Elsevier

Renewable Energy

Volume 43, July 2012, Pages 157-164
Renewable Energy

A comprehensive study of the impact of green roofs on building energy performance

https://doi.org/10.1016/j.renene.2011.12.004Get rights and content

Abstract

Green roofs have several environmental benefits, such as improving building energy efficiency. The present paper provides a comprehensive study of the impact of a green roof on building energy performance. A model of green roof thermal behavior was coupled with a building code to allow the evaluation of green roof foliage and soil surface temperatures. Simulations were conducted for a single-family house with conventional and green roofs in a temperate French climate. In the summer, the fluctuation amplitude of the roof slab temperature was found to be reduced by 30 °C due to the green roof. The heat flux through the roof was also evaluated. In the summer, the roof passive cooling effect was three times more efficient with the green roof. In the winter, the green roof reduced roof heat losses during cold days; however, it increased these losses during sunny days. The impact of the green roof on indoor air temperature and cooling and heating demand was analyzed. With a green roof, the summer indoor air temperature was decreased by 2 °C, and the annual energy demand was reduced by 6%. The present study shows that the thermal impact of green roofs is not functionally proportional to the leaf area index parameter. It also shows the high dependency of this impact on the roof insulation. Finally, the simulations suggest that green roofs are thermally beneficial for hot, temperate, and cold European climates.

Highlights

► A green roof thermal model was coupled to a building simulation code. ► The impact of a green roof on the roof temperature and heat flux is studied. ► Green roofs reduce building energy demand and improve thermal comfort. ► The effects of the leaf area index, the roof insulation, and the climate are discussed.

Introduction

Green roofs are considered to be an effective contribution to the resolution of several environmental problems at the building and urban levels. In addition to the creation of a pleasant environment, green roofs offer several benefits in comparison to conventional roofs. They improve storm water management [1], [2] as well as reduce air pollution [3], [4] and noise [5]. Green roofs increase vegetal and animal biodiversity in cities [6], [7], and they also reduce a city’s carbon footprint by converting carbon dioxide to oxygen through photosynthesis [4], [8].

Green roofs improve building energy efficiency by enhancing the heat transfer through roofs [2], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. The reduction of the summer temperature around green roofs improves the efficiency of HVAC systems by providing a local free cooling effect to the fluid before it returns to the chiller. This reduced temperature also improves the efficiency of surrounding photovoltaic panels [20]. Green roofs improve the longevity of roofing membranes by limiting the thermal stress to which they are subjected [2], [15], [17], [21], [24], [25], [26]. Finally, at the city level, green roofs contribute to the mitigation of the urban heat island effect [14], [21], [27], [28].

Two types of green roofs are generally identified: extensive (with soil thickness less than 10–15 cm) and intensive (with soil thickness more than 15–20 cm) [2], [8], [13], [16], [20], [23], [24], [29], [30], [31]. Because of their low additional loads, extensive green roofs are suitable for building retrofitting, i.e. they do not require any additional strengthening [20]. By calculating the net present value (NPV), Carter and Keeler [31] suggest that green roofs become more economic than traditional roofs if their cost decreases by 20%.

The choice of green roof characteristics depends greatly on climate. For instance, in Australia, solutions for green roofs (plants, substrate, etc.) may be different from those used in the European climate [29]. A study on the choice of suitable plant species for green roofs in the midwestern US climate is presented in [30].

The surface temperature of conventional roofs can reach very high values in the summer. For instance, a temperature of 90 °C was recorded in Australia [29]. Green roofs have a large impact on this temperature because of several effects (foliage shading, soil thermal resistance, evapotranspiration, etc.). The heat flux through the roof is therefore affected, which influences the building energy demand and the indoor thermal conditions. The summer and winter temperatures on the exterior surface of the roof slab are less extreme, and their fluctuation amplitude is lower than that of a conventional roof. Thus, the thermal stress applied to roofing membranes is substantially limited, which improves their longevity [2], [15], [17], [21], [24], [25], [26].

Although there are many works dealing with the impact of green roofs on building energy performance, many aspects are still not well understood, and more studies on this subject are necessary. For instance, quantifying the impact of green roofs on indoor air temperature has not yet been examined with detailed models. In addition, the variation of the building energy demand and indoor conditions as a function of key parameters, such as the roof insulation, climate, and green roof configuration, requires further investigation.

In this study, a comprehensive analysis of the impact of green roofs on the thermal performance of buildings is presented, including consideration of the foliage and green roof soil temperatures, the indoor air temperature, and the energy demand. For this purpose, a green roof thermal model was coupled with a building model, and a comparative study was made between the energy performance of conventional and green roofs on a single-family house.

Section snippets

Green roof modeling

Modeling the thermal behavior of green roofs requires the study of several interacting phenomena, such as heat and mass transfer and plant physiology. Many green roof models are available in the literature, ranging from simple to detailed. The simplest model considers only the decrease of the roof U-value [9], [10], [11]. Many other studies have presented more detailed models, with a heat balance that considers additional influencing phenomena, such as solar shading by foliage and cooling by

Mathematical model

The presented model divides the green roof heat balance into two parts: the balance at the foliage and at the soil surface. The heat balance equations are based on the models developed and validated in the works of Sailor [13] and Frankenstein and Koenig [32]. The main heat fluxes that describe the heat balance of the canopy are the following.

  • The solar radiation absorption by the foliage.

  • The long-wave radiation exchange between the foliage and the sky as well as between the foliage and the soil

Case study

The developed approach is applied to the evaluation of the impact of a green roof on the energy performance of a single-family house with an area of 96 m2. The window-to-wall ratio is 0.18. The mean value of the internal heat gain is 5 W m−2. The house is located in La Rochelle (France), where the climate is considered to be temperate oceanic. Comparisons were made for the energy performance of the house with a conventional roof and an irrigated extensive green roof. The green roof considered in

Description of the impact of the green roof

The energy performance of the studied house with both conventional and green roofs is presented in Fig. 3, Fig. 4, Fig. 5 and Table 1, Table 2. The summer results are presented with a free-floating temperature (Fig. 3, Fig. 4, Fig. 5 and Table 1) and with a set-point temperature of 28 °C (Table 2). The results suggest that the roof temperature (and thus, the heat flux through the roof, the indoor air temperature, and the cooling and heating demand) changed in a significant way because of the

Conclusions

In this paper, a study of the impact of green roofs on building energy performance is described. A green roof model is presented and integrated into a building thermal program in a way that allows an objective comparison with conventional roofs. The impact of a green roof on the energy performance of a single-family house was analyzed. The temperatures of the foliage, the soil, the roof slab, and the indoor air were evaluated. The heat flux through the roof, the cooling demand, and the heating

Acknowledgment

This work was supported by the French National Research Agency (ANR) through the “Habitat intelligent et solaie photovoltaïque” program (project AGROBAT n° ANR-09-HABISOL-001) and the Poitou-Charentes region through the “Excellence environnementale” project.

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