INTRODUCTION
Problem statement, Potentiality, Gaps
Addressing traditional energy consumption and exploring alternative resources are crucial for sustainable development Recent advancements in thermal insulation materials have enhanced energy efficiency and minimized environmental impact These materials, characterized by low density, high thermal resistance, biodegradability, and cost-effectiveness, have demonstrated significant benefits in building applications Numerous studies have examined the thermal performance of insulation materials, particularly those made from open-cell foam and inorganic fibers Additionally, research on polymer composites utilizing natural fibers from plant-based sources has revealed superior thermal properties compared to conventional materials While many experiments have focused on mechanical properties, thermal conductivity, and thermophysical analysis, factors such as ambient temperature, moisture absorption related to humidity, and airflow velocity's impact on heat convective conductance remain underexplored.
Research gaps in existing literature highlight the need for a comprehensive overview of the factors affecting the thermal properties of building insulation materials Notably, there is a lack of empirical data on the thermal conductivity coefficients of insulation materials, with insufficient focus on the effects of relative humidity Additionally, many natural fibrous insulating materials are produced as polymer composites reinforced with fibers and synthetic adhesives, offering benefits such as high strength and durability, which are vital for sustainable industrial applications However, safety concerns arise from the recycling of composites containing formaldehyde-based adhesives that release volatile organic compounds Consequently, binderless thermal insulation materials are gaining interest and are being prioritized as a key research objective in Ph.D studies.
Energy consumption in the building sector
In the third decade of the 21st century, global energy expenditure in construction has emerged as a critical concern, with building construction and raw material processing being major sources of greenhouse gas emissions Buildings, as significant energy consumers, contribute substantially to global warming, exacerbating climate change and endangering countless lives and ecosystems The European Directive 2010/31/EU mandates that new constructions must achieve nearly zero energy consumption, primarily sourced from renewable resources, as the construction sector accounts for up to one-third of global greenhouse gas emissions and 40% of energy use With global energy consumption projected to increase by 64% by 2040 due to rising residential, industrial, and urban development, addressing energy efficiency in construction is more crucial than ever.
Environmental disasters and climate change are increasingly evident, with global warming driven by the greenhouse effect—largely attributed to the construction industry, which accounts for 45% of carbon dioxide emissions Projections indicate that the Earth's average surface temperature could rise between 1.1 °C and 6.4 °C by 2100 The growing demand for energy in commercial buildings, fueled by urbanization, leads to significant energy expenditures for lighting, heating, cooling, and ventilation To address these challenges, it is essential to adopt renewable resources and implement sustainable energy strategies in the construction sector.
The use of thermal insulation materials
As energy becomes increasingly valuable, the enforcement of insulation materials in buildings is on the rise Thermal insulation effectively slows heat transfer through conduction, convection, and radiation, reducing reliance on HVAC systems for comfort and conserving energy This not only diminishes dependence on traditional energy sources like coal and natural gas but also offers benefits such as increased profits, environmentally friendly options, enhanced indoor thermal comfort, noise reduction, and fire protection Insulation materials are crucial for achieving energy efficiency and have diverse applications, including food cold storage and LNG pipelines The demand for sustainable insulation products with lower embodied energy and emissions is growing, with numerous innovative options entering the market Thermal insulation materials can be categorized into four main groups: inorganic, organic, combined, and advanced, available in various forms like porous, rigid, and reflective structures Inorganic materials, such as glass wool and rock wool, dominate the market at 60%, while organic materials make up 27% Conventional options like polyurethane and polystyrene are favored for their low thermal conductivity and cost-effectiveness in buildings and thermal energy storage.
Figure 1.1 Classification of common insulation materials used in buildings
Mineral wool encompasses various inorganic insulation materials, including rock wool, glass wool, and slag wool, with thermal conductivity values ranging from 0.03 to 0.04 W/(m·K) Glass wool typically has λ-values between 0.03 and 0.046 W/(m·K), while rock wool ranges from 0.033 to 0.046 W/(m·K) These materials are characterized by low thermal conductivity, non-flammability, and high moisture resistance, although they may cause skin and lung irritation In contrast, organic insulation materials, sourced from renewable natural resources, are increasingly favored for their aesthetic appeal, high thermal resistance, and lower energy requirements for production Advanced insulation technologies, such as vacuum insulation panels (VIPs), gas-filled panels (GFPs), aerogels, and phase change materials (PCM), offer superior heat retention capabilities VIPs achieve exceptionally low thermal conductivity values of 0.002–0.004 W/(m·K) under specific pressure conditions, although their performance can degrade over time due to gas diffusion Aerogels, with thermal conductivity between 0.013 and 0.014 W/(m·K), are highly efficient but face limited commercial availability due to high production costs GFPs and PCM represent the future of thermal insulation, boasting low thermal conductivity values of 0.013 W/(m·K) and 0.004 W/(m·K), respectively, with GFPs utilizing a reflective structure to insulate gas, while PCM facilitates heat storage and release through phase transitions.
