Previous research works suggest that buildings with green roofs are eco-friendly. For example, green roof i.e. green vegetation on roof top retain storm water during storms and heavy rain, which reduces amount of water entering city runoff system. First, the green roofs retain excess water, and later they return this water to atmosphere by evapotranspiration. Consequently reduces runoff helps in reducing amount of pollutant which enter a non-regulated storm water system.These benefits have been demonstrated in many experiments which were conducted across many sites, cities, and landscape. In addition, green roofs help in regulating internal temperature of buildings, which reduces overall energy consumption of a given building. Buildings™ energy consumption reduces because green roofs provide shading, increase thermal mass, and cool the buildings™ internal temperature. However, amount of energy saving depends on various factors such as design of green roofs, climatic season, and level of insulation or thickness of vegetation provided on the roof. Research statistics suggests that amount of irrigation on the green roofs also influence amount of energy reduction. For example, during summer proper irrigation of green roof reduces demand of buildings™ air conditioning requirement from 98.90% to 90.80%; similarly, during winter buildings™ heating requirements comes down from 38.30% to 27.40%. Therefore, it can be concluded that benefits of green roof vegetation contribute to a sustainable development towards combating climate change. Developing green roof buildings are better alternative to other greening strategies such as street trees and green city gardens which is a challenging task for city administration in cities with higher building fraction i.e. lesser land availability for planting trees due to presence of large number of buildings.

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Researchers conduct different type of green roof experiments mainly on building scale; however, with growing concern over climate change, there is a need to conduct city scale experiments. Till date only few studies have attempted to study effect of green roof on a city scale. In this regard, one can start to use numerical models to evaluate the performance of green roofs on a city scale. Numerical or computer modelling helps in reducing cost of experiment at the same time reduces investigation time that ranges over various seasons. Few research studies have investigated impact of green roofs over city scale. These studies represent the green roofs with evaporating surfaces such as grass at roof tops and ground levels. Parameterized and calibrated experiments are conducted to simulate the hydrodynamic and energy transfers. In other words, numerical models are convenient to study the effects of city scale green roof infrastructure on the urban heat island (UHI). Simulation study suggests that in a given city 2°C cooling can be achieved by covering half of the city buildings with irrigated green roofs; and cooling of 0.5°C can be achieved by covering half of the city infrastructure with dry (not irrigated) green roofs. In addition, investigators study effect of green roofs on the water runoff and surface temperature. Consequently, these two parameters help in reducing the amplitude of urban morphology classes (UMC). Greener roofs at town center reduce storm-water runoff by 17.60% and surface temperature by 6.60°C, especially around green roof buildings. Literature review indicates that much needs to be done to study city scale impact of green roofs. While, some simulation works have focused on the city scale green roofs and its impact on temperature and storm water runoffs; future models should study its impact on residents™ comfort level and overall building energetics. Further, these models should incorporate urban canopy, extreme climatic conditions and related meteorological data associated with the climate of city.

Types of Green Roofs

Installation of green roofs help in reduction of surface urban heat islands and provides numerous environmental benefits. Green roofs are classified into two sub-groups namely, extensive system and intensive system.

Extensive system

Extensive type green roofs are simple and easy to maintain; they comprise of plants that are suitable for alpine environment such as succulent and hardy plants (Fig 1). For example, these roofs do not require regular irrigation. Thus, they require little maintenance and less intervention after plantation. In addition, extensive roofs are cost effective because of their lighter weight. Therefore, rugged plants suitable for extreme climatic conditions are suitable for extensive type green roofs.An extensive green roof easily grows on roofs with high slopes. For example, extensive green roofs can grow on roofs with steepness ranging from 9.5° to 30°. Steepness of roof determines, if it would require additional support to hold growing load of plantation. Further, the water holding capacity of roofs depends on the steepness of roofs.

Intensive System

Intensive types of green roofs are heavier than extensive green roofs and they accommodate almost all type of plants which are available in gardens (Fig 2). It also includes large trees. Intensive green roofs provide natural roof top garden and save energy. It requires heavy initial and maintenance cost unlike extensive green roofs. In addition, intensive type roofs require support structures due to presence of humans and growth of big to medium trees. Further, the intensive green roofs require regular irrigation to maintain green and large roof top environment. Nevertheless, they have higher retention ability and provide additional benefits such as clean air, lower temperature and energy saving.

Green roof photo

Fig 1 A type of extensive green roof.

Green roof photo-2

Fig 2 A type of intensive green roof.

3. Advantages of Green Roof

Green roofs provide similar benefits that normal vegetation provides. Green roofs provide planting opportunity in the densely built-up areas of city. Following sub-sections discuss major advantages of green roof buildings.

3.1 Reduces Energy Consumption

Green roofs shelter building; therefore, they save energy required for cooling and heating the internal units. For example, the wet green roofs help absorb heat from building which reduces the amount of energy required for cooling the building. Similarly, dry green roofs insulate internal units which reduce heat flow from the internal units to atmosphere. This implies that during winter less energy is needed to heat buildings with green roofs. On the other hand in summer green roofs reduce external heat from flowing into the building which also reduces amount of energy required to cool internal units. Though green roofs act as insulators but they cannot be considered replacement for insulations.

Fig.3 shows trend of average daily energy demand of a building with conventional and green roof. During summer a building with conventional roof requires 6 to 8 kWh energy to cool. On the other hand building with green roof requires 1.5 kWh lesser energy to cool the building. Thus, during summer a green roof reduces the energy demand by 75 percent. Though green roofs reduce energy demand both during summer and winter; nevertheless, its actual performance depends on the local climatic conditions and other design characteristics such as size, insulations and usage of roof.

Green roof photo-3

Fig 3 Monthly energy demand of a building with green and conventional roof.

