Thermal condition and heat exposure within buildings: Case study of a tropical city

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Case Studies in Thermal Engineering

journal homepage:www.elsevier.com/locate/csite

Thermal condition and heat exposure within buildings: Case study of a tropical city

Ayansina Ayanlade

^a,∗

, Oluwakemi M. Esho

^a

, Kehinde Olayinka Popoola

^b

, Olajumoke D. Jeje

^a

, Bode A. Orola

^c

aDepartment of Geography, Obafemi Awolowo University, Ile-Ife, Nigeria

bDepartment of Urban and Regional Planning, Obafemi Awolowo University, Ile-Ife, Nigeria

cDepartment of Architecture, Obafemi Awolowo University, Ile-Ife, Nigeria

A R T I C L E I N F O Keywords:

Temperature within buildings Humidity

Louvres window Sliding windows houses

A B S T R A C T

This study aims at examining the thermal relationship between indoor-outdoor temperature and relative humidity using houses with two different window types in Ile-Ife, Nigeria. Indoor and outdoor temperatures and humidity were measured simultaneously using digital mobile weather stations placed in different residential houses with sliding and louvre window type.

Measurements were taken within the houses for the periods of three years, both day and night at 30 min interval. Box plots, scatter plot, correlation and inferential statistics were used for the analysis of the data collected and conclusions were drawn. The results showed significant variations in mean temperature between the indoor and outdoor temperature in the two house types.

The house with the sliding window had a higher temperature, with a mean temperature of 28 ≤

°C ≥ 31, compared to the house with the louvres window, with a mean temperature of 21 ≤

°C ≥ 28. Though indoor temperature appears higher than the outdoor throughout the seasons but much more during the months of the dry season (November to April), with 26≤ °C ≥ 33 for indoor temperature and 25≤ °C ≥ 29 for outdoor temperature. The mean temperature within the houses with sliding windows type was found to have nearly 2 °C more than the louvres window.

The correlation results show the high negative relationship between temperature and humidity with 0.60 ≤ R²≥ 0.88 but a very low relationship was observed between outdoor temperature and humidity with R²≤ 0.48 in all the houses for the entire period. The study concluded that materials used to provide ventilation in houses especially the window type have a significant influence on the indoor temperature and comfortability of its occupants.

1. Introduction

Over the last decades, the interest in the assessment of indoor and outdoor temperatures in relation to urban thermal comfort has greatly increased both in developed and developing countries [1–3]. Most of the concerns although, have been mainly on indoor thermal comfort as there have been relatively few studies on the outdoor environment. The majority of recent studies have shown that increase in indoor and outdoor temperature might be caused by climate change, thus increasing heat stresses in many cities in the world [3–5]. Although it is evident that indoor temperatures may perhaps be modified by fans or air conditioners, measurements of the outdoor thermal environment are limited [6,7]. This is because the outdoor thermal conditions in a city essentially depend not

https://doi.org/10.1016/j.csite.2019.100477

Received 15 April 2019; Received in revised form 13 May 2019; Accepted 1 June 2019

∗Corresponding author.

E-mail addresses:sinaayanlade@yahoo.co.uk,aayanlade@oauife.edu.ng(A. Ayanlade).

Available online 06 June 2019

T

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only on the city population and housing density, but also depends on amongst others, the intensity of vehicular movements, building arrangements, and availability of green park in such a city [8,9].

What is obvious from the previous studies is that thermal comfort in any city depends not only on the adequacy of any building space and the suitability of the built environment but also the variations between the indoor and outdoor temperature which are an essential characteristics when considering the functional environmental systems within cities. This is because thermal comfort is a function of indoor/outdoor air temperature and quality within a city, although, these factors are independence, since they have a collective impact in relation to other environmental and design factors [10–12]. Appah-Dankyi and Koranteng [13]further estab- lished, as also reported in most of the studies of thermal comfort, that a greater part of the variation in response can be attributed to change in temperature rather than to changes in either humidity or air movement. It must also be emphasized that the concept of adaptive thermal comfort is the basis of the thermal experience in the urban spatial environment. This is because the urban climate and the building indoor climate are both parts of a climatologically continuum, thus it is obvious that the comfort levels are usually of low quality as a result of lack of proper ventilation due to poor construction in cities.

