A passive skylight system is a significant building design element that provides an ideal condition for interior spaces. However, the use of this system is limited to specific climatic regions because of its considerable effect on the indoor environment. Malaysia is a tropical country that has favorable natural benefits, such as solar geometry and natural light, which can brighten building interiors throughout the year. However, harnessing this benefit affects spaces, especially those in single-story buildings, because of excessive natural loads. This study reviews a concept to understand the passive behavior of solar radiation in the form of light and heat that falls on, interacts with, and is emitted from a skylight system in a single-story building. The study method is theoretically based on descriptive analysis to assess design requirements. The review shows that designs grounded on the physical aspects of climate (influenced variables), materials (design variables), and human comfort (affected variables) in one process (ESI) can develop the architectural way of thinking rather than estimate the condition based on a limited perspective. This assumption indicates that the adoption of this concept in the preliminary design stage will enable designers to balance the building environment effectively.
Skylight system ; Tropics ; Solar radiation ; Light ; Heat ; Malaysian built environment
The roof skylight system is a construction technique that enables natural light from the sun to pass through roofs or horizontal surfaces of building interiors with limited openings from walls. This construction system is a critical part of a building envelope, which is in contact with most external environmental conditions compared with any other building element, such as windows, walls, and roofs. The roof skylight system is generally prevalent in temperate and cold climates to capture the heat from sunlight under cold weather conditions, such as in late autumn and winter, to reduce energy consumption.
However, the requirement is reversed in tropical regions that have only two seasons: hot and wet. This feature is also a difficult design restraint that tropical architects neglect in their designs, especially for domestic buildings. Additionally, studies on the tropics are unclear regarding the theoretical evaluation of the environmental performance of roof skylights systems because of incomplete research on this approach. A considerable number of studies and books on different climatic regions have not clearly addressed any systematic concept to evaluate the environmental load imposed on skylight systems. Most existing books and guidelines (McNicoll and Lewis, 1994 ; Heschong and Resources, 1998 ; Muneer and Kinghorn, 2000 ; Ruck, 1982 ; Edmonds and Greenup, 2002 ; Boyce et al ., 2003 ; Mardaljevic, 2007 ; MS 1525, 2007 ; Boubekri, 2008 ; Szokolay, 2008 ; Natural Daylight Design Through Rooflighting, Amendments, 2009 ; Kittler et al ., 2012 ) have only discussed strategies and types, but a holistic approach toward tropical architecture remains lacking.
As a result, research on with the assessment of the effect of sun energy on tropical roof skylight systems can be viewed as a contribution, not only to the quality of building interiors, but to the improvement of the architectural way of thinking to go beyond the estimation of conditions based on a limited perspective.
A roof skylight system under solar radiation is subjected to the diverse effect contributing to varying environmental behavior. Such behavior will therefore either increase or decrease the strength of the indoor load. This observation gives rise to the need to identify the effect of the sun load that falls on, interacts with, and is emitted from a skylight system. Figure 1 shows the basic theoretical concept of the load process influenced by the external environment, modified by the mediator (roof skylight system), and transferred from the system to the indoor environment, which consequently affects performance outcomes. To clarify a critical part of the concept, we should understand that the direct load from the sun imposed on the external environment differs from the indirect load modified by the roof skylight in the indoor environment because these loads represent actions and reactions in buildings that are controlled by a medium. Targeting the characteristics of each parameter will therefore enable the identification of the keys for evaluation.
Theoretical concept of the load process.
The approach hypothesizes that the external environment, roof skylight system, and indoor environment are the targeted keys for evaluating the load that affects performance outcomes. Each key clearly represents a unique environment characteristic that is totally different from others. Thus, this approach needs to specify the conditions separately by determining the variables.
Mardaljevic (2007) and Szokolay (2008) address climate as an external parameter that controls design strength. Heschong and Resources (1998) and Kittler et al. (2012) specified material to be the most important aspect in daylighting design. McNicoll and Lewis (1994) and Boubekri (2008) identified human comfort as the basis for evaluating indoor conditions in day-lit indoor spaces. In fact, these three independent aspects of climate, material, and human share one common dependent variable, that is, solar radiation, which embodies light and heat. Solar radiation is an electromagnetic spectrum given off by the sun mainly in three wavelengths: visible light radiation in the form of light, as well as infrared (IR) and ultraviolet (UV) radiations in the form of heat. Therefore, these two elements of light and heat that come from the sun are discussed as the targeted variables in this paper.
To comprehend the effects of these variables, the ESI approach is used to coin the terms External , Skylight , and Indoor , as shown in Figure 2 , which identifies the environment load process that relates to any roof skylight system.
ESI process classifying independent and dependent variables.
According to the ESI approach, the characteristics of the environmental loads, i.e., natural light and thermal heat, will be evaluated in three different stages:
The EEC represents the substantial load that has various effects. Although EEC is beyond the control of the architect, its effect should be considered for assessment. The sun is paramount to the entire ESI approach because it is a remarkable source of energy. Vitruvius (1999) highlights the sun as the main source that controls building environments. Szokolay (2008) stressed the issue for architects to understand two significant aspects, namely, solar geometry and the amount of energy that it reaches and how it can be handled.
Solar geometry describes the relationship between the sun and the earth, which is easier to identify than the energy itself. The position of the sun clearly varies in the sky constantly during the day and throughout the year. Therefore, the latitude, angle of declination, solar time, solar noon, as well as the seasonal and hourly changes of the sun at specific locations, should be determined (Kukreja, 1982 ; Wright, 1984 ; Lam, 1986 ). Furthermore, specifying the angle of direct sunlight helps aids in the estimation of the energy that will directly strike any system on Earth.