3.2 Reduces Air Pollution

Green vegetation on roofs decreases air pollution and emission of greenhouse gases by carbon sequencing and dry deposition. In addition, buildings with green roofs require lesser energy for air conditioning which also reduces greenhouse gas emissions during energy production. Further, green roofs reduce formation of ozone at lower level i.e. at ground level by reducing air temperature in and around a building. The process of dry deposition removes some pollutants such as particulate matter and gaseous pollutants. Furthermore, green roofs also reduce amount of carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen oxides (NOx). Research studies have estimated that 93 m2 of green roof can remove forty pounds of particulate matter over a year time. This amount of particulate matter is equivalent to the amount of particulate matter emitted by fifteen cars annually. Similarly, if green roofs are installed to cover 20% of a building with roof area of 930 square meters then they can remove six tons of particulate matter and equivalent amount of O3 from the air. Research suggests that installation of green roofs at this scale will help reduce air pollution equivalent to 25,000 to 30,000 trees. Therefore, it can be concluded that if 20% of the city roofs are covered with green roofs then it would reduce air pollution by 10 to 20 percent. Plantation of trees on green roofs should be encouraged as it can store more carbon than grass.

3.3 Increase in Comfort level

Comfort level of city dwellers improve with installation of green roofs, as it provides them suitable temperature by reducing heat flow in and out of building. Researchers support installation of green roofs on non-air conditioned infrastructure because it helps regulate air temperature both in winter and summer. In addition, green roofs improve quality of life by providing trees and garden on roof which is equivalent to traditional ground vegetation. Therefore, city dwellers get access to greenery on roof to enjoy. In addition, some researcher works look at green roofs as a place to provide living habitat to endangered species as planting these species on roofs would protect them from natural predators that might endanger their growth.

3.4 Enhances Quality and Management of Water

In urban dwellings, green roofs absorb excess rain and storm water, which reduces amount of runoff into water management system. In addition, green roofs absorb some of the pollutants from the storm water that helps to purify it. Green roofs work similar to normal ground vegetation and help reduce runoffs, at the same time help purify storm water. Architects suggest that the thickness of green roof and slope of roof governs that ability of green roof to absorb and purify water. Research findings show that the green roofs can absorb 50 to 100% of the rain water with proper design and thickness of vegetation. In other words, intensive green roofs with higher thickness will absorb more water than extensive green roofs. Ten centimeter thick vegetation would reduce water runoff by 75% over a time period of fifteen month. Similarly, at peak rainfall green roofs can reduce water runoff by 75 percent. Green roof vegetation releases this excess water to atmosphere through evapotranspiration; thus, decreasing rainfall runoff entering water management systems of a city. It has been observed that over time green roofs have shown increasing capability to absorb water. Therefore, it can be concluded that green roofs help in reducing investment into water management systems.

In addition to the green roofs™ design parameters, local conditions and climatic conditions regulate the retention capability of green roofs. Studies have demonstrated that green roofs can detain water runoff during intense rain and help reduce water runoff by 95% at peak time. This detained water can be released later into water management systems.

Further, green roofs act as a filter and help reduce pollutants in the runoffs i.e. they retain pollutants. For example, one study found that from the storm water green roofs can remove 95% harmful pollutants such as copper, lead and cadmium. A monitoring program found that 80 to 95% purification can be achieved with green roofs which eliminate harmful elements such as polycyclic aromatic hydrocarbons and suspended particles from the storm water. However, green roofs may increase concentration of phosphorus and nitrogen in storm water due to presence of soil in the green roof installations. Further, this increase in phosphorus and nitrogen content is primarily due presence of compost materials in the soil. Nevertheless, most research studies indicate that green roofs reduce amount of pollutants in storm water in comparison to the conventional roofs. In addition, green roofs also help in reducing pH; therefore, it can be concluded that green roofs help reduce pollutants from water runoff compared to the conventional roofs.

4. Installation

Available resources and purpose determine installation of green roofs. Builders and owners need to select among the extensive and intensive type green roofs. For example, extensive type of green roofs are suitable for those builders and owners who look to spend minimal on its maintenance and at the same time want to address energy saving. On the other hand, city centers with no space for recreational park at ground level would look to develop intensive type of green roofs. In addition, some green roofs are semi-intensive or semi-extensive based on the need and requirement of those sites. Therefore, site selectin governs type of green roof installation. This section highlights major findings from research studies on installation of green roofs. Discussion is provided based on site selection, installation and fire safety, which are areas of major concern.

4.1 Characteristics of Site

Site selection for installation of green roofs is first step towards harvesting the benefits of green roofs. Developers across the world are focusing to build green roofs in the old central part of cities, which have limited land availability due to rapid development at those sites. These city centers are now left with no space for green vegetation and plants. Therefore, green roofs are the only option for providing greener environment at these city centers. Further, these green roofs will help in providing space for recreational activities and improve the ambience of city centers.

Builders are focusing to develop green roofs at those locations which suffer from excess energy usage. For example, big buildings consumer lot of energy for providing cooling and heating facilities. These sites can be used for developing green roofs which has shown to reduce energy consumption by regulating inner and outer temperature. Michigan has one such green roof facility in less dense area on the building of Ford™s Dearborn Truck Plant. Therefore, factories and commercial buildings which consume lot of energy for air conditioning are also candidate site for installation of green roofs.

Sites that receive heavy rainfall are also candidate site for green roof installation because green roofs would help in managing excess storm water. Further, due to structural and constructional challenges, roofs with low to flat slopes are better suited for installing green roofs. Steep slopes require additional structural support for developing green roofs which discourages use of such sites for installing green roofs. In addition, green roofs are easily incorporated into new ongoing construction than older buildings. This is because it is easier to incorporate design and construction changes during the construction of new buildings. Nevertheless, sites such as old city centers are increasing using retrofitting supports to facilitate green roofs. Further, recent development in civil engineering makes it possible to develop green roofs in old existing building with large roofs. For example, a building of a University that had stone ballast roof was augmented with green vegetation layer as shown in the Fig 4. Department of conservation and management funded this project.

Green roof photo-4

Fig 4 University office with stone ballast was developed into green roof.

 

4.2 Maintenance and Installation

Maintenance and installation of green roofs comprises of almost similar major components. Fig 5 show major components of a green roof facility. Major components of green roofs are following:

  • Vegetation.
  • Growing medium,
  • Filter membrane,
  • Drainage
  • Barrier for roots
  • Waterproof layer,
  • Cover,
  • Thermal insulator,
  • Vapor barrier, and
  • Structural support

Green roof paper

Fig 5 Components and various layers in a green roof installation.

