Green Roof Layers Research Articles

Green Roofs on Shipping Containers: How Substrate Thickness Affects Thermal Performance

Green roofs have become a popular sustainable solution in urban areas, and in recent years, shipping containers have gained popularity as a sustainable alternative for housing. A promising proposal is to combine these two solutions. This research aims to analyze the thermal behavior of experimental modules of scale constructions. Four modules were constructed with different substrate thicknesses (4, 6, 8, and 12 cm) to verify the impact on thermal behavior and provide guidance for this technology. Additionally, another module was built without a green roof for control purposes. The indoor and outdoor air temperatures and humidities, soil moistures, and temperatures between green roof layers were recorded in a tropical climate in summer. The behavior was similar between the different thicknesses for the whole period but with significant differences in the indoor temperature amplitudes (13.8 °C for the thinner substrate, 9.7 °C for the thicker one, and 38.7 °C for the bare roof). This study also revealed considerable heat conduction between the side walls and the slab, which resulted in an upward heat flow to the substrate during a day with a clear sky, which is the opposite of what is observed in conventional roofs. During the night and rainy periods, temperatures tend to become closer between the roof’s layers when the substrate dissipates the energy absorbed throughout the day.

Sensitivity analysis and weather condition effects on hygrothermal performance of green roof models characterized by recycled and artificial materials’ properties

This study conducted a sensitivity analysis and assessed the effects of long-term weather conditions on green roof models, including recycled and artificial materials. Climate conditions can affect the hygrothermal performance of green roof materials, but this important issue has hardly been evaluated for drainage and substrate layers made of recycled and artificial materials. Climate change makes it unclear how well green roofs will perform hygrothermally. Moreover, the heat flux sensitivity to the thickness and physical characteristics of green roofs with artificial and recycled materials has received less attention. This study applied three weather scenarios on green roof models with artificial and recycled materials: the beginning, middle, and end of the 21st century. As per the results, at the beginning and middle of the 21st century, substrate layers' water content was roughly nine times more than the drainage layers'. At the end of the 21st century, the comparable difference was 6.5 times larger. During the summer and the beginning of autumn, the green roofs’ thermal performance with recycled and artificial materials was improved until the end of the 21st century. The entire parameter change demonstrated the scatter of thermal conductivity, density, and thickness effectively influenced the dispersion of heat flux for the green roof layers. Also, the scatter of density was more effective in heat flux dispersion for substrate layer than drainage layer.

Water permeability, water retention capacity, and thermal resistance of green roof layers made with recycled and artificial aggregates

Substituting natural aggregates in the green roof substrate and drainage layers with lightweight artificial and recycled coarse materials is an eco-friendly alternative for applying lower load to the rooftops and preserving natural resources. However, a lack of precise understanding of the thermal resistance, water passing ability, and water holding capacity of green roof materials, including recycled and artificial materials, has raised a demand for measuring their Rc-value, water permeability, and water retention capacity as three main indicators for green roof systems. This study comparatively evaluated the thermal resistance, water permeability, and water retention capacity of green roofs with substrate and drainage layers, including coarse recycled and artificial materials. Different kinds of coarse granular aggregates were separately used for the drainage layer, including Natural Coarse Aggregate (NCA), Recycled Coarse Aggregate (RCA), Incinerated Municipal Solid Waste Aggregate (IMSWA), and Lightweight Expanded Clay Aggregate (LECA). The substrate layers were made with coarse recycled materials (SC) and without coarse recycled materials (SP) in wet and dry states. The outcomes revealed the highest thermal resistance and the lowest weight were obtained for 20-cm green roofs with a 15-cm substrate layer and 5-cm drainage layer of LECA. The water permeability of NCA was obtained 1.5 times more than that of LECA, whereas there was no significant difference between the result of the former, RCA, and IMSWA. The water retention capacity of the LECA was two times higher than that of the NCA. SC and SP satisfied the water passing and retention criteria given for green roofs.