Table 1.1 Classification of the commonly used insulation materials and uncertainty about their thermal conductivity
Thermal conductivity values for inorganic, organic, and advanced materials range from 0.03–0.07 W/(m.K), 0.02–0.055 W/(m.K), and below 0.01 W/(m.K), respectively Porous materials typically have thermal conductivity between 0.02 to 0.08 W/(m.K), while natural fiber-based insulation materials show values from 0.04 to 0.06 W/(m.K) Conventional insulation materials like mineral wool and foamed polystyrene are favored for thermal energy storage due to their cost-effectiveness and durability In contrast, natural fiber insulation from agricultural waste, such as coconut and rice straw, is gaining traction for its eco-friendly properties, despite its high water absorption leading to increased thermal conductivity Innovative materials like aerogel and vacuum insulated panels (VIPs) offer superior thermal insulation with conductivities of approximately 0.017–0.021 W/(m.K) and 0.002–0.008 W/(m.K), respectively The effectiveness of fibrous insulation relies on fiber fineness and orientation, while foam insulation performance is influenced by cell structure and gas composition For wood fiber insulation, density significantly impacts thermal performance, and manufacturers provide specific service temperature ranges Insulating materials can behave differently in varying temperatures, and a standardized testing method for direct comparisons is lacking.
Natural fibrous insulation materials
Recent research highlights the growing interest in natural fibrous materials for composite manufacturing due to their eco-friendly characteristics, low cost, lightweight nature, and renewability Natural fibers, derived from plant-based resources, offer excellent mechanical strength and are increasingly replacing synthetic fibers in various applications, including automotive and textiles Despite their advantages, challenges such as reduced mechanical properties due to low interfacial bonding and hydrophilicity, which leads to incompatibility with hydrophobic polymers, pose limitations These issues can result in decreased thermophysical properties and stress concentration within the composite structure.
Figure 1.2 Common natural fibers used in reinforcement polymer composites.
Thermal conductivity coefficient
Insulation materials are designed to minimize heat conduction, thereby reducing heat loss in buildings The effectiveness of an insulation layer is primarily determined by its thermal conductivity value (λ-value), which measures the rate of heat flow through a material Factors influencing thermal conductivity at the microscopic level include cell size, fiber arrangement, and gas type, while macroscopic factors include temperature, moisture content, and density Standards such as EN 12664:2001 and ASTM C518 are used to test thermal conductivity, but no single method applies to all materials due to their varying properties Effective insulation not only lowers energy loss and greenhouse gas emissions but also impacts energy efficiency for heating and cooling, as well as health concerns Heat transfer in insulation occurs mainly through conduction, gas molecule interaction, and radiation, with convection being negligible due to small air bubble sizes.
To create environmentally friendly insulation materials, understanding their apparent thermal conductivity is crucial According to DIN 4108, materials with a λ-value below 0.1 W/(m·K) are classified as thermal insulating materials, with values under 0.03 W/(m·K) considered very good Innovative nanotechnology materials exhibit λ-values between 0.01 and 0.015 W/(m·K), while those above 0.07 W/(m·K) are less effective Manufacturers typically specify thermal conductivity values based on standard laboratory conditions, such as a mean temperature of 23.8 °C and relative humidity of 50±10% Although these values facilitate comparative evaluations of different materials, actual thermal performance can vary significantly when insulation materials are used in building envelopes due to differing temperature and humidity levels This study investigates the dependence of thermal conductivity on operating temperature and moisture content, highlighting the discrepancies between predicted and actual thermal performance in real-world conditions.
Factors influencing thermal conductivity of insulation materials
Examining the thermal properties of insulation materials is crucial for understanding their impact on heat transfer in building envelopes Key thermal properties include thermal conductivity, specific heat, thermal diffusivity, thermal expansion, and mass loss Notably, the thermal conductivity coefficient is vital for assessing a material's capacity to manage heat flow in insulation At the macroscopic level, thermal conductivity is influenced by three primary factors: operating temperature, moisture content, and density.
