Thermal Performance Analysis of Rooftop Greenery System in

subtropical climate of Australia

ANWAR, M., RASUL*, M.G. & KHAN, M.M.K

School of Engineering and Technology

Central Queensland University

Bruce Highway, Rockhampton, Queensland

AUSTRALIA *Email: m.rasul@cqu.edu.au

Abstract: - Green roof application on real commercial building in Australia is very limited and mostly

concentrated in major cities mainly for visual purposes. There is not enough research has been conducted to

boost up the benefits of green roof system in hot and humid weather in Australia. Before implementing the roof

top greenery systems, it is really important to investigate its thermal performance which can be obtained by

experimental data collection and analysis during different seasons so that seasonal variation can also be

observed. An experimental study of an extensive green roof is presented in this paper. The paper investigates

the thermal performance of a green roof and without green roof over a 6 meter long steel structured standard

shipping container which locates in Central Queensland University, Rockhampton campus, Australia. A

temperature difference of up to 4 oC was observed in a typical summer (February) day.

Key-Words: - Green Roof, Shipping container, Extensive, Thermal, Subtropical

1 Introduction Rooftop Greenery System is a promising technology

that has been used in different countries to improve

thermal performance of building. The system is

made up of suitable plants planted in soil having

several layers and then affixed in a structure. It acts

as an insulation barrier, reducing the heat flux

through the roof. There are many advantages of

using rooftop greenery systems in buildings such as

lower energy consumption, cooling of buildings in

summer and longevity of roof and energy efficiency,

mitigation of urban heat island effect, retention of

the storm-water runoff from roof surface, air

pollution and noise.

A green roof or rooftop greenery system is

layered system consists of a waterproofing

membrane, growing medium and the vegetation

layer itself. Green roof also include a root barrier

layer, drainage layer and an irrigation system

depends on the climate [1]. There are two main

classifications of green roofs: intensive and

extensive. These two are appropriate for different

purposes. Intensive green roof can support plants

and bushes as it has a deep soil layer but requires

maintenance in the form of weeding, fertilizing and

watering. Generally intensive green roof can support

complex vegetation like groundcovers, small trees

and shrubs which has deeper rooting. Intensive roof

is generally heavy weight (more than 150 kg/m2)

and have more than 200 mm of substrate with

higher amount of organic material than extensive

systems. It requires strong roof structure and that’s

why it is very hard to retrofit in existing building.

Intensive green roof is relatively expensive than

extensive system. Extensive green roof has a thin

substrate layer (up to 100 mm) with low level

planting, typically sedum or lawn. Extensive roof is

expected to be self sustaining and minimal

maintenance required. It can be distinguished by

being low cost and lightweight (50-150 kg/m2).

This paper investigates the practicality of using a

green roof in Rockhampton, Queensland by

comparing a building with and without a green roof.

This was done by creating an experimental bed of

green roof over a standard 6 meter long shipping

container. Two containers are used for this

comparison-one with green roof and other without

green roof. Both are well decorated, insulated and

can be used as normal office space. A window type

air condition is running in both containers for 24

hours at a set temperature (24°C) to keep the interior

temperature under control.

2 Design of Extensive Green Roof Designing a green roof in subtropical climate is not

easy task as limitation of both researches and

resources. Although the extensive green roof is the

most fundamental form of Green roof, Williams [2]

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showed a poor uptake of extensive green roof

technology in Australia due to technical difficulties

for growing plants in shallow substrates in a dry and

variable climate. Figure 1 shows a typical design of

extensive green roof.

Fig. 1: Typical green roof system [3]

For successful green roof setup, several factors

considered were as follows:

2.1 Site Selection The experimental investigation has been conducted

in an experimental facility located at the Central

Queensland University, Rockhampton.

