**1. Introduction**

Global warming has become one of the greatest challenges facing humans in modern times, causing serious problems such as heat waves, drinking water shortages, and the spread of disease [1]. This phenomenon is mostly caused by the increase in the concentration of greenhouse gases in the atmosphere due to the use of fossil fuels to satisfy the growing demand for energy, which is, in turn, driven by population growth, the increase in the number of vehicles, and the development of new information technologies, among other reasons. According to the Intergovernmental Panel on Climate Change (IPCC), in each of the last three decades, the Earth's surface has been successively warmer than any previous decade since 1850 [2]. To alleviate this problem, it is necessary to develop new energy sources and to find ways to reduce society's energy consumption. The latter is a major challenge for the building sector, which, together with industry and transport, is one of the most energy-intensive sectors in the world, and is increasing due to the growing use of ventilation, air conditioning, and heating systems [3]. It is estimated that buildings consume about 40% of the world's energy, 25% of the world's water, and 40% of the world's resources, and are responsible for 1/3 of the world's greenhouse gas emissions [4–6]. For this reason, there has been a grea<sup>t</sup> deal of interest in improving their energy efficiency around the world, which can be seen in the increase in research on the subject [7] and in the emergence of regulations [8,9].

The use of thermal insulation is recognized as one of the most efficient ways to reduce energy consumption in buildings [3,9–11]. For this purpose, materials obtained from petrochemical products (mainly polystyrene) or from processed natural sources with high

**Citation:** García Sánchez, G.F.; Guzmán López, R.E.; Gonzalez-Lezcano, R.A. Fique as a Sustainable Material and Thermal Insulation for Buildings: Study of Its Decomposition and Thermal Conductivity. *Sustainability* **2021**, *13*, 7484. https://doi.org/10.3390/ su13137484

Academic Editor: Antonio Caggiano

Received: 11 June 2021 Accepted: 2 July 2021 Published: 5 July 2021

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energy consumption (such as rock wool and glass fiber) are often used. This has negative effects on the environment, both in terms of energy consumption and the generation of waste and emissions, mainly in the production stage [3,12], as well as causing serious problems for human health [13]. In fact, some of the materials used as insulation have recently been banned because they may pose health risks [14,15]. These problems could be addressed by using insulation made from recycled or natural materials (biomass) that do not require such a high degree of processing. Unfortunately, these materials are still in the development stage and, in many cases, their most important properties have not ye<sup>t</sup> been fully determined [12]. The study of biomass as an insulating material (natural insulators) began in 1974 and remained a topic of little interest, with few scientific articles published until 1998, when the number of studies published on the subject began to increase. This trend progressed even more after 2003, and especially between 2010 and the present day. Currently, there is a grea<sup>t</sup> deal of interest in the development of this type of thermal insulator [3,16–18]. This interest is due to the increased environmental awareness of society, the increase in air conditioning systems—which require methods to reduce energy consumption—and changes in the use of biomass, for which other alternatives are required for its efficient use, as it is being replaced by hydrocarbon fuels in applications such as heating [3]. Studies on the subject have mainly been carried out in France, the UK, Italy, Turkey, and Algeria, which are the countries with neither the largest arable land area nor the largest number of forests; this can be explained by the greater environmental awareness of European countries compared with the rest of the world. Additionally, as expected, the most researched biomasses are from crops grown in these countries, i.e., hemp, straw, flax, wood, coconut, maize, and sunflower [3]. A comprehensive review of the state-of-the-art work on the main natural insulators around the world is presented in the works of Liu et al. [3], As-drubali et al. [12], Hurtado et al. [19], Mangesh et al. [20], Ingrao et al. [21], and Kymäläinen and Sjöberg [22].

The feasibility of using fique as a thermal insulating material is a subject that has rarely been studied in the world, which is evidenced by the low number of scientific articles published on the subject. However, there has been growing interest in the subject since 2003, which has been accentuated since 2010. Some of the most relevant works on the subject are presented in Table 1. The superficial characteristics of fique fibers were studied by Guzmán et al. [23]. An expansion in the field of research towards the manufacture of composite materials (biopolymer/fiber, aerogel/fiber) is shown in the works of An et al. [24] and Dou et al. [25]. Fique is a crop of grea<sup>t</sup> importance for Colombia, which is the world's main producer, and the livelihoods of more than 70,000 Colombian families depend on its production [26]. In the country, the plant's fibers are widely available throughout the year [27], making it ideal for uses beyond the manufacture of ropes and garments in order to contribute to our knowledge of the properties of fique as a thermal insulator and provide a more holistic overview of the environmental impact during the whole specific life cycle phase [28,29], from raw material extraction to end of life stages; hence enabling justified decisions on the suitability of using fique fibers in thermal insulation [30]. This paper reports the measurement of its thermal conductivity, by means of the guarded hot plate method (ASTM C 177), and a study of its thermal decomposition by means of the analysis of thermogravimetric data using the Coats–Redfern method.


**Table 1.** Studies on the characterization of fique as a thermal insulator.