Type of vegetation depends on multiple factors such as design of building, regional climate, availability of water, purpose of roof, and type of roof i.e. extensive or intensive roof. For example, hardy perennial plants are preferred in extensive type of roofs. These plants do not require deep soil, maintenance, and need lesser minerals for growth. Further, these plants are tolerant to extreme weather conditions and severe wind conditions. Similarly, succulents such as sedums are also suitable for green roofs as it is resistant to drought and it has higher moisture content, which makes it resistant to fire. On the other hand, intensive type of green roofs provide deeper soil that allows growth of plants and trees with deeper roots such as, shrubs, designer trees, and bushes. Further, intensive type of green roofs that support bigger plants have better irrigation facility. The green roof plants come in different size, color and textures; building owners can select type of vegetation based on type their purpose and budget.

The growing mediums of green roofs are kept light weight to reduce the load on building. In this respect, growing medium may not only consists of only soil; engineers use tailored soil for green roofs. The tailored growing medium has special composition so that it can support plants and at the same time impart less weight to the building. Further, the composition can be altered provide organic material that would retain more water from rain and storms. Life of growing medium should be ideally equal the life of roof. Generally, 80% of the growing medium consists of inorganic mineral and 20 percent comprises of organic matter such as topsoil. Thickness of growing medium depends on type of green roof i.e. extensive green roofs have 15 cm thick growing layer; while, the intensive type of green roofs have thickness greater than 20 cm.

A filter membrane is provided in the green roofs that remove excessive water from the soil at the same time it prevents loss of finer soil particles from eroding with the rain or storm water. A drainage layer prevents overloading of roof top with excess water collected during heavy rain or storm. In addition, a good drainage layer maintains enough moisture in the vegetation and helps in healthy growth of plants on the roof.

A root barrier is provided at lower layer in the green roofs to prevent the plant roots from penetrating concrete water proofing layer, which may cause leakage and serious damage to the water proofing layer of roof. The water proofing membrane protect roof from water penetration. Generally, hard ply waterproof materials are used; however, some roofs are resistant to water and root penetration due to stringent construction. A cover board is used that protects, supports, and separates the roofing membrane.

Thermal insulation is placed above or below the soil i.e. growing material. Though, growing material provides some insulation but it cannot replace traditional insulation materials. Nevertheless, growing materials insulation increases with increasing water or moisture, and insulation material complements or adds to the insulation capability of green roofs. Further, a barrier is installed at lower level in green roofs that prevents moisture from penetrating roof. Discussion in this section clearly indicates that green roofs are heavier than traditional roofs. Therefore, green roofs require additional supporting structure. For example, an extensive roof weighs around 15 to 30 pounds per square foot. Therefore, engineers need to ensure sufficient support to the green roof even with water saturation. Additionally, builders need to ensure meeting the building code of conduct for region with snow and heavy rainfall. It is easier to provide support on a new building than adding support to existing roofs. Engineers include design and cost of additional support structure.

Generally, intensive roofs require more support in comparison to the extensive roofs. This is primarily because intensive roof have more water retention capability and support larger plants which require deeper soil. In addition, the extensive roofs support recreational activities and there is constant presence of city dwellers. Furthermore, extensive roofs require some maintenance; however, an extensive roof which does not provide recreational space requires lesser maintenance.

Both extensive and intensive green roofs require constant pest and weed control during early days. Constant weed control help plants develop and grow faster. Researchers recommend that weeding should be carried out monthly or quarterly during the first few years, and slowly it can be decreased after plants have grown to their full length. The standard guides support constant maintenance is important for health of green roofs. Following are some of the recommendations of healthy plantation:

  • Fertilizers should be applied slowly once in a year to avoid acidification of soil.
  • Extensive roofs relay on natural rainfall for irrigation; however, green roofs in dry climatic zones require regular irrigation. Irrigation is also important to reduce risk of accidents such as fire due to dryness and extreme weather. On the other hand, all intensive type roofs require regular irrigation because they facilitate bigger plants. Therefore, it can be concluded that extensive type green roofs are inexpensive and require less irrigation.
  • In addition, replantation is recommended at regular interval of time to facilitate even distribution and constant density of vegetation at green roofs.
  • Further, gutters should be cleaned at regular interval of time to prevent water logging and decrease chances of standing water. This would prevent blockage and maintain healthy plant growth.

Thus, it can be concluded that green roofs require maintenance and life of green roof is larger than regular black roofs. In addition, precise installation of various membranes of green roof, ensure a life of 30 to 50 year.

4.3 Safety

In addition to structural failure, green roofs can cause fire in case it is left dry. Though, green roof that are properly moisture and watered would prevent fire, but improper moisture and too much dryness can support fire. Therefore, following steps should be followed to prevent fire:

  • Use of fire resistant plants such as sedums and a growing medium with sufficient moisture prevent fire hazards. Further, builders should not plant that would dry in the heat of summer.
  • Standard code recommends construction of fire breaks-2 foot in width- at regular intervals prevents fire.

4.4 Cost of Installation

Green roof installation charges can range from $10 to $25 per square foot depending on type of green roof, vegetation, planting medium, drainage, and type of side fencing. Availability of skilled workforce is another deciding factor that regulates price of installation. For example, cost of installation is less in Germany where green roofs are readily available; on the other hand, green roofs are less prevalent in the United State. Therefore, cost of installing green roof is higher in the United State in comparison to Germany as it is easier to find contractors in Germany than in the United State.

5. Experimental and simulation studies

Research studies are being conducted to improve the existing technology of green roof installation. In this regard, researchers and engineers carryout experimental and numerical models to investigate response of green roofs. Further, these numerical model help reduce cost of research studies; consequently, they are gaining popularity for investigating the performance of green roofs.

5.1 Field Experiment

Research organizations have installed facilities to quantify the efficiency and thermal performance of green roofs.European countries such as Switzerland, Germany, and France have installed green roofs. Due to multiple benefits green roofs are gaining popularity in other parts of world too. Government is encouraging installation of green roofs over commercial, academic, and residential buildings. Consequently, researchers have started to conduct experimental studies to access and improve the thermal performance and efficiency of roof top vegetation. This section quantifies findings from the experimental studies that assessed performance of green roofs to identify its sensitivity towards changing climatic conditions.

