Role of building material in thermal comfort in tropical climates – A review

Author’s Accepted Manuscript Role of building material in thermal comfort in tropical climates-A review PK. Latha, Y Darshana, Vidhya Venugopal

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To appear in: Journal of Building Engineering Received date: 28 March 2015 Revised date: 27 June 2015 Accepted date: 28 June 2015 Cite this article as: PK. Latha, Y Darshana and Vidhya Venugopal, Role of building material in thermal comfort in tropical climates-A review, Journal of Building Engineering, http://dx.doi.org/10.1016/j.jobe.2015.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Title: Role of Building Material in Thermal Comfort in Tropical Climates - A Review

First Author- PK Latha, Sri Ramachandra University, Chennai

Second Author - Darshana Y, Indian Institute of Science Education and Research, Kolkata

Corresponding Author Dr. Vidhya Venugopal Professor Department of Environmental Health Engineering Sri Ramachandra University Porur, Chennai-600 116 Phone: +91 44 4592 8547. Fax:

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[email protected] [email protected]

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ABSTRACT Changing climatic scenario and raising temperature is likely to subject nearly 60 % of the working populations in India to thermal discomfort in their workplaces. Half of the total energy produced in the developed world is used to heat, cool, ventilate and control humidity in buildings, to meet the increasing thermal comfort demands of the occupants. Indoor workplaces many times cannot offer thermal comfort to workers attributable to the location, processes involved and resource constraints that may pose a negative effect on worker’s health, their ability to function effectively 1

and also on their work productivity. In most situations, mechanical cooling devices offer solutions that are neither environment friendly nor energy sustainable. These mechanical devices are nonfunctional and cannot offer thermal comfort without energy input. Hence utilization of advanced building materials and passive technologies in buildings may offer the solution for thermal comfort demands, substantially reduce the energy demand, impact on the environment and carbon footprint of building stock worldwide. This also could offer a sustainable solution in the context of predicted raising temperatures and constraints in energy availability especially in the developing world. The review particularly identified certain materials such as VIPs, PCMs, ACC, polymer skin, with good thermal properties with a potential to be incorporated in different parts of the building envelope to enhance thermal comfort. Light coloured external surfaces, reflective paints, window treatments and roof gardens are also discussed as preferred options to help reduce the heat load of the building. Keywords: Thermal comfort; building material; energy conservation; health risks; productivity; climate change

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Role of Building Material in Thermal Comfort in Tropical Climates - A Review

ABSTRACT Changing climatic scenario and raising temperature is likely to subject nearly 60 % of the working populations in India to thermal discomfort in their workplaces. Half of the total energy produced in the developed world is used to heat, cool, ventilate and control humidity within buildings, to meet the increasing thermal comfort demands of the occupants. Indoor workplaces many times cannot offer thermal comfort to the workers owing to the location, processes involved and resource constraints. This lack of thermal comfort in workplaces may pose a negative effect on worker’s health, their ability to function effectively, their psychology and consequently their work productivity. In most situations, mechanical cooling devices are the easy and the most preferred solution in providing thermal comfort, but become non-functional without energy input. The mechanical devices offering thermal comfort are neither environment friendly nor energy sustainable for the fast growing industrialized world. Hence utilization of appropriate building materials and passive technologies in buildings may offer the solution for thermal comfort demands, substantially reduce the energy demand, impact on the environment and carbon footprint of building stock worldwide. This also could offer a sustainable solution in the context of predicted rising temperatures and constraints in energy availability especially in the developing world. Locally available natural building materials, light coloured external surfaces, reflective paints, window treatments and roof gardens are discussed as preferred options to help reduce the heat load of the building. The review particularly identified certain advanced materials such as VIPs, PCMs, ACC, polymer skin, with good thermal properties with a potential to be incorporated in different parts of the building envelope to enhance the thermal comfort within workplaces. Keywords: Thermal comfort; building material; energy conservation; health risks; productivity; climate change 3

1. Introduction Buildings are large consumers of energy in all countries, especially in regions with extreme climatic conditions and a substantial share of the energy goes towards heat and cool buildings. Though there are multiple ways of reducing the heat and air-conditioning load in the buildings, notable among them are proper design and selection of building envelope and its components. The increase of the thermal loads in the building is primarily due to the arrival of office computers and lighting requirements that has made the installation of air conditioning systems necessary to neutralize these loads and to create a comfortable indoor thermal environment. The new European energy regulation now considers a high standard of thermal protection in buildings with reasonable energy consumption, satisfactory thermal comfort conditions and low operational costs [1]. American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) Standard 55 defines thermal comfort as “that state of mind which expresses satisfaction with the thermal environment.” It involves the well-being of the occupants in a particular environment for a particular climate about their capacity to adapt to thermal equilibrium, physiological, psychological and behavioral changes [2]. Thermal comfort is dependent and influenced by

a range of

environmental factors viz. air temperature, radiant temperature, humidity, air movement, metabolic rate or human activity, clothing [3,4] and other personal factors such as; metabolic heat, state of health, acclimatization, expectations, and even access to food and drink [5]. Hot indoor thermal environment at workplaces may lead to a range of to heat related symptoms or illnesses like heavy sweating, weakness, dehydration through sweating, low blood pressure, salt imbalance leading to sharp muscle pain or cramps, fainting or reduced mental ability and even death [3] and could be aggravated/influenced by high metabolic work load, radiant and air temperatures or relatively impermeable protective clothing [6-9].

