THERMAL INSULATION PERFORMANCE OF GREEN ROOF SYSTEMS

THERMAL INSULATION PERFORMANCE OF GREEN ROOF SYSTEMS

1

Mechanical Engineering, Southern Illinois University Edwardsville, USA, e-mail: scelik@siue.edu

2

Civil Engineering, Southern Illinois University Edwardsville, USA, e-mail: smorgan@siue.edu

3

Biological Sciences, Southern Illinois University Edwardsville, USA, e-mail: wretzla@siue.edu

4

Mechanical Engineering, Southern Illinois University Edwardsville, USA, e-mail: oonce@siue.edu

ABSTRACT

This study is on the theoretical and experi mental analysis of thermal insulation properties of different green roof systems based on experi mental measurements. Tes ts were carried out on 12 different green roof samples, invol ving four types of growth media (lava, arkal yte, pumice , haydite ) matched with three sedum types (kamtc haticum, spuri um, s exangulare). Temperature readi ngs at the bases of each s ample were rec orded for three years continuousl y at every 15 minutes. The data was processed and the i nsul ation behaviors of each green roof application were anal yzed using relevant theory. The base temperature variations among the tested samples showed that the right selection of growth media and vegetati on can yield significant energy savings fo r air-conditioning applications.

Keywords: Green roofs, thermal insulation, air-conditioning, energy savings. INTRODUCTION

In recent years, building insulation has become more significant as energy costs have been increasing continuously. Parallel to the rising costs, insulation technologies have been improving, as well. Today, most of the insulation materials available in the market are synthetic. However, there also exists a natural insulation technique that is sustainable and environmentally friendly. This new revolution is called the green roof technology, where the roofs of building envelopes are covered with plants. This application is considered to deliver several benefits such as reducing heating and cooling energy costs, decreasing storm water run-off, filtering pollutants and CO2 out of the air, decreasing the heat-island effect in highly populated cities, and increasing the lifespan of roofing materials.

One of the main advantages of green roofed buildings is the energy savings due to the reduction in required heating and air-conditioning loads in winter and summer seasons, respectively. Thermal benefits of green roof systems have been covered in numerous studies (Eumorfopoulou and Aravantinos, 1998; Feng et al. 2010; Liu, 2003; Niachou et al. 2001; Palomo and Barrio, 1998; Sidwell et. al., 2008), experimentally. Another study involves numerical modeling investigating the green roof system in a dynamic state with a uni-dimensional analysis using the finite differences method. (Lazzarin et al., 2005). A mathematical model was also developed for evaluating cooling potentials of green roofs (Kumar and Kaushik, 2005). An energy modeling study investigated all forms of heat transfer including radiation effect (Gaffin et al. 2005). The suggested model used a control volume approach based on the finite differences method. Different case studies from around the world, focusing on the temperature fluctuations and energy savings, have also been presented (Spala and Bagiorgas, 2008; Teemusk and Mander, 2009). Evaporative cooling aspects of green roofs have also been analyzed (Onmura et al. 2001). Economic analysis of green roofs is also crucial. Financial observation of green roof systems along with the effect of this newly technology on the local economy of a particular region was studied (Celik et al. 2010). The study not only focused on the cost of these systems, but also the impact of this application on the local economy including employment analysis for a period of 10 years.

Although many aspects of the thermal benefits of green roofs have been evaluated recently, not much has focused on the effects of the types of vegetation and the resulting potential benefits. In this study, experimental data of different vegetation types with varying growth media are compared in terms of their isolative behavior and energy savings.

HEAT TRANSFER

For the heat transfer analysis, the plant canopy and the growth medium regions can be combined to form a single domain. This roof model is illustrated in Fig. 1. An energy balance can be defined between the plant canopy-growth medium coupling and the remaining roof layers. This analysis involves radiative and convective heat transfer to/from the upper surface of the vegetation, and conduction through the coupled system and the below roofing system layers such as the roof membrane, insulation, and lumber.