5.1.1 Experimental setup

Fig 6 shows one such experimental roof top with low slope and high roof to wall ratio. It has area of 72 m2. The experimental roof comprises of two areas; the left region represents a green roof and right part represents a generic extension of green roof with bituminous membrane which is a grey color membrane that can reflect heat unlike dark colored roofs. The green roof comprises of same components with additional support for planting vegetation. Fig 7 represents the internal components of both type of roofs used in the experimental setup i.e. a) Reference roof and b) green roof. The reference roof comprises of a structural support, vapor barrier, thermal insulator, supporting panel, and waterproof membrane (Fig 7). The green roof has all the components of reference roof; in addition, it has drainage layer, filter membrane, and a growing medium to support green vegetation as shown in the Fig 7.

Instruments are installed in both type of roofs to measure major parameters for assessing its performance and energy efficiency. Experimentalists measure following parameters:

  • Moisture content of soil,
  • Heat flow through the installation,
  • Water runoff
  • Solar reflectivity of roof,
  • Temperature distribution or profile across the roof, and
  • Microclimate above the green roof vegetation.

Researchers planted a wild flower meadow during the first year of the experiment; while, a Kentucky blue grass was planted during the second year. First, the green roof parameters were measured. In addition, the meteorological station measured the meteorological parameters such as daily rainfall, temperature, relative humidity, and solar radiation within 60 m radius of the experimental site. Section below summarizes the results of the experiment conducted over a period of two year.

Green roof paper-1

Fig 6 Experimental green roof and median roof facility.

Green roof paper-2

Fig 7 Main components of reference roof and green roof.

5.1.2 Temperature distribution

Temperature of exposed roof membrane raises during the day due to solar radiation. Color of membrane determines extent of temperature rise due to absorption of solar radiation. Membranes with light and color reflect most of the solar radiation. Therefore, they are cooler in comparison to the membranes with darker color. Results from this experiment suggests that green roofs are cooler in comparison to the reference membrane i.e. reference roofs experience much higher raise in temperature than green roofs. Fig. 8 shows temperature profile of both roofs during a summer day. Instrument measured a maximum temperature of 70°C for the reference roof; while, the green roof reported a maximum temperature of only 25°C on the same day.

Statistical analysis shows that for 33% of the observed time period temperature of reference roof exceeded 50°C i.e. 219 days out of 660 days showed temperature greater than 50 degree Celsius. Further, only 13% – 89 out of 660- days temperature of reference roofs exceeded 60°C. Consequently, the room temperature exceeded 30°C for 10% and 3% for reference roof and green roof buildings respectively. Experimental measurements show that exposure to solar radiation reduces the life of bituminous material, which reduces its performance over time. This is due to continuous exposure of bituminous material to the solar radiation which changes its chemical properties and reduces its mechanical performance. Though, much research needs to be conducted to find response of these materials to prolonged exposure to solar radiation, but preliminary results suggest that green vegetation reduces direct exposure of these membranes by preventing ultra-violet rays and increases life of the roofing membranes.

Green roof paper-4Fig 8 Temperature distributions over the green roof and reference roof as measured by the measuring instruments.

5.1.3 Efficiency and performance

Results from the experiment indicate that green roofs maintain a cooler temperature during summer. In addition, roofing membrane is shaded by the green vegetation and soil, which prevent direct exposure of membrane to solar radiation. Further, evapotranspiration from the plant leaves reduce surface temperature of growing medium and roof membrane.

In a building with normal tradition roof, heat flows beneath roof into the building which increases air conditioning and energy demand. Fig 9 presents the measured heat flow into the building from roof on a summer day. The data was obtained from three transducers installed in each roof section, which measured the heat flux. The positive values in the graph represent heat entering the building through the roof and the negative values represent the heat radiating out of the building. Results show that membrane of reference roof absorbed heat radiation during day (positive values) and radiated the absorbed heat during night time (negative values). On the other hand green roofs helped in regulating heat flow in and out of building through roof during the same period. Further, on a winter day when the roof was covered with snow, the green roof helped in partially insulating the building and kept the heat in the building, which in turn reduces energy demand for heating.

Fig 10 shows the average monthly air conditioning energy demand due to heat flow or loss through the reference and green roofing systems. It can be observed that green roofs are slightly efficient than reference roofs with only insulating membrane. During fall and early winter green roofs outperform the reference roof; while, in the peak of winter there performance are comparable. In addition, during summer too green roofs perform much better than the reference roofs with membrane. In summer the reference roof with membrane absorb the solar radiation during the day time and at night it radiates the heat out to the surrounding. This creates a high energy demand for air conditioning. On the contrary, the green roofs are covered with green vegetation and help insulate the building and membrane. Therefore, during summer the green roofs absorb lesser solar radiation in the day time and reduce energy demand. Measurements show that 6.00 to 7.5 kWh/day energy was needed for air conditioning a building with reference roof. On the other hand, less than 1.5 kWh/day of energy was required for air conditioning a building with green roof.Results show that 75% reduction in energy demand can be achieved by using green roof installations.

This experimental study shows that during spring and summer green roofs efficiently reduce the heat comparison to the heat loss during winter. This is due to fact that during summer green roofs provide shade and insulation from solar radiation at the same time it provides cooler atmospheric temperature through evapotranspiration. Nevertheless, green roofs reduce heat loss only by insulation and by lowering heat radiation. These techniques are efficient during summer but not during winter, as in winter snow provides better insulation by reflecting solar radiation efficiently. According to the experimental study green roofs block 95% of the in flowing heat and 26% of the out flowing heat. On the other hand, reference roofs reduce 47% overall heat flow. Therefore, it can be concluded that green roofs are better alternative to traditional roofs and other hybrid insulating roofs.

Green roof paper-6

Fig 9 Average monthly energy demands through the reference roof and green roof.

Green roof paper-7

Fig 10 Heat flow into a building through roof for a given day in summer.