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Numerous studies across the world have shown the impacts of hot working environments on the working population [10-15]. In the context of climate change, and in the view of predictions made by Intergovernmental Panel on Climate Change (IPCC) [16-19], the rise in temperatures across the globe is further expected to adversely affect the thermal comfort in the work places, health of the workers, consequent productivity losses and other related issues. Planning ahead of time to protect the workers from the future risks of Climate Change and implementing protective measures is one of the adaption strategies that is needed at this juncture. The Health and Safety Executive (HSE) previously defined thermal comfort in the workplace, as “roughly between 286.1 K and 303.2 K (13°C and 30°C), with acceptable temperatures for more strenuous work activities concentrated towards the bottom end of the range, and more sedentary activities towards the higher end”. The thermal comfort of occupants in workplaces is their wellbeing in a particular environment for a particular climate with their ability to adapt to the thermal equilibrium, physiological, psychological and behavioral changes [20]. Environmental thermal stress imposes health and productivity consequences in the occupants’ especially working individuals [21, 22]. According to the study conducted by Nag et al., in 2011, 80 % of the occupational groups in India exposed to higher indoor temperatures reported excessive sweat, thirst, tachycardia and dryness of mouth, 70 % reported feeling of elevated body temperatures and 33 % reported reduced urination and itchy skin. Thermal discomfort shall also impair a person's ability to do physical and mental work [23] with consequent reduction in worker productivity. In the United States, from 2008 through 2010, 99 deaths related to environmental heat stress were recorded by the US Bureau of Labor Statistics (BLS) Census of Fatal Occupational Injuries (CFOI). Thermal discomfort includes thirst, fatigue, and decrements in vigilance, visual tracking, response time, short-term memory, and auditory discrimination [24-26]. The results of a series of studies (1919 to 1927) by Vernon’s [27] in hot manufacturing industries like glass, steel, tinplate and munitions as 5

well as coal mining clearly indicated a decline in work rate/output and increased accident rates with increasing temperatures that is substantiated by observations made by Weston, 1922 [28] in the linen weaving industry [3]. In tropical settings and hot regions, the issue of heat stress is a much bigger problem in terms of thermal comfort and heat related deaths and illnesses [29,30], that especially affects working population who have physical exertion [17, 22, 23, 31]. The HSE, 2005 has stated that “engineering controls should be the first choice to reduce or eliminate such hazards” [32]. Although the initial cost of engineering controls maybe high, it has been found that the implementation cost is often offset by the resulting improvements including better working environment, workers’ health, reduced productivity losses and sustainable economic growth. To offset the hot working environment and provide thermal comfort to the workers, many workplaces have installed mechanical devices including air conditioners, fans and various kinds of ventilation devices that are energy intensive, costly to maintain and are operational with only electricity. Earth Tube Heat Exchanger System Coupled to a Space Model has been used to Achieve Thermal Comfort in Rural Areas [33] which could make a sustainable and energy efficient solution, but it applicability in large work spaces is yet to be tested. Buildings by itself can contribute to thermal discomfort and if this is not addressed while choosing materials for its construction and applying them appropriately in the design stage (apart from the location and orientation of the building), the issue of thermal comfort could be costly to handle at a later stage. A sustainable option is still debated and have not be widely used is a passive method to achieve energy efficiency and thermal comfort, that could be in the materials that are used to build the buildings itself. Use of eco-friendly and low thermally conductive building materials for living spaces/work places while at design and construction stage shall offer a sustainable solution to address the problem of heat stress due to Climate Change depending on the choice of materials and the intended use of the building space. 6

In recent years, the concept of green buildings has emerged that primarily aims to use ecofriendly materials and reduce the resource usage including energy demand that is gaining focus in the western world. In developing country setting and tropical regions, traditional knowledge in use of eco friendly and thermal resistant building materials has been passed on through generations which are being practiced in rural areas. But many workplaces have not been able to adopt the practices as a solution to address the issue of thermal comfort for various reasons. Eco-friendly materials used for construction such as bamboo, straw, timber, grass, linoleum, sheep wool, panels made from paper flakes, compressed earth block, baked earth, rammed earth, clay, vermiculite, flax linen, sisal, sea grass, cork, expanded clay grains, coconut, wood fibre plates etc. are both natural materials and also reduce the energy usage within buildings [34-36]. To benefit from the use of these materials in building, however, one needs to check their material properties like specific heat, thermal conductivity, transmissivity and heat transfer coefficients (for convective and irradiative surfaces) to ensure their fair thermal performance and load bearing properties in the building envelope. The present review focuses on materials used in green buildings and other materials, compounds or molecules that have the properties to respond to heat and may be incorporated into the workplace built environments itself to provide thermal comfort to its occupants that will have positive health and productivity consequences. 2. Factors Influencing Thermal Comfort of Building The thermal comfort of a building in addition to energy saving is influenced by various factors, including the thermo physical properties of the building materials, building orientation, ventilation, building space usage and integration of modern and passive energy saving technologies. The envelope of a building is not only a separator from the external environment but also a protection from climatic elements affecting the building directly [37]. The internal thermal comfort is dependent on the properties of the building materials used and that are affected by the external 7

