State-of-the-art analysis of the environmental benefits of green roofs

Published Date
Applied Energy
15 February 2014, Vol.115:411–428, doi:10.1016/j.apenergy.2013.10.047

• Author 

• Umberto Berardi ^a,,
• AmirHosein GhaffarianHoseini ^b,
• Ali GhaffarianHoseini ^c,

• aCivil and Environmental Engineering Department, Worcester Polytechnic Institute, MA, USA
• bDepartment of Civil and Architectural Engineering, College of Science and Engineering, City University of Hong Kong, Hong Kong
• cDepartment of Architecture, Faculty of Design and Architecture, Universiti Putra Malaysia (UPM), Malaysia

Received 13 August 2013. Revised 23 October 2013. Accepted 28 October 2013. Available online 7 December 2013. 

Highlights

• •
  Cross-disciplinary review is performed for showing benefits of green roofs.
• •
  Green roofs have several benefits for energy, water and pollution management.
• •
  Experiments show that green roofs must consider specific climatic conditions.
• •
  Lifecycle analysis ensures the economic feasibility of green roofs.
• •
  Quantification of the green roof performances needs to consider technical design.

Abstract

Green roofs have been proposed for sustainable buildings in many countries with different climatic conditions. A state-of-the-art review of green roofs emphasizing current implementations, technologies, and benefits is presented in this paper. Technical and construction aspects of green roofs are used to classify different systems. Environmental benefits are then discussed mainly by examining measured performances. By reviewing the benefits related to the reduction of building energy consumption, mitigation of urban heat island effect, improvement of air pollution, water management, increase of sound insulation, and ecological preservation, this paper shows how green roofs may contribute to more sustainable buildings and cities. However, an efficient integration of green roofs needs to take into account both the specific climatic conditions and the characteristics of the buildings. Economic considerations related to the life-cycle cost of green roofs are presented together with policies promoting green roofs worldwide. Findings indicate the undeniable environmental benefits of green roofs and their economic feasibility. Likewise, new policies for promoting green roofs show the necessity for incentivizing programs. Future research lines are recommended and the necessity of cross-disciplinary studies is stressed.

Keywords

Green roofs

Sustainable design

Low-energy buildings

Environmental benefits

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Nomenclature

C_e,g

latent heat flux bulk transfer coefficient at ground layer

C_f

bulk heat transfer coefficient

C_hg

sensible heat flux bulk transfer coefficient at ground layer

C_p,a

specific heat of air at constant pressure

F_f

net heat flux to foliage layer (W/m²)

F_g

net heat flux to ground surface (W/m²)

h

effective heat transfer coefficient with convection + radiation

h_fg

latent heat of evaporation

H_f

foliage sensible heat flux (W/m²)

H_g

ground sensible heat flux (W/m²)

total incoming short-wave radiation (W/m²)

total incoming long-wave radiation (W/m²)

l_f

latent heat of vaporization at foliage temperature (J/kg)

l_g

latent heat of vaporization at ground temperature (J/kg)

K

total thermal conductivity

L

characteristic depth of green roof

L_f

foliage latent heat flux (W/m²)

L_g

ground latent heat flux (W/m²)

LAI

leaf area index (m²/m²)

m

evaporation flow rate

q_af

mixing ratio for air within foliage canopy

q_f,sat

saturation mixing ratio at foliage temperature

q_g,sat

saturation mixing ratio at ground temperature

Q_cond

conduction heat

Q_irr

radiation heat

Q_conv

convection heat

Q_evap

evapotranspiration heat

r″

surface wetness factor

T_af

air temperature with in the canopy

T_f

foliage temperature

T_g

ground surface temperature

T_C

temperature of cold space

T_S

temperature of green roof surface

T_∞

ambient temperature

V_∞

air velocity

W_af

wind speed within the canopy

α_f

albedo (short-wave reflectivity) of the canopy

α_g

albedo (short-wave reflectivity) of ground surface

ε_f

emissivity of canopy

ε_g

emissivity of the ground surface

ε₁

ε_g + ε_f – ε_g·ε_f

φ_∞

relative air humidity

ρ_af

density of air at foliage temperature

ρ_ag

density of air at ground surface temperature

θ

moisture content

σ

Stefan–Boltzmann constant

σ_f

fractional vegetation coverage

Fig. 1.

 Table 1

Table 1.

 Table 2

Table 2.

Fig. 2.

Fig. 3.

Fig. 4.

 Table 3

Table 3.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

 Table 4

Table 4.

 Table 5

Table 5.

Fig. 9.

 Table 6

Table 6.

 Table 7

Table 7.

Fig. 10.

 Table 8

Table 8.

 Table 9

Table 9.

 Table 10

Table 10.

 Table 11

Table 11.

• ⁎ 
  Corresponding author. Tel.: +1 508 831 6545.

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