Solar radiation, as an entire frequency spectrum of electromagnetic radiation from the sun, is of considerable interest. Solar radiation is a pure energy that carries various forms of loads, such as light and heat. Page (2006) specified three types of solar radiation wavelengths that reach the ground and affect the human level, viz., UV , visible light , and IR radiation . Identifying the wavelength of radiation effectively aids the architect to understand natural loads. For instance, a short wavelength, such as ultraviolet, is an extremely high-energy load that is very harmful, whereas long wavelengths, such as IR, have the lowest load. The distribution of sunshine (solar spectrum) that arrives on Earth is divided into 51% IR, 47% visible light, and 2% ultraviolet, which are distributed as direct and indirect loads ( Robertson and Mortgage, 2003 ; Wausau Window and Wall Systems, 2010 ). According to Szokolay (1992) , the amount of solar radiation that strikes the ground in a normal day is divided into 24% direct beam radiation and 22% diffuse radiation, with 23% being absorbed by the atmosphere.
Light represents almost half of the transferred solar radiation with wavelengths between 380 and 700 nm. This energy reaches the Earth׳s surface as two types: (i) sunlight as direct component and (ii) diffuse component , commonly known as daylight , both of which comprise the total light from the sky dome. These types have different amounts of energy. According to Baker and Steemers (2002) , direct sunlight has higher, more concentrated energy and is five to 10 times stronger than daylight. Meanwhile, the invisible part, which can be detected as a warm sensation on the skin, represents more than half of the solar radiation in the form of UV and IR. IR is literally heat, and almost all sources that emit UV and visible-light radiation will probably emit IR (Moss et al., 1982 ). This finding indicates that heat is the dominant factor in solar radiation that has to be targeted.
According to Oral et al. (2004) , light and heat as part of solar radiation are the most important variables that affect the built environment. Understanding the behavior of these factors will help elucidate the conditions that affect their intensity and distribution.
Understanding the medium that captures all sun radiations is important in identifying the external load. Given that we are dealing with an architectural concept, the sky is the main embodiment of the roof skylight system. The sky generally lies at a certain distance above the surface of the earth, which is only known as the denser portion of the atmosphere. The sky is the main medium for solar radiation in the external environment, and its condition is difficult to characterize because of fluctuations in the weather and climate over a very short time (Pages et al., 2003 ).
Although sunshine comes directly from the sun, the sky will appear as a light source because sunlight is scattered by dust, air, and water vapor. The length of the day is significantly influenced by the appearance of both sunlight and skylight because both depend on atmospheric turbidity and cloudiness changeability throughout the year. Therefore, ceaseless changes in solar altitude with various scattering and absorption features of the atmosphere result in unique non-repeating conditions that change the sunshine levels continually (Kittler et al., 2012 ).
Perez et al. (1993) identified 15 types of sky models. The Commission Internationale de l׳Eclairage (CIE) divides the skies into three simple models, namely, clear , intermediate , and overcast . Each model has different characteristics according to Baker and Steemers (2002) :
Ruck, in his Ph.D. Dissertation “Daylight availability in Australia” (1982) , states that sky illuminance relies on solar altitude and the percentage of cloud cover, which are essentially identified based on latitude and climatic conditions. Kittler et al. (1997) and Kittler (1999) affirm that different standard skies in daylighting designs can possibly be used, even for disorderly sky patterns appearing in tropic regions.
Furthermore, sky color temperature is another important issue to note because of different changing conditions. The values depend on the distribution in the sky vault, which highly relies on sky conditions: below (3000 K) 2726 °C at sunset near the sun, from (5000 K) 4726 °C for cloud patches, to more than (20,000 K) 19,726 °C for blue-sky patches (Chain et al., 1999 ). Admittedly, these types of skies serve a significant function in controlling the amount of thermal radiation and luminous intensity. These skies are affected by several factors, including climate, latitude, and air quality, which influence the intensity and duration of daylight (Page, 2006 ).
Climate is the main parameter that controls the amount of light and heat in the external environment and is also the parameter that captures different levels of both based on climate types. Climate averages the weather and the set of atmospheric conditions at a particular time and place (IPCC, 2001 ). That is, climate is an aggregate of all meteorological occurrences in the atmosphere synthesized at a specific location over a long time period. Climate is one of the location features where no two places on the earth׳s surface have the same characteristics. The combination of properties, such as latitude, sea level, closeness to the sea or other large bodies of water, wetness of the ground and the nature of its vegetation and hills or mountains, can result in significant climatic varieties (Koenigsberger et al ., 1974 ; Lippsmeier and Tropenbau, 1980 ). However, the climate is continuously changing, either by geomorphological transformations or by the intervention of man. Climate is divided into three types: macro , meso , and micro . Microclimate is concerned with limited spaces, such as cities, towns, small landscapes, streets, or rooms ( Lippsmeier and Tropenbau, 1980 ; Ayoade, 1983 ). The fact that climate is not static, but rather fluctuates and varies over time, should also be noted.
The astronomer Ptolemy, during the 2nd Century AD, believed that climate differs around the world because solar altitude influences air temperature. He developed the most fundamental classification for the earth (Oliver, 1997 ). He divided the earth into three zones: (i) the tropics, (ii) the temperate zones, and (iii) the cold zones around the poles. The tropical zone refers to parts of the earth that lie between the Tropic of Cancer (23°27′N) and the Tropic of Capricorn (23°27′S) (Ayoade, 1983 ). This zone receives a large amount of solar radiation throughout the year because the sun lies at the zenith around these latitudinal boundaries. Atkinson (1954) developed a classification for tropical climate based on two factors: temperature and humidity. He defined three major zones in the tropics: (i) warm/humid equatorial , (ii) hot/dry desert or semi-desert, and (iii) composite or monsoon . Atkinson (1954) and Edmonds and Greenup, (2002) specified the equatorial zone that has a hot and humid climate within 10° to 15° (North and South) off the equator, including such locations as Brazil, Central Africa, and South-East Asia.