Rockhampton is the capital city of Central

Queensland, situated 40 km inland from the Coral

Sea. It is located at 23°22'S, 150°32'E and has a

population of 70,000 people [4]. Rockhampton,

being on the Tropic of Capricorn experiences a

Subtropical Climate with an annual average rainfall

of 800 mm [4]. It has a usual wet season of

December to March and a dry season between June

and September. Temperature in summer ranges

from 32°C- 22°C and 23°C- 9°C in the winter [4].

Both mean maximum and minimum temperature

has been computed from average of maximum and

minimum outdoor temperature recorded by the

Bureau of Meteorology (BOM) in Rockhampton of

Australia. Figure 2 shows the temperature profile of

Rockhampton during 2009-2011. It has been

demonstrated that the variation between maximum

and mean minimum temperature is around 6-10°C

from January to April, 8-14°C from May to

September and 7-10°C for October to November.

2.2 Experimental Setup Two standard 6 meter long shipping containers are

used for this experiment. Both containers are

identical and used as normal office space. One

container has green roof and other one is bared roof.

The dimensions of both containers are 6 meter long,

2.44 meter wide and 2.44 meter high. The roof area

of the container is approximately 15 m2 with a 3

degree pitch. The schematic diagram of the

experimental green roof is shown in Figure 3.

Fig. 2: Temperature profile of Rockhampton for last

couple of years

Fig. 3: Schematic diagram of experimental green

roof

The site of the facility is on open and flat ground,

therefore has minimal shading effect. The location

of the facility is convenient to the main water and

electricity supply. The nearest BOM station is

around 13 km away from the location of the facility.

Both the containers have single sliding door and

window. Dimensions are 2133 mm x 1828 mm and

914 mm x 914 mm respectively. A 5 mm dark grey

float toughened glass with aluminum frame has

been used for both door and window. Australian

Standard handrail with stairs is used to ensure safety

while working on the rooftop. Figure 4 shows the

experimental setup along with handrail and stairs.

2.3 Extensive Green Roof Green roof system of the experimental set up

consists of the following materials, used layer by

layer as follows:

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2.3.1 Roof Structure A corrugated steel sheet is affixed with the container

roof ensuring water runoff at a 3° angle. A gutter is

used to collect the runoff water. 230 mm galvanized

aluminum bracket has been used for holding the

green roof materials.

Fig. 4: Experimental setup

2.3.2Waterproofing membrane

GRT HP 700 is used as waterproofing membrane. It

is a single component, UV stable, polyurethane

waterproofing membrane system. It forms a

seamless, durable and flexible membrane which

protects structure from water penetration. One 22 kg

pail of GRT HP 700 covers approximately 15 m2

based on a dry film thickness of 1.5 mm.

Waterproofing membrane is applied in Figure 5.

Fig. 5: Waterproofing membrane applied on

corrugated rooftop

2.3.3 Drainage System

Elmich Versicell® lightweight drainage module has

been used. High strength and interlocking modules

not only captures high volume water but also

protects waterproofing membrane. Elmich

Versidrain® 25P drainage sheet is also used for its

cost effective water management purpose. High

capability of water storing into the cells coupled

with high discharge capacity which ensures

effective capillary irrigation. It also reduces

irrigation frequency and minimizes fertilizer runoff

and usage.

2.3.4 Filter

A needle punched geotextile filter fabric is used to

prevent fine particles of growing media entering

into the drainage layer which shows in Figure 6.

Typically it is made from polypropylene or

polyester.

Fig. 6: Geotextile filter fabric on top of drainage

system

2.3.5 Growing Media

Enviroganics® Bioganic Earth substrate has been

used. It is lightweight, natural, biological organic

based product. It is a combination of long lasting

sustainable media that does not slump and remain

robust for 2 years. It has high water storage capacity

components such as peat moss, composted sawdust,

coco peat, washed sand, fertilizer and water

retaining crystals. Substrate dry weight is 300 kg/m3

and fully irrigated green roof weight is 450 kg/m3

[5].

