Idea Transcript
International Journal on Architectural Science, Volume 8, Number 4, p.114-121, 2011
COOLING EFFECTS OF NATURAL VENTILATION OF A HOME IN HOT DRY CLIMATE O.M. Idowu Department of Architecture, Modibbo Adama University of Technology, Yola, Adamawa State, Nigeria (Received 3 January 2013; Accepted 22 February 2013)
ABSTRACT Whereas natural ventilation design in hot climates is intended to mitigate discomfort effect of thermal transmission into building interiors, the uncertainty of the aggregate effect of the strategy on interior air temperature in hot dry climates has been expressed. This study investigated how air temperature in rooms of a residential building in a hot dry climate is affected by some natural ventilation strategies. A combination of expost-facto and experimental designs, the study employed instrumentation to observe temperature, wind speeds and relative humidity in around a residential building in Yola in the months of March and April. Mean temperature values were compared to establish any significant difference between spaces and periods of the day. Spaces were paired and their mean-temperatures compared using the t-statistics (at 0.05 level of significance) to draw inference. Natural ventilation in the building resulted in lower temperatures indoors compared to outdoors; the reduction was however majorly insignificant. Variation in wind direction and location of spaces was found to have significant effect on cooling of spaces.
1.
INTRODUCTION
It is believed that there are three different ways of cooling an interior to create thermal comfort during the overheated period of the year [1,2]. These cooling strategies can be captured as follows: (i)
Minimising heat gain by appropriate building orientation, use of shading devices, insulation, colour, vegetation, etc; (ii) Passive cooling by natural ventilation, radiant cooling, evaporative cooling, earth cooling, and dehumidification; (iii) Mechanical cooling by means of fans and air conditioners. A rational design process is expected to maximise the effects a combination of the first two strategies and minimise the application of mechanical means. Such a combination required for passive cooling is dependent on the climatic conditions of the building site. It is therefore believed that hot dry climates would require a combination of design strategies that are different from that of hot humid climates [1-3]. In the hot dry climates, it is believed that buildings should have their longer sides orientated facing north and south. Such buildings are traditionally characterised by few and small windows, light surface colours and massive enclosure fabrics. The massive materials are expected to retard and delay heat transmission through the fabrics, especially when the fabrics are well insulated [1,4]. Its ability to retard or delay heat transmission is believed to depend on the density and moisture content of the materials [5]. The massive fabrics also act as heat sink during the
day. The cool nights, characteristic of hot dry climates, is expected to significantly dissipate the stored heat of the massive fabrics. Thus the fabrics are cooled at night and prepared to act as heat sink the following day. Roof insulation can be enhanced by maximising the volume of its enclosed air. The use of light and reflective surface finishes on such roofs has also been shown to reduce buildings heat gain in hot dry climates by as much as 60 per cent [6]. Vegetation could be employed in passive cooling in form of grasses, shrubs and trees. This has been corroborated thus [7]: “Shade trees reduce solar heat gain by transferring the active heat-absorbing surface from an inert building envelope to living foliage. Because the heat capacity of leaves is low, most of this energy is transferred to the surrounding air. If ample soil moisture is present and environmental conditions are suitable, water in the leaves evaporates in a process known as evapotranspiration and the air is cooled.” It is believed that cooling of an interior space can be effected by air exchange with the exterior. In other words, natural ventilation has cooling potentials. The cooling ability of a ventilation strategy is dependent on the following variables [5]: (i)
Volume of the interior space;
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(ii) Rate of air exchange, usually expressed as number of air changes per hour (this is also dependent on the areas of air inlet and outlet); (iii) Temperature difference between the interior and exterior spaces.
draw some inspiration from, is: What is the cooling effect of these contemporary home-designs in hot dry climates? This study attempts to provide an answer.
1.1 Some Similar Empirical Results
3.