Abdou and Budaiwi investigated the thermal conductivity of inorganic materials at mean temperatures between 4 °C and 43 °C, focusing on rock wool and fiberglass of varying densities Their findings revealed a linear increase in thermal conductivity with rising temperatures, particularly noticeable in lower density materials In subsequent research, they analyzed rock wool, mineral wool, and fiberglass, confirming that higher operating temperatures correlate with increased λ-values, represented by a linear regression for most insulation materials Their third study examined the effects of moisture content on thermal conductivity in fiberglass and rock wool, revealing that both temperature variations (14 °C to 34 °C) and moisture levels affect conductivity, consistently showing that higher temperatures lead to greater thermal conductivity Additionally, they studied the effective thermal conductivity of conventional materials like mineral wool and foam glass, demonstrating a linear increase across a temperature range of 0 °C to 100 °C using a protected heating plate device.
The thermal conductivity (λ-value) of various insulation materials measured at a mean temperature of 10 °C was found to be 0.04 W/(m·K), 0.045 W/(m·K), and 0.05 W/(m·K) In some cases, inorganic open-cell materials like fiberglass and rock wool exhibit a linear temperature-dependent behavior, showing reduced thermal conductivity at lower temperatures.
The thermal conductivity of polystyrene (PS) and polyethylene (PE) was assessed in relation to mean temperatures, revealing that PE's heat exchange rate is more sensitive to temperature changes compared to PS, with values of approximately 0.000384 (W/(m·°C)/°C) for PE and 0.0001 (W/(m·°C)/°C) for PS Research by Gnip et al demonstrated that the thermal conductivity of expanded polystyrene (EPS) can be calculated across a temperature range of 0 °C to 50 °C, using data from 10 °C, indicating a slight increase in thermal conductivity with rising temperatures This study confirms that variations in temperature consistently impact thermal conductivity.
A study revealed that elevated temperatures enhance the thermal conductivity of three types of polystyrene materials Additionally, the researchers found a linear increase in thermal conductivity across four polyethylene insulation specimens, varying from low to super high densities, as temperatures rose.
A study by Song et al [56] utilized the hot wire method to examine how temperature affects the thermal conductivity of EPS and polyurethane (PUR) materials The findings indicate that, for materials of the same density, the thermal conductivity coefficient rises as the ambient temperature increases.
Empirical observations indicate that the effective thermal conductivity of insulation materials such as EPS, XPS, and PUR is significantly influenced by temperature, demonstrating a linear relationship between the λ-value and temperature Studies have shown that as temperatures rise from 10 °C to 40 °C, alternative insulation materials like sheep wool also exhibit a linear increase in thermal conductivity Koru's research on closed-cell thermal insulation materials, adhering to standards EN 12664, 12667, and ASTM C518, confirms that thermal conductivity increases within a temperature range of -10 °C to 50 °C, reinforcing the linear relationship between λ-value and temperature Similar findings by Berardi et al and Zhang et al further support this trend, with Zhang's investigation revealing changes in thermal conductivity of five PUR foams across temperatures from -40 °C to 70 °C Additionally, Khoukhi's study on expanded polystyrene insulation materials highlights the incremental rise in thermal conductivity with increasing operating temperatures, affirming that thermal conductivity increases linearly with temperature across various insulation materials.
Recent studies highlight the potential of natural fiber-based insulation materials, such as hemp, cotton, rice straw, and wood waste, which exhibit high thermal resistance Research by Manohar et al demonstrated that the apparent thermal conductivity of coconut and sugarcane fibers increased with temperature, with values ranging from 0.046 to 0.049 W/(m·K), significantly lower than conventional insulation materials Additionally, wood-based fiberboards are recognized for their low density and high thermal resistance; however, their porous structure makes them sensitive to environmental changes, leading to a 50% increase in thermal conductivity as temperatures rise from -10 °C to 60 °C.
Bio-based materials serve as effective alternatives in building construction, contributing to reduced energy consumption and optimized fossil fuel use for sustainable development Research by Rahim et al indicates that the thermal conductivity of these materials increases slightly and linearly with temperature, ranging from 10 °C to 40 °C Similarly, Srivaro et al confirmed this linear relationship in their empirical tests on rubberwood specimens Additionally, thermal conductivity in samples of sheep wool, goat wool, and horse mane significantly rises by approximately 55% with increasing temperatures.