In recent years, the intensity of indoor temperatures experienced in residential buildings, especially in tropical urban centres, is on the increase and thermally uncomfortable, due to extreme temperature event resulting from climate change. Many studies have reported that the outdoor environment is deteriorating in many tropical and sub-tropical cities, resulting from rapid urbanization and this has led to a number of problems related to the health and well-being of residents. Developing optimum thermal conditions in residences is therefore essential for health and comfort [14–16]. In the tropical part of the world, thermal comfort was found to be far above the recommended comfort level. This is resulting from the combination of intense solar radiation and high temperatures especially during clear sky days; consequently, the establishment of thermally comfortable microclimates in urban environments is very imperative. Unfortunately, the housing conditions in some urban areas in African countries, such as Nigeria, are worrisome as many of them are of poor standards and poor air quality [17]. Thus, this study aims at examining the relationship between indoor- outdoor temperature and humidity in residential houses. The study focuses on the impact of window types on the relationship between indoor-outdoor temperature and humidity in residential houses. This study was developed based upon the assumption that the indoor and Outdoor temperature is a range of temperatures that is defined as comfortable for humans, over the temperature range a person is either hot or cold when wearing ordinary clothing and it is regarded as a component of personal exposure associated with various effects on health. The major hypothesis of this study is that thermal comfort plays a significant role in human performance at both physiological and mental levels [18–20] since the majority of these depend on indoor/outdoor temperature [5,21]. Also, the study is set to sensitize and equip the general public especially occupants of urban areas of the possible factors responsible for the thermal comfort of the indoor and outdoor environment. This study is based on the assumption that the intensity of indoor temperatures experienced in residential buildings, especially in urban centres are thermally uncomfortable for a substantial period of time.

2. Materials and methods 2.1. Study area and data acquisition

This study was conducted within houses with two types of windows in Ile-Ife, Nigeria (Fig. 1). The city has an estimated population of about 501,000 people with humid tropical and a recognizable amount of rainfall during the rainy season followed by a dry/harmattan season. The average annual temperature in Ile-Ife is 26.2 °C, with an average precipitation of about1340 mm. The mean annual rainfall is bimodal in nature [22] with peaks in July (average of 350 mm) and September (average of 200 mm). The driest month is January, with 9 mm of rain with an average of 211 mm, the most precipitation falls in September. There is a difference of 202 mm of precipitation between the driest and wettest months. During the year, the average temperatures vary by 4.4 °C.

In this study, indoor and outdoor temperature and relative humidity were measured simultaneously using digital mobile weather station placed in different houses. Measurements were taken within houses with two types of windows over a period of three years.

The measurements were taken in Louvers and Sliding window types for both day and night at a 30-min interval. Permission to use the houses was obtained from the house owners enabling access within and around their homes, with the consent form signed. The data used in this study were collected from the archive of the database of an ongoing research looking at the indoor and outdoor temperature variability in South-Western Nigeria. Out of this database, a 3-year data set from 2015-2017 was used in this study to examine variation in the indoor and outdoor temperature and humidity within two sets of houses. Digital mobile weather stations loggers were placed indoor and outdoor of six houses, with three of the houses having sliding windows and the other three having louvre windows (also known as jalousie window). The houses are identical in term of material used for their construction, but the only difference is the window types, in the sense that their window to wall ratio and/or window to floor ratio is similar. The indoor temperature measurements were taken mainly in the bedrooms, which are installed different window types.

2.2. Data analysis

Both quantitative and qualitative research methods were used in this study. Box plots were used to assess the variation in mean for indoor and outdoor temperatures and relative humidity for three years for the two window types used in the houses. The essence is to understand how the indoor temperature varies with outdoor temperature for both day and night for the period of three years.

Correlation analysis was used to examine the relationship between temperature and humidity (indoor and outdoor) in the houses with different window types. Scatter plots were used to establish variations in the relationship between temperature and humidity

A. Ayanlade, et al. Case Studies in Thermal Engineering 14 (2019) 100477

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between the two window types in three seasons of the year. The seasons are Rainy season (April to October), harmattan season (November to January) and dry season (February to March). This was done to determine how the various seasons affect the outdoor temperature and how this, in turn, affects the indoor environment with different window types. Inferential statistics were also used to draw conclusions based on the results of the analysis. This study focused on comparing the similarities and dissimilarities between indoor and outdoor temperatures and heat stress in houses with two different window types in Ile-Ife, Nigeria.

3. Results and discussion

The detailed context of the houses used in the study, the locations of the data loggers inside the house, occupancy patterns, dimensions, dynamic data over three years are presented inTable 1. General temporal variations in both indoor and outdoor temperature and humidity were assessed. Three major findings are notable from these results: (1) there appear to be significant variations between indoor and outdoor temperature; (2) that the indoor humidity and indoor temperatures revealed an inverse relationship; and (3) that there are variations in indoor temperature between houses with louvres and sliding window, but the indoor temperature in the houses with sliding window was found to be higher.