Empirical studies have been carried out on the cooling ability of natural ventilation in life buildings. One of such works was an apartment in Port ocean residence, La Rochelle, France [8]. A large window (5.l m x 2.22 m) in four sliding sections provided light and ventilation to the living room of the said apartment. The third section of the sliding window was opened and the black-bulb temperature was measured. This was compared with the measurement when window was closed. It was discovered that the indoor temperature was lower by 3°C when window was opened. In another study, the ventilation effect of the design of a School of Architecture in Lyon, France, was investigated [9]. The usually-closed windows of the school’s studios were opened for natural ventilation. It was reported that the temperature of the west studio varied from 30°C in the morning hours to 36°C in the evenings. The values for unventilated situations were 29°C and 39°C. The east studio experienced the lowest temperature of 30°C in the morning and highest value of 35°C in the evening when windows were opened for natural ventilation. The corresponding values for unventilated situations were 28°C and 37°C respectively.
2.
STATEMENT OF THE PROBLEM
These empirical studies [8,9] and other known ones on the cooling impacts of natural ventilation were either in temperate or hot humid climates. If the assertion that natural ventilation cooling strategies would differ with climates [1], is anything to reckon with, then these studies may be limited to their contextual climates. It is not certain whether there is any of such studies in hot dry climates, from which inspirations for designs can be drawn. Whereas, it has been noted that buildings in hot dry climates were traditionally of insulated mass construction and small openings [1] contemporary homes in the study area seem to deviate from this norm. Majorly characterised by 225 mm sandcrete block walls, timber and metal roofs, and window opening areas up to 30% of floor area or more, these contemporary buildings appear not different from those found in hot humid climates. A question, the answer to which future designs may
AIM AND OBJECTIVES OF THE STUDY
The study is aimed at enhancing the cooling effects of designed spaces in hot dry climates by recommending appropriate design guides. It is guided by the following objectives: (i)
To determine the effects of space enclosure fabrics on cooling; (ii) To determine the effects of wall openings on cooling; (iii) To examine the effects of space orientation on cooling of spaces.
4.
MATERIALS AND METHODS OF STUDY
4.1 Materials A residence occupied by the author, was the subject of this study. The residence is one of four buildings in a lot, beside the House of Assembly in the southern part of Jimeta, the capital of Adamawa state. It is bounded by a duplex at the northern, a gate-house at the north-eastern, and boys-quarters at the south-western side. The three bedroom-house has the following spatial features: An entrance foyer of about 2.4 m by 5.4 m at the northern side; a living/dining room of about 40 m2 floor area; kitchen and a store measuring about 2.4 m by 3.6 m; a guest room and toilet/bath linked by a lobby to the living room on one side; a master bedroom and another bedroom (each with a toilet/bath) also connected to the living room by a lobby on the other side; the rooms have ceiling height of about 3 m. The walls are 225 mm sandcrete blocks finished with cement-sand plaster, except the toilets/baths with ceramic tiles. Living/dining room, kitchen and toilet floors are finished with ceramic tiles, while p.v.c tiles are on the bedroom floors. All the rooms are ceiled with hardboards except the living/dining room with plywood. The roof of the house is double pitched sloped less than 20o and with metal roofing on timber carcass. Each of the bedrooms has two windows, each 1.2 m by 1.2 m on different walls; four windows of the same size, two on opposite sides open to the living/dining room. One window of size 0.6 m by 0.6 m opens to each of the toilet/baths; the size of
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the only window that opens to the kitchen is 0.9 m by 0.9 m. All the windows are 50 percent operable, sliding aluminium framed glass, bounded with burglary bars inside and mosquito nets outside. The burglar bars are 25 mm square iron pipes at 160 mm spacing.
A digital hand-held climatic data meter was employed in observing wind speeds and air temperatures. The wind directions were observed with an improvised wind vane designed and constructed by the investigator.
At the southern and north-western sides are trees that provide shades for outdoor sitting etc.