Greening of roofs as an adaptive measure for climate change

AbstractThe negative impact of climate change, especially in urban areas, requires the use of new construction technologies and materials, harmonized with environmental aspects. One such approach is the construction of green areas on flat roofs of buildings. The positive impact of greening roofs is reflected in several important aspects: environmental, economic, and social. The pilot area that was researched is characterized by a high coefficient of construction. All this affects the creation of the so‐called heat island and bad microclimatic conditions, as well as the lack of areas that can be arranged for the rest of the local population. In addition to the impact of this type of urban planning on the preservation of the city's environment, the costs of building green roofs are also presented. In the final part of the article, the life cycle impact assessment (LCIA) of green roof layers is presented, using the Eco‐indicator method to show the environmental impact of a materials based on data from a life cycle assessment.

Experimental investigation of summer thermal performance of the green roof system with mineral wool substrate

Green roof systems have become an important part in creating sustainable cities. They can provide a wide range of economic, environmental, and social benefits. The goal of this research was to quantify the thermal performance improvements from a green roof with mineral wool substrate installed on a school building in a humid subtropical climate. In-situ measurements during a summer period included heat fluxes through the green and reference roof, vertical temperature profile through both roofs, the local air temperature above roofs, and local meteorological parameters. Furthermore, the summer thermal performance of green and reference roof and the green roof cooling effect were evaluated concerning meteorological parameters using the Pearson correlation analysis. The results indicate that the green roof layers have improved thermal performance of the roof with respect to reduced conductive heat flow by 57% and delayed heat transfer. The maximum and averaged reference roof to green roof outdoor surface temperature difference was 27.5 °C and 5.5 °C, respectively. It was found that ambient temperature and relative humidity have a dominant role on the thermal performance of the green and reference roof, while solar radiation and ambient temperature present the key meteorological determinants of the green roof cooling effect.

Coarse recycled materials for the drainage and substrate layers of green roof system in dry condition: Parametric study and thermal heat transfer

The replacement of natural materials with recycled materials for green roof systems with drainage and substrate layers has scarcely been evaluated while it can be considered as a potential solution to reduce the overuse of natural resources. Moreover, optimizing green roof layers' thickness and evaluating the thermal resistance of drainage and substrate layers in dry state have rarely been taken into account. Therefore, the aim of this work was to assess the green roof layers' thermal resistance concerning ISO 9869-1 where the drainage and dry substrate layers are substituted with coarse recycled materials. A sensitivity analysis was conducted using WUFI software to optimize the thickness of different layers. As per the results, there was a narrow difference between the Rc-value of the proposed green roof made with coarse recycled materials and that of the control green roof without coarse recycled materials, (5.3%); so, recycled materials are recommended to be used for the rooftops due to their low bulk density and light weight. Moreover, the dry substrate layer's Rc-value was about twice that of the drainage layer of coarse aggregates due to deeper thickness of the former than the latter; however, considering the same thickness for the coarse aggregates and dry substrate layers, the Rc-value of the coarse aggregates was higher. Meanwhile, by simultaneously increasing the thickness of both layers, the model with an 18-cm substrate layer and a 6-cm drainage layer adequately provided the heat retention capacity required for the green roof system.

Modelling hygrothermal conditions of unsaturated substrate and drainage layers for the thermal resistance assessment of green roof: Effect of coarse recycled materials

Alternative materials exist for green roof layers: secondary resources like coarse or fine recycled aggregates may be used as a substitute to natural materials. For using these new types of materials, it is needed to assess their heat resistance which is performed according to ISO 9869-1 standard. Moreover, the initial hygrothermal conditions of unsaturated substrate and drainage layers have also to be modelled and assessed for optimizing the layers’ thickness. In this study, the green roofs with unsaturated substrate and drainage layers incorporating coarse recycled materials were tested and assessed. The hygrothermal conditions of unsaturated substrate and drainage layers were simulated using WUFI software. A small difference (4.2%) was observed between the Rc-value of the green roofs with and without coarse recycled materials, confirming that these materials provided a sufficient thermal resistance, similar to soil particles for the substrate layer. Considering a constant thickness for the substrate layer (15 cm), a 6-cm drainage layer with coarse aggregates was considered as the optimum design for green roof systems. Besides, 18-cm unsaturated substrate layer was the optimum design when the drainage layer’ thickness was considered constant (5 cm). The 6-cm drainage layer and 18-cm unsaturated substrate layer were definitely the best design for the roofing systems with the simultaneous change in the substrate and drainage layers’ thickness.

