A Review of Skylight Glazing Materials in Architectural Designs for a Better Indoor Environment

Glazing materials have a significant impact on the energy efficiency of the building interior. Poorly designed skylights, windows and glazed surfaces can create an uncomfortable environment. The effect of heat gain through an un-shaded glass can be 100 times higher compared to the same area of an insulated wall. Currently, various alternative glazings with different performances are being used daily. There are many choices and alternatives to consider, and careful selection is significant to meet the design requirements. The optical features of glazing materials influence daylighting quality, the thermal characteristics and the potential of energy savings. The increasing interest of skylights as an effective strategy among building professionals and building owners implies that they are keen to know about the performance and energy efficiency of various glazing materials used on skylights. The aim of this study is to review the glazing performance and glazing types that are applied in the skylight roofing system. The review targets at two important factors in glazing performance, namely, solar light and solar heat. The review emphasises this general relationship during the design stage to ensure greater visibility in the product selection, also to ensure various other advantages improved comfort in perimeter zones, reduced mechanical loads and improved energy performance.


Introduction
Energy consumption has become the current topic of interest particularly that in building designs.The indications show that buildings consume about 57% of total primary energy requirement of Singapore, 47% of Switzerland, 42% of Brazil, 40% of the USA, 39% of Great Britain, 28% of China, 25% of Japan and 23% of Spain (Dong et al., 2005;Levermore, 2008;Masoso & Grobler, 2010).Much of the energy in buildings is consumed by two significant elements as air conditioning systems and artificial lighting fixtures.In actual fact, the relation of these elements in most cases is connected, as most of lighting energy from the electric light systems are converted to thermal energy; consequently cooling systems are required to add more loads in summer time.Thus, designing buildings to be more environmentally friendly can lead to energy savings (Hassid, 2011).
Lighting building interior with natural light could be one of the contributors to help reduce energy consumption that provides a healthy indoor environment.Based on Edward (2005) there is an indication that the incident of sunlight rays on 1000 square centimetres of horizontal glass in sunny days can deliver efficiency almost twice that of the fluorescent lamp.Natural light, as clean lighting energy when it passes glazing materials, improves tenant's well-being and productivity, as well as decreases the energy used for artificial lighting.Nevertheless, it also brings with it thermal energy that adds a heavy load to the cooling system.
An appropriate design of skylight can minimize the cooling loads and can utilize the natural light as the most efficient source of renewable energy.There are several factors which control the design such as the size, the orientation, the placement, the amount of shading, as well as the glazing types.In reality, glazing types consider that the most essential factor must be addressed seriously in the skylight design (Carmody, 2007;Adelaide City Council, 2013).
Generally, the glazing material transfers various levels of environmental loads based on its properties.Understanding these properties will help the designer to control the excessive impact of natural light and guide the designer to select the most appropriate options.There are different studies carried out by Laouadi et al. (2002Laouadi et al. ( , 2003)), Gueymard and DuPont (2009), Freire (2011) and Kim and Todorovic (2013).However, all these studies are based on empirical researches which do not cover any studies relating to the physical glazing condition in the skylight design, as well as those which have not identified the performance of different glazing types for building professionals.Therefore, the aim of this study is to review the most important physical factors of glazing materials that influence the process of the architectural design especially those that relate to the performance of solar light and solar heat in various types of the existing materials.

Glazing Overview
Glass is one of the commonest man-made materials that have been in use for a long time.It was first discovered around 1500 B.C in Mesopotamia and Egypt, then the Romans further developed its production in the first century A.D. In 1665, Royal Glassworks for the first time manufactured mirrors for the 'Palace of Versailles' in France.In 1773, England became the world's leading manufacturer for quality glass for windows.In the 1950s glass manufacturing was transformed for good when Alastair Pilkington invented the float glass.It cut down the cost of glass drastically and formed innovative applications such as the exterior of high rise buildings (Garg, 2007).
Glass is a physical substance that has special features to prove the functional demands, which create the interior and exterior of a skylight.Therefore, skylight systems require some design considerations especially on the choice of materials for achieving durability and sustainability.According to Crisman (2010) the physical properties of glazing materials are divided into several factors: thickness, colour, mass, physical and visual density, texture and temperature or thermal behaviour.For instance, the different thickness of glazing and various sorts of colours influence the amount of daylight and solar heat being transmitted (Garg, 2007;Kittler, 2011).
Generally, there are several questions that have become debatable among the architects such as; How much light is transmitted through the glazing?; How much of the direct beam sunlight is diffused?;How much of the sun's radiant heat is transmitted through the glazing?; How much heat from the air will pass through the glazing?(Heschong, 1998).To answer these questions, the identification of the basic physics concepts of sunlight characteristics is required.