2.3.6 Green roof Plants

In subtropical climate, green roof plant should

tolerate harsh sun and hot temperatures along with

humidity and moisture. As very limited research

resources are available for extensive green roof in

subtropical climate, a variety of local native plants

have been chosen. Such as Rhoeo, Scaevola,

Grevillea Obtusifolia, Helichrysum Italicum,

Callistenon Captain Cook, Dianella little jess,

Eremophila Maculata both purple and aurea. All

those plants shown in Figure 7 are planted on early

November 2012. There is a time controlled

irrigation system which ensures everyday garden

watering once. This experiment is continued for last

three months during summer. Figure 8 shows the

green roof at the end of January 2013.

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Among all the plants Rhoeo, Helichrysum Italicum

and Eremophila Maculata are doing really well.

Scaevola and Dianella little jess got nice canopy but

not suitable for this harsh summer. Some leaves are

dried and growth very slow in summer. Callistenon

Captain Cook is stable but slow grower and has

smaller canopy. Grevillea Obtusifolia is totally dead

and not a green roof plant at all.

Fig. 7: Variety of green roof plants

Fig. 8: Present condition of green roof (31st January

2013)

2.4 Instrumentation and Data collection The data has been collected from both containers

(green roof and non green roof) during December

2012 to February 2013. Both temperature and

relative humidity data has been collected from

inside of the containers at 10 minutes intervals.

Following tools have been used in the experimental

facility to record data.

2.4.1 HOBO U10 Temp & RH logger

HOBO U10 temperature data logger is used for

indoor temperature and relative humidity

monitoring. It is small size and can store large

amount of data. Each container got one HOBO

logger.

2.4.2 BTM-4208SD 12 CH Temperature

Recorder

12 Channel temperature recorder has been used to

measure surface temperature of the green roof

structure. K type different shape probes have been

used to measure temperature at different layers.

3 Results and Discussion Due to the geographic location of Rockhampton, the

difference between indoor and outdoor temperature

is relatively small in winter but varies in summer.

The average indoor temperature profile for both

green roof and non green roof is shown in Figure 9,

which illustrates that the temperature varies from

25° C to 27° C for green roof and 25.5°C to 29° C

for non green roof. The test was conducted while a

box type air condition (power input 1.91 KW and

capacity output 5.70 KW) set at 24°C constantly.

The mean maximum outdoor temperature was

32.1°C and mean minimum outdoor temperature

was 21.2°C [4]. In Figure 9, the highest temperature

difference occurs in 4th and 5

th December as

maximum outdoor temperature was 39.1°C and

38.3°C respectively [4].

Fig. 9: Average temperature difference between

green roof and non green roof in December 2012

Indoor humidity level for green roof varies from

45% to 75% whereas non green roof varies from

37% to 73 % in summer. Figure 10 shows the

average humidity level for both green roof and non

green roof in December 2012. The humidity of

green roof is higher than non green roof container.

At some points humidity levels are very close due to

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rainfall. The average outdoor humidity maximum

and minimum in December was 90% and 43% [4].

Fig. 10: Average relative humidity difference

between green roof and non green roof in December

2012

The temperature profile for a heavy wet day (25th

January, 2013, 348 mm) has shown in Figure 11.

Although air condition had been set for 24°C, the average indoor temperature difference was almost

0.5°C. The outdoor maximum temperature for

that day was 27.5[4].

Fig. 11: Temperature profile of Green roof and non

Green roof on a heavy wet day, 25 January 2013

The indoor relative humidity ranges from 72% to

91% which is shown in Figure 12. An average of

85% humidity level was recorded in both containers.

The indoor temperature and humidity of both green

roof and non green roof containers without the air

condition has been recorded. Figure 13 illustrates

the changes in temperature of green roof and non

green roof alters at certain time of the day.