Warm humid climates generally have special weather parameters that control the amount of daylight and heat. The important and significant factors are temperature , sunshine duration , cloudiness , humidity , rainfall , and wind , which are normally measured by meteorological stations.
Malaysia, a country with a tropical climate, is located at 3°N of the Equator. As shown in Figure 3 , the country has hot and humid characteristics. Malaysia has rainy and sunny seasons, is characterized by higher temperatures, is exposed to a very large amount of solar insolation, has a high level of humidity, has plenty of rainfall throughout the year, and has unpredictable wind movement.
Map of Malaysia (Map of Malaysia 2013 ).
Air temperature is generally high at all times in the humid tropics, but is extremely low compared with that in the dry tropics or sub-tropics (Jarrett, 1977 ). In fact, the temperature differences are negligible between seasons and the diurnal temperature range during a day is insignificant (Faniran and Jeje, 1983 ). Additionally, the temperature does not differ considerably within places because the amounts of net radiations received by these places have very slight differences. Furthermore, most of the tropics have ocean surfaces, which are enormous heat storage reservoirs that cause significant temperature differences over short distances. Therefore, temperature decreases at an average rate of approximately 1 °C per 150 m in altitude (Senior, 1979 ; Reading et al ., 1995 ; McGregor and Nieuwolt, 1998 ). According to a 10 day Agromet Bulletin issued by the Malaysian Meteorological Department (MMD, 2013a ) diurnal air temperature ranges from 22 °C to 33 °C have been recorded; whereas MMD (2013b) recorded a significantly higher normal temperature, especially with diurnal air temperature ranges of 19 °C to 37.8 °C. Malaysia is exposed to a very large amount of solar insolation, ranging between 1400 and 1900 kWh/m2 (Ahmad et al., 2011 ), with an average of approximately 1643 kWh/m2 per annum (Haris, 2008 ) and more than 10 sun hours per day (Amin et al., 2009 ). The position of the sun in Malaysia, especially in Penang, peaks in the orbit at 1:15 pm to 1:30 pm throughout the year, with the altitude reaching a minimum of 60° around December and January and a maximum at approximately 90° at the end of March, April, and September, depending on location.
Undoubtedly, more heat will be generated with more intense levels of the sun rays. Thus, air temperature will fall as air rises into the atmosphere, which causes cloud formation. Clouds are abundant in the tropics and form in different sizes, from small isolated cumuli to large cloud ensembles (Hastenrath, 1985 ). The appearance of clouds depends on seasonal variations attributed to the changing locations and intensity of the Inter-Tropical Convergence Zone (ITCZ), relative to the low surface pressure, rising air movements, and convergence of air masses (McGregor and Nieuwolt, 1998 ). Furthermore, diurnal changes vary in terms of coastal versus inland location. Coastal stations experience the highest cloud cover during the early morning and clearer conditions in the early afternoon, whereas inland stations encounter the highest cloud cover in the late afternoon (Reading et al., 1995 ). Bittencourt (1993) affirmed that in the warm humid climates, the typical sky condition is partially cloudy. Clear sky appears to be rare (approximately 4.5% on average), whereas overcast skies are almost above 15%. Zain-Ahmed and Sopian (2002) measured the Malaysian sky and classified it as intermediate mean and overcast sky with illumination between 60,000 and 80,000 lux at noon during months when solar radiation is the highest. Field measurements revealed that the illuminance values can exceed 100,000 lux in Shah Alam and 140,000 lux in Bangi. Zain-Ahmed and Sopian (2002) indexed the characteristics of Malaysian skies as predominantly intermediate 85.6% of the time (2.3% intermediate overcast, 66.0% intermediate mean, and 16.3% intermediate blue) and 14.0% overcast.
Additionally, the MMD (Sunshine and Solar Radiation, 2013) stated that having a full day with a totally clear sky, even in times of severe drought, is very rare. The amount of solar radiation that penetrates depends on the cloud factor; less than 20% may be diffused on a very sunny day, but 100% of the energy available may serve as diffuse radiation on a cloudy day, which affects the level of natural light (Szokolay, 2008 ). Hopkinson and Kay (1972) further asserted that the hot-humid tropical climate is characterized by skies with very high brightness because of the increased variability of cloudiness, along with regular seasonal sunshine. Oakley (1961) stated that the atmosphere contains a high amount of water vapor, which makes skies typically cloudy. Large clouds and high humidity will increase the intensity of radiation through the reflection of bright sunshine. The sky is therefore the main source of heat, rather than the sun, under overcast conditions because the sky reduces the outgoing radiation.
Water vapor in the air generally indicates that air can become warmer and ultimately cause weather conditions. In the humid tropics, most areas have a high level of relative humidity, which is close to the saturation point, especially at night, but this amount is rapidly minimized during the day. The vapor pressures can generally exceed 25 mb, and relative humidity levels are constantly higher. The RH in Malaysia is very high and normally exceeds 80% sunshine and solar radiation (MMD, 2013c ). Furthermore, humidity with pollution has significantly affects the air, which tends to diffuse direct sunlight. This condition causes skylight illumination levels to rise at the expense of the intensity of the sunlight beam (Baker and Steemers, 2002 ).