Glazing and Solar Radiation
Generally, the sun radiates solar energy by electromagnetic waves known as the solar spectrum.The solar spectrum that reaches the ground is divided into three bands: Ultra-violet light, Visible light and Infra-red (James, 2001;Kittler, 2011) as shown in Figure 1(a).Solar light represents about 47% of the total solar radiation that falls in the skylight systems.Natural light is divided into sunlight and daylight, and both of them have different levels of energy and impact.The velocity of light is approximately 300,000km/s in space when it travels in the same medium; however when it passes from one medium to another, it will change its speed such as in glass to be 199,861km/s.This deceleration in velocity is caused by several factors as the light is transmitted, absorbed, reflected and refracted in the glazing materials (Taylor, 2000) as shown in Figure 1(b).This means, the total amount of natural light is changeable inside a building based on the materials' properties that create different bands.However, this coating has a problem with heat gain and daylight, yet it may prove to be beneficial especially in winter time (Cardinal Glass Industries, 2011).

Insulating Glass
There are several types of insulating glass under this category, Transparent Insulation Materials (TIM) and Aerogel windows.Transparent Insulation Materials (TIM) have been developed primarily as insulating materials for wall structures, but over the years, this glass has become famously and broadly implemented in skylight systems as they can function as a glazing material as well.Commonly, this type of glazing consists of glass or plastic capillaries, fibre and gel or honeycomb structures sandwiched between two glass panes.Light transmission in this type ranges from 45% to 80%, with a decrease of roughly 8% for each sheet of protective glass used.In fact, the TIM can be combined into purpose-made skylight assemblies; however the costs tend to be about three times the cost of conventional double-glazed windows (Muneer & Kinghorn, 2000).
The second type is Aerogel windows.This type of glass is used in preventing heat loss from the interior.The criteria of these windows are that they are transparent, have low density, are extremely solid, have low thermal conductivity and are fragile.They fill the air space between window panels with a micro-porous silicate foam material which decreases thermal transmission.The daylight transmission is moderate, being in the range of 50% for 12mm glazing (The European Commission Directorate-General for Energy, 1994).There are two types, namely granular (in the form of granules) and monolithic (in the form of continuous slabs).

High Tech Glazing
There are few types of glass under this category.The most developed technique is the photovoltaics, where the glass sandwiches the solar cells designed to produce electricity from the solar radiation.Another type of high tech glass deserves an indication but is not at present, economical.They are the photochromic glasses, which respond directly to an environmental stimulus.These types of glass are designed to respond indirectly by the application of an electrical current which changes their visual and thermal features (Phillips, 2004).This type of glass is divided into three types namely Electrochromic, Thermochromic and Photochromic.The first model changes its optical absorption properties in response to an externally applied electric field and becomes dark or cloudy.The second type switches between a heat-reflecting and a heat-transmitting state at selected temperature thresholds.Finally, the last type of glass responses to changes in light intensity and gradually darkens and lightens.The function of all these glasses types is evident in the selective blocking of radiation (The European Commission Directorate-General for Energy, 1994).
Other types that could also fall into this category is the holographic film which is a technique used to diffract solar radiation instead of blocking it.The advantage of this type is that it is able to deal directly with direct incoming sunlight.This type of glazing can be tuned to reflect and diffract solar radiation at any waveband in the solar spectrum, while allowing 75-80% transmittance in the visible waveband (Muneer & Kinghorn, 2000).

