in cooled buildings or improve indoor thermal conditions in not cooled buildings. Cool materials keep the roof cool under the sun by reflecting the incident solar radiation away from the building and radiating the heat away at night. Roofs covered with vegetation take benefits of the additional thermal insulation provided by the soil and of the evapo-transpiration to keep the roof cool under the sun. These two technologies are different in: structural requirements, initial and lifetime maintenance costs, impact on the overall energy performance of buildings. This paper presents a numerical comparative analysis between these solutions, taking into account the several parameters that affect the final energy performances. By means of dynamic simulations, the paper depicts how cool and green roofs can improve the energy performance of residential buildings in different localities at Mediterranean latitudes.