3.1. Temperature and humidity variations between the two houses based on window types

Indoor humidity and indoor temperatures revealed a similar pattern in all housing types, having inverse patterns, with an increase in temperature led to a decrease in humidity for the two window types in the houses studied for the period of three years (Table 2).

The indoor and outdoor temperature for the house types was found to have a direct relationship for the two window types. Any significant change in the outdoor temperature led to a change in the indoor temperature. It was also found that the house with the sliding window had a higher temperature, with a mean temperature of 28 ≤⁰C ≥ 30, compared to the house with the louvres window, with a mean temperature of 21 ≤⁰C ≥ 25 (Fig. 2). Thus, the house with the louvres window appears cooler all year round for the three-year period studied in this research. In all, the results showed a considerable difference in the indoor temperatures of the Fig. 1.Map of Nigeria showing the location of study sites (Red points in map) within Ile-Ife Osun state. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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two window types, houses with the sliding windows had a higher mean temperature of about 33 °C (Table 2), and those with the louvres window was about 30 °C. The minimum indoor temperatures were 23 °C and 21 °C respectively (Fig. 2). These results suggest that houses with louvres windows may be better ventilated than the houses with sliding windows. This is because the structure of the louvres allows for more ventilation indoor than that of the sliding windows.

Unlike the annual results, the results show much of seasonal variations in the degree and intensity of both indoor and outdoor temperature and humidity. FromFig. 3Indoor and outdoor temperature show a much seasonal pattern with highs during the months of dry season and lows during the wet season months. Generally, three major findings are obvious from seasonal analysis presented in Fig. 3andTable 3: (1) the intensity of indoor temperature appears higher than the outdoor temperature during all seasons; (2) outdoor humidity was significantly higher than the corresponding indoor levels during all seasons in all house types; and (3) the mean temperature appears much more low during the wet season month (dray bar) compare to dry season months for both louvres window and sliding window house types. Though indoor temperature appears higher than the outdoor throughout the seasons but much more during the months of the dry season (November to April), with 26≤ °C ≥ 33 for indoor temperature and 25≤ °C ≥ 29 for outdoor temperature (Fig. 3). The highest indoor temperature was observed in the month of March, with 31.1 °C for louvres window and 33.0 °C for sliding window house types (Table 3). The reason for this is obvious, March is the pick of the dry season in the study area with general high temperature due to low or some year no rainfall during this month. The lowest monthly average indoor and outdoor temperatures were July 24≤ °C ≥ 26 (Table 3).

On the other hand, indoor humidity was suggestively lower than the corresponding outdoor humidity level in every month, except the month of March for the sliding window (Fig. 3C) house types. The average humidity has the same seasonal patterns with the mean temperature in that the lowest humidity was in the month of July for both indoor and outdoor, with humidity 49≤ % ≥63 in all house types. The lowest and highest monthly average indoor humidity was 49% and 57% in July and 64% and 72% in March, for both louvres window and sliding window house types respectively. These results imply that the average humidity appears much more in sliding window house types than louvres window. Despite that the measurements were measured simultaneously, the average temperature within sliding window house types is nearly 2 °C higher than that louvres window types, and the average humidity is nearly 10% varies between the house types (Table 3).

Table 1

The Setting of the houses used in the study.

Context of the houses Percentage (%)

Occupancy patterns

Apartment 20

Single family house 80

Location of the data loggers inside the house

Seating room 90

Bedroom 10

Kitchen None

Cooling system

Central air conditioning 5

Window air conditioning 5

Fan 90

Use of a humidifier

Yes None

NO 100

Window Type

Sliding 50

Louvers 50

Table 2

The daily characteristics of outdoor and indoor temperature and humidity in the study sites over the study periods.

Mean S.d. Min Max

Houses with Sliding Window Type Temperature (°C)

Outdoor 27 1.44 25 30

Indoor 30 1.04 28 33

Relative humidity (%)

Outdoor 67 3.94 59 71

Indoor 62 4.87 57 72

Houses with Louvers Window Type Temperature (°C)

Outdoor 27 1.34 25 30

Indoor 28 1.02 27 31

Relative humidity (%)

Outdoor 67 3.94 59 71

Indoor 54 4.12 49 64

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3.2. Relationship between indoor humidity and indoor temperature

The trend lines,Fig. 4, explain a negative relationship between indoor temperature and humidity. It obvious that an increase in temperature leads to a decrease in humidity and a decrease in temperature leads to an increase in humidity. There appears to be a strong and high degree of relationship between indoor temperature and humidity within louvres, with R²= 0.93 compared to sliding window houses, with R²= 0.78 (Fig. 4). These results imply that an increase in temperature leads to a decrease in humidity and a decrease in temperature leads to an increase in humidity. This has been explained in several publications and the result from the present study is similar to the findings of many of these studies which an inverse relationship between temperature and humidity [3,5,23,24].