Fig. 1: Inventory of the studied building Source: Sahal M. Junaid and Author’s survey (2012)
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4.2 Methods The study is a combination of ex-post facto and experimental strategies. The climatic conditions and some of the spaces were studied ‘as they were’ without any attempt to manipulate their variables. In some other spaces, window and door openings were manipulated to determine the effects of such manipulations on indoor/outdoor temperature differences. Some spaces within and around the building were studied in the months of March and April, 2011 when it is usually hot. Air temperatures were observed in the morning and evening hours at the entrance, living/dining room, the three bedrooms and two toilets. Temperature and wind speeds were also recorded within the same periods in the tree-shaded outdoor space at the southern side of the building. An experiment was conducted to determine the effect of ventilation opening on interior air temperature. Normally, all windows were opened; but in the experiment, windows of certain rooms were closed for about seven days. These rooms were the guest bedroom and its toilet/bath, as well as the other bedroom and master’s toilet/bath. Mean values of temperature were compared to establish any significant difference between spaces and periods of the day. Spaces were paired and their mean-temperatures compared using the tstatistics (at 0.05 level of significance) to draw inference.
5.
RESULTS AND DISCUSSION
The observed air temperature of eight spaces of the case study residence, their dates of observation and the spaces’ mean temperatures are shown in Tables 1 and 2. While Table 1 indicates observations in the morning hours (0900 – 1100 hours), values obtained in the evening periods (1530 – 1730 hours) are shown in Table 2. In the morning hours, daily space temperatures range from 31.1oC obtained in the living room on the 20th, to 36.8oC on the 31st of March and 2nd of April recorded at the entrance porch. The master toilet has the lowest mean temperature of 33.6oC in the same morning hours; the highest mean value of 34.9oC was obtained at the entrance porch (refer to Table 1). In the evening hours, daily space temperatures range from 35.1oC obtained in the master toilet on the 16th, to 41.6oC on the 13th of March recorded at the entrance porch. Again the master toilet has the
lowest mean temperature of 36.3oC in the same evening hours; the highest mean value of 38.8oC was obtained at the entrance porch (see Table 2). Four spaces (rooms) were involved in the experimental study in which their windows were initially closed; the observed air temperatures then are indicated in Table 3. Table 4 shows the spaces’ air temperatures when their windows were later opened. When windows were closed, space meantemperatures range from the lowest value of 32.9 oC (in the morning hours) in bedroom I, to the highest value of 37.5oC (in the evening hours) obtained in the guest bedroom. The resultant mean range is thus 4.6oC. When windows were opened, the lowest meantemperature of 34.5oC (in the morning hours) was obtained also in bedroom I; the highest value of 38.0oC (in the evening hours) was obtained in the guest bedroom. Thus the mean range was 3.5 oC for this session of the experiment.
5.1 Spatial-Variation of Temperatures Table 5 highlights the differences between mean temperatures of spaces by comparing values in the two periods at the tree-shaded backyard with those of every other space in turn. The table also shows the effect of space location on temperature by comparing the observed values in the experimented spaces in pairs: guest bedroom and bedroom I; guest toilet and master toilet. In absolute terms, the temperature difference between the tree-shaded backyard space and the other spaces vary between 0.1 oC and 2.3 oC. This suggests that the insulating properties of the walls, ceilings and floors of the spaces are discernible. The mean-temperatures of interior spaces compared to the exterior backyard are reduced with the exception of that of entrance porch with increased values. This may be due to poor exposure of the porch to wind or low thermal insulation compared to the tree-shaded space. These variation in temperatures are however found to be insignificant (at 0.05 level of significance) in all observed spaces and periods except for the evening-hour observations at the bedrooms and toilets. These spaces of significant temperature differences are majorly those involved in the experiment where their windows were initially closed and then opened. It appears that natural ventilation has significant effect on evening-hour temperature of spaces. Such effect may however be negative because observed temperatures are generally lower when windows were closed than when they were opened (see Tables 3 and 4).
