The potential of the reed as a regenerative building material—characterisation of its durability, physical, and thermal performances

energies Article The Potential of the Reed as a Regenerative Building Material—Characterisation of Its Durability, Physical, and Thermal Performances Raphaele Malheiro 1, Adriana Ansolin 1, Christiane Guarnier 2, Jorge Fernandes 1, Maria Teresa Amorim 3, Sandra Monteiro Silva 1and Ricardo Mateus 1,*   Citation: Malheiro, R.; Ansolin, A.; Guarnier, C.; Fernandes, J.; Amorim, M.T.; Silva, S.M.; Mateus, R. The Potential of the Reed as a Regenerative Building Material— Characterisation of Its Durability, Physical, and Thermal Performances. Energies 2021,14, 4276. https:// doi.org/10.3390/en14144276 Academic Editor: Paulo Santos Received: 9 June 2021 Accepted: 12 July 2021 Published: 15 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Institute for Sustainability and Innovation in Structural Engineering (ISISE), University of Minho, 4800-058 Guimarães, Portugal; [email protected] (R.M.); [email protected] (A.A.); [email protected] (J.F.); [email protected] (S.M.S.) 2Federal Center for Technological Education “Celso Suckow da Fonseca” (CEFET/RJ), Rio de Janeiro, RJ 20271-110, Brazil; [email protected] 3Centre for Textile Science and Technology (2C2T), University of Minho, 4800-058 Guimarães, Portugal; [email protected] *Correspondence: ricar[email protected] Abstract: Knowing the properties of vernacular materials is crucial to heritage conservation and to develop innovative solutions. Reed, considered to be a carbon-neutral and a carbon dioxide sink material, has been used for centuries for diverse uses. Its high availability and properties made it a popular building material, including in Portuguese vernacular architecture. An experimental investigation was conducted to evaluate the physical performance, thermal performance, and durability of the reed found in Portugal since the characterisation of this material was not found in previous studies. The influence of geometric characteristics and the presence of nodes on these properties were also analysed, and the results showed that they are irrelevant. The studied reeds were found to have an adequate thermal performance to be used as thermal insulation. Their thermal resistance (1.8 m 2·◦ C/W) and thermal conductivity (0.06 W/m ·◦ C) are under the requirements defined by Portuguese regulations on thermal insulation materials. Overall, the physical characteristics (moisture content, density, and retraction) are compatible to its use in the construction. Concerning durability, there was only a trend for mould growth in particular environments. The results provide valuable data to be considered in the development of new construction products based on this natural and renewable material. Additionally, considering the studied samples, the reed found in Portugal has characteristics suitable for use as a building material, especially as a thermal insulation material. Keywords: reed (Arundo donax); material characterisation; sustainability; natural materials; vernacular architecture 1. Introduction Reed has been used for thousands of years in diverse uses by many cultures [ 1 – 3 ]. The reed has been a conventional construction material since ancient times. It was used to make baskets, fences, windbreakers, building walls, roofs, floors, shading barriers, and temporary shelters for men and animals; music instruments; paper; and bio-fuel. Its characteristics, such as its high availability, lightweight stem, and fair mechanical strength and high flexibility (due to the tubular shape of the stem), has allowed for different uses of reed and made it a popular component as a construction material [2–5]. Its low mechanical strength and easy combustion have made it difficult to use in building structures, being more commonly used in ceilings or supports for covering roofs, wall panelling to improve their thermal performance, or to complement the earthquake resistance of internal and external walls [ 3 ]. However, the solution adopted in the construcEnergies 2021,14, 4276. https://doi.org/10.3390/en14144276 https://www.mdpi.com/journal/energies Energies 2021,14, 4276 2 of 19 tion of the external walls of earthquake-resistant buildings with mats of Arundo donax L. in Calabria, Italy, has shown good durability since many examples are still perfectly intact [ 3 ]. Around the world, it is possible to see vernacular buildings entirely built out of reeds, just with some construction elements (walls and ceilings), and using reeds together with other materials [ 6 ]. In Portuguese vernacular architecture, the reed was also used as a construction material, particularly in the southern region, probably due to its high availability. However, since it is spread throughout the Portuguese territory [ 7 ], its use in vernacular architecture can also be found in other regions of mainland and island Portugal [8,9]. Reed was used mainly as an element for thermal insulation in walls [ 10 ] and roofs. The encaniçado is an example of the use of reeds in the construction of roofs (Figure 1). This technique allows for a roof to achieve better thermal performance [ 11 ], and it is widely used in the vernacular buildings of the Alentejo and Algarve regions in noble and common buildings. The encaniçado consists of rows of reeds tied together [ 12 ] and to the structural timber beams. In some cases, a layer of mortar is applied over the reeds to flatten the surface and connect them to the roof tiles that are placed over the mortar. Figure 1. Use of reed for roof construction: (a) common gable building [8] and (b) section of a roof [8]. Concerning the use of reed as an element for thermal insulation in walls, the walls of palheiros, and the tabique technique are important examples of Portuguese vernacular architecture using reeds. Palheiros are palafitic timber buildings, and although their timber walls usually just have an air cavity, there are records of palheiros in Leirosa (near Figueira da Foz) in which the cavity of the external wall was filled with reeds to improve thermal insulation [ 8 , 13 ] (Figure 2). Reed is also found in earth-filled timber frame walls such as the tabique. The tabique technique can be found almost everywhere in