temperature and humidity [38]. Heat and cold can enter the building through transparent and translucent materials, windows and the indoor conditions are influenced by the thermo physical properties of the materials. Materials having lower thermal conductivity, thermal diffusivity and absorptivity has been shown to have less temperature swing on the inside surface of the walls compared to the materials with high thermal conductivity [39]. Some building materials that have low thermal conductivity such as nylon, polystyrene foam, polyurethane foam might not provide the optimal thermal comfort especially when used for flooring in hot and humid environments, could be incorporated in other parts of the building to achieve the thermal comfort. Ventilation is a very important attribute in enhancing thermal comfort, with or without use of advanced building materials. Building envelopes that lack ventilation have the heat trapped in the building that could further worsen the thermal comfort of the workers. An attempt has been made in this review to discuss materials with good insulation properties and that have been reported to improve the thermal comfort inside the buildings. Some conventional materials have higher thermal conductivities and diffusivities in comparison to some advanced materials and hence increase heat transfer that does not favor energy saving or thermal comfort [40]. Improvisation of some conventional materials by modifying and adopting a better composition, design and/or integration of the technology have been suggested for addressing future energy needs with added co-benefits of thermal comfort [40]. Several studies have proved that the most optimal building orientation is the key critical energy efficient design that could have an impact on the energy performance within the building envelope to provide thermal comfort to the occupants [41-44]. In addition to the building orientation, research shows that natural ventilation improves thermal comfort in buildings that are located in hot and humid climates [45-50]. Natural ventilation can be become an integral part of the building envelope by introducing any one of the following ventilation elements viz., wind scoop, wind tower, chimney, 8

double façade, atrium, ventilation chamber, embedded duct and/or ventilation opening in the facade [51, 52]. A successful architectural design used in Persian buildings that helped in natural ventilation of the interior spaces in buildings is a dome with openings at the peak and was reported that the curve shape produced pressure difference influenced the air to flow from the outside to inside that provided direct cooling to the interiors [52] according to Bernoulli equation that could be applied in certain conditions [54]. There is a growing interest in the application of natural ventilation in buildings due to energy requirement, indoor air quality and environmental concerns associated with mechanically ventilated buildings. Many studies have reported human thermal sensation in a naturally ventilated environment being much better than in a mechanically controlled thermal environment [55, 56], a theory that holds well in buildings in fairly clean localities. In many workplaces, the orientation of the building and consequent natural ventilation may not be an option owing to industrial zoning, spacing issues or other regulatory reasons, where building materials might be the next sustainable and easy to adopt option. A study by Energy Consumption Guide (ECG 19) (1993) [57] stated that the final energy cost of a naturally ventilated building is 40 % less compared to an air conditioned building which definitely makes natural ventilation a more sustainable way to solving the problem of thermal comfort with co-benefits of energy saving. Though there are not many studies in tropics, the control of humidity in the supplied fresh air and consequent humidity gradient in the building must also be considered as an option for energy efficient cooling and thermal comfort of inhabitants [58-61] The usage of building space plays a major role in thermal comfort and the energy usage of the building. High heat processes and radiant energy release from machinery decreases both the thermal comfort and increases the energy usage to maintain an optimal level of thermal comfort inside building, especially in hot climates. New technology that help with the reduction or suppression of 9

air conditioning has been successfully demonstrated to achieve energy savings by using passive or low-energy techniques like Earth-to-Air Heat Exchanger (EAHE) and the solar chimneys (SC), by ventilating air to the indoor spaces, using the ground’s potential thermal capacity [62-64]. Solar chimneys are reported to be very effective in hot climates with their high cooling capacities and in conjunction with natural ventilation they can help generate electricity [65-67]. 3. Building materials providing thermal comfort Conduction, convection and radiation of heat transfers from buildings predominantly occur through the building envelope, windows and the ceilings [68]. Materials like marble, gravel concrete, asphalt are good conductors of heat and must be avoided in external construction, whereas materials that transfer minimum heat from outside to inside viz., certain kind of glass materials, wood may be chosen for walls, ceilings and windows for a cool interior. Windows also act as a medium of heat transfer where the glass absorbs and traps the heat within the room. Mutual radiation between the wall and the ceiling also affects the internal temperature, and subsequently the thermal comfort of the occupants in the building. Study shows that materials that reflect rather than absorb radiation and more readily release the absorbed quantity as thermal radiation will cause to the lower temperature within the structure [69-72]. The excessive heat transferred through the roof inside the building is one of the main causes of thermal discomfort in warm humid climatic conditions, which prevail in the tropical zones [73]. A study by Kunzel etal., suggest that building materials should be non-hygroscopic and capillary-inactive (hydrophobic) as water can be a silent culprit that can damage the thermal insulation lining of a building [74]. However, research into dynamic moisture storage in hygroscopic building materials has renewed the interest in the moisture buffering capacity of building materials and has shown the potential for these materials to improve indoor humidity, thermal comfort and indoor air quality in buildings [75].