5.2 Simulation of UHI

Studies suggest that Urban Heat Island (UHI) develops with removal of vegetation of a region to develop cities, which replaces the natural vegetation with non-porous and head concrete structures. The natural vegetation plants and trees act as a source of water i.e. known as evapotranspiration. Through the process of evapotranspiration, plants and trees give the water back to atmosphere in form of water vapor. During the evapotranspiration green vegetation absorbs solar heat and uses it to convert the water into water vapor that is sent back to atmosphere in the form of water vapor. Thus, the solar energy is stored into the water molecules and later it is released in the upper atmosphere during condensation. However, when natural vegetation is replaced by hard non-porous concrete structures, the solar heat is not used for evapotranspiration due to absence of green vegetation. This solar heat then gets absorbed by hard non-porous infrastructure, which increases the urban heat by radiating the heat back to the atmosphere. In addition, the city infrastructure does not absorb water, whereas green vegetation allows water absorption. In absence of green vegetation solar heat is not used for evaporating water and keeps accumulating in the atmosphere.

Different regions of a city infrastructure can be represented by their surface temperature. In a study researchers studied the surface temperature of different type of surfaces. Fig 11 shows variation of surface temperature of transect, forest, wood shelter and road. Results show that surface without vegetation show higher surface temperature in comparison to the surface covered with vegetation. For example, roads and clear cut ground show surface temperature in the range of 45 to 60°C; while, green grass and forest show average surface temperature of 30°C.

 

Green roof paper-10

Fig 11 Surface temperatures of different geographical features.

Thus, above discussion indicates that roof surface temperature can be reduced my implementing green roof techniques. Therefore, it can be concluded that green roofs can reduce urban heat island. Urban heat island exists in the urban boundary layer; this layer ranges from the city rooftop to the level in atmosphere where influence of urban heat becomes zero. Results suggest that canopy layer gets affected by the boundary layer temperature. Consequently, increasing urban heat island affects people living in cities. This phenomenon is also reflected from the fact that roofs at higher elevation show higher surface temperature due to effect of increase in the temperature of boundary layer. Fig 12 shows the difference in surface temperature of roof tops with different elevation.

Builders should plan roof tops properly because roof tops increase the temperature of UHI, and it also radiates more heat into the building. This increases the room temperature inside the buildings, especially in the top floors. These temperature variations increases the inmates discomfort level and in some cases causes heat and respiratory problems. Further, as discussed in the introduction, normal exposed roof tops increases the load on refrigeration and air conditioning. Therefore, traditional roof tops also increase greenhouse gas emission due to increase in air conditioning and refrigeration.

Green roof paper-11

Fig 12 Thermal image of roof tops showing variation in its surface temperature.

Few decades back most cities had less dense residential housing i.e. regions within cities supported green vegetation. For example, Fig 13 shows a chart of land usage for Toronto city. It comprised of less dense residential apartments. It was observed that upon increasing the density of residential housing i.e. by applying zoning restrictions leaf area density decreased. Further, larger or maximum restriction can reduce the potential of urban forestry to almost two-third (Fig 14). Therefore, it can be concluded that even with increasing density of residential and commercial apartments, city must find alternative area for revegetation, such as roof tops.

Green roof Royal

Fig 13 Chart showing land usage of Toronto city in 1990s.

Green roof Royal-2

Fig 14 Bar-graph showing effect of land restriction on the leaf density.

Building and the urban environment get benefited by the installation of green roofs. Green roofs help in mitigating the extreme climatic swing in the city temperature which prolongs the life of roofs. For example, green roofs reduce energy required for cooling and heating a unit during summer and winter respectively. In addition, it provides green space for the building inhabitants in the form of horticulture and garden space.

5.1.1 Process of modelling UHI

The area of urban heat island spreads over two to three times the size of city in the horizontal direction. UHI and other atmospheric process interact among themselves and modify each other. For example, wind breezes and flow over mountain directly interact with UHI of neighboring cities. Therefore, UHI model should be able to resolve mesoscale circulations to investigate the green roof and its effect on UHI. Mesoscale Compressible Community Model (MC2) is a numerical model, which can be used for modelling UHI of a city with green roofs. These numerical models implicitly solve the Navier-Stokes equations using semi-lagrangian scheme. A soil vegetation atmosphere transfer (SVAT) scheme is used for parameterizing the natural environment of the city.

Researchers use two methods to model the urban land use flux into mesoscale model. First method natural terrain represents the urban land usage. Building elevation is represented by adjusting the roughness length. Second approach incorporates more details in the model i.e. more parameters are used in the model to define the urban land usage flux. Later model defines drag in the air flow due to presence of buildings, turbulent kinetic energy and flux due to solar radiation and trapped heat. Further, the second model computes heat budget for the city roads, parking area, building roofs and walls. In addition, this model considers long and short radiation fluxes for reflection, shadowing and cloudiness which indirectly influence representation of street canyon. Fig 15 represents a city atmosphere and various parameters that affect it.

Green roof Royal-3

Fig 15 Parameters considered during modeling of urban heat island.

These models represents he earth city surface with either ground or rooftop depending on the land usage. An average flux value is given to each cell that represents an area of one kilometer. The average flux is mean of flux from the road, parking, rooftops, and walls. These cells with average surface flux are placed at the bottom of urban heat island model. Roof top temperature of a cell with mixed land usage i.e. equally distributed roads and roof tops, is best represented by the average temperature of roads and rooftops.

Use of canyon model restricts representation of green roof building, though it provides a realistic representation of natural conditions. For example, this model does not account for the effect of building for that part of building which has green roofs. Therefore, to compensate for the effect of building this model considers only half roof in the simulation as green. Further, building height is a vital parameter in simulation model. The variations in the height of buildings are well incorporated in the model which is represented by canyon with different height. Results of simulation studies suggest that representation of building height effects model results; therefore, one needs to carefully estimate the building height using reliable data sources.

Land use data for a given city is obtained from survey maps and local data sets. These data are used for calculating share of green roofs, concrete, asphalt, natural vegetation, and building for each computation cell i.e. per square kilometer. This method assumes equal share for roof and road for each given computational cell. Green roofs on the other hand is assumed from 5% of the total land and is distributes evenly. Further, a decrease in land usage is assumed going out of city especially near the boarders. Despite detailed layout of land usage, one needs to carefully apply land usage to reduce uncertainties.