Correlation tables were used to assess the relationship between indoor temperatures and humidity and outdoor temperature and outdoor humidity for the period of study, for sliding window house type (Table 4) and louvres window house type (Table 5). The results show a relatively higher negative relationship between indoor temperature and indoor humidity for both house types with R²≥ 0.60 (Tables 4 and 5). But a very low relationship was observed between outdoor temperature and humidity with R²≤ 0.48 in all house types (Tables 4 and 5). Though, the relationship between indoor temperature and humidity was likewise higher in louvres window house type 0.70 ≤ R²≥ 0.88 (Table 5). These results imply that the window types do not have an effect on outdoor

Fig. 2.Variation in indoor and outdoor temperatures of two types of windows over the years.

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temperature and humidity, rather much more on both indoor temperature and indoor humidity.

However,Fig. 3shows a positive relationship between the indoor and outdoor temperature for the two house types. From the trend line it shows that there is a direct relationship between the indoor and outdoor temperature, such that as the outdoor temperature increases, the indoor temperature increases likewise. Trend value showed a strong positive relationship between the two variables. A very strong linear correlation was noted between the indoor and outdoor temperature R²> 0.90 for both house types (Fig. 5). Comparing the two window types, the difference in the correlation values at 0.01 showed that the relationship between the indoor and outdoor temperature of the louvres window was greater than that of the sliding window for this period.

In all, the results from this study suggest that the house with the sliding window appeared to be hotter than the house with the louvres window. This result implies that the thermal property of sliding window appeared superior to the louvres window which has the existence of air space between glasses, which increases thermal resistance and reduce heat intensity in the house. The major findings of the present study are that both indoor temperature and humidity varied between the types of the window used in residence [25–28]. The study further showed that indoor mean temperature in the different house types was significantly higher than Fig. 3.Seasonal pattern of indoor and outdoor temperature and humidity during the Wet season Month (dray bar) and Dry season months for both louvres window (A&B) and sliding window (C&D) house types.

Table 3

Monthly averages of indoor and outdoor temperature and humidity.

Source: author's analysis

Louvers Window Sliding Window

Average indoor humidity

Average indoor

temperature Average outdoor humidity

Average outdoor

temperature Average indoor humidity

Average indoor

temperature Average outdoor humidity

Average outdoor temperature

January 52 27.5 59 25.9 60 29.4 59 25.9

February 60 29.5 68 27.9 68 31.4 68 27.9

March 64 31.1 71 29.5 72 33.0 71 29.5

April 62 30.1 71 28.5 70 32.0 71 28.5

May 55 29.1 69 27.5 63 31.0 69 27.5

June 51 27.5 65 25.9 59 29.4 65 25.9

July 49 26.7 63 24.9 57 28.6 63 24.9

August 49 26.5 62 25.1 57 28.4 62 25.1

September 51 26.7 69 25.1 59 28.6 69 25.1

October 52 27.4 70 25.8 60 29.3 70 25.8

November 53 28.7 70 27.1 61 30.6 70 27.1

December 53 27.9 70 26.3 60 29.8 70 26.3

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outdoor levels, while indoor humidity was lower than the outdoor level in all seasons. The degree of indoor mean temperature appeared much more in sliding window house types compared to houses with louvres windows. These results imply that window affects indoor temperature is mainly through window type. Compared with sliding window, louvres window has the advantage of opening area and windowpane is larger and this favour of much ventilating in all seasons. Besides, it is obvious from the results that the ranges of monthly average indoor temperature and humidity were constricted than the corresponding outdoor values. The study revealed a lower indoor mean temperature during the wet season. The reasons for this might be because this time is a period of monsoon season experiences heavy rainfall. This brings about differences in precipitation between the driest and wettest months in the study area. The indoor mean temperature was highest in the month of March. This is because the month of March is the peak of the dry season and is the warmest month of the year. The results from this study thus imply that indoor mean temperature does not only vary from one house to another based on their window types but also there is much more seasonality in their degree, varies from wet months to dry months of the year. Earlier studies have reported similar results in some cities in other part of the world [2,25,28,29]. Previous studies have shown that urban indoor temperature intensity varies with seasonal variation and time of day [30–32]. What are noticeable from these studies are higher degree indoor air temperatures and percentage lower relative humidity recorded in the cities residential buildings during the daytime. The results of the indoor thermal conditions presented in the present study may be useful for understanding personal thermal exposure in the two types of residence studied.