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`Natural building materials have been used from time immemorial and the building materials made with natural ingredients have its pros and cons in today’s constructed environment, including issues with the load bearing capacity and durability of these materials. A wide range of conventional and natural materials have thermal conductivities that could be suitable to be used in buildings to have optimal thermal comfort [40]. In the recent years, lot of advancement and technology innovations have happened in building material and such materials have a great potential to be used in tropical regions for achieving thermal comfort and energy saving. Contemporary research reports several types of advanced materials such as gas-filled panels, polymer skins, aerogels and vacuum insulation panels which have the potential to provide better thermal insulations depending on the locations and the thickness of the material [76, 77]. Thermal performance of advanced materials [40] has been claimed by various researchers to play a lead role in energy efficiency and in providing thermal comfort. However in low-resource settings, the use of materials that are abundantly available locally would the most sought after materials for construction, with due consideration for the durability, ease of maintenance and cost. Often a combination of natural and synthetic construction materials have been used to get the desired thermal comfort without compromising on the structural integrity [68, 79]. While modifying the composition and using a mix of natural and synthetic materials, it must be borne in mind that the selection of the characteristics of the materials used must be dependent on the usage of the building and the weather conditions to achieve optimal thermal comfort.

3.1 Building materials made with Natural / Indigenous materials: Cork insulation has been used as a rigid insulation material for decades in Europe. Cork granules are compressed at high temperature and pressure to provide low thermal conductivity [80] and are widely used for construction applications, flooring, flooring-underlayment, interior or exterior wall 11

buildings and ceilings, to provide thermal and acoustic insulation in North American and European buildings [81]. Depending on the cost and the local availability, expanded cork may find its application in tropical settings for providing insulation, especially in the roof. Wood and Timber are known to be good thermal insulators and is acceptable in terms of thermal insulation properties that are suitable for all kinds of applications in windows, doors, roofing, flooring etc. [82, 83]. Since wood is a hygroscopic material, the thermal properties of wood are functions of moisture content and the type of wood. The treatment of wood and its application is predominantly dictated by its usage in the building and the climatic condition of the locality. Wood products such as fiberboard and hardboard panels made from fibers or fiber bundles have thermal conductivity values less than solid wood due to the large number of air spaces in the fiber based panel [84]. According to the Healthy Building Workshop 2000 [85] wood and wood-based products in the building envelope and furnishings control the indoor climate by moderating the diurnal changes in indoor humidity. Wood has higher heat capacity (1.6-3 kJ/kgK) and relatively low density compared with other building materials such as glass, rubber, plastic, concrete, brick etc., [106], and find suitable use as thermal insulators due to its low thermal conductivity [86, 87]. Straw, as a fiber, has been used as part of building materials for several years is biodegradable and has minimal environmental consequences which became popular technology in Mexico, France, Finland and Australia. A carefully constructed straw-bale building has excellent thermal performance because of the combination of the high isolative value of the bales and the thermal mass provided by the thick plaster coating of the interiors [88]. Straw is a thermally resistant material and according to Oak Ridge National Laboratory, it has a high thermal resistance value ranging from 6.51 to 7.82 W/m2 K for 55 cm thick straw bale [89] and can be most effective insulators when kept under dry condition [90]. Plastered straw-bale construction creates a longlasting, super-insulated buildings offering thermal comfort and are being used in building load12

bearing and in-fill straw-bale construction [89, 91]. Studies show that straw-bale building have better thermal performance than other materials used for walls in tropical climatic conditions, but with inherent disadvantages like less density to store heat in the building fabric and low load bearing properties making it unsuitable for taller structures [91]. Loose straw is highly flammable, however, bales of straw are compacted tightly enough that they deprive any would-be flame of needed oxygen. In addition, finishing products typically used on straw bale walls (plasters and stuccos) are fire resistant, and mold resistant to avoid moisture infiltration [92]. Rockwool insulation, constructed out of real rocks and minerals, can be used to make a broad collection of goods due to its outstanding capability to obstruct sound and heat and finds widespread application in building assembly [93]. Rock wool is a superior conductor of warmth, but rolls and sheets of this insulation are extremely proficient at stopping heat movement and also meet the principle of sustainability, power protection and recyclability [93] with added advantages of being environmental friendly and non-ozone-depleting properties [94]. Bricks are one of the most important construction materials that have high heat capacity [95]. Clay brick has a high thermal conductivity of 0.82 (W/m. K) and provides high thermal mass performance [96]. The commonly available red bricks require comparatively less energy to sustain thermal comfort conditions and the internal temperatures in the red brick buildings remained fairly stable despite external diurnal fluctuations which make it a good choice for building material [97], especially in hot climates. Reports say that fiber reinforced mud brick keeps the indoor temperature cooler during summer [98, 99]. Use of Fly Ash Bricks (FAB) is also gaining momentum and finds major use as building materials in Green Buildings. FAB not only solves the problem of disposal of the byproduct from power industry but also has better thermal conductivities (0.90-1.05 W/m K) compared with the conventional bricks (1.25 – 1.35 W/mK) [100, 101].