Estimating Carbon Sequestration of Green Roof Plants in Tropical Climate

A green roofs is one of the recommended methods of practice in improving the sustainability of urban areas. Green roofs can be used as a passive technique to reduce carbon dioxide (CO2) emissions from the atmosphere. This is due to the role of green roof layers which are vegetation and soil to process the photosynthesis activities and to capture CO2 from the atmosphere. The objective of this paper is to quantify and to compare the amount of CO2 sequestration by ten potential green roof plants, in order to estimate the CO2 reduction within the surrounding atmosphere. It is found that, by planting the Alternanthera Paronychioides (Alternanthera P.) on the roof surface could reduce the indoor air temperature as well as the CO2, thus enhance the quality of air. This study also revealed that the Alternanthera P. generates the highest photosynthesis rate or CO2 uptake compared to the other nine plants, as high as 23.59 μmol CO2 m-2s-1. It is predicted that if the Alternanthera P. is to be installed on a 0.64 m2 test bed roof, the CO2 could be reduced almost 0.010 tonnes, annually. This suggested that by planting Alternanthera P. on the roof surfaces may potentially give advantages on mitigating the greenhouse gas emission, particularly CO2, and reducing the impact of global warming.

Use of a thermo-module as a soil heat flux sensor: Applications in the evaluation of extensive green roof thermal performance

Green roofs are a component of energy-saving architecture. Building energy savings due to green roofs are a function of both vegetation and substrate properties. Direct empirical measurements of heat flux through green roof layers represent a method for comparing green roof vegetation or substrate types, but these methods are limited by the expense of heat flux sensors. This paper proposes to use an inexpensive thermo-module for heat flux measurements. The thermo-module heat flux sensor had a big advantage for both expense and measuring sensitivity compared to a commercial heat flux meter: two orders of magnitude less cost and exceeding three times higher sensitivity. Then the thermo-module heat flux sensors were installed in a replicated extensive green roof, comparing heat flux measurements during winter conditions on a roof in western Japan among seven different vegetation type treatments. Vegetation had strong effects on both temporal mean and range of heat flux values. The strongest performing plant type was Luzula capitata, a low-growing graminoid with dense leaf cover even in winter, showing up to 50% less heat loss than the poorest performing species. This kind of sensor is recommended for further replicated empirical comparisons of green roofs or other energy-saving architectural technologies.

Analysis of roof greening technology impact on rain and meltwater retention

Over the past 20 years, a large number of studies have been published on reducing storm runoff by various types of green roofs. This article analyzes the results of experimental studies presented in 39 publications on green roof runoff reduction in a climate similar to the climate of Russia: in Canada, the USA, Finland, Norway, France. An analytical review found that the ability of green roofs to retain rainfall varies from 20 to 99.5% depending on climatic conditions (duration and intensity of rains, duration of dry periods, solar radiation, temperature and humidity, wind conditions), the properties of green roof layers (moisture capacity of the substrate and a drainage layer, the substrate thickness), the type of vegetation, the geometry of a green roof (slope and orientation). Green roofs can be a useful tool for reducing urban storm water runoff. However, in order to ensure high efficiency, it is necessary to use green roof technology with other measures to reduce runoff.