Recent advancements in technology and materials have led to the development of cutting-edge thermal building insulation, including vacuum insulation panels (VIPs), aerogels, gas-filled panels (GFPs), phase change materials (PCM), and closed-cell foam VIPs stand out due to their exceptionally low thermal conductivity, ranging from 2 to 4 mW/(m·K) at pressures between 20 and 300 Pa, allowing for thinner insulation layers compared to traditional materials Research by Fantucci et al demonstrated that the thermal conductivity of fumed silica-based VIPs increases by up to 45% as temperatures rise from 2 °C to 50 °C, indicating a significant impact of temperature on performance Additionally, subsequent studies revealed a 53% increase in thermal conductivity of raw VIPs under varying conditions.
0.0049 to 0.0075 W/(mãK), and from 0.0021 to 0.0028 W/(mãK) in fumed silica over the range of temperatures between -7.5 °C and 55 °C [60]
Aerogel is emerging as a promising thermal insulation material for future building applications, thanks to its low density, high porosity, and minimal thermal conductivity Its potential uses span across thermal insulation systems, energy storage, and construction Research has demonstrated that silica aerogels exhibit effective thermal conductivities ranging from 0.014 to 0.044 W/(m·K), with a nonlinear increase in conductivity observed as temperatures rise from 280 to 1080 °K Similar findings were noted in studies involving silica aerogel samples of varying densities within the 300 to 700 °K temperature range Additionally, aerogel blankets showed a linear increase in thermal conductivity from 0.0135 to 0.0175 W/(m·K) across mean temperatures of -20 °C to 80 °C, corroborated by findings from Nosrati et al.
1.6.1.5 Influence of mean temperature in thermal conductivity values
Table 1.2 shows practical equations to illustrate the temperature-dependent thermal conductivity of different insulation materials according to data collected from published articles
Table 1.2 Linear relationship between thermal conductivity and mean temperature of some commonly used insulation materials
Insulation Materials λ-T relationship Mean temperature
Figure 1.3 illustrates the linear relationship between the thermal conductivity of four insulation material groups and temperature increases from -10 °C to 50 °C Fibrous insulation materials, including fiberglass, hemp, flax, cellulose, and sheep wool, are particularly sensitive to temperature changes, with lower density samples showing a quicker rise in thermal conductivity as temperature increases This is attributed to their larger pore volume and higher air content Open-cell materials, such as fiberglass and rockwool, start with significantly higher thermal conductivity than closed-cell materials like XPS, EPS, and PUR, primarily due to higher initial moisture content from water penetration Advanced insulation materials like aerogel and vacuum insulation panels (VIPs) can have thermal conductivity values up to ten times lower than traditional options The thermal conductivity of these innovative materials is influenced by elevated temperatures, moisture levels, and aging Additionally, combined insulation materials, such as wood wool and wood fibers, also demonstrate temperature-dependent behavior due to their density and moisture absorption capabilities.
Figure 1.3 Effect of mean temperature on thermal conductivity of various building insulation materials: (a) inorganic materials; (b) organic materials; (c) advanced materials; (d) combined materials
Higher operating temperatures typically lead to increased thermal conductivity in most insulation materials, resulting in a rise in the λ-value as heat conduction rates increase; however, this effect is generally observed within a specific temperature range.
10 °C to 50 °C and typically up to 20–30 % This is the case with inorganic fiber insulation and some petrochemical insulating materials which show lower thermal conductivity at lower
T he rm al c on d u ct iv it y (W /( m K ))
EPS (b) Sheep wool Bagasse Hemp XPS PUR
T he rm al c on du ct iv it y (W /( m K ))
Wood wool exhibits a nearly linear relationship between thermal conductivity and temperature, as measurements are conducted under steady-state conditions while isolating various influencing factors According to ASTM C518 standards, thermal conductivity is reported under standardized conditions, with most published values derived from laboratory experiments by manufacturers However, since weather conditions, exterior temperatures, and moisture levels fluctuate throughout the day, it is crucial to assess the thermal conductivity of insulation materials and understand how it varies with temperature.
