Download cooling effects of natural ventilation of a home in hot dry climate

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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REFERENCES
1.
N. Lechner, Heating, cooling, lighting: Design
methods for architects, New York: John Wiley &
Sons (1991).
2.
D. Heerwagen, Passive and active environmental
controls: Informing the schematic design of
buildings, New York: McGraw-Hill Companies
Inc. (2004).
3.
V. Gupta, “Thermal efficiency of building clusters:
An index for non air-conditioned buildings in hot
climates”, in: D. Hawkes, J. Owens, P. Rickaby
and P. Steadman (editors), Energy and urban built
form, London: Butterworths (1989).
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.
T.A. Markus and E.N. Morris, Buildings, climate
and energy, London: Pitman Publishing Limited
(1980).
6.
D. Parker and S. Barkaszi, “Saving energy with
reflective roof coatings”, Home Energy Magazine
(1994). Retrieved October 22nd 2010 from:
http://www.homeenergy.org/subinfo.html
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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