Precipitation that can fall in the form of rain results from a large amount of moisture absorbed by clouds. Seasonal variations in the tropical climate are mastered by changes in precipitation. Rainfall is the main determinant of the season in the tropics (Reading et al., 1995 ). A humid tropic has a higher intensity of rainfall that occurs over short periods of time. Extreme rainfall tends to occur in places near the sea coasts and equator (Senior, 1979 ; Faniran and Jeje, 1983 ). The humid tropics additionally receive various amounts of rainfall from year to year. Rain is an important factor to deal with. Skies generally become overcast during rainy days, thus affecting the light level. Moreover, rain is always associated with wind movement (Bittencourt, 1993 ).
Humid tropics are generally distinguished by light and variable wind movement. This area is considered a sensitive issue relative to climate, such that different places in the humid tropics are given various values. According to the MMD (Wind flow), Malaysia is considered a maritime country with a general wind flow pattern that depends on the effects of land and sea breezes. According to Azusa (2009) , this parameter will be considered only in coastal and mountainous areas, but not in urban areas. Additionally, Al-Obaidi et al ., 2014a ; Al-Obaidi et al ., 2014b found general wind flow in urban areas to be insignificant and unpredictable.
As a result, the Malaysian environment is restricted and cannot rely only on passive practices. Most passive designs adopt a combination of several combinations of passive and active methods (Rahman et al., 2013 ).
The roof skylight system is the most sensitive part of the overall building envelope that mediates transparent and opaque attributes. Transparent elements represent approximately 90% and are among the most advanced elements that influence both daylighting quality and thermal conditions; whereas opaque components, such as mullion and sash assemblies, constitute almost 10% (Heschong and Resources, 1998 ). In fact, regardless of location, these attributes will be under the effect of solar radiation, which influences behavior that can maximize or minimize internal load. Therefore, the characteristics of light and heat that interact in a roof skylight system and how these factors behave environmentally should be considered and understood. This part is divided into two sections, namely, lighting and thermal interactions.
Visible light represents 47% of the total solar radiation that falls on the roof skylight systems (Robertson et al., 2003 ). Visible light is composed of only relatively narrow band wavelengths between 760 (Red) and 380 nm (Violet). Visible light is divided into two types according to the external condition, direct sunlight and indirect daylight , both of which have different effects on the roofing system. Einstein׳s theory of relativity has proven that nothing can accelerate beyond the velocity of light and that traveling is always at the same speed. Light transfers by photon and the speed of such photon depends on the amount of energy radiated by the source ( Roberts et al., 2007 ). When light incidence exists on a surface, the light passes through different stages as transmitted , absorbed , diffused , reflected , and refracted in the material, which affects its speed and amount ( Taylor, 2000 ). Therefore, the velocity of light can be noted as approximately 299,792 km/s in space when it travels in the same medium, whereas its speed obviously changes directly as in glass at 199,861 km/s when it crosses from one medium to another.
From the architect’s perspective, Szokolay (2008) identified the light incident on the surfaces in three ways: reflected, absorbed, or transmitted, the corresponding properties of which are equal to 1. Meanwhile, light behaves widely according to material properties that induce varying behavior, as in Szokolay׳s assumptions. According to Taylor (2000) and Vandergriff (2008) , light can be reflected directly, but not all reflections obey the standard law. Real reflection appears in varied forms: regular or specular , spread , diffuse , mixed , and scattered . Moreover, the absorption factors vary, whereas materials absorb some wavelengths while transmitting others.
In fact, refraction is considered as the most important factor that affects reflection, dispersion, and transmission, which bends and changes the light velocity. To clarify, the refraction factor relies on two aspects: the incident angle (θ ) and the refractive index of the material ( David and Hunter, 2010 ). The incident angle depends on the sun path, whereas the refractive index is the ratio of the light speed in a vacuum to the light speed in a specific material. Snell’s law indicates that light traveling from a medium with a low index to one with a high index bends light toward the normal angle, whereas light traveling from a medium with a high index to one with a low index bends away from the normal angle. In case a beam of the light׳s angle of incidence increases away from normal, the beam reaches an angle called the critical angle at which light is refracted along the boundary between the materials instead of being reflected or passing through the boundary (Taylor, 2000 ). This observation suggests that different refracting wavelengths will generate diverse speeds as a result of the occurrence of various wavelengths in a particular system.
Additionally, the refractive index directly affects surface reflections and total internal reflection. For example, a translucent medium enables the transmission of the majority of lights but reflects only a minimal amount from each of its two surfaces. This reflection appears whenever light transfers through a change in the refractive index (Davis and Wyman, 2007 ). Furthermore, the refractive index instantly affects dispersion, which occurs as the difference of the refraction index and light wavelengths. Materials that have a higher index of refraction generally tend to bend shorter wavelengths, such as blue light, by traveling more slowly than red light, which has a longer wavelength (Taylor, 2000 ).
Therefore, understanding the lighting performance of the skylight is important. Heschong and Resources (1998) highlighted two important aspects that should be identified for light transmission: how much light can pass through transparent materials and how much light diffuses. People generally assume that natural light is more efficiently received with a higher percentage of visible transmittance of the glazed material. This assumption is incorrect. The two properties that are indirectly related, namely, frosted or patterned glass that scatters light, exhibit a low percentage of visible transmittance while still permitting a very high level of light to pass through. Highly diffusing materials have to achieve uniform illumination to eliminate a hot spot that is extremely bright and uncomfortable. According to Boyce et al. (2003) and Al-Obaidi et al. (2013b) , understanding glazing material properties can have a significant effect on the quantity and quality of light, which can meet efficiency requirements.
