Assoc. Professor of Architecture and Env. Health, Enugu State University of Science and Tech.,Enugu. And

Transcription

1 STUDIES ON COMFORT LEVELS OF NATURALLY VENTILATED ROOFS OF TROPICAL BUILDINGS By ODIM O. ODIM, PhD, RN, MNIA, Intl.Assoc.AIA Assoc. Professor of Architecture and Env. Health, Enugu State University of Science and Tech.,Enugu. And ABSTRACT NWANGUMA O. EPHRAIM, MNIA Dept. of Architecture, Imo State University, Owerri. Natural roof ventilation in buildings particularly in tropical environments in most developing countries has been generally neglected by stake holders in the building industry. Most architects and environmental designers tend to over look the provision of simple structural devices for the enhancement of adequate roof ventilation which contributes to more acceptable comfort standards of buildings. This paper through controlled experiments examines the comfort standards of buildings with and without provisions for roof ventilation. Thermal comfort data were obtained using the Space Syntax Research Methods (SSRM). Data were analysed using statistical tools involving the central tendency and dispersion of the recorded data. Hypotheses were also tested. Results showed that roof ventilation has significant effect on indoor comfort level of buildings in the tropics. Provisions for adequate roof ventilation at the design stage of buildings were therefore recommended. Keywords: Building, Temperature, Thermal Comfort, Tropical Climate, Ventilation. 1

2 Introduction Koenigsberger, Ingersoll, Mayhem, & Szokolay (1973) described natural ventilation and air movement in buildings as the supply of fresh air necessary for convective and physiological cooling. It is the supply of outdoor fresh air to indoor by passive means thereby replacing vitiated air which is necessary for the achievement of thermal comfort and consequently reducing environmental stress in tropical environments (Odim, 2007). A building is any structure with walls, floor and roof, a physical enclosure with some means for creating a comfortable internal environment for human habitation (Odim,2008). The maintenance of acceptable comfort level posses a major problem for buildings in tropical regions characterized by high air temperature and relative humidity almost all through the year. Thermal comfort according to Givoni (1987) is the absence of irritation and discomfort due to heat or cold. Markus and Morris (1980) and Koenigsberger et al (1973) gave six major comfort factors necessary for the determination of thermal comfort (which include factors of the thermal environment and subjective factors) as: Air temperature of the occupied space; Radiant temperature between the body and the environment; Relative humidity of the occupied space; Air velocity within the space; Intrinsic clothing and Level of activity. Comfort ranges for these comfort factors have been identified after prolonged studies by authorities and numerical values produced for these parametric ranges. Heerwagen (2004) gave the parametric range for air temperature to be 20 to 27 C. Koenigsberger, et al (1973) gave it as 22 to 27 C. The comfort parametric ranges for the comfort factors can serve as targets to be sought after by architects during the design stage of buildings in 2

3 tropical environments. The importance of proper roof ventilation in tropical buildings cannot be overemphasized, especially as it helps in the reduction of indoor thermal comfort problems in this climate. The warm humid climate presents thermal comfort problems due to the associated high temperature and relative humidity which makes it difficult for comfortable building occupancy. This peculiar problem needs to be addressed at the design stage of buildings by the employment of passive devices for the inducement of natural ventilation in buildings. This can be achieved by consciously employing and ordering structural members of the building such as wind orientation, fenestration, form, building fabric and location among others. The use of environmental control devices if properly applied can contribute to the reduction of thermal comfort problems, particularly heat transmission indoors through heated roofs in tropical regions. This paper therefore employs controlled experiments for the comparism of buildings with and without provision for roof ventilation with the aim of assessing the comfort levels in buildings in warm humid climate of the tropics. Study Area The warm humid climate is one of the major climatic zones of the tropics. It is characterized by high air temperatures and relative humidities almost all through the year. According to Koenigsberger et al (1973) the warm humid climate is found in a belt near the Equator, and extends to about 15 N and 15 S of the Equator. The mean maximum temperature is C, relative humidity is %, vapour measure N/M². The sky is frequently overcast. The mean annual rainfall is between 1185 to 2788mm with intensity over 500mm/hour. The major wind direction is South-West both for wet and dry seasons (NBRRI,1983). 3