Green Roof Design: State of the Art on Technology and Materials

In order to consider green roofs as an environmentally friendly technology, the selection of efficient and sustainable components is extremely important. Previous review papers have mainly focused on the performance and advantages of green roofs. The objective of this paper is to examine the primary layers: The waterproof and anti-root membranes; the protection, filter, and drainage layers; the substrate; and the vegetation. First, the history, modern applications, benefits and classification are analyzed in order to present a well-defined state of the art of this technology. Then, the roles, requirements, characteristics, and materials are assessed for each green roof layers. This technology was compared to a conventional roof technology, Mediterranean climate conditions and their influence on green roof design were assessed, also comparing them with Tropical area and focusing on irrigation systems, examples about the commercial materials and products available in the market were provided and innovative materials coming from recycled sources were analyzed. Future research should evaluate new materials for green roof technologies, in order to enhance their performance and increase their sustainability. The information provided in this review paper will be useful to develop Mediterranean green roof guidelines for selecting suitable components and materials during the design and installation phases.

Green roof thermal buffering: Insights derived from fixed and portable monitoring equipment

Thermal buffering of the 27,316m2 Jacob K. Javits Convention Center Green Roof (JJCC Green Roof) was investigated in the field, as well as in the laboratory in order to determine its ability to insulate a building. An instrumented replica of the green roof cross section was developed to elucidate the inner-workings of the system. During the course of a year a total of twelve field monitoring campaigns were conducted to measure temperatures with 249 images collected at sixteen different locations (eight exterior and eight interior). During this first sequence of measurements, the north green roof had already been completed, while the south green roof was gradually being installed. Additional field imaging was performed on two separate days in August (on 08/08/15, 08/22/15), and on one day in January (on 01/22/16) between sunrise and sunset (generating an additional 850 images). To isolate the thermal performance of individual components of the roof structure, a physical model of the roof was constructed in a laboratory, instrumented with thermistors, and subjected to a heat lamp. A one-dimensional heat conduction model was used to investigate physical relationships revealed by the field and laboratory observations, and represents the experimental data well. The results indicate that the construction of the green roof significantly reduces heat flux through the convention center roof. The mean internal and external temperatures on the north and south sections of the roof on the first monitoring day (when the north green roof was complete, but the south green roof had not yet been installed) and the last monitoring day (when both north and south green roof sections were complete) are provided to support the insulating value of the green roof layers. The mean exterior temperature on the south roof on the first day of monitoring was 53.9°C and 37°C on the north roof. The mean exterior temperature on the south roof on the last day of monitoring was 24°C and the north roof 28°C.

Thermal behavior of green roofs under Nordic winter conditions

To understand how green roofs affect building energy performance under cold climatic conditions, a proper thermal analysis of the roof and its components is required. To address this, we measured the thermal conductivity of each layer of experimental green roofs, as well as equivalent thermal resistance of the complete green roof system during winter conditions in southern Finland. Three experimental green roof platforms (1 m × 2 m) with heated boxes and three identical bare roof platforms (without substrate, vegetation and other green roof layers) were equipped with thermocouples that continuously measured a vertical temperature profile through the roofs. A steady-state heat transfer analysis was performed to assess the functioning and relative thermal performance of the green roof systems. Layer analysis at various intensities of frost penetration showed that the thermal conductivity of each layer decreased when penetrated by frost. In particular, thermal conductivity of the substrate and vegetation layers decreased from 0.41 Wm−1K−1 and 0.34 Wm−1K−1 prior to freezing, to 0.12 Wm−1K−1 and 0.10 Wm−1K−1 after freezing, respectively. This phenomenon is explained by a reduction in bridge-water connectivity during freezing and a volumetric water content that was below the critical threshold value. Overall, a frost depth that extended through the complete green roof yielded the greatest equivalent thermal resistance. During times of snow cover, snow acted as an insulator and reduced the relative energy saving benefits achieved by green roofs. These results provide information for designing the substrate and vegetation layers of green roofs for optimal insulation.