Heat from solar radiation is considered as the greatest load that falls directly onto any roof skylight system, especially in the form of thermal energy (IR), which represents approximately 51% of the total radiation that falls on the ground (Robertson et al., 2003 ). However, the load amount can be increased when other types of radiation, such as UV or visible light (short wavelengths), are converted to IR as reflection, diffusion, and absorption in the external environment (Moss et al., 1982 ). Heat load represents a complicated issue in the tropics and involves the direct effect from the sun and indirect effect from the sky dome, as well as the surroundings. Heat generally transfers from warmer to cooler surfaces by three methods: conduction , convection , and radiation . Each method has different physical quantities that affect its surroundings. For instance, Frank and Klar (2011) indicated that the physical quantities of radiation field depend on space, time, area, solid angle, direction, and frequency; whereas conductive and convective heat fluxes depend only on space and time. They also stated that for any amount of solar radiation that strikes a surface, part of it will be reflected, a fraction will be absorbed, whereas the rest will be transmitted.
Various types of materials respond to the solar energy incident in different ways (Garg, 2007 ). The Energy Efficiency Fact Sheet, 2008 identified transmission, reflection, absorption, and emission as properties of a material’s surface that influence radiation heat transfer. Each property has a special response; for example, the levels of transmission are diverse in materials; some substances enable most of the radiant energy to cross, whereas others absorb more energy (Heschong and Resources, 1998 ). Undoubtedly, the amount of heat transmitted through the transparent material depends on the transmission property, which is based on two factors: reflectance and absorption. The reflective factor behaves differently at diverse wavelengths; for instance, ordinary glass transmits a very small proportion of the sun’s ultraviolet rays at 315 nm, which is less than 1%, but at 340 nm, the value reaches 41% (James, 2001 ). This finding indicates that a surface that reflects one type of electromagnetic spectrum may be unreflective to various sorts of the radiation. Indeed, many surfaces, even that of the shiniest metals, that reflect visible light are poor reflectors of IR or UV radiation, (National Radiation Laboratory, 2011 ).
Meanwhile, absorptivity and emissivity are special properties that have a strong working relationship with each other and interact with both external and internal conditions. A convenient relationship indicates that a surface that immediately absorbs radiation just as readily remits the absorbed energy back to the environment. This finding suggests that surfaces with high emissivity are very absorptive, and vice versa (Frank and Klar, 2011 ). In fact, absorption refers to the capability to absorb shorter and longer wavelengths, whereas emissivity refers to the emission of only longer wavelengths in the form of heat. However, these factors depend on the type of material and the medium temperature (James G Group, 2001 ).
By contrast, convective and conductive conditions are totally different. The heat transfer in natural convection depends on the surface geometry, orientation, variation temperature, and thermo physical properties of the fluid (Energy Efficiency Fact Sheet, 2008 ). According to Szokolay (2008) , the rate of convection heat flow and the velocity of fluids depend on the convection coefficient that relies on the position of the surface. The heat that flows up from horizontal surfaces (air to ceiling, floor to room air) is always higher than heat that flows down (air to floor, ceiling to room air) because of the feature of hot air, which rises upward. However, heat conduction represents the final load that forms under the effects of radiation and convection when they fall and interact in a material. Obviously, heat behavior relates directly to the material properties, which require special consideration. These properties include thermal conductivity, specific heat, density, thermal transmittance (U value), surface exchange coefficients, and thermal mass ( Oral and Yener, 2004 ; Szokolay, 2008 ). These factors are very important to identify, regardless of the size and shape, because they determine the amount of energy that can be stored and released. As a result, comprehending the interaction of heat load in the material properties will aid the architect in evaluating heat characteristics easily.
The indoor environment refers to the overall aspects of the building׳s interior and the occupants׳ health issues. Various factors may contribute to the quality of the indoor environment. These factors include heating and cooling, daylighting, natural ventilation, indoor air quality, and the presence of volatile organic compounds released by certain building materials. This paper focuses on the roof skylight system. Thus, architects need to perform the assessment based on the linkages between daylight illuminance levels and solar heat gains with the eyes and skin as body stimuli (Kittler et al., 2012 ). The effect of light and heat on the internal load represents the main factors affecting human comfort in a built environment. The significance of these loads stems from the modification by the roof skylight system, which gives rise to unique characteristics. As a result, evaluating the behavior of these factors in terms of the final load will help estimate the actual impact level that controls the performance outcomes. This part is divided into two sections: lighting and thermal load.
The attributes of the roof skylight system generally cast natural light over an area in a more uniform manner that is less likely to be obstructed either externally or internally (McNicoll and Lewis, 1994 ). Thus, skylight admits more light per square meter than vertical windows, thus being three times more effective. This finding is attributed to the admittance of sunlight and daylight entering directly from the sky dome. In fact, natural light that passes the roof skylight supplies high illuminance levels, which enable perfect color discrimination and rendering to satisfy the conditions for good vision (Lester, 2010 ). However, this condition can also create uncomfortable solar glare, high contrast levels, very high luminance reflections on surfaces, and non-uniform lighting conditions, thus conflicting with good vision (Steffy, 2001 ).
The natural light conditions in the indoor environment vary and change every minute, day, and year (Pages et al., 2003 ) according to the climate situation and mediator attributes. To identify the load capacity, human requirements need to be evaluated to estimate the light intensity. Visual comfort is an effective criterion used to detect comfort conditions in building interiors. AFE (1995) defined visual comfort as the satisfaction of the visual system and the absence of physiological pain, such as headaches, eyestrain, fatigue, and irritability (Dubois et al., 2007 ).