4 Some of the cities in Nigeria found in this climatic zone according to Odim (2008) include: Lagos, Calabar, Benin City, Umuahia, Owerri, Warri, Port Haarcourt, Aba among others. The study was carried out in Owerri, the capital city of Imo State of Nigeria. Methodology This study was designed to use the Space Syntax Research Methods (SSRM). This involved experimental tests on experimental buildings, and the collection and analysis of temporal morphological data on the experimental units, and their applications on indoor and outdoor environmental conditions and spaces. Primary and Secondary data Comfort factors for air temperature were obtained from the model buildings which were statistically designed for this purpose. Sybron Taylor type thermometer was used to obtain air temperature readings in degree Celsius ( C). Stevenson s screen was used to mount the thermometers at a height of 1.2metres above the floor level. This was done to reduce the effect of radiant temperature (tmrt) which was assumed to be equal to the air temperature (ta) of the model buildings (ta = tmrt). A total of 204 data points were obtained from the randomized experiments between January to December, The air velocity was measured when the air velocity was less than 0.10m/s (almost still air). Activity rate was assumed to be 1.00MET. Others include intrinsic clothing which was taken as 0.60 CLO, and relative humidity which was not statistically different was ignored (Odim, 2008). 4

5 The data was physically collected from the experimental units which were designed and constructed for this purpose at the premises of the Faculty of Engineering and Environmental Sciences, Imo State University, Owerri, Nigeria. The secondary data were obtained from Nigerian Metrological Agency (NIMET). They include: Air temperature, Relative humidity, and Wind speed (including major direction) among others. Other secondary data obtained include the Thermal Comfort Index (TCI), comfort scales, and parametric ranges for air temperature already established and validated by authorities. Other possible variables not required were kept constant through control. The Model Buildings The experimental units were designed to have typical elevations, sections, plans among others. The dimensions of the floor plans were 3.60m x 3.60m. The sectional height was 3.0m. The window opening were 1.2x1.2m and windows made of transparent ordinary transparent float glasses on aluminum profile. The doors were made of wooden panels on wooden frames of 0.9x2.1m. The typical construction methods and building materials normally used for the warm humid climatic zone were employed for the model buildings. Some of these materials include hollow sandcrete blocks 0.150x0.225m for the walls. Corrugated zinc roofing sheets on wooden roof trusses were used for the roof. Asbestos cement ceiling boards (0.6x0.6m) on soft wood noggins and batons were used for the ceilings. The following treatments were given to the models: Model A (ModelNRVO) had no roof ventilation openings Model B (ModelWRVO) had roof ventilation openings. The models were sited at an open field at the premises of the Faculty of Engineering and Environmental Sciences, Imo State University, Owerri, Nigeria (See Fig 1 and Plate 1). 5

6 Model A (Model NRVO) Model B (Model WRVO) Fig 1: Plans and sections of model buildings A &B 6

7 Model A Model NRVO Model B Model WRVO Plate 1: Model Buildings A & B Analysis, Results and Discussion Statistical tools established by Moore (2000) were used for the analysis of data. They include summary statistics involving the central tendency and dispersion of the recorded data. Results were obtained for the mean values, standard deviation, and variance among others. These are shown in Table 1 below. 7