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Buildings with FAB have better thermal performance and provide cooler indoor environment than buildings built with clay bricks [102, 103]. Central Public Works Department (CPWD) has used 8.8 million FAB during the years 2003-2004 and currently using FABs it in all their construction projects in Tamil Nadu in many parts of India [104]. New emerging technology bricks have better thermal performance and Paki Turgut and Bulent Yesilata showed that the physio mechanical and thermal performances of the rubber added bricks fared 5-11 % better than their traditional counterparts [105]. An analysis by the simulation performed by Alahabad et al., revealed that indigenous materials have better thermal properties as compared to contemporary building materials [91]. Similarly, other natural products like raw or recycled sheep’s wool or cotton rolls with additives like boric acid and flame retardants with supporting polyester fibers, Hemp mats, and fleece from flax fibers also serve as good insulation materials [106]. Loose fill insulation material obtained from rye grain, rye pulp, and other additives can be useful when cavity walls in wooden constructions have to be insulated [85, 107]. Another versatile insulation material that is widely used in masonry is Tuff stones and act as good heat insulators owing to their porosity. Walls made with tuff stone are biodegradable and have been found to be more durable compared with other contemporary building materials with high embodied energy [108, 109]. While they are also more appropriate and affordable by the common man, who is the most affected by thermal stress, one of the disadvantage lies in their higher maintenance requirements. Although fired bricks are versatile and fares well in all seasons, the stone rock provides the best and the fired brick provides the worst thermal comfort during summer [97]. Stone rocks especially granite and marble can be used for flooring, providing cooler indoor temperature [109, 110] and is very popular in tropical settings.

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3.2 Building materials made with Synthetic substances: Concrete is the most common building material with a high thermal mass and it has the property to store heat and delay the conduction of heat through structural elements and thereby provide thermal comfort. Studies show that cement paste has low thermal conductivity and low volumetric heat capacity and the addition of sand and aggregates significantly increases both these properties [111]. Regular concrete has a thermal conductivity range from 1.3 to 1.5 W/m K with a relatively high moisture supplement of 8 % [112]. Use of rubber from scrap tires in Portland cement concrete (PCC) mixtures resulted in significant benefits that included lowered density, increased toughness, ductility and efficient heat and sound insulation [113]. Aerated Autoclaved concrete/Autoclaved Cellular concrete (AAC) are considered highly sustainable building material requiring low energy and raw material consumption, are non-toxic and are durable, that have good thermal performance. AAC has been incorporated throughout the Metropolis Center (MC) facility recently completed at LANL (Los Alamos National Laboratory Sustainable National Guide) [114] for its thermal properties and versatility. AAC has high thermal capacity and absorbs large quantities of radiant heat and does not transmit it through the structure rapidly [115] and using coal Bottom Ash (BA) in AAC formulations also has been shown to reduce thermal conductivity of the conventional cement [116]. A typical 20.34 cm thick AAC wall without adding insulation can provide an R-value of 13.28 with the added benefit of its ability to retain the coolness from mechanical air-conditioning for a longer time [115, 117] which makes it a preferred material for use in hot humid tropical locations for thermal comfort and energy saving [118]. For 5 specimens of aerated concrete with densities varying from 390 to 900 Kg/m3, an increase in conductivity was noticed with increasing moisture content in the material [119]. The modern use of AAC began in the US in 1990 for residential and

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commercial complexes [120] and now is being used in structure, envelope and cladding applications in residences, hotels and in high-rise buildings of Mexico. Vermiculite Concrete is hydrous, silicate mineral that is classified as a phyllosilicate, expands on heating, is light-weight, absorbent, non-combustible and an excellent insulator that is environmentally beneficial and has low thermal conductivity of 0.047-0.058 W/m K [121]. The shape of vermiculite concrete influences the thermal conductivity properties and the plate-like vermiculite concrete was found to have 20-30 % lower conductivity, for any given density compared with the granular type [121]. Insulation materials such as Rigid foams (Expanded Polystyrene, Extruded Polystyrene, Polyisocyanurate, Polyurethane, etc.), Flexible foams (Polyethylene, etc.) and Spray foams (sprayin-place foams dominated by spray polyurethane foams, can be open or closed cell) also contribute to indoor thermal comfort. Expanded polystyrene foam (EPS) is the cheapest and least used foam board product on the market having an R value of 3.6 to 4.0 m2K/W per inch of thickness. Extruded polystyrene foam one of the most widely used foam board insulation products in the residential construction industry having an R value of 4.5 to 5.0 m2K/W per inch of thickness. Polyiso is typically used with a foil facing and it has an R value of 7.0 to 8.0 m2K/W per inch of thickness. The reflective foil facing makes it an excellent insulation board when radiant heat is involved [122]. Phase change materials (PCM) have emerged as key role players as thermal comfort materials and trap heat by storing the thermal energy better when impregnated in the building materials. PCMs can be encapsulated in concrete, gypsum wallboard, ceiling and floor to capture solar energy directly and increase human comfort by decreasing the amplitude of internal air temperature swings and maintaining the temperature closer to the desired temperature for a longer period of time [123] and may minimize the need for air conditioning [124]. A search for technically feasible PCM has brought the focus on organic materials like paraffin waxes, like Paraffin (R20) that have been used 16