4. Conclusion

In this study, we examined the impact of window types on the relationship between indoor-outdoor temperature and humidity in Fig. 4.Scatter plot showing indoor temperature and humidity for houses with sliding window (A) and louvres window (B).

Table 4

The relationship between temperature and humidity in houses with sliding window.

Year Temperature

Indoor/Hum Indoor Temperature

Outdoor Hum Outdoor/

2015 −0.67 −0.32

2016 −0.68 −0.48

2017 −0.62 −0.34

Table 5

The relationship between temperature and humidity in houses with louvres window.

Year Temperature

Indoor/Hum Indoor Temperature

Outdoor/Hum Outdoor

2015 −0.88 −0.31

2016 −0.71 −0.42

2017 −0.70 −0.33

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residential houses within Ile-Ife, Nigeria. The indoor temperature and relative humidity were measured over a period of three years to determine indoor thermal conditions within houses with two different window types. Scatter plot, box plots and correlation coef- ficient were used in this study, to show the extent to which the indoor and outdoor thermal conditions are related and the differences of these conditions in the two sets of houses. Three major findings are obvious from this study. (1) The indoor temperature and humidity varied between the types of houses due to the differences in window types. (2) The indoor/outdoor temperature, and relative humidity could be good indicators of indoor conditions, but the outdoor relative humidity was not useful for determining the indoor level. (3) The materials used to provide ventilation in houses especially the window type have a significant influence on the indoor temperature, though the intensity varies from dry to wet seasons. Nevertheless, this study has some limitations. This is because this small study only measured indoor thermal conditions in two sets of houses that were selected according to the type of windows used. The small number of houses might not be enough to represent each window type. Still, the results from this study could provide basic information of annual tendency in each window type as previous measurements of thermal conditions have mainly focused on commercial buildings and measurements undertaken in residential buildings have been of short duration. It has been discovered from the literature that the most common and widely used indicator of thermal comfort is air temperature, both indoor and outdoor. This is because it is easy to use and people can rate it without any form of difficulty, although it has vital importance, it is not the only parameter that can be used to define thermal comfort accurately, air temperature should always be considered in relation to other environmental and design factors [33,34]. Another set of factors affecting thermal comfort are known as the human thermo-regulatory factors and change in landuse in cities that affects the temperature, leading to urban heat island [35,36]. Although these factors are independent of each other, they have collectiveimpacts [11,37–39]. Though, the thermo-regulatory factors are not considered further in the present study, because it is outside the scope of the study.

Based on the findings of this research, it was observed that the materials used to provide ventilation in houses especially the window type has a great influence on the indoor temperature and comfort of occupants. Correspondingly, the study might not be conclusive as various occupants, some other houses might modify their indoor temperatures using artificial means such as fans and air conditioners to make their environment conducive, and so we cannot conclude that the indoor temperature of different houses is a function of the window types only. Other factors such as materials used for construction, materials used to provide ventilation, number of occupants in the house, materials used for roofing and the type and color of paints used were not put into consideration in this particular study so it might not be conclusive to say that only window types affect the thermal conditions of a house. Hence, stakeholders in the built environment should not be concerned with the aesthetic values only but also use materials that would make the house habitable for its occupants. The outdoor temperature has a great influence on the indoor environment, so measures of climatic modifications such as tree planting would help to make the thermal conditions of the indoor environment pleasurable. It further obvious from the present study that temperatures in residential buildings during the dry season in tropical regions like our study sites is becoming almost a major concern. This is because the high levels of solar radiation influence the heat produced in this region which in turn increases the indoor temperatures felt by residents within the buildings. Although findings from this study might not be able to draw conclusions for all houses, it can be used as a guide for further studies; the window types used in houses should be selected taking into consideration the prevailing climate of the environment where the structure is to be sited.

Conflicts of interest

The authors declared that there is not conflict of interest in this paper.

Fig. 5.The relationship between the indoor and outdoor temperature of louvres window (A) and sliding window (B).

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Acknowledgement

This research work was funded and supported by the Department for International Development (DFID) of the United Kingdom (UK) through The Climate Impact Research Capacity and Leadership Enhancement in Sub-Saharan Africa programme (CIRCLE), Grant Cohort 2.

Appendix A. Supplementary data

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.csite.2019.100477.

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