It can be concluded that there is limitation in representing the features of a city using the mesoscale model, which is an inherent modelling drawback. Nevertheless, this model incorporates rural environment and solves circulation patterns that determines daily weather conditions. Results of research studies suggest existence of similar temperature above and at the urban canyons. However, it is unscientific to generalize such results; nevertheless, it can be concluded that similar temperatures exist above surface and at lower boundary layer.

MC2 model uses three nested domain, generally comprising of 151 x 151 grids where each grid represents 635 km2. A nested grid is the used for representing internal geographical feature where each grid would represents 25 km2. For example, consider a nested grid representing Great Lake; further, another inner domain is used for representing city area with a resolution of 1 km2. Large number of computation cells consumes more CPU time; therefore, a compromised is made on number of grid cells such that enough precision can be achieved at the same time enough resolution is achieved. Vertical resolution of 10 m is maintained in the lower five layer, which stretches to 8000 m approximately. A time scale of 2 to 3 days can be selected with variable boundary conditions available for each 6 hour. The larger domain results are used for updating inner boundary conditions.

Results of a study are discussed here, which simulated the urban heat island for the city of Toronto. Simulation was run the month of May, June, and August. Simulation results satisfactorily match with actual observations from the Buttonville and Pearson airports. Surrounding region and the Toronto city experience warmer temperature due to synoptic pattern. Fig 16 shows that except for geographical locations such as lakes most of the area experiences warm temperature. The city of Toronto is within the blue boundary, as represented in the Fig 16. However, green vegetation towards south reduces the temperature when moving from north to south.

Green roof Royal-5

Fig 16 Wind and temperature distribution of the city domain.

Availability of moisture in the soil limits the daily evapotranspiration; therefore, model was simulated for irrigated and non-irrigated roofs. This helps to better understand the effect of green roofs on the cooling of surrounding area. Results indicate that distinct cooler temperature was observed in the lower layer with green roofs, where a temperature difference of 1 to 1.5°C was observed. Further, much more cooling was achieved with irrigated roofs, where temperature of lower layer showed decrease of 2°C.

6. Discussions

Green roofs perform better than the traditional roofs as they utilize benefits of soil, water and plant. Detail discussion is provided here, to understand the thermal system of green roofs. Two major phenomenon are discussed here i.e. heat storage and heat transfer with respect to the properties of soil.

Thermal mass of materials determines its ability to store heat. Green roof technology uses soil and water, and both the materials have high thermal mass. Therefore, green roofs help regulate temperature of building by storing heat in soil and water. Fig 17 shows various means of heat transfer from green roof. Soil transfers the heat using four phenomenon a) conduction, b) convection, c) radiation, and evapotranspiration.

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Fig 17 Different modes of heat transfer from green roof installation.

Soil helps the process of conduction and water present in the soil transfers the heat using the process of convection. Convection is the process in which liquid transfers heat from one point to another point. Therefore, roof-temperature is regulated by the moving water and drainage system within the vegetation layer of soil. Further, blowing wind also helps in regulating temperature of green roofs. Similarly thermal conduction of soil also regulates the temperature of roofs. Higher thermal conductivity prevails in materials having higher thermal mass. Soil is one such material which have higher thermal mass and hence possesses higher thermal conductivity. This helps in conduction of heat from material at higher temperature to region with lower temperature through the process of conduction. Since, the thermal conductivity governs rate of heat transfer, a material with good thermal conductivity such as soil is suitable for regulating heat in and out of buildings.

In addition to conduction and convection, radiation, evapotranspiration and thermal mass help in the process of heat flow or heat transfer. Radiation is defined as electro-magnetic flow of heat from hot materials to cold materials. Different type of roofs absorb and emit solar radiation by the process of radiation too i.e. green roofs emit heat radiation in the form of long heat waves that flows into atmosphere. Further, the process of evaporation helps in cooling the temperature of soil. In the process of evaporation heat is absorbed from the soil and it is used for converting the water or moisture into gas. In addition, plants absorb water through roots and transpire it to atmosphere through the process of evapotranspiration that further reduces the temperature of green roofs. Thus, discussion in this section makes it clear that soil plays an important role in governing heat flow; the next section discusses major characteristics of soil suitable to green roofs.

6.1 Heat Transfer in Soil

Soil facilitates plantation of vegetation on the green roofs. It comprises of solid material, air and water, which makes it a porous medium. Amount of heat transfer through soil is governed by multiple factors such as water content, particle size of solid matter, composition of minerals in the soil, bulk density of soil, and its temperature. Researchers have conducted in depth research on soil. Following are the main findings on the parameters that govern thermal conductivity of soil:

  • Water and minerals in the soil are efficient medium for heat transfer i.e. they are even better mediums than air.
  • Dry soil transfers heat through point of contact. Wet soil are better thermal conductors than dry soil as water increases the surface area of soil particles, which increases the point of contact. Therefore, water in soil increases its thermal conductivity.
  • Rate of increase in thermal conductivity of soil slows down after a threshold value, as increasing water content in soil does not add to thermal conductivity of soil.
  • Heat energy does not travel through shortest available path in the soil layer, but it travels through the path which has least resistivity i.e. the path having greatest thermal conductivity.
  • Thermal conductivity of soil is not constant; instead, it depends on the local geological conditions and soils™ composition.

6.2 Soil for Green Roof

Soil used for green roofs have different thermal conductivity than naturally occurring soil. Therefore, it is important to specify standard properties of soil that can be used in green roofs. According to researchers the green roof soil can be made of three main components namely, sand, compost, and inorganic aggregates. The inorganic aggregate is light in weight which keeps the green roofs lighter and prevents higher pressure due to presence of soil on roof tops. In addition, these light-weight inorganic aggregates can hold larger amount of water. For example, expanded shale is one such aggregate that is lighter in weight and holds water. Soil selection for green roof aims to address two key issues i.e. less mass and more water holding capability. In this respect, peat soil provides an excellent choice for organic matter as it is lighter in weight. Efficient water drainage feature of green roof soil helps during heavy rains and storms. Though, water drains faster through green roof soil but the soil retains sufficient water between its pores to regulate thermal heat flow.