owing to its desirable thermal and physical attributes including its melting temperature of 293-295 K (20-22°C) that is close to the human comfort temperature [124]. While there is an opportunity to harness the full potential of energy saving by the use of PCMs in industrial setting, it also finds wide application in residential buildings. Polymer skins form a pneumatic cushion assembly with skins held in-between structures. The skins inflate or deflate on the basis of thermal conductivity requirements. These have been successfully applied in large construction projects like the Allianz football Arena in Munich, Germany and the Beijing National Aquatic Center in China. However, further studies need to be done on their applicability in smaller buildings [125]. Aerogels are synthetic porous solid materials and one of the lightest among building materials that have the highest success rate in providing thermal insulation by limiting the major mechanisms of thermal transport. The disadvantages are that they possess extremely low tensile strength and inhibitive production cost [104], which could be offset by providing a composite structure for a rigid aerogel panel, a sandwich panel with a truss the core filled with monolithic aerogel [126]. Vacuum insulation panel (VIP), another advanced insulation material [127] has extremely low thermal conductivity and has an insulation performance that is a factor of 4-8 times better than conventional insulation materials [128]. A typical VIP system has an inner core material, barrier envelope, a getter or (and) desiccant and a heat seal [104] and has a thermal conductivity as low as 0.002 - 0.004 W/m K depending on the core material used. Core materials act as the main fillers of the VIPs from which air is evacuated. Barrier films act as protectors from environmental and physical damages resulting from the panel handling. Desiccants/getters absorb water vapor and other gases that break through the barriers. The major core materials used are fumed silica, silica aerogel, expanded polystyrene and polyurethane foams, fiberglass and staggered beam, all of which are non-biodegradable and non-recyclable. A study by NRC-ICR reveals that fiber powder 17

composites made with traditional fiber insulation materials and volcanic powders have promising potential to be used as core materials in VIPs [129]. In the last decade, VIP have been used in building applications, especially in those projects that aim at passive houses, zero energy buildings, and zero emission buildings such as in small spaces requiring high thermal efficiency. The application areas include insulation of ceilings, walls, floors or roofs [130]. One analysis indicated that a traditional mineral wool foam insulation board with a thickness of 185 mm is equivalent to a VIP of only 20 mm thick [ 86]. This serves as an advantage by increasing internal space allowing significant space economy and by reducing construction energy and resources. VIPs are durable, but once they get damaged they may increase the indoor temperature and cause damages to the construction. The risk for moisture damages decreased when VIP was added to the exterior of an old exterior wall. A way to avoid unnecessary risks on the construction site is to integrate the VIP in prefabricated constructions and by providing proper training to the VIP handlers. Despite the growing number of studies on these potentially auspicious materials, the research on their applications for buildings has remained somewhat disorganized and localized [125]. Shape Memory Polymers in ventilation valves: Shape Memory Polymers (SMPs) that respond to temperature that act as a sensor for opening and closing of the ventilators used in conjunction with vents have been found to control the inner room temperature [126, 127]. SPMs could be combined with sensors or controllers connected to Heating, Ventilating, and Air Conditioning (HVAC) systems like air conditioners that can automatically switch off when the windows open which could help in energy conservation. SMPs are fairly easy to produce using many conventional polymer foaming techniques and complete shape recovery is observed in a 94 % pre-compressed polyurethane SMP foam even after a couple of months of storage [128]. The major area of SMPs application in improving thermal comfort could be as insulation in walls due to its ability to the

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contract in the presence of high heat, disallowing heat transfer by the gas molecules, which could also mean thinner walls and more interior space apart from lower energy costs. Though the synthetic materials discussed above stand-up to the thermal comfort component, the bearing of the embodied energy of these non-recyclable materials on the environment aspect is to be considered. PCM, AAC have lower embodied energy, as they are made using simple chemical processes, compared to aerogels which are found to be less than most alternative insulators. Though the embodied energy for VIP, SMP, and reflective paints are higher, the environmental cost may be recovered by energy savings by the use of these materials [129-132]. 3.3 Other building components Windows and the type of materials it is made up of have a large influence on the thermal comfort inside the building. Glass windows affect the building by heat transmission and compromised thermal comfort [133-139] and studies by Chaiyapinunt et al, showed that the thermal comfort was dependent on the optical properties (total transmittance and total absorbance) and the overall heat transfer coefficient of type of glass used in windows [140]. Singh et al, have also studied the impact of different glazing systems on human thermal comfort in an Indian scenario [141]. Window glazing like double or triple glazing, specialized transparent coatings, insulating gas sandwiched between panes, and improved frames provide lower heat exchange and air leakage that improve indoor comfort [142]. All of these features reduce heat transfer; thereby cutting the energy lost through windows. The discomfort from the surface temperature effect for double pane glazed glass window with a film is also dependent on the glass thickness other than the absorbance and the change in overall heat transfer coefficient [143-144]. Paul Baker in his study on thermal performance of the traditional windows showed that, providing shutters over the windows is the most effective option of the available traditional methods, reducing the heat flow by 51 % [145]. Adding an additional