Quality assessment of three warm-season turfgrasses growing in different substrate depths on shallow green roof systems

In an effort to increase the accessibility and functionality of shallow green roof systems, the ability of warm-season grasses to provide acceptable growth needs to be further investigated. In the current study, which was conducted during 2011 and 2012, three warm-season grasses (hybrid bermudagrass, Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt-Davy ‘MiniVerde’; seashore paspalum, Paspalum vaginatum Swartz ‘Platinum TE’ and zoysiagrass, Zoysia japonica Steud. ‘Zenith’) were established in outdoor lysimeters. The lysimeters were equipped with all necessary green roof layers placed below a coarse-textured substrate that comprised pumice, thermally treated attapulgite clay, peat, compost and zeolite. Half of the lysimeters had a substrate depth of 15cm, while the other half had a substrate depth of 7.5cm. Irrigation was applied at crop evapotranspiration (ETc). Measurements included determination of substrate moisture content, green turf cover (GTC) and leaf stomatal resistance. Significant differences were observed in the values of GTC among the three turfgrass species and the two substrate depths. Zoysiagrass exhibited the best adaptation at the lower depths of shallow green roof systems. At 15cm substrate depth, zoysiagrass managed to sustain green coverage for the two study periods. In addition, it was the only turfgrass species that managed to perform well at the substrate depth of 7.5cm. Seashore paspalum exhibited limited green cover at both substrate depths, while hybrid bermudagrass could provide acceptable green coverage only at 15cm substrate depth.

The use of reactive material for limiting P-leaching from green roof substrate.

The aim of the study is to assess the influence of drainage layer made of reactive material Polonite(®) on the water retention and P-PO(4) concentration in runoff. A column experiment was performed for extensive substrate underlined by 2 cm of Polonite(®) layer (SP) and the same substrate without supporting layer as a reference (S). The leakage phosphorus concentration ranged from 0.001 to 0.082 mg P-PO(4)·L(-1), with average value 0.025 P-PO(4)·L(-1) of S experiment and 0.000-0.004 P-PO(4)·L(-1) and 0.001 P-PO(4)·L(-1) of SP experiment, respectively. The 2 cm layer of Polonite(®) was efficient in reducing P outflow from green roof substrate by 96%. The average effluent volumes from S and SP experiments amounted 61.1 mL (5.8-543.3 mL) and 46.4 mL (3.3-473.3 mL) with the average irrigation rate of 175.5 mL (6.3-758.0 mL). The substrate retention ability of S and SP experiments was 65% and 74%, respectively. Provided with reactive materials, green roof layers implemented in urban areas for rain water retention and delaying runoff also work for protection of water quality.

Building thermal-insulation effect on ambient and indoor thermal performance of green roofs

Building thermal insulation (BTI) may influence indoor cooling by green roofs. The hypothesis that buildings with poor BTI may benefit more from green-roof passive cooling is tested. An experiment was designed in humid-tropical Hong Kong to investigate two BTI states (good and poor) and three treatment plots (control-bare, succulent Mexican Sedum, and broadleaved Perennial Peanut). Temperature sensors were installed along a holistic vertical profile from outdoor air to green-roof layers and indoor ceiling and air. On summer-sunny day, poorly-insulated control roof allows notable daytime peak thermal intrusion, counterpoised by comparable nocturnal thermal discharge. It is the only roof with bidirectional heat fluxes and nearly-balanced diurnal heat-energy budget. Other roofs are predominantly unidirectional (downward) with considerable indoor heat gain. Well-insulated control roof experiences extended (morning to night) thermal intrusion, with depressed but delayed and prolonged heat influx, and thermal insulation breaching. At night time, it creates nocturnal thermal barrier to reduce heat escape from indoor space. Simple Sedum green roof on poorly-insulated roof establishes green-roof heat-sink effect to incur daytime and nocturnal aggravated thermal intrusion. On well-insulated roof, it generates a synergistic-dual thermal barrier to suppress thermal insulation breaching. However, nocturnal thermal discharge has been constrained by persistent positive thermal gradient throughout the day. Thicker and more-elaborate Peanut green roof brings considerable evapotranspiration cooling and synergistic-dual thermal barrier to generate daytime subdued thermal intrusion and mask inherent BTI-differences. As omission of BTI for both Sedum and Peanut green roofs does not earn more indoor cooling, the hypothesis cannot be accepted. To optimize passive cooling, green roofs with thicker substrate and denser foliage should be installed on buildings with good BTI.