Excessive moisture in buildings, whether in solid, liquid, or gas form, can lead to several significant issues, including deteriorated habitation quality, reduced thermal resistance, and material decay These problems arise from various factors, such as moisture intrusion from liquid water, deposition from water vapor, and built-in moisture In particular, moisture can compromise the thermal efficiency of insulated walls and roofs, while also contributing to poor indoor air quality High levels of ambient moisture foster microbial growth, posing serious health risks, including allergies and respiratory issues.
[70] As the thermal conductivity of water is about 20 times greater than that of stationary air, water absorption is always connected with an increase in thermal conductivity [16]
Research rationale and objectives
The emergence of plant-based natural fibrous insulation materials presents a promising solution for thermal insulation in buildings, addressing global energy preservation needs A thorough review highlights the key factors affecting the thermal conductivity of these materials and their interrelationships It is crucial to understand the quantitative relationship between effective thermal conductivity and its influencing factors to assess thermal performance and energy consumption in buildings Additionally, lignocellulose insulation materials can be produced without synthetic resins, leading to lower costs, reduced health hazards, and minimized environmental impacts associated with the disposal or recycling of bio-based fiberboards.
In the light of the comprehensive review, the following research objectives were proposed:
Development of binderless thermal insulation materials from natural fiber resources
Determination of the thermal conductivity coefficient of natural fiber insulation materials and their values regarding the variations of temperature and relative humidity.
Experimental examination of the water absorption regarding the variations of relative humidity
Experimental examination of the influence of temperature and humidity in the thermal conductivity of binderless insulation materials
Characterization of natural fiber insulation materials using advanced analytic techniques (SEM, FTIR, TGA)
Numerical simulation of the heat and moisture transfer in the multi-layered insulation materials used as an exterior wall for building envelope.
Dissertation outline
This dissertation has been structured into four chapters as follows:
Chapter I – outlines the problem statement and the research outcomes It presents the comprehensive review on the factors influencing the coefficient of thermal conductivity of insulation materials It also discusses the relationship between the thermal conducitivity values and mean temperature, moisture content, density Overall, the chapter provides the following insights for current and future research to examine the dependence of thermodynamic parameters on unavoidable influencing factors
Chapter II – works with the materials, sample preparation, instrumentation and empirical methods employed for thermal conductivity measurement, the water absorption test, and the changes in thermal conductivity values under influence of temperature and relative humidity have been discussed Other practical analyses are also presented
Chapter III – presents and discusses the thermal conductivity of samples made from natural fiber including coconut fiber, rice straw fiber, energy reed fiber, and sugarcane bagasse fiber The empirical results can be used for comparison or reference with other commonly used insulation materials in buildings and constructions This chapter also examines the dependence of thermal conductivity of insulation fiberboard on temperature and humidity Their relationship is also explored using the linear regression technique The numerical simulations of the heat and moisture transfer in the multi-layered insulation materials have been investigated to evaluate the potentiality of the next generation thermal insulation materials used in building envelopes
Chapter IV – presents the conclusions of the research work and recommendations for future research.
Summary
This chapter provides an in-depth analysis of various building insulation materials and their thermal conductivity coefficients It addresses key research questions regarding the factors that affect the thermal conductivity of insulation materials in building envelopes, as well as the potential correlations between mean temperature, moisture content, density, and thermal conductivity Additionally, the chapter presents the λ-values of both traditional and innovative insulation materials utilized in construction.
Lignocellulose is a highly appealing material due to its abundance, low cost, and biocompatibility Cellulose-based insulation materials are particularly promising for sustainable building applications, offering eco-friendliness, lightweight properties, durability, high strength, and effective heat retardation Research in the development of natural fiber-based insulation materials has focused on their thermal characteristics, specifically examining the relationship between thermal conductivity, temperature, and humidity, forming the basis for Ph.D studies.
The research aims to address the current demand for natural resource utilization by focusing on reducing energy consumption from traditional sources and improving energy efficiency in the construction sector at the building level.
MATERIALS AND METHODS
Materials
Coir fiber, derived from raw coconut husk (Cocos nucifera L.) collected in Vietnam, undergoes a thorough cleaning process to remove pollutants, followed by a two-day sun drying and an additional 24-hour oven drying at 70 °C The chemical compositions of the coir fiber are detailed in Table 2.1.
