Energy demands continue to rise and are at an all time high. As a result of increasing population, industrialisation,urbanization, technological advancements and climate and seasonal changes, solving the matter of traditional energy consumption and finding the proper alternative resources are vital keys to a sustainable development policy. Maintaining a comfortable indoor temperature accounts for a significant portion of energy use worldwide, and innovative new insulative and efficient materials for building structures are at the forefront of energy conservation.
Use of thermal insulation materials
In recent years, many different thermal insulation materials have been developed for better energy efficiency and less environmental damage. These products have confirmed their usefulness in buildings due to their benefits such as low density, high thermal resistance, and cost effectiveness. The efficiency of thermal insulation depends on their thermal conductivity and their ability to maintain their thermal characteristics over a period of time. This study presents factors influencing the thermal conductivity coefficient of three main groups including conventional, alternative, and new advanced materials. The most common factors are moisture content, temperature difference, and bulk density. Other factors are explained in some dependent studies such as airflow velocity, thickness, pressure, and material aging. The relationship between the thermal conductivity values with the mean temperature, moisture content, and density which were obtained from experimental investigation has also been summarized. Finally, uncertainty about the thermal conductivity value of some common insulation materials is also reviewed as the basis of selecting or designing the products used in building envelopes.
Energy consumption in the building sector
The global energy expenditure in industrial and residential construction has become one of the most important concerns in the third decade of the 21st century. Building construction, raw material processing, and product manufacturing are the largest sources of greenhouse gas emissions. Carbon dioxide compounds are the main by-products of fossil fuel consumption, and since buildings are among the biggest consumers of energy, they are also major contributors to global warming which is accelerating climate change and threatening the survival of millions of people, plants and animals.
What is Thermal Conductivity ?
Thermal conductivity is a property that describes how efficiently a material can conduct heat. It determines how quickly heat energy spreads through a substance when a temperature difference exists. Imagine holding a metal spoon and a wooden spoon over a flame. The metal spoon heats up quickly, while the wooden spoon remains relatively cool. This happens because metal has a high thermal conductivity, meaning it allows heat to pass through easily, whereas wood has a low thermal conductivity, making it a good insulator.
Factors Affecting Thermal Conductivity(k)
1.Material Type – Metals have high k, while insulators have low k.
2.Temperature – In metals, k decreases with temperature; in gases, it increases.
3.Density & Structure – Denser materials usually conduct heat better.
4.Moisture Content – Water improves conductivity in porous materials.
Thermal conductivity measurement is crucial in the construction industry because it determines how well materials can conduct or resist heat transfer. This directly impacts energy efficiency, comfort, safety, and durability in buildings. Cement and concrete play major roles in the construction industry and researchers are seeking ways of creating better materials that provide high levels of insulation without sacrificing structural strength. Thermal conductivity is critical in the development of these materials as lower thermal conductivity values correlate to better insulative systems.
Temperature, moisture content, and density are the most important factors
Moisture : Changes in thermal conductivity due to moisture variations can be explained by a shift in the division of the thermal and humidity distribution in the structure, which alters the sorption properties of the material. The rate of change in thermal conductivity with moisture content is higher at higher initial moisture content. The lower the density of open-cell insulation materials, the higher the effect of moisture content on the thermal conductivity. An increasing in thermal conductivity of aerogel with increasing moisture content reflects that the hydrophilic properties of the materials need to be investigated for use in building and construction.
Density: For closed-cell materials, it can be stated that high density shows low thermal conductivity. Higher density means smaller pores and less than air volume, resulting in the heat flow through the materials is mainly governed by the thermal conduction of the solid particles, while the effect of the heat transfer by convection and radiation becomes insignificant. Besides, the effect of density on thermal conductivity of open-cell materials is generally lesser than that of closed-cell materials. It is because the diffusion of gas in the foam materials; the higher the apparent density the lower the proportional of gas filling up the cells, and thus the lower the impact of other influencing factors like mean temperature, moisture content, or aging. On the other hand, fibrous materials such as bagasse, palm, coconut fibers showed the variation in which the thermal conductivity decreased to a minimum and then increased. It is also shown that an increase in solid fiber density reflected an increase in thermal conductivity values of the fibrous batt after reaching the minimum value and it is consistent with loose fill materials having higher thermal conductivity than closed-cell insulation materials.
In further investigations, it would be interesting to compare the calculated thermal performance and the real performance on the site of an insulated structure. This finding provides the following insights for future research to examine the dependence of thermal properties including thermal conductivity, thermal diffusivity, heat specific capacity, coefficient of thermal expansion on their moisture content, temperature differences, density, and air velocity of the thermal insulators derived from natural fibers.
The significance:
- Energy Efficiency & Insulation: •Buildings lose or gain heat through walls, roofs, and floors. •Low thermal conductivity materials (insulators) (e.g., fiberglass, foam, bricks) reduce heat transfer, improving energy efficiency by minimizing heating and cooling costs. •High thermal conductivity materials (e.g., metals) allow rapid heat transfer, which may be undesirable for insulation but useful in heating systems. Example: •Using insulation with low k (~0.03 W/m·K) in walls reduces air conditioning costs in hot climates. •Concrete (~1.7 W/m·K) is used in floors to retain heat in colder climates.