Generally, visual comfort suffers because of the changing condition in building designs attributed to the lighting preferences of people, which tend to vary with gender, age, and the time of day or year (McNicoll and Lewis, 1994 ). Visual comfort evaluation is subjective, contextual, and relative. Therefore, different factors could increase or decrease the criteria, such as the type of activity, dimensions of space, and light reflection coefficients of the internal surfaces. To reach an optimum daylight level, the roof skylight system should provide the appropriate light quantity for the specified visual task, suitable direction and uniform distribution, proper color rendering, absence of glare, and comfort range in lighting quality and intensity from area to area over time (McNicoll and Lewis, 1994 ; Ruck et al ., 2000 ). Most of these levels and values are identified in international investigations, and they are published as guidelines, standards, and regulations (Oral et al., 2004 ). Therefore, these requirements need to be evaluated by specific variables to estimate and identify the final load amount. Heschong and Resources (1998) , Ruck et al. (2000) , and Muneer and Kinghorn, 2000 identified illuminance, distribution, glare, direction, and color as visual comfort performance variables to specify the quantity and the quality of lighting.
To identify the quantity of light, luminance, illuminance, and brightness measure the capacity of the lighting load. However, these factors tend to confuse many designers. Illuminance and brightness are properties of the light source, which could be the sun and the sky. Both are difficult to observe, but illuminance as an observable property is an effective target to evaluate the light quantity for a task over time. In addition, the illuminance level is a standardized property of all the standards and guidelines worldwide. Guidelines, such as (IES 1993 , CIE-29.2, 1986 ) indicated that the amount of illuminance required for special functions varies according to the task, user age, speed and accuracy requirements, and task background reflectance. The comfort level can exceed the recommended level for tasks, such as reading and writing, by two or more factors without glare and heat gain.
Carlos et al. (2012) and Nabil and Mardaljevic (2006) identified that useful illuminance could range from 100 lux to 2000 lux, to provide the variability in a space while avoiding thermal discomfort at the same time. Moreover, Mardaljevic (2007) evaluated the illuminance range between 500 and 2500 lux. The high values in this range are more beneficial and highly desirable in terms of the well-being and productivity of building occupants (Heschong, 2002 ). Malaysian MS 1525 (2007) specified the illuminance range between 300 and 700 lux for the skylight area as shown in Figure 4 . However, the Malaysian Green Building Index for the Residential New Construction (GBI-RNC) is unclear on the range in which no lux levels are specified. Only the daylight factor ranging between 1% and 3.5% is indicated (Green Building Index-RNC, 2011a ). Even the GBI for the Non Residential New Construction (NRNC) has the same issue, indicating a level below 2000 lux to eliminate glare from all direct sun penetrations (Green Building Index-NRNC, 2011b ).
Illuminance range in skylight area as specified in MS 1525 (2007) .
By contrast, light quality evaluates the characteristics of the total amount of light that falls on a surface. Direction and diffusion, distribution, glare, and color are variables that specify the quality of light. Light direction and diffusion are considered as the main properties that identify the light attributes in the space condition. Obviously, direct light is more noticeable by the sharp shadows in an object; whereas the high amount of light diffusion creates less shadow, which decreases the ability of users to identify the depth, texture, and shape of a surface (Ruck et al., 2000 ).
However, controlling between directional and diffusion lighting enables a user to evaluate the specularity, iridescence, smoothness, grain, and other features on a surface. According to Ruck et al. (2000) , no standard exists to evaluate the direction and diffusion of the light, but the skylight system will produce a typically diffused omni-directional light.
Therefore, these variables affect the uniformity and diversity of light distribution from point to point over a surface or plane. The glare issue, which occurs as a sensation of high reflection and irregular distributions of brightness from a very bright source, also has an effect. The assessment of discomfort glare depends on luminance, size, number of glare sources, and background luminance. Moreover, glare could occur even without extreme contrast when illumination levels exceed 25,000 cd/m2 (Illumination, 1995 ; Szokolay, 2008 ).
Finally, the color of natural light is the last variable that affects light quantity and quality, which is very acute as it is intense. Daylight color affects the perception, and the quality of space also affects the perception of brightness contrast (Chain et al., 1999 ). Based on the continually changing daylight color from sunrise to sunset, as well as from day to day (Rasmussen, 1962 ), the need to visualize true color is important for tasks that involve color matching, quantity, quality control, and accurate color perception. For instance, a smooth and brilliant-white wall may reflect 85% of the light that falls upon it. A cream wall might reflect 75% of the light, whereas a yellow wall reflects approximately 65% of the light. Bright colors, like orange or vermilion, absorb as much as 60% of the light, but may create an impression of warmth in places where the sunlight cannot reach (McNicoll and Lewis, 1994 ). Furthermore, shading devices and glazing materials tend to modify the color of daylight (Chain et al., 1999 ). Even less daylight penetrating from a skylight will change the colors visually from their true state (Ruck et al., 2000 ).
Heat by solar radiation is the most noticeable load that transfers from the roof skylight system and one of the heat loads that affects the thermal sensation of occupants in a building. Solar radiation is a very critical issue in the building envelope with regard to its thermal mass effect where the heat stays longer. The amount of heat that comes from a roof skylight system is diverse and variable during a typical day and a typical year and relates to the climate condition and the influence of the material properties (Muneer and Kinghorn, 2000 ). To identify the amount of heat load, human satisfaction is used as a basis to evaluate its intensity. Thermal comfort is one of the general criteria which are currently gaining increasing interest among designers especially for daylight design (La Gennusa et al., 2007 ). This interest results from the awareness of the indoor performances that directly affect the energy demands.