Figure 2.1 Coir fiber extracted from coconut husk resources
Table 2.1 Chemical compositions, physical properties of coir fiber
Compositions and Properties Unit Value Ref
Figure 2.2 Bagasse fiber extracted from sugarcane waste resources
Bagasse fiber, a byproduct of the sugar cane plant (Saccharum officinarum L.), is the residual stalk left after juice extraction To obtain bagasse fiber, the sugarcane waste is oven-dried at 70 °C for 24 hours to eliminate remaining juice, followed by defibration using a grinding machine The resulting materials consist of long stems and particles, which are then sieved with a sieve analyzer to produce a homogeneous fiber with dimensions ranging from 0.1 to 2 mm The chemical compositions and physical properties of the bagasse fiber are detailed in Table 2.2.
Table 2.2 Chemical compositions, physical properties of bagasse fiber
Compositions and Properties Unit Value Ref
Sample preparation
2.2.1 Binderless coir fiber insulation boards
Binderless coir fiber insulation boards (BCIB) were developed at the University of Sopron's Department of Timber Architecture The production process involved placing an equal number of mats into a 250 mm × 250 mm forming box, which were then hand-formed into a homogeneous single layer Following this, the mats were compressed to achieve the desired thicknesses of 30, 40, and 50 mm To ensure accurate testing, the samples were enclosed in a polystyrene specimen holder, facilitating one-dimensional heat flow measurement over the specified area.
Figure 2.3 (a) Tested sample; (b) Schematic of polystyrene specimen holder
2.2.2 Binderless bagasse fiber insulation boards
The binderless bagasse insulation fiberboard is produced using a wet-forming process that leverages the natural self-bonding properties of lignocellulose, activated by hydrogen bonding and the adhesive characteristics of lignin and cellulose during heating and drying Bagasse fibers are soaked in tap water and defibrated by adjusting the grinder's disc distance from 5 to 0.1 mm to achieve a consistency of 3-9% The mixture is then molded into a 50 cm diameter round shape, dewatered overnight using gravitational force, and dried in an oven at 70 °C until a constant weight is achieved The resulting dry specimens are sanded flat and stored under ambient laboratory conditions for further processing For thermal conductivity measurements, the specimens are cut into dimensions of 250×250×20 mm³, 250×250×25 mm³, and 250×250×30 mm³.
Figure 2.4 Fabrication of binderless bagasse insulation materials: (a) hydrodynamically treated fiber; (b) disc shape wet mats; (c) dry sample
The Faculty of Wood Engineering and Creative Industry at the University of Sopron developed several natural fiber-based polymer biocomposites for thermal conductivity measurement These include three samples of rice straw and reed fiber reinforced phenol formaldehyde biocomposites (REPC) measuring 400×400×12 mm and three samples of coir fiber reinforced phenol formaldehyde polymeric biocomposites (CFPC) measuring 400×400×8 mm, produced using hot-pressing technology Additionally, four cross-laminated specimens made from coconut wood panels (CTCP) measuring 200×200×60 mm were provided by the Center of Excellence in Wood and Biomaterials at Walailak University, Thailand, with each specimen formed by binding three panels of 200×200×20 mm using melamine formaldehyde glue.
Figure 2.5 (a) Rice straw and energy reed fiber reinforced PF biocomposites (REPC) ([129]); (b) Coir fiber reinforced PF biocomposites (CFPC) ([130]); (c) Cross-laminated made with coconut wood insulation panels (CTCP) ([131])
Table 2.3 Experimental design for rice straw and reed fiber reinforced PF biocomposites, [129]
Table 2.4 Experimental design for long and short coir fiber reinforced PF biocomposites, [130]
Methods
2.3.1 Determination of thermal conductivity coefficient
The coefficient of thermal conductivity was determined using the heat flow meter (HFM) method, adhering to the EN 12667:2002 and ISO 8301:1991/Amd 1:2002 standards for steady-state heat transfer Heat flux was measured with sensors (120 mm × 120 mm, accuracy of 0.1 W/m²) positioned at the center of the heating plate A constant temperature difference of 10 °C was maintained between the cold and hot sides of the specimen during testing Thermal conductivity (λ) was then calculated using the established formula.
q/ dx dT (2.1) where q is the heat flow rate (W/m) and dT/dx is temperature gradient (K/m)
Figure 2.6 Transversal cut of a typical single heat flow meter apparatus
2.3.2 Examination of temperature-dependent thermal conductivity coefficient
The study investigated the impact of varying ambient temperatures on thermal conductivity by measuring the λ-value at 11 different mean temperatures, ranging from -10 °C to 50 °C in 5 °C increments Following European certified reference materials for lambda measurement, the temperature difference was maintained at 10 °C The thermal conductivity values were determined using the heat flow meter method, as detailed in Table 2.5.