- Thermal Comfort: •Buildings should maintain a comfortable indoor temperature without excessive reliance on heating or cooling systems. •Materials with the right thermal conductivity help stabilize indoor temperatures by preventing rapid temperature fluctuations. Example: •Brick walls (thermal mass) store heat during the day and release it at night, maintaining stable indoor temperatures.
- Fire Safety & Heat Resistance: •High thermal conductivity materials spread heat faster, which can be a fire hazard. •Low thermal conductivity materials act as fire-resistant barriers, slowing heat propagation in case of a fire. Example: •Concrete (~1.7 W/m·K) and fire-resistant gypsum board (~0.17 W/m·K) are used in fireproofing structures.
- Material Selection for Structural Stability: •Extreme temperature fluctuations cause thermal expansion and contraction, leading to cracks and structural damage. •Properly selected materials help prevent thermal stress and improve durability. Example: •Bridges use expansion joints to accommodate thermal expansion and avoid cracks.
- Sustainable & Eco-Friendly Construction : •Measuring thermal conductivity helps in selecting sustainable materials that reduce energy consumption and lower carbon footprints. •Green buildings use materials that balance insulation, thermal mass, and recyclability. Example: •Aerated concrete (AAC) is lightweight, insulates well, and reduces energy use in buildings.
Conventional materials such as polyurethane (PUR), polyisocyanurate (PIR), extruded polystyrene(XPS), expanded polystyrene (EPS) are preferred in many buildings and thermal energy storage applications due to their low thermal conductivity and low cost.
WHY IMPORTANT ?
Global energy consumption is predicted to grow by 64% until the year 2040 from the considerable increase in residential, industrial, commercial, and urban construction due to the industrial development and growth of population, according to the Energy Information Association in 2018. As a result, environmental disasters and climate change are becoming more apparent. For instance, global warming from the greenhouse effect (45% carbon dioxide emissions in which buildings and construction industry are major contributors) is predicted to raise the Earth’s average surface temperature from 1.1° to 6.4 °C by the end of 2100. The increased consumption of natural resources for lighting, refrigeration, ventilation, recycling, heating, and cooling system in commercial buildings due to the acceleration of urbanization, causes an enormous expenditure for energy. Therefore, it is necessary to use insulation materials for better energy conservation, and to enhance sustainable energy strategies in the building sector.
Thermal conductivity values of different building materials
There is uncertainty about the thermal conductivity values for inorganic, organic, and advanced materials which are 0.03–0.07 W/(m.K), 0.02–0.055 W/(m.K), and lower than 0.01 W/(m.K), respectively. Generally, the nominal thermal conductivity of porous materials range from 0.02 to 0.08 W/(m.K), while the thermal conductivity values of alternative insulation materials made from natural fibers vary from 0.04 to 0.09 W/(mK). Conventional materials such as mineral wool, foamed polystyrene are mainly used in thermal energy storage systems due to long term usage, and low cost. Natural fibers-based insulation materials derived from agricultural waste such as coconut, rice straw, bagasse, etc., currently applied in some building applications due to the environmentally friendly properties. However, the main disadvantage is their relatively high-water absorption, resulting in high thermal conductivity. Additionally, there are efforts to make thermal insulation from wood waste products , but they should be protected against saturation, especially when used externally. In fibrous insulating materials, the fineness of the fibers and their orientation play a main role. In foam insulating materials, the thermal conductivity is determined by the fineness and distribution of the cells and particularly by the gases in those cells. Insulating materials made from wood fibers or wood wool, the density factor is critical for the insulating capacity. The range of temperature shows the minimum and maximum service temperatures based on manufacturers information. Insulating materials can react very differently to hot and cold environments and there is no uniform test method that enables a direct comparison between insulating materials.
This provided a detailed discussion on the factors influencing the thermal conductivity of insulation materials. Temperature, moisture content, and bulk density are the primary factors significantly affecting the thermal conductivity coefficient, depending on the material. Other factors such as thickness, airflow velocity, pressure, aging also influence the thermal performances. In most cases, the thermal conductivity shows a linearly increasing trend with temperature. The moisture content strongly affects the thermal conductivity of both organic and inorganic materials. Bulk density also plays an important role in determining the thermal conductivity with opposite trends, it may exhibit a linear decrease for conventional materials and nonlinear variation for organic materials. This function seems to be linear decrease in types of conventional materials or shows a non-linear with natural fiber materials. Depending on the relationship between thermal conductivity and influencing factors, fibrous insulation materials show more sensitivity to temperature, moisture, and density compared to the other conventional materials. Understanding the quantitative relationship between the effective thermal conductivity and the influencing factors of the insulation materials is fundamental in determining the envelope heat, mass transfer, and building energy consumption.
The world’s energy consumption is at an unprecedented rate of rising. The countries worldwide are shifting to renewables such as solar, wind and hydro energy and with consistent efforts to reduce fossil fuel use. Governments and businesses are increasingly increasing to newer and state of the art technologies that help to lower consumption rates. Energy efficient measures are also being deployed. So to conclude, measuring thermal conductivity in construction materials are highly significant to improve energy efficiency, enhance thermal comfort of the present generations, increase fire safety,prevent thermal stress and damage to our living spaces and thereby supports sustainable construction. Energy demand is highly crucial and imperative in today’s world for economic growth, sustainability and energy security.