ANSI/ASHRAE Standard 55 defined thermal comfort as the “condition of mind that expresses satisfaction with the thermal environment.” Thermal comfort can be achieved when the heat generated by human metabolism can be dissipated while the thermal comfort level stays in equilibrium with the surrounding environment (Mid-Atlantic Masonry Heat, 2011 ). Generally, thermal comfort is specified in two categories, namely, subjective factors that relate to the characteristics of a person (metabolic rate and clothing level) and environmental factors, which define comfort as the absence of any type of thermal stress. Subjective factors constitute the unstable condition because of different variables that affect heat dissipation from the body and fluctuate based on gender, age, race, activity level, consumption habits such as food and drink, body shape, body surface area, health condition, skin color, acclimatization, and environmental conditions ( Smolander, 2002 ; Thermal Comfort Chapter, 2005 ; Toftum, 2005 ; Khodakarami, 2009 ). To identify an optimum environment for everyone in a specific space is difficult because each person differs in terms of psychological and physiological satisfaction measures. Therefore, this paper focuses on the environmental issues.
Numerous environmental factors affect the thermal load, such as air temperature, radiant temperature, air velocity, and humidity, which are considered as the most reliable factors to evaluate the conditions. Nevertheless, one has to understand the thermal load types that cause heat gains before specifying the load capacity requirements. Szokolay (2008) identified six types of heat gains, namely, (i) conduction gains through the fabric, (ii) indirect solar gains from sun on opaque fabric (walls and roof), (iii) direct solar gains from sun through transparent fabric (windows and skylight), (iv) ventilation gains from ventilation and air infiltration, (v) internal gains from people and equipment in the zone, and finally (vi) inter-zone gains from adjacent zones.
This paper focuses on identifying the heat that comes only from solar energy. Therefore, understanding the behavior of solar energy crossing a medium and its distribution inside is more appropriate. According to Bradshaw (2010) , the solar radiation that encounters a mass has three possible outcomes. First, the radiation continues its travel unaffected (in which case, radiation is said to be transmitted) as a direct effect. Second, radiation is deflected from its course (in which case, radiation is said to be reflected or diffused) as an indirect effect. Third, radiation travel comes to an end (and is said to be absorbed) as the indirect effect. Direct solar heat gains, which penetrate through the glazing area and indirect solar heat gains that reflect, diffuse, re-radiate solar heat from absorption surfaces affect temperature of air mass directly and are considered to be the most important loads.
Generally, direct and indirect solar gains have strong relations, in which the first is variable based on the sky and weather conditions, and the second depends mostly on the period of direct radiation that affects the indoor environment longer. Both are more noticeable in the calculation for the indoor thermal conditions, which are mainly based on indoor air and inner surface temperatures (Tredre, 1965 ; Oral and Yener, 2004 ) as shown in Figure 5 . Our thermal comfort in a building is associated with the influence of surface temperature and the dry air temperature in that space. Consequently, when these two factors stay within the required level of comfortable temperature, the related humidity and indoor air flow become secondary parameters that influence the thermal comfort.
Calculation of the indoor thermal conditions (Oral et al., 2004 ).
Indoor air temperature is a more simplified factor that mostly relies on the air mass. Various studies have been done to standardize its level. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) recommended an indoor temperature within the range from 22.5 °C to 25.58 °C. ASHRAE proposed that most people would feel comfortable with these conditions regardless of the climatic zone (ASHRAE, 2004 ). Regarding the acclimatization factor, many researchers (Sharma and Ali, 1986 ; Nicol, 2004 ; Bradshaw, 2010 ) indicated that people who live in hot-humid tropical climates over long periods generally feel thermally comfortable at temperatures higher than those specified by ASHRAE standard 55 (1992) for summer comfort requirements in temperate climate. Regarding the physiological factor, the warm conditions sensed mentally in the brain react stronger than the cold receptor that exists in the human skin (Mayer, 1993 ). Therefore, the studies in Malaysia have different indications. According to Zain-Ahmed et al. (1997) , the optimum comfort temperature ranges from 24.5 °C to 28 °C; whereas Abdul Rahman and Kannan (1997) indicated that the optimal comfort temperature ranges from 23.4 °C to 31.5 °C with neutral temperature at 27.4 °C. In addition, Ismail and Barber (2001) found that the comfort temperature for Malaysian office workers in Penang ranges between 20.8 °C and 28.6 °C.
By contrast, the inner surface temperature can be identified by the mean radiant temperature (MRT), which is expressed as the total surface temperature of a building enclosure. MRT represents a unique and complex factor, which is independent from the air temperature, but is felt in combination with air temperature (Szokolay, 2008 ). As a second factor, the MRT is the most significant issue which must be evaluated in buildings, especially in spaces where the conventional indoor temperature and humidity control cannot guarantee indoor comfort. Its effects occur clearly even when the room air is maintained at a comfortable temperature, and occupants may experience considerable discomfort as a result of radiant heat exchange (Richmond, 1963 ). Furthermore, its effect is increasing particularly in spaces with glazing area because the temperatures of these surfaces fluctuate more significantly than the opaque fabric. MRT is an unstable factor which varies at a point in a room related to the effect of radiation types (diffuse or direct), intensity radiating from the boundary surfaces, and the temperature of the surrounding surfaces. Usually, the response of radiation in an interior fabric is a combination of temperature, emissivity (emitting characteristics), absorptivity, reflectivity, and transmissivity of materials (Richmond, 1963 ). However, to control these factors efficiently, the dimensions and the shape of a room, open area, orientation, as well as the features of the selected transparent and opaque materials should be considered. Based on Bradshaw (2010) , MRT is a weighted average of the temperatures of all the surfaces in direct line, which tends to stabilize air temperature near the room (La Gennusa et al., 2007 ).