Table 2.5 Temperature variation between cold and hot sides
Mean temperature (°C) -5 0 5 10 15 20 25 30 35 40 45 Cold plate (°C) -10 -5 0 5 10 15 20 25 30 35 40 Hot plate (°C) 0 5 10 15 20 25 30 35 40 45 50 Temperature difference
2.3.3 Investigation of water absorption of natural fiber based insulation material
The water absorption of binderless fiberboard was assessed by measuring the total mass change of a sample exposed to a controlled environment, following the ISO 12574:2021 standard for determining the hygroscopic sorption properties of building materials and products using a climatic chamber and desiccator.
Water absorption of binderless coir fiberboard
The binderless coir fiberboard specimens were initially dried to determine their dry weight before being placed in a climatic chamber with controlled humidity levels set at 15%, 40%, 60%, 80%, and 95% relative humidity (RH) Continuous weight measurements were taken until the samples reached a constant value in equilibrium, allowing for the calculation of moisture content using a specified equation.
(2.2) where m(t) is the weight as a function of time (g), md is the dry weight (g)
Water absorption of binderless bagasse fiberboard
Binderless bagasse fiberboard samples were conditioned at various humidity levels within a sealed desiccator containing saturated salt solutions, as outlined in Table 2.6 The sorption test involved preparing saturated solutions by mixing salt with distilled water, which were then placed in a glass plate at the bottom of the desiccator to maintain the desired humidity A ceramic mesh was positioned 5 cm above the plate to support the samples Prior to placement in the desiccator, the samples were dried in an oven at 70 °C to determine their initial weight (md) Vacuum oil was utilized for sealing to prevent air leakage, and the setup was stored under room conditions Sample weights were periodically measured until they reached equilibrium, defined as a change in mass of less than 0.1% over three consecutive weighings Water absorption was calculated using Equation (2.2).
Table 2.6 Solutions used for water absorption test and respective relative humidity
Solubility at 20 °C (g/g) Magnesium chloride MgCl2.6H2O 1.569 33 54.57
Figure 2.7 Photograph of water absorption test using a desiccator
The moisture content in cross-laminated coconut wood panels (CTCP) was assessed using the Hydromette M4050 device, a multifunctional meter designed for measuring wood moisture, structural moisture, humidity, and temperature, as illustrated in Fig 2.8.
Figure 2.8 Photograph of testing the moisture content percentage of CTCP specimen
2.3.4 Determination of moisture-dependent thermal conductivity coefficient
Natural fibrous materials, due to their hygroscopic nature and porous structure, can absorb moisture from the air This moisture penetration into their internal pore system at higher relative humidity levels significantly influences temperature distribution and thermal conductivity.
This study investigates the moisture-dependent thermal conductivity of binderless coir and bagasse fiber insulation boards For coir fiber, three samples measuring 250×250×30 mm³, 250×250×40 mm³, and 250×250×50 mm³ were placed in a climatic chamber at five humidity levels (15%, 40%, 60%, 80%, and 90%) until equilibrium was achieved, with thermal conductivity measured at a mean temperature of 20 °C using the heat flow method In contrast, binderless bagasse fiber samples of 200×200×20 mm³, 200×200×25 mm³, and 200×200×30 mm³ were tested in a desiccator with four solutions to create humidity levels of 33%, 57%, 75%, and 96% Thermal conductivity for these samples was also measured at a mean temperature of 20 °C after reaching saturation, utilizing the heat flow meter method.
2.3.5 Surface morphology and morphological analysis of binderless bagasse fiber insulation boards
The surface morphology of binderless bagasse fiber insulation boards was examined using a Tagarno FHD Prestige digital microscope and scanning electron microscopy (SEM) with a Hitachi S-3400N at magnifications of 100x and 450x, and a voltage of 20kV These advanced techniques revealed detailed insights into the surface morphology, composition, crystallography, and topography of the bagasse particles and tested binderless samples.
Figure 2.9 Photograph of digital microscope Targano FHD equipment
Figure 2.10 Photograph of SEM Hitachi S-3400N equipment
Fourier transform infrared spectroscopy (FTIR) is a powerful technique utilized to identify functional groups and bonding patterns in materials by analyzing the absorption of infrared radiation In this study, FTIR was employed to examine the structural composition of binderless bagasse fiberboard using the transmission mode The Jasco FT/IR-6300 was used to collect full scan spectra in the mid-infrared range of 4000–400 cm -1 under ambient conditions, and the resulting spectra were analyzed with OriginPro 2018 software.