Miscellaneous Glazing
This category of glass includes a number of different types of glazing, largely due to the difficulty of classifying them together into a single group.They comprise of the following; laminated glass and wired glass (safety glass), vacuum windows and super windows.The laminated glass is commonly used for security reasons as resistance to impact.Generally, it is found in museums where exhibits are exposed to daylight, and therefore, it will be necessary to control the entry of the UV light.However, this type reduces the transmission of daylight.On the other hand, wired glass is also one of the types used commonly for security reasons as resistance to impact, especially for the protection of skylights.It consists of wire mesh applied in sandwiched panels within the thickness of the glass (The European Commission Directorate-General for Energy, 1994).
Furthermore, vacuum windows are a combination with a low-emissivity coating on one of the internal surfaces.This approach aims to eliminate the radiant heat transfer and cavity gas convection (Green building advisor, 2009).Finally, Super windows comprise of multiple panes of glass or plastic films, with gas cavity fillings, insulating frame and spacers and one or more low-emissivity coatings.Super windows are commercially available now and the range will continue to increase (Muneer & Kinghorn, 2000).
Figure 7 gives a clear picture of the appearance of the glazing group that helps to generally identify the condition of the selection.Al-Obaidi et al. (2013) indicates that Polycarbonate is a very useful material for skylight systems in the tropical region.It also represents highly diffused light transmission and Low U-factors which make it a good daylighting material.It is many times stronger than glass and a great deal stronger than Acrylic which has good weathering characteristics.Polycarbonate has a great range of different colours like Acrylic that comes in clear, opal and various tints (Queensland Skylight Installation, 2013).
Based on National Association of Rooflight Manufacturers (2009), The American Architectural Manufacturer Association (2010) and Queensland Skylight Installation (2013) Polycarbonate is divided into sheet forms: Solid -Flat or Domed, Profiled and Multiwall.Solid polycarbonate provides superb workability and good optical clarity.It can be moulded into numerous shapes such as pyramids and domes.Profiled polycarbonate is a feature popular with many designers, as it matches profiled roof cladding and has very good profile accuracy.Multiwall polycarbonate is a glazing system which incorporates two layers of polycarbonate which is considered as an insulating glazing material that provides the best thermal properties.Thicker sheets with more walls achieve the highest thermal performance.Structured polycarbonate is generally applied in most domestic and commercial buildings.The visible transmittance ranges from 0.85 for clear single glazed, 0.50 for Bronze however in double glazed, 0.73 for clear and 0.43 for Bronze (Heschong, 1998).

Acrylic
Acrylic is an alternative to glass.It is a tough, transparent polymer of poly (methyl methacrylate) known as (PMMA) commonly known under the well-known trade names, Plexiglass and Lucite.Acrylic sheet is an ideal material for a wide variety of skylights systems.It has been the material of choice among manufacturers for years, all around the world due to its clarity, durability, weather resistance, impact resistance and low weight.Many colours are available but white, gray and bronze are the most common types.In fact, white is usually characterized as low, medium or high with the light transmission level of the coloured acrylic.Medium white is the most frequently used because of light and diffusion.Low white will typically produce 100% diffusion and 25% light transmission.Medium generally produces 95% to 100% diffusion and 50% light transmission.High produces 20% diffusion and 75% light transmission.However, gray and bronze colours are referred as tints and a standard medium level of tint will transmit light around 30% and produce 0% to 5% diffusion.Generally, double glazed skylights are the most common glazing used for industrial and commercial purposes (Queensland Skylight Installation, 2013).Furthermore, Solar Heat Blocker Acrylic is one of the acrylic materials which are spectrally selective Low-E coatings that have a unique feature which allows high levels of visible light transmission while decreasing solar heat transmission.The thickness of this coating delivers various levels of light transmission, infrared and ultraviolet.Solar Heat Blocker Acrylic can transmit visible light from 70% to 20%; block 50% to 95% of infra up to 99.9% of the UV light (Bristol daylighting systems, 2013).