Support for the integration of green roof constructions within Chinese building energy performance policies

Green roofs could act as a thermal buffer in buildings and offer potential energy savings. However, the energy benefits from green roofs are not usually properly recognised by traditional building energy regulations. Building energy regulations are traditionally over-simplistic during the assessment of the energy performance of complex building constructions. In the case of green roof designs, it is essential that the assessment mechanisms should not ignore the complex heat and moisture balances within the green roof layers. In this paper, dynamic energy modelling that considers the complexity within the green roof layers is adopted to guide policy makers in China on the relationship between using specific thicknesses of roof insulation against green roof layers. Simulations are run for a residential building type by also considering different thermal envelope characteristics across eight large Chinese cities and within the five main climatic zones of China. Results that link the green roof characteristics with respective traditional insulation layers are produced for all cities and it is found that optimising the plant and soil characteristics of green roofs in some climates could substitute more than 125 mm of roof insulation, while less optimum green roof types could only replace about 25 mm of roof insulation.

A combined experimental and simulation method for appraising the energy performance of green roofs in Ningbo’s Chinese climate

A passive means of lowering the energy demand of buildings is the application of green roofs. The complexity between heat and moisture exchanges in green roof layers and the large variations of green roof types make the need for experimental or simulation assessments necessary for quantifying the energy benefits from green roofs. The current treatment of green roofs in simulation programs is either over-simplistic, for example by ignoring heat and moisture exchanges such as evapotranspiration, or the more advanced models have limitations and require inputs that are rarely available in practice. In this paper a combination of experimental and modelling techniques are used to assess the potential heating and cooling load reductions from the application of green roofs in the subtropical climate of Ningbo in China. The method provides a generalised energy performance assessment of green roofs in Ningbo by overcoming the limitations of existing gree

Development and validation of a coupled heat and mass transfer model for green roofs

This paper describes a dynamic model of transient heat and mass transfer across a green roof component. The thermal behavior of the green roof layers is modeled and coupled to the water balance in the substrate that is determined accounting for evapotranspiration. The water balance variations over time directly impact the physical properties of the substrate and the evapotranspiration intensity. This thermal and hydric model incorporates wind speed effects within the foliage through a new calculation of the resistance to heat and mass transfer within the leaf canopy. The developed model is validated with experimental data from a one-tenth-scale green roof located at the University of La Rochelle. A comparison between the numerical and the experimental results demonstrates the accuracy of the model for predicting the substrate temperature and water content variations. The heat and mass transfer mechanisms through green roofs are analyzed and explained using the modeled energy balances, and parametric studies of green roof behavior are presented. A surface temperature difference of up to 25°C was found among green roofs with a dry growing medium or a saturated growing medium. Furthermore, the thermal inertia effects, which are usually simplified or neglected, are taken into account and shown to affect the temperature and flux results. This study highlights the importance of a coupled evapotranspiration process model for the accurate assessment of the passive cooling effect of green roofs.

Modeling the heat diffusion process in the abiotic layers of green roofs

Green roofs have been increasingly installed to alleviate some common environmental problems. The thermal benefit of living vegetation on rooftop has been extensively studied. The individual and joint contribution of the non-living green roof layers, namely soil, rockwool (water storage) and plastic drainage layers, to thermal performance of green roof has seldom been assessed. This study evaluates the insulating and cooling effects of these abiotic materials. A one-dimensional theoretical model was developed to assess the heat diffusion process in the layers. The model was validated with empirical results from three experimental plots. A calibration procedure was successfully applied to determine key model parameters. The model can capture the most critical features of temperature variations and thermal performance of common abiotic green roof materials. The appreciable water-retention capacity of rockwool plays the dual role of supplying water to the soil to enhance evaporative cooling, and increasing the specific heat capacity of the green roof. The plastic drainage sheet with ample air spaces serves as an excellent thermal insulator. The model remains robust despite seasonal and weather variabilities. Our research findings contradict with some researches in the temperate region that the thermal dissipation in green roofs with dense vegetation is lower than thermally insulated bare roofs. The theoretical model could be used to simulate the micro-environmental conditions and predict the thermal performance of different materials to improve green roof design.