Figure 2.11 Photograph of FT/IR-6300 equipment
2.3.7 Thermogravimetric analysis and the first derivative thermogravimetric
Thermogravimetric analysis (TGA) and first derivative thermogravimetric analysis (DTG) were conducted using the Labsys evo STA 1150 (Setaram, France) following ASTM D3850 standards The samples, weighing between 17–21 mg, were heated from ambient temperature to 800 °C at a rate of 20 °C/min in a nitrogen atmosphere with a flow rate of 50 mL/min.
Figure 2.12 Photograph of TGA equipment
2.3.8 Numerical simulations of heat and moisture transfer in the multi-layered insulation materials
Study I – Heat and moisture transfer in the multi-layered insulation materials in the static boundary conditions
The geometrical model of a multi-layered insulated wall, depicted in Fig 2.13, features three layers: oriented-strand board, cellulose fiber board, and oriented-strand board (OSB-CFB-OSB) Given that the cellulose fiber board serves as the insulation layer, it is crucial to assess its thermal performance, particularly due to the sensitivity of natural fibrous insulation materials to variations in temperature and humidity under real environmental conditions.
Figure 2.13 Modelled image of multi-layered insulation materials with three layers (Oriented strand board-Cellulose fiber board-Oriented strand board)
The interaction between heat and moisture transfer in multi-layered insulation materials for building envelopes is interconnected, as heat conduction leads to water evaporation or condensation Conversely, moisture influences latent heat changes during phase transitions To enhance the specificity of the simulation, certain assumptions are made to exclude unnecessary factors.
The wall is characterized as a continuous, homogeneous, and isotropic medium that remains unaffected by compression deformation during heat and moisture transfer, ensuring that its porosity remains constant.
The effect of radiation and capillary hysteresis during moisture absorption and desorption on heat transfer is not considered
The thermal properties of the wall matrix such as density and heat capacity do not change when temperature and moisture content change
There is no dissolution of chemical substances during the process of moisture transfer
Due to the wall's thickness being significantly less than its height and weight, with a thickness ranging from 50 to 200 mm compared to dimensions of 500 x 500 mm, the heat and moisture transfer model can be effectively simplified to a one-dimensional analysis.
Based on the above assumptions, the governing equations of the dynamic modeling of heat transfer and moisture transport of the wall are defined in the Norm EN 15026:2007 [134]:
Q is the heat source (W/m 3 ãs)
G is the moisture source (kg/m 3 )
(ρCp)eff is the effective volumetric heat capacity at constant pressure (J/K)
T is the thermodynamic temperature (K) λeff is the effective thermal conductivity (W/(mãK))
Lν is the latent heat for evaporation (J/kg) δp is the vapor permeability coefficient of material (s) ϕ is the relative humidity psat is the partial pressure of saturated vaporation (Pa)
Summary
This chapter details the extraction of fiber from raw plant materials and introduces the fabrication of binderless fiberboard insulation and bio-based polymer composites from natural resources It also discusses experimental methods for investigating thermal conductivity, including the dependence of the thermal conductivity coefficient on temperature and moisture content, water absorption related to humidity levels, thermogravimetric analysis, and morphological analysis.
Plant-based insulation materials and biocomposites reinforced with natural fibers are emerging as effective solutions in the building and construction industry to reduce thermal load and energy consumption Binderless insulation materials demonstrate excellent heat retardant capabilities due to their low thermal conductivity and minimal impact on human health When incorporated as an additional layer in multi-layered installations, natural fiber insulating materials enhance thermal performance and comply with low-energy building standards Furthermore, natural fibrous materials are proving to be valuable raw materials for reinforcing polymeric biocomposites, offering a sustainable alternative to traditional resources for the future.
A m b ie n t re la ti ve h u m id it y
A m b ie nt t em p er at u re ( ° C )
Ambient temperature Ambient relative humidity
This study explores the numerical simulation of heat and moisture transfer in multi-layered insulation materials for building envelopes It assesses their thermal performance in relation to fluctuations in temperature and humidity levels, as well as the potential risk of condensation within natural fibrous insulation materials due to daily changes in outdoor environmental conditions.