6.3 Resistance of Green Roof Soil

Thermal resistance of soil depends on the moisture content. Thermal resistance of soil is an important characteristic as it determines the thermal conductivity. Thermal conductivity of dry soil is 0.18 W/ mK. On the other hand thermal conductivity of soil with 82% moisture is 0.41 W/mK. Similarly, a soil with 17 and 33 percent moisture content have thermal conductivity of 0.26 and 0.30 W/mK. Therefore, it can be concluded that the thermal conductivity of soil increases with increasing water content.

Based on previous research works sample No DH08 is best composition for green roof soil. This soil sample has shale, compost, and sand with 75%, 10%, and 15% share respectively. This sample was tested and found to be best soil for green roofs. Table 1 lists the thermal property of DH08.

Table 1 Thermal property of DH08 soil sample suited for green roofs with different moisture content.

Water Content (%)

0

17

33

82

Thermal Resistance (W/mK)

0.18

0.26

0.30

0.41

7. Comparison between Green, White and Black Roofs

Research finding indicates that black roofs (i.e. low reflectance roofs) add to urban heat and water runoff. Consequently, there is growing attention towards white and green roofs which have more reflection capabilities. This section compares seasonal and thermal performance of three type of roofs namely, a) Green Roof, b) a black traditional roof, and c) white roof with high reflectance membrane. In this regard, surface temperature of these roofs are compared and analyzed to compare their performance.

7.1 Measurement tools

First, selected instruments were installed on three roofs to monitor and collect desired data. A computer logs all the data securely from the installed instruments. Fig 18 show the test roof with three types of roofs and installed instruments. One instrument known as radiometer measures solar radiations i.e. net flux that roof absorbs. The second instrument measures humidity, temperature, and wind speed and direction i.e. it is a mini-weather station. The other roofs namely black and white roofs are made of EPDM (Fig 19); it is a material that provides resistance to the UV rays. The reflective membrane of white roof is capable of reflecting 80% of the solar radiations. Two set of thermistors were installed along the upper and lower layer of roofs to measure actual temperature gradient generated for the flowing heat in and out of the building.

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Fig 18 Instruments installed on the green roof.

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Fig 19 Roof sensors installed on the white and black roof.

7.2 Comparison of Temperature

First, temperature of roof membranes are presented and compared in the Fig 20. This temperature is measure of temperature at the contact surface i.e. for green roof it is measure of temperature below the vegetation layer; on the other hand, for black and white roofs it is measure of surface temperature.

Green roof Royal-11

Fig 20 Comparison of temperature of three roofs i.e. green, white and black roofs; indicated by green, white, and black lines.

Upon comparing the peak surface temperature one notices that maximum temperature of 80°C was recorded on the black roof. Contrastingly, the peak surface temperature of white roof was lower by 17°C on an average. Further, the green roof reported least peak temperature that was lower on an average by 33°C when compared to the black roof. Further, during the night the surface temperature of white and black roof were observed to be much below the air temperature. This colder roof temperature may be due to efficient convection and radiation from the white and black surface. On the other hand, green roof showed median decrease in the surface temperature due to presence of vegetation. Further, there is no cooling due to latent heat as there was no water on the smooth white and black roofs. Similarly, during winter time temperature of black roof reported -20°C. On the other hand, the green roofs maintained a temperature above freezing point due to presence of thick vegetation layer which prevented heat loss during winter. Thus, results indicate that black roof underwent extreme temperature changes due to expansion of surface membranes reporting maximum and minimum temperature during summer and winter respectively. Therefore, it can be concluded that green roofs perform much better than black and white roofs during summer and winter i.e. outperforming other roofs by a factor of 2 or 3 time.

7.3 Comparison of Heat Fluxes

It is vital to compare the energy benefits that can be achieved by replacing the traditional roofs with green or white roofs. In this regards, this section compares the heat flux of black, white and green roofs. Modelling of heat flux through the roof is a complex process and regardless of amount of heat being transferred in and out of building, the temperature gradient on the roof determines amount of heat flowing in and out of building. Therefore, it is efficient to compare the temperature gradient generated over the green, white, and black roof. Thus, by comparing the temperature gradient above and below the three type of roofs one makes the comparison process efficient.

Green roof reported least heat flux losses due to radiation and convection; while, the black roof reported maximum heat loss. Approximately, there was 37% decline in heat loss in green roof when compared with black roof. Upon comparing heat loss per unit area, one noted that during winter green roof lost 5 W/m2 while the white roof lost 6.10 W/m2, and black roof lost 6.3 W/m2.Thus, results show that on an average white and black roofs lost same amount of heat during winter. This is in contrast to their performance during summer where white roofs are cooler than black roofs. This under performance of white roof during winter is called winter heat penalty. This phenomenon can be explained with the help of emissivity value. Emissivity coefficient is measure of emission capability of a material i.e. how efficiently a given material emits heat radiation. It has been observed that emissivity coefficient ranges from 0 to 1, and a perfect emitter has value of one. In this regard, organic matters are considered to be good emitters of heat waves and their emissivity coefficient ranges from 0.90 to 1. On the other hand emissivity values of metals are lower in the range of 0.1 to 0.3, which makes them excellent cooking wares. Thus, ideally black roofs should have higher emissivity than white roofs. However, manufactures of white roof reported that the material used on white roof has higher emissivity in the range of 0.90-0.95. Therefore, higher emissivity coefficient of white roof explains heat loss during winter. These findings are further supported by the readings of radiometer that measures emissivity. Reading of radiometer show that black roof reported emissivity of 0.45 to 0.50, while white roof reported higher emissivity of 0.90.

7.4 Comparison of Seasonal Energy Cost

One can compare seasonal energy cost of three types of roofs i.e. green, white and black roof by converting the heat losses and heat gains into energy saving to provide air conditioning. In this regard, a uniform area of 1000 m2 was considered for the three roofs. This analysis compares the cooling cost of summer and heating cost winter. Heating cost was estimated assuming the building uses natural gas and heating fuel to provide heat during winter. Similarly, it was assumed that cooling during winter was provided using electric air conditioning facility.