8 Table 1: Summary Statistics of the Recorded Data Statistics Temp. C Model A Temp. C Model B Mean Std. Dev Variance Results obtained from the analysis of data obtained from the experiment showed air temperature values of C for Model A ie model without provision for roof ventilation (Model NRVO), and C for model B ie model with provisions for roof ventilation (Model WRVO). The results were compared to known standard according to Heerwagen (2004) and Koenigsberger et al (1973) of 27 C (upper limit of comfort factor for air temperature). Model B with C was closer to the upper limit of the parametric range than Model A with C. Therefore within the limitations of this experimental programme, Model B (Model WRVO) had a more acceptable comfort level than Model A (Model NRVO). To further confirm the results, two sample Z-statistics was used to test if the differences obtained were statistically significant to justify the results. Since the sample size is large and the population standard deviation was not known, the appropriate normal distribution Z-table was therefore used for test of hypothesis. The null hypothesis Ho and the alternative hypothesis Ha were represented as follows: 8

9 Ho: Roof ventilation has no effect on indoor air temperature (ta). Ha: Roof ventilation has effect on indoor air temperature (ta). Mathematically expressed as: Ho: µ Model NRVO = µ Model WRVO: There is no difference Ha: µ Model NRVO µ Model WRVO: A difference exists. A 5% level of significance was assumed for the purpose of these tests. The hypothesized difference between the experimental units was assumed to be zero (0). The Z-test procedure involves the determination of the difference of the estimated standard errors of the experimental units. The decision to accept or reject Ho lies on the calculated and expected values of Z for a fixed level of significance. Considering the comfort factor air temperature, the calculated (Zcal) value = The expected (Zexp) value = Since Zcal > Zexp, the null hypothesis of no difference was rejected and the alternative accepted. The implication of this is that the indoor air temperature of the model buildings (A and B) differs significantly. 9

10 Conclusion and Recommendation This paper, through comparative controlled experiments studies the indoor comfort level of buildings with and without provision for roof ventilation respectively in warm-humid climatic zone of Nigeria. Results obtained showed that there exists a relationship between the indoor comfort conditions of buildings with provision for roof ventilation and those without provisions for roof ventilation. The model with provision for roof ventilation had a more acceptable level of comfort than the one without provision for roof ventilation. Furthermore, it was observed that the comfort conditions of both buildings do not conform to comfort standards despite the treatments given to the experimental units. From the research findings, it is therefore recommended that provisions for adequate roof ventilation should be considered at the design stage of buildings as this contributes to a more acceptable indoor thermal comfort level in buildings in tropical climates. The conclusion and recommendations are however limited to the scope and duration of the experimental studies. 10

11 Acknowledgements Thanks are due to Professor Nkwogu and Professor Awuzie respectively. They provided a piece of land at Imo State University premises where series of experiments on thermal comfort and energy usage of buildings were performed. References Dagostino, F.R. (1978). Mechanical and Electrical Systems in Construction and Architecture. R. Vergina, Reston Publishing Co. Inc. Heerwagen, D (2004). Passive and Active Environmental Controls. Informing the Schematic Designing of Buildings. The Mc Graw - Hill Companies Inc., N.Y Koenigsberger, O.H., Ingersoll, T.G., Mayhem, A, & Szokolay, S.V. (1973). Manual of Tropical Housing and Building. Part 1, Climatic Design. Longman, London. Markus, T.A. & Morris, E.N. (1980). Building, Climate and Energy. Pitman Publishing Ltd. Moore, D.S. (2000). Basic Practice of Statistics. W.I.I., Freeman and Company, USA. 11

12 Nigerian Building and Road Research Institute (1983). Nigerian Climatic Zones and Building Design Guild lines. NBRRI Report, No 4, Lagos. Odim, O.O & Okereke. P.A. (2007). Potentials of Passive Solar Design in Energy Conservation in Buildings in the Warm-Humid Climates. Advances in Materials and Systems Technology (Ed). Akii Ibhadode, Trans Tech Publications. Vol.18-19, Pp Zurich, Switzerland. Odim, O.O (2008). Experimental Study on Comfort levels of East-West and North South Solar Oriented Building in Warm-Humid Climates. Architectural Science Review. Vol 51.4, Pp Sydney, Australia. 12