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Table 2: Evening-hour temperatures, oC
Table 1: Morning-hour temperatures, oC
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Table 3: Experimented spaces mean-temperatures: windows closed
Table 4: Experimented spaces mean-temperatures: windows opened
Whereas Table 5 showcases the differences in mean temperatures between spaces, Table 6 reflects the differences in mean temperatures within the experimented spaces as a result of manipulations of their windows. The differences in meantemperatures range between 0.2oC and 1.9oC. This seems to reinforce the earlier view that ventilation has some effect on temperature of the spaces. The variations in temperature are however found to be insignificant in the evening-hour observations in all the spaces. The trend is reversed in the morninghour temperatures with significant variations in all the spaces except in bedroom I. The observed exception may be ascribed to factors such as: variation in wind directions; differences in location of spaces; and nature and size of space ventilationopenings. The observed wind speeds have mean (average) value of 0.4 m/s and mean (maximum) value of 0.8 m/s. Its direction was majorly (59% occurence) from and around the west (NWW, 23%; W, 16%; SW, 10%; SWW, 7%; NW, 3%) in the morning hours; and equally majorly (59% occurence) from and around the east (E, 25%; NEE, 19%; NE, 7%; SEE, 7%) in the evening-hour observations. Bedroom I and guest bedroom have similar crosssided opening areas (11% of floor area) but are located on opposite sides of the building. The guest bedroom is in the north-west while ‘bedroom I’ is
in the north-east of the building. The cooler morning-hour, predominantly western- and nearwestern-wind may have more cooling impact on the guest bedroom than on bedroom I.
6.
SUMMARY, CONCLUSION RECOMMENDATION
AND
Whereas, it has been noted that buildings in hot dry climates were traditionally of insulated mass construction and small openings [1], contemporary homes in the study area seem to deviate from this norm. Majorly characterised by 225 mm sandcrete block walls, timber and metal roofs, and wide window opening areas (up to 30% of floor area), these contemporary buildings appear not different from those found in hot humid climates. The extent to which these home-designs provide cooling effects in their interiors has hitherto not been ascertained. The study revealed that the insulating properties of the walls, ceilings and floors of the spaces are discernible. The mean-temperatures of interior spaces compared to the exterior backyard are reduced even though the reductions in temperatures are found to be insignificant (at 0.95 confidence level).
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The study also reinforces the earlier view that ventilation has some effects on temperature of the spaces. The effects are found to vary with the time of the day: effect significant in the morning-hour but insignificant in the evening-hour observations.
It is also observed that variation in wind directions and differences in location of spaces have significant effect on cooling of spaces.
Table 5: T-statistics: Comparison of spaces mean-temperatures
MH = Morning-hour; EH = Evening-hour
Table 6: T-statistics: Comparison of experimented (opened- and closed-window) spaces mean-temperatures
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2.
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3.
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4.
M.P. Ternes, K.E. Wilkes and H.A. McLain, “Cooling benefits from exterior masonry wall insulation”, Home Energy Magazine (1994). Retrieved October 22nd 2010 from: http://www.homeenergy.org/subinfo.htm
5.
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6.
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7.
G. McPherson and J.R. Simpson, “Shade trees as a demand-side resource”, Home Energy Magazine (1995). Retrieved October 22nd 2010 from: http://www.homeenergy.org/subinfo.html
8.
K. Liman and M. Abadie, “Naturally ventilated buildings—Porte Oceane Residence”, in: F. Allard (editor), Natural ventilation in buildings, London: James and James (Science Publishers) Ltd. (1998).
9.
G. Guarracino and V. Richalet, “Naturally ventilated buildings―A School of Architecture at Lyon”, in: F. Allard (editor), Natural ventilation in buildings: A design handbook, London: James (Science Publishers) Ltd. (1998).
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