233 Fig. 1. Coupled green roof model.

Total net radiation heat transfer to the green roof sample is

)

(

)

(











A

G

G

G

A

T

T

q





where α is the absorptance, A is the exposed surface area, GND, Gd, and GR are the normal direct, diffuse, and reflected irradiations, respectively, ε is the emittance, σ is the Stefan-Boltzmann constant, Ts is the sample surface temperature, and T∞ is the outside ambient temperature. Convective heat transfer from the surface of the green roof sample can be calculated by

)

(





hA

T

T

q

Convective heat transfer coefficient, h, is a function of wind speed, domain geometry, and surface roughness. Conduction heat transfer through the green roof samples and the rest of the roofing materials is a function of the thermal conductuvity (k) and thickness (L) of each layer.

total r s conduction

R

T

T

A

q



(



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where Tr is the interior design temperature and Rtotal is the unit thermal resistance through all layers which is given by

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k

L

k

L

k

L

R

Based on the law of conservation of energy, assuming adiabatic side walls (neglecting heat loss/gain from the sides) and 1-D heat transfer from the upper surface to the base of the green roof system, one can obtain

convection radiation

conduction

q

q

q





(5)

Hence by obtaining varying daily outside temperatures, irradiation values and convective heat transfer coefficient, heat fluxes through different roof systems can be computed and compared for energy analysis. The results of this experimental study will be given in the results section.

Assuming equal conduction heat flux values due to same outside air conditions and geometric propoerties, one can compare the unit thermal resistance values through the coupled plant canopy-growth media employing

ij g p b s ij

R

T

T

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where Tb is the base temperature underneath the growth medium, Rp+g is the unit thermal resistance, and i and j are the indices referring to the type of the growth medium and the vegetation type.

EXPERIMENTATION AND PROCEDURE

Experimental setup involves green roof block samples with temperature probes and weatherproofed data loggers. Different growth medium and plant types yielding 12 combinations are listed in Table 1. The plants used in the experiments are illustrated in Fig. 2.

Table 1. Letter codes of tested samples.

Fig.2. Tested plants.

The temperature readings were obtained using HOBO TMC6-HD high accuracy soil/water probes and were recorded by HOBO U12-008 weatherproof outdoor data loggers. The accuracy of the probes is declared as ±0.25°C at 20°C by the manufacturer and the accuracy of the U12 data loggers is given as ±2 mV ±2.5% of the absolute reading. For the green roof system, probes were located underneath the growth media at the center of the aluminum trays. Data was collected continuously from each channel for three years, every 15 minutes. Each data logger can store 43,000 measurements which is equivalent to three months of data. Fig.3. represents the schematic of the experimental setup.

Growth Medium Vegetation Species

L – Lava S – Sedum spurium

A – Arkalyte K – Sedum kamtchaticum

P – Pumice A – Sedum sexangulare

235 Fig. 3. Schematic of the experimental setup.

RESULTS

To better understand the effect of growth media on the thermal performance of green roofs; density, porosity, and water holding capacity of the rocks used were determined. These values are listed in table 2 (values marked with an asteriks are obtained from pertinent literature). It was found out that red lava rocks have the highest density although they have porosity value close to that of pumice whose porosity is 80%. The reason for these two rocks having high porosity is due to them being volcanic rocks. During their formation, as they solidate rapidly, air pockets are formed within and around the rocks. On the other hand, the other two rocks, arkalyte and haydite, have comparitively low porosity values. These are clay based rocks. In terms of the water holding capacity, which is an important parameter due to it being related to evaporative cooling ability of the system, lava and pumice are again found advantageous over arkalyte and haydite due to their porosity values. In addition to the above mentioned propoerties, thermal images of the rocks were also captured using a thermal camera. The images represent the reflective properties of the growth media along with their thermal storage behavior. These images are illustrated in Fig. 4.

Table 2. Properties of growth media (values with * are obtained from literature). Growth medium Density (g/l) Porosity (%) Water holding

capacity (%)

Lava 1840.9 65* 26

Arkalyte 971.9 8.8 9.9

Pumice 641* 80* 30*

Haydite 1327.4 7.1 5.7

Fig. 4. Thermal images of four different growth media on the same day.

Raw data collected from the data loggers were processed. For comparison of base temperatures, August 2007 was selected among the 32 months of experimetation. In this month, all compared samples were fully vegetated hence plant health, which could be a significant parameter, was eliminated in the analysis. Fig. 5 illustrates the base temperatures of twelve different tested combinations on a selected day which is August 15th, 2007. A daily heat flux representation of three red lava applications with varying plant selections for the selected day is given in Fig. 6. In this figure, negative heat flux implies reverse heat leak during the night.

For comparison, ambient and reference surface temperatures for the same day are presented, as well (Fig. 7). Reference temperature values are of a black, non-reflective EPDM (ethylene propylene diene Monomer) roofing membrane which is very commonly used on flat roofs in most of the buildings in the midwest USA. Heat flux variation of the EPDM roof is compared with that of the green roof applications employing Sedum Spurium plants.