Fig. 12: Relative humidity profile of Green roof and

non Green roof on a heavy wet day, 25 January

2013

Fig. 13: Temperature profile of green roof and non

green roof without air condition in a typical summer

day

Temperature of green roof is higher than non green

roof from 12:00 am midnight to 8:40 am in the

morning. The difference is ranges from 0.5°C to

0.7°C. When sun rises in the morning, the non

green roof starts to warm up. Due to the high

thermal conductivity of steel, it not only get hot

quickly and but also cold down fast. The green

roof plant absorbs significant quantities of solar energy through biological function; the remaining

solar radiation that would affect the internal

temperature of the building is much less than that of

non green roof [6].

From 8:50 am to 7:50 pm, non green roof

absorbs much more solar energy than the green

roof. Although the maximum temperature of that

day is 31.7°C [4] but non green roof got maximum

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42.75°C which indicates direct solar energy

absorption. The maximum temperature difference

occurs at 3:10 pm which is 4°C. From 8:00 pm the

non green roof starts getting cooler than the

green roof and maximum difference of

temperature was recorded 0.8° C. Relative humidity profile of green roof and non

green roof is shown in Figure 14 which illustrates

relative humidity inside the green roof container is

much higher than non green roof container.

Fig.14: Relative humidity profile of green roof and

non green roof without air condition in a typical

summer day

4 Conclusions and Recommendation Rooftop greenery systems for buildings represent a

promising technology to provide multiple private

and public benefits. It can pay back its costs in

energy savings alone and thus earn money. The

main purpose of this study was to determine the

thermal performance of a green roof over a non

green roof structure. Any indoor temperature

difference may reduce air conditioning electricity

consumption. The recorded data may vary in

different areas of Australia due to variation in solar

radiation and weather condition. The result

demonstrates the effectiveness of extensive green

roof for cooling energy saving and reducing indoor

air temperature in subtropical climate of Australia.

For any typical summer day in Rockhampton, the

difference in indoor temperature of green roof and

non green roof ranges from 0.5°C to 4°C. Any

single indoor temperature reduction may save

thousands of dollars over the year.

To draw a conclusion based on one month

summer data is not enough. For detailed

analysis over the year (all season) data is

required. Still it is a good understanding of

thermal performance of green roof. As both

containers are identical and made of steel; a fair

thermal profile has been recorded. Although

both containers are shaped as office space, but

in reality there is no occupancy at any time. To

understand the thermal comfort, more

temperature and humidity data points are

required. Modelling and simulation should also

be done in order to extensively study the

applications of roof top greenery system. The model can be created in Designbuilder [7] and

validated with the measured performance. Authors

are working on this.

References:

[1] M. Anwar, M. G. Rasul and M. M. K. Khan,

Rooftop Greenery Systems in Subtropical

Climates for Environmental Sustainability: A

Review, International Conference of WREC-

Asia & SuDBE2011, Chongqing, China 28-31

October 2011.

[2] N.S.G. Williams, J.P Rayner, Green roofs for a

wide brown land: Opportunities and barriers for

rooftop greening in Australia. Urban Forestry

& Urban Greening 9, 2010, pp. 245-251.

[3] M. Anwar, M. G. Rasul and M. M. K. Khan,

Green roofs for storm water management: a

review. J Chongqing Univ: Eng Ed [ISSN

1671-8224], 2012, 11(1): 5-11.

[4] Bureau of Meteorology, (BOM), Australia,

Climate data online retrieved from

http://www.bom.gov.au/climate/data.

[5] Bioganic Earth Podium Indoor (2013),

retrieved from

http://practicalaquaponics.shopfactory.com

[6] M. Anwar, D. Steele, M. G. Rasul and M. M.

K. Khan, Performance Assessment of Rooftop

Greenery System in an Institutional Building in

Subtropical Climate in Australia, Recent

Researches in Environmental and Geological

Sciences, Proceedings of the 7th

IASME/WSEAS International Conference on

Energy and Environment (EE’12), pp. 127-132,

14-17 July 2012, Kos Island, Greece.

[7] DesignBuilder Software Homepage, Retrieved

from http://www.designbuilder.co.uk.

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Relative humidity profile of Green roof and Non

Green roof at 9th February 2013

Green Roof

Non Green

Roof

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