Skylights deliver various lighting and thermal loads that provide the designer the opportunity to improve source placement to provide more uniform illumination from the sky and enhance privacy and security (Kroelinger, 2005 ). The Lighting Guide LG10 publication (1999), “Daylighting and Window Design,” demonstrated a number of categories. The shed, the monitor, and the saw-tooth are the most common types of roof systems (Phillips, 2004 ). According to a study conducted by Chps Best Practices Manual (2002) , the performance of the most common types of toplight is shown in Figure 6 .
The performance of the most common types of toplight systems (Chps Best Practices Manual, 2002 ).
Several studies cover the skylight design in several climatic conditions. The research is summarized in several studies on different aspects and sorted in various locations all over the world, especially in places that have high level of solar insolation.
Lee et al. (1996) conducted a study on the skylight system in California, USA/ North America (hot and usually dry climate). The study design skylight comprises three systems, namely, skylight opening and light well, a reflector array, and a low diffusing panel. The design improved the light redirection and balance throughout a deep perimeter space by employing geometry and a special prismatic film to reflect direct sunlight throughout the year. In addition, Beltran (2005) conducted a research in Lima, Peru (Tropical climate), and examined the daylight performance of traditional and innovative toplighting systems. Skylights with diffusing glazing are integrated with high performance glazing. The findings indicated that the skylights with reflectors have the best overall daylight and thermal performance among all the systems. They provide uniform light throughout the space. Toplighting systems with sufficient solar control and proper use of reflective surfaces can deliver both good daylight illumination and energy savings.
Yunus et al., (2011) conducted a study in Malaysia (tropical hot and humid climate) and examined the effects of roof forms and internal structural obstructions on daylight levels in the atria. The inclined roof creates various patterns of daylight reduction levels. For high angle and complicated profiles, the daylight level in the east and west surfaces declined more than half compared with the unobstructed atrium, and the atrium facing west had received more illumination at all floor levels. Yildirim et al. (2012) conducted a study on the roof skylight systems in Ankara, Turkey/Asia (continental climate). The study included five roof skylight systems, such as the single layer one-way roof skylight system, the single layer two-way roof skylight system, sunshade with double layers, the no sunshade with double layers, and moving sunshade with double layers. Light should be restricted in order to avoid glare, but with limited sunlight, light should be admitted at a greater rate. Based on these conditions, the double-layer roof system demonstrated superior performance and delivered a more uniform and sustainable lighting under all conditions compared to the other four roof skylight systems.
Most skylight designs all over the world focus more on the lighting outcomes than investigating the thermal or another sort of environmental loads except for several studies such as the simulation study conducted by Al-Obaidi et al. (2013a) to identify the performance of the roofing system integrated with skylight design. Therefore, the aim of this research is to introduce the ESI concept to comprehend the methods and environmental load behavior that interact in every stage in the architectural design. Generally, the evaluation process clarified by the theoretical analysis is based on three different parameters, namely, EEC, SRS, and IEC. Every parameter has been identified by specific variables to evaluate the light and heat performances. The ESI concept specifically analyzes three particular fields, namely, direct load (sender), mediator, and indirect load (receiver) to target the hidden issues. Figure 7 shows that the direct loads as external environment represent the dominant parameter that is beyond the control of the architect which circulates the entire process. The roof skylight system, the mediator, works as a filter that is responsible for most of the environmental behavior inside a building. This system consists of various features that could alleviate or aggravate the direct load based on the material properties. Finally, indirect loads which appear as part of the indoor environment represent the final and the most critical load that interacts directly with human comfort, as well as on energy needs. These loads are controlled by the external environment, roof skylight system properties, and indoor condition that form its total forces, i.e., quantity and quality.
Therefore, the ESI approach proposes the physical aspects in the climate, materials, and human comfort in one process to develop the architectural way of thinking rather than to evaluate the condition based on a limited point of view.
In general, the research study opens the possibility of delivering an overall picture of the hidden issues that limit the application of skylight systems in Malaysia. The Malaysian climate is sunny all year round, which makes it as an ideal place to use skylight systems. In addition, the hot and wet weather conditions throughout the year will identify only specific design system. However, the hot-humid tropical environment especially solar levels and sky conditions are considered as the most significant restrictions in this region. Furthermore, indoor requirements (light and heat) associated with solar energy require the alleviation and control of the excessive effects in order to provide a proper comfort range. As a result, these boundaries force any building system in this region to rely on several strategies to overcome the severe weather. These strategies should deal with these environmental restrictions by combining a number of design techniques to meet the requirements of the Malaysian built environment. Skylight design adopts a combination of several approaches, such as passive and active solar methods, because the Malaysian climate condition cannot solely rely on passive practices.
During the review of several skylight systems, these systems were found to be inappropriate for direct application in the tropics to balance the thermal and lighting loads. Therefore, we recommend that these systems should be integrated by using shading, glare protection, proper use of reflective surfaces, reflectors, prisms and multi-pane, using splaying and wells for skylight, as well as double-layered roof system, and taking advantage of different geometries, roof angles, orientations, and complicated roof profiles. These systems can deliver both good daylight illumination and reduce relative heat gain.
As a result, the study is presented as a sub-process of the design procedure that provides the opportunity to simulate a condition before the actual construction. This evaluation is significant to determine the optimal values for the design parameters by understanding the load stages that affect the architectural design. Moreover, the outcome from this analysis study can enhance the future of skylight design in Malaysia, and especially in domestic buildings which preserve the usefulness of the natural gift while exhibiting a different set of features.