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pane of glass and modifying the surface of the glass with a low-E coating, with low emissivity, will improve the insulation properties of the glass when compared with ordinary non-coated glass [146]. Building color and reflective coatings are among the factors that contribute to the thermal load in buildings and studies showed that use of light envelop color provides reduced indoor temperature and consequent improvement in thermal comfort [147-152]. Therefore use of a high emissivity, high albedo paint is an easy measure that reduces the indoor summer time temperatures [153-155]. Synnefa studied fourteen types of reflective coatings and found that white colored coatings performed better than aluminum-pigmented coatings [156]. A study by Akridge showed that the installation of a thermal control coating on a building reduced the peak roof temperature by 306 K (33°C) [157]. The surface temperatures of structures coated with white elastomatric coatings with a higher reflectivity were found to be cooler than surfaces coated with black coatings with lower reflectivity [158]. The roof surface temperatures were cut by about 293.15K (20°C) by coating a gray cool paint with titanium dioxide with excellent reflective qualities [159]. Hui Shen et al., studied the relationship between the thermal resistance of materials and coatings effect and reported that coatings perform better when applied on materials with smaller thermal resistance [160]. Solar Reflective Paint (SRP) by the virtue of its formula provide better thermal insulation [160] and cool colored paint formulations have higher near infrared radiation reflectance than conventional paints of similar colors and reduced surface temperatures by more than 283 K (10 °C) [161]. A multi-mineral eco hydro-paint based on a mixture of milk and vinegar, obtained from Mediterranean grapes formulated in a research lab in Cellini, Italy exhibited high solar reflectance (even for non-white colors), and was found suitable for use in all the construction materials including wood, concrete, plaster, metal, glass and roofs (both flat and tilted) [162] for added thermal comfort.

20

Roofs with high solar reflectivity (SRI) and high emissivity play a significant role in cooling the building. Akbari et al., compared cooling energy and peak power consumption of two identical school bungalows with different roof reflectivity and found a 3.1 kWh (35 %) savings in cooling energy as well as 0.6 kW peak demand savings from high-albedo roof [157]. Traditionally used red brown roof tile in India has a low brightness reference value of 10-20 % and short wave absorptivity of 0.6-0.8, making it a suitable building material for hot climates. Similar such material with thermal performance are lime silica brick (0.45 absorptivity, 55% brightness) and spruce wood (0.4 absorptivity, 50% brightness) [162]. Cool roofs with both high SRI and high emittance are good choices for hot climates as it heats up the roof only up to 316-319 K (43-46°C) [163]. Another study showed that use of Hollow Clay Tile (HCT) for the roofs had better energy saving and thermal comfort in comparison to the conventional tile [163-166]. School building in Kaisariani, Athens, Greece used white elastomeric coating over their concrete roof and found that after the cool roof application, the indoor air temperature was reduced by 274.7-275.2 K (1.5-2.0°C) during summer and 273.6 K (0.5 °C) during winter. In another success story, the roof of a Public building in Trapani, Italy was covered with eco-friendly paint where the indoor air temperature was 274 K (0.9°C) cooler than the outdoor temperature and reported a reduction energy consumption by 54 %. In an office building at Brunel University, UK the roof was coated with cool roof paint and it was found that internal air temperature was reduced by an average of 276-277 K (3-4°C ) [167]. The thermal properties of the various building materials discussed in this article that provide thermal comfort are summarized in Table 1.

21

Table 1: Thermal performance of building materials providing thermal comfort Sl. No

Thermal properties and performance

1.

Low Thermal Conductivity

Wood, cork, straw bale, Vermiculite Concrete, Vacuum insulated panel, Hollow clay tile

73, 74, 77, 102, 112, 119, 121, 150-153

2.

High Heat Capacity

Wood & timber, brick, Phase Change Material

86, 96, 114, 115

3.

High Thermal Mass

Straw, Concrete, Autoclaved Cellular concrete, Phase Change Materials

79,80, 105, 106,

4.

High Thermal resistance

Rockwool, straw, Vermiculite Concrete, Polymer skins, Aerogels, Shaped Memory Polymers

84, 116, 117, 119

5.

High Solar Reflectivity

Structures coated with white elastomatric coatings, gray cool paint with titanium dioxide, red- brown roof tiles

144, 146, 149

6.