While lava seems to be the most advantageous, and EPDM the most energy-consuming, pumice and haydite applications seemed to be better than eachother during different parts of the day. This can be explained by the higher thermal storage capacity of hadite compared to pumice. Heat flux comparison of EPDM and three green roof samples are illustrated in Fig. 8.

237 Fig. 6. Heat flux values with lava samples.

Fig. 7. Ambient and reference surface (EPDM membrane) temperatures on August 15th_.

Fig. 8. Heat flux values of green roof samples vs. EPDM roof. CONCLUSION

Different combinations of growth media and sedum types in a modular green roof system were tested for comparing their insulative performance. The varying parameters of the tests were the growth media and the vegetation type. The results showed that the haydite and sedum sexangulaire combination (HA) had the best

insulating characteristics. The daily temperature difference between this combination and the inside air was the minimum among all combinations tested. On the other hand, at the end of the peak hours pumice rocks with sedum kamtchaticum (PK) and lava with sedum spurium (LS) showed a steeper cooling trend than the HA application due to their thermal mass being larger than that of the HA sample. One of the advantages of these two rocks over the other two rocks is that they have high porosity values (lava ~70%, pumice ~90%). However due to the alkaline nature of pumice, algae formation on the rocks fill the pores of the growth medium and hence the thermal performance of these rocks will decrease faster in time than that of others. Although arkalyte seemed to be coolest in terms of the thermal images, when it comes to thermal measurements of green roof samples with the vegetation, it did not show such a competitive performance. This could be due to the interaction of the plants with the growth media.

Although particular combinations could be selected among all samples due to their isolative performances, it was observed that the same type of growth media with a different type of vegetation can also yield significantly different results. Hence, further studies on the interactions of growth media and the plant roots should be conducted.

REFERENCES

Celik, S., W. A. Retzlaff, S. Morgan, A. O. Binatli, C. Ceylan, 2010, Energy evaluation and economic impact analysis of green roofs applied to a pilot region in Aegean coast of Turkey, Society for the Study of Emerging Markets – EuroConference 2010, Milas, Turkey, July 16-18.

Eumorfopoulou E., D. Aravantinos, 1998, The contribution of a planted roof to the thermal protection of buildings in Greece, Energy and Buildings, 27, pp 29-36.

Feng C., Q. Meng, Y. Zhang, 2010, Theoretical and experimental analysis of the energy balance of extensive green roofs,Energy and Buildings 42, pp. 959-965.

Gaffin S., C. Rosenzweig, L. Parshall, D. Beattie, R. Berghage, G. O’Keefe, D. Braman, 2005, Energy balance modeling applied to a comparison of white and green roof cooling effeiciency,Proceedings of the 3rd North American Green Roof Conference: Greening rooftops for sustainable communities, Washington D.C., pp. 583-597,

Kumar, R., S.C. Kaushik, 2005, Performance evaluation of green roof and shading for thermal protection of buildings”, Building and Environment, vol.40, pp.1505-1511.

Lazzarin, R. M., F. Castellotti, F. Busato, 2005, Experimental measurements and numerical modelling of a green roof, Energy and Buildings, vol.37, pp.1260-1267.

Liu, K., “Engineering performance of rooftop gardens through field evaluation”, National Research Council Canada: Institute for Research in Construction, 2003.

Niachou, A., K. Papakonstantinou, M. Santamouris, A. tsangrassouls, G. Mihalakakou, “Analysis of the green roof thermal properties and investigation of its energy performance”, Energy and Buildings, vol.33, pp.719-729, 2001. Onmura S., M. Matsumoto, S. Hokoi, “Study on evaporative cooling effect of roof lawn gardens,” Energy and Buildings 33, pp. 653-666, 2001.

Palomo, E., D. Barrio, 1998, Analysis of the green roofs cooling potential in buildings,” Energy and Buildings, 27, pp.179-193.

Sidwell, A., J. Gibbs-Alley, K. Forrester, V. Jost, K. Luckett, S. Morgan, T. Yan, B. Noble, and W. Retzlaff, 2008, Evaluation of the thermal benefits of green roof systems, Proceedings of the 6th Annual Greening Rooftops for Sustainable Communities Conference, pp.12, Baltimore, MD.

Spala A., H. S. Bagiorgas, 2008, On the green roof stystem. Selection, state of the art and energy potential investigation of a system installed in an office building in Athens, Greece, Renewable Energy, 33, pp. 173-177. Teemusk A., U. Mander, 2009, Green roof potential to reduce temperature fluctuation of a roof membrane: A case study from Estonia, Buildings and Environment, 44, pp.643-650.