Fiberglass
Fiberglass has been used for nearly forty years and has some superior performance qualities compared to other common skylight materials.It comes with different profiles such as monolithic, flat, dome or various other profiles and can come in two pieces with an air space for insulation.
Fiberglass has lower solar heat gain coefficient and comparable insulating (U-Factor) properties in comparison to polycarbonates, copolyesters and acrylics.In fact, it absorbs the UV B radiation efficiently, while permitting the UV A radiation.The increasing in obscuring the warm part of the light spectrum will curb the needing of air conditioning.It has a significantly different appearance than other skylight materials.Its colour is soft brown and with age, it tends to turn into various shades of yellow.In fact, in terms of the light transmission, the colour in the skylight is a matter of perception rather than reality.Of all colours, yellow negatively affects the perception of any plastic's clarity or light transmission (The American Architectural Manufacturer Association, 2010; Bristol daylighting systems, 2013).The visible transmittance ranges from 0.30 for Crystal and 0.20 for White with SHGC 0.30 and 0.23, respectively (Heschong, 1998).

Copolyester
Copolyester is similar in composition to the plastic used in water bottles, but is more impact resistant known by the trade names of UltraTuf-CX and TiGlaze ST.Copolyester has an excellent clarity, high diffusion that provides impact resistance and thermoform requiring less energy and lower temperatures compared to other plastic glazing materials.It is normally supplied for glazing applications in clear and translucent white sheet.It  -Glass material can deliver different degrees of energy efficiency; however it is an expensive decision to make.Meanwhile, plastics cost less and this is very practical when lighter loading is a concern.
-The visible transmittance to control the glare should be taken into consideration with moderate visible transmittance of (50-70% is a good starting point) this decision rely on the visual tasks, opening size and glare sensitivity.
-The size of glazing material especially for larger opening area requires lower visible transmittance and lower solar heat gain coefficient and vice versa.
-The differences between the dark glass types as modified glass and coating glass have to be accounted for.
The earlier glass types provide good solar control; however many dark glazings block light more than heat.Dark glass creates gloomier interior atmosphere that might be the cause of absenteeism, reduced productivity and increased occupant discomfort on a sunny day.However, coating glass is designed to have high daylight transmission.
-It is not possible that most of the plastic materials have sealed insulating glass (SIG) configurations and protective solar (low-e) coatings due to the inherent permeability of plastics.However, the U-factor can be decreased through the use of multiple layers of plastics.Recently, new technological advances have led to the reduction of solar heat gain by as much as 20 to 40% by using special plastic formulations and coatings.
-UV resistance is very important in glass and particularly in plastic.The review shows that Polycarbonate represents the best plastic materials however fiberglass is the worst due to its ability to absorb UV B radiation efficiently, while permitting UV A radiation.as Also, Copolyester can be damaged by the UV radiation -The colour of glazing materials plays an important role for tinted glass, as every colour delivers different levels of light and different impacts even with the same thickness.On the other hand, as far as the changeability of material colour such as fiber glass is concerned, the review shows that the colour soft brown changes with age and it tends to turn into various shades of yellow.
-Glazing is not the only element that can endure the front of the solar beam.There are other methods that could be used to overcome this such as the orientation of skylight design, shading elements etc.
-Understanding the concept of an efficient glazing material for daylighting or the light-to-solar gain ratio (LSG) is very significant as this term refers to the low SHGC in conjunction with a comparatively high visible transmittance.
-It is desirable to evaluate the manufacturers' literature for the desired visible transmittance and the lowest possible solar heat gain coefficient.

Conclusion
The review highlights the most important concern relating to the physical issues connected with the architectural designs.It identifies the most significant factors required during the design stage.The research shows that by comprehending the harmony of physical factors and glazing types, it helps to meet the requirements of most new energy-efficient buildings.Glazing materials must first be understood environmentally before any further decision is made.As architects, building designers and owners, this review offers the opportunity to evaluate the performance and energy efficiency of various glazing materials used on skylights.Even more so, it also offers greater flexibility in product selection, improves comfort in perimeter zones, reduces mechanical loads and improves energy performance.
Figure 8 a to identify Based on the Bristol daylighting systems (2013) Polycarbonates are 100% recyclable material where most manufacturers use up to 25% and it is one of the Leadership in Energy and Environmental Design (LEED) projects.According to

Table of SS
REINFORC PLASTIC