High Thermal Emissivity

Low-E coating glass, High- albedo paints

140,141, 142

Building materials

References

A much more advantageous solution for cheap and effective thermal comfort would be to make a roof garden. A mathematical model giving a simplified representation of the dynamic thermal behaviour of green roofs concludes that green roofs act both as cooling devices and insulators to reduce the heat flux through the roof [168]. Green cover also helps the environment and existing simulation studies show that green roofs when applied on a city scale, may reduce the average ambient temperature between 0.3 K and 3 K [169]. Man made shades (natural and synthetic) for eg. building pavilion on top of roof using interwoven coconut leaves, palm leaves, cool roof sheets and single-ply membrane sheets [170, 171] and natural shades like vegetation in the surrounding and roof tops of the building will improve the thermal comfort conditions and energy performance of a 22

building [172, 173]. A model developed by Rakesh Kumar in Yamuna nagar (average outdoor temperature 308.2 K – 312.2 K (35-39 °C) in India [174] showed that the cooling potential of green roof combined with solar roof shading was found adequate to maintain an average room air temperature of 298.9K (25.7 °C). A thermodynamic model study by Ouldboukhitine et al., 2011 comparing green roof model and roof slab concrete model showed a significant difference (of up to 303.1 K (30 °C) in temperatures between the outer surfaces of the two roofs in summer [172].

4. Conclusion The basis for this review is the growing concern of rising temperatures owing to Climate Change, the consequent adverse changes in the thermal environment and the thermal comfort of the inhabitants of workspaces in tropical climates. Thermal comfort has a significant implication on the health, psychology and productivity of the working population who form the foundation of a country’s economy. We have reviewed work on the use of building materials as a passive mechanism for improving thermal comfort inside buildings in hot & humid climates, focusing on factors that make certain building materials more favourable for providing thermal comfort to the indoor environment. One of the ways to achieve thermal comfort while minimizing energy use is to minimize the cooling requirement of the living space. In tropical countries with low-resource setting and shortage of energy availability, application of building materials as passive technique shall make an effective and easy method to control and improve thermal comfort. A range of materials that have the properties for passive cooling techniques may be used to help achieve thermal comfort. This could be an adaptation strategy that may offer a sustainable solution to tackle the issue of rising temperature and consequent thermal comfort impacts on the living spaces. The review has highlighted the means of achieving this objective by use of various materials, notably, the materials used for building envelope, natural materials that have inherent properties to provide thermal insulation from the exterior environment, advanced materials with high insulation 23

properties, reflective paints and green roofs. With the view that appropriate insulation is required to minimize the thermal impact from solar radiation, we have looked into materials that have properties that could provide thermal comfort to the occupants, including materials that have been traditionally used for centuries, eco-friendly materials and new emerging technology materials. It is apparent that materials with lower thermal conductivity, thermal diffusivity and absorptivity may be suitable as envelopes for building, especially workspaces that are occupied primarily during the day. The review particularly identified certain materials like VIPs, PCMs, window glazing, ACC, polymer skin, with good thermal properties with a potential to be incorporated in different parts of the building envelope to enhance thermal comfort indoors. Light-colored external surfaces and reflective paints are recommended options in tropical climates, as they help minimize the surface temperature and the heat load of the building. Providing shading for both glass and opaque surfaces in windows, balanced with daylight strategy shall also significantly improve thermal comfort inside buildings. Use of vegetation, a traditional time-tested and proven method, should be encouraged in tropical climates to provide shading for buildings, roofs and the surrounding areas as indirect evaporative cooling by vegetation shows a promising performance in improving thermal comfort within building. For environmental sustenance, locally available, low-cost, recyclable natural materials and simple low-energy consuming techniques such as heat exchange pipes & latent thermal storage systems must be preferred options in industries and encouraged and promoted by government for tackling both the future energy needs and the changing climatic scenario. New concepts in construction materials that offer co-benefits of energy efficiency and thermal comfort is likely to gain momentum, and efforts to adopt new ideas in use of building materials is needed for protecting the future community from the risks of thermal stress owing to the predicted rise in temperatures due to Climate Change. Further research to investigate the use of 24

various building materials for the optimal thermal comfort of the living spaces and workplaces is an urgent need to tackle the health and productivity implications of rising temperatures due to Climate Change.

Acknowledgements This study was carried out as a part of HOTHAPS project. Prof. Kjellstorm whose encouragement and support has been very instrumental in our work is appreciated. The authors highly acknowledge and thank the Department of Environmental Health Engineering, Sri Ramachandra University, Chennai for providing them this platform for carrying out the work. Authors also appreciate the support extended by Rekha, Vennila and Manikandan of EHE Department, SRU for their support. References References 1.

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Highlights: 1. Thermal comfort is a growing concern in workplaces with rising temperatures in the face of Climate change. 2. We have reviewed building materials that provide thermal comfort at workplaces. 3. Natural and synthetic building materials that provide thermal comfort are discussed. 4. Use of building materials as a passive method for thermal comfort has co-benefits of energy saving and efficiency.

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