Table 2 and 3 show winter and summer cost estimation for the three type of roofs. It clearly indicates that during winter black roofs require 21800.89 KWh of energy for heating that consumes $940.47 worth of natural gas; on the other hand, green roof require lesser heating and consumes 13838 kWh of energy that costs only $ 599.22. Similarly, during summer black, white, and green roof require air conditioning which costs $267.77, $ 72.40, and $ 42.20 respectively. Therefore, it can be concluded that both during summer and winter green roof reduces the cost of cooling and heating in comparison to white and black roofs.

Table 2 Winter heat energy analysis and heating cost for 1000 m2 roof.

 

Black Roof

White Roof

Green Roof

Natural Gas (kWh)

21800.89

19124.58

13838.45

Cost ($)

940.47

835.70

599.22

Heating Oil (kWh)

15923.80

14017.60

10205.63

Cost ($)

876.34

775.60

566.72

 

Table 3 Summer air conditioning energy analysis and air conditioning cost for 1000 m2 roof.

 

Black Roof

White Roof

Green Roof

Cooling Energy (kWh)

1488.16

402.55

234.57

Cost ($)

267.77

72.40

42.20

Table 4 summarizes the annual cost of heating a building with a roof of 1000 m2. Black, white and green roofs annually require heating worth $2901.32, $2547.78, and $2002.61, respectively. Further, annual cost of cooling the same building with black, white, and green roof is $294.21, $95.01, and $38.08, respectively (Table 5). Therefore, it is clear that green roofs perform better than black and white roofs and involve less cost for heating and cooling.

Table 4 Annual heating cost for 1000 m2 roof.

 

Black Roof

White Roof

Green Roof

Natural Gas (kWh)

34854.70

29435.33

23860.44

Cost ($)

1500.98

1276.33

1043.61

Heating Oil (kWh)

25455.60

21500.54

17346.09

Cost ($)

1400.34

1271.45

959.00

 

Table 5 Annual air conditioning cost for 1000 m2 roof.

 

Black Roof

White Roof

Green Roof

Cooling Energy (kWh)

1232.77

657.74

243.57

Cost ($)

294.21

95.01

38.08

8. Conclusion

Green roofs are growing alternative to traditional roofs, as they help in management of storm water, reduce energy consumption, clean air, and add to the aesthetic look of a city. Nevertheless, technology of green roofs is at early stages of its development and much need to be done. Therefore, researchers and engineers study different aspects of green roof infrastructure to increase its performance under different climatic conditions.

Green roofs™ thermal performance has improved over the year with development and research activities.In this regard, many studies have investigated the thermal performance of green roof for different climatic and local conditions. It has been successfully proved that the green roofs reduce the energy requirement for heating and cooling. Both experimental and simulation models are used to study and investigate thermal performance and efficiency of green roofs. It has been found that a green reduces heat flow from a building by 8.2%; and it reduces energy requirement for cooling by 75%. In this way, green roofs save money with respect to traditional sources of energy. Further, simulation study suggests that in a given city 2°C cooling can be achieved by covering half of the city buildings with irrigated green roofs; and cooling of 0.5°C can be achieved by covering half of the city infrastructure with dry (not irrigated) green roofs. Green roofs serve public in following manner:

  • The green vegetation help in retaining storm water, which reduces overflow of city drainage at time of heavy rainfall.
  • In addition, green roofs reduce the amount of heat being absorbed by roof, which is achieved by reducing direct exposure of given surface to sun.
  • Further, green roofs increase life of roof and reduce wearing of roof with time.
  • Furthermore, green roofs reduce the air conditioning cost in big buildings. This is due to presence of additional insulating layer.
  • Aesthetically, green roofs are much superior to white and traditional black roofs. Intensive type of green roofs provides roof top parks where public and building dwellers can spend time on recreational activities.
  • Vegetation on roof top also help in cleaning air of city. This is especially useful means of plantation in area with almost reduced air quality and negligible ground level area for planting vegetation.

In this study we highlight different type of green roofs and provide detailed information on different instruments used for measuring thermal flow and other related meteorological parameters. Research studies have analyzed the heat flow through various roof configurations; and results suggest that growing medium and different layers of a green roof installation plays vital role in retention of water and regulation of building temperature. Further, it was found that thermal properties of green roofs are superior to the traditional roofs.Growing medium selection and composition is important for healthy growth of plants and proper functioning of green roofs.

Soils™ thermal performance depends on its particle size, moisture content, density, composition of mineral, and its temperature. The green roof soil consists of three main component namely, sand, compost, and inorganic aggregates. The inorganic aggregate is light in weight which keeps the green roofs lighter and prevents higher pressure due to presence of soil on roof tops. Green roofs reduce the heat loss due to convection and radiation. For example, a 37% reduction in heat loss can be achieved with green roofs in comparison to the black roofs. Analysis shows that heat loss per unit area during winter from the green roof is 5 W/m2, while the white roof loses 6.10 W/m2, and black roof loses 6.3 W/m2.

Builders should plan roof tops properly because roof tops increase the temperature of UHI, and it also radiates more heat into the building. This increases the room temperature inside the buildings, especially in the top floors. These temperature variations increases the inmates discomfort level and in some cases causes heat and respiratory problems. Research work also indicates that UHI of city can be reduced by employing green roofs. The area of urban heat island spreads over two to three times the size of city in the horizontal direction. UHI and other atmospheric process interact among themselves and modify each other.In this regard, green house can help in regulating and reducing UHI because greener roof installation will ensure reduced temperature in and around building.

Thus, this study concludes that green roof installation should be encouraged and incorporated in bother residential and commercial buildings. In this regard, builder and stake holders need to properly select suitable site for installing green roofs. The site selection criterion includes availability, function, and cost of installation. Green roofs may require maintenance from time to time depending on the density and type of vegetation; the life of green roof is larger than regular black roofs. In addition, precise installation of various membranes of green roof, ensure a life of 30 to 50 year.