Portugal [ 14 ]. It is part of the Portuguese heritage, and similar techniques are applied worldwide [ 15 ]. In brief, a tabique is formed by a regular timber frame covered with an earth mortar [ 14 , 15 ]. In Algarve, more specifically in the cities of Lagos and São Brás de Alportel, there are examples where the wooden structure of the tabique walls was replaced by reed panels [ 16 ]. Energies 2021,14, 4276 3 of 19 Figure 2. Use of reed for wall construction: palheiro in Leirosa [13]. Similar solutions are used worldwide. For example, in Italy, there is a building system uses a timber-framed supporting structure and a sheathing composed of two mats made of Arundo donax L. fixed to the walls and then covered with a layer of lime and cement plaster [3]. Knowing the properties of the natural materials used in vernacular architecture is crucial to ensure successful heritage conservation, optimise the use of these materials, and develop innovative solutions. In general, the characterisation of natural materials is a challenge for researchers. The high variability in their properties and the absence of specific standards are the main barriers to these materials characterisation [ 17 ]. In the particular case of reeds, from the literature review, it was verified that only a few studies have presented their characteristics. Regarding physical characteristics, the research results are even scarcer. Phragmites australis (PA) (common reed) and Arundo donax (AD) (giant reed) are frequently mentioned in these studies on natural building materials. However, it is important to note that these species have a marked difference in their geometry, particularly in average diameter: around 1.0–2.5 cm for PA [ 18 , 19 ] and 2.5–5.0 cm for AD [ 19 , 20 ]. In this sense, the comparison of results from different studies should be made with caution. Concerning thermo-acoustic properties, different approaches to characterisation have been identified. Some studies have evaluated the thermo-acoustic potential of panels where the reed is the main material, and others have evaluated panels made only with reed [6,21–23]. Both approaches highlight the thermal insulation potential of the reed. Regarding the panels made only with the reed, the review research carried out by Asdrubali et al. [ 24 ] showed thermal conductivity values for the reed of between 0.045 and 0.056 W/m ·◦ C. Asdrubali et al. [ 22 ] presented a thermo-acoustic characterisation of reed panels (PA) while considering different geometries, densities, humidity rates, and stems shapes in experimental research. The maximum diameter of the used reed was 1.5 cm. The thermal characterisation was carried out in a guarded hot plate and hotbox apparatus. According to the authors [ 22 ], the layout and characteristics of the reeds did not strongly influence the equivalent thermal conductivity, reaching values between 0.055 and 0.065 W/m ·◦ C. However, the acoustic behaviour was strongly affected by the stem configuration. Since there was a significant difference in the average diameter of the different reed species, it is essential to understand whether these conclusions apply, for example, to panels made with AD. Regarding a panel where the reed is the main material, the AD harvested in Portugal was used in a prototype of a building solution based on earth and reeds (stem and fibres) [ 25 ]. The prototype built in Lisbon had its indoor and outdoor temperatures monitored during the different seasons. The researchers [ 25 ] concluded that the solution contributed to controlling the interior air temperature, given the thermal amplitudes that were registered outside. Energies 2021,14, 4276 4 of 19 Molari et al. [ 26 ] used the same bamboo standards to evaluate the mechanical properties of reed (AD). According to the results, the reed’s compressive strength (127.4 MPa) was 20% higher than the tensile strength (103.7 MPa) and almost twice the axial compressive strength (57.0 MPa). On the other hand, the axial Young’s modulus under tension (15.3 MPa) was similar to that under compression (13.4 MPa) and about three times higher than the shear modulus (2.96 MPa). According to the authors [ 26 ], the results showed good mechanical properties of the reed that were similar to those of various species of bamboo used in constructions. However, it is important to note that, given the small reed dimensions compared to bamboo, variations concerning the bamboo standards were carried out. Therefore, caution is needed when comparing these results with the results of other research. Furthermore, the water content was not considered in the discussion of the results. Conte et al. [ 27 ] studied the effect of water content on the structural and mechanical properties of reed (AD). The researchers used a combination of different analytical techniques (such as calorimetry and fast field cycling NMR relaxometry). They concluded that reed bending properties are strongly affected by the presence of bound water. Thus, it is important to consider the physical properties of the reed when studying its mechanical properties. There is an important gap in the knowledge of the physical properties of reed. In recent literature, there has been no research focused on these properties. Some research with mainly thermo-mechanical objectives included the determination of some physical characteristics of the reed. Soliman [ 28 ] studied reed (AD) use in thermal insulation and, in parallel, also determined its water absorption and moisture content. After three months of drying the reed, they found 12.11% of moisture and 52.60% of water absorption (after immersion in water for 24 h at a room temperature of 23 ◦ C). The moisture content was higher than values achieved in other research [ 26 , 29 ]. This difference could have been related to the test methodology. In their study focused on mechanical characterisation, Molari et al. [ 26 ] determined the moisture content of reed, reaching values between 7.09% and 8.96%. Ortunõ[ 29 ] carried out a reed (AD) characterisation for use as a construction material and determined its moisture content and density. After one year of drying, they determined the reed density as received (583 kg/m 3 ), the density of anhydrous reed (537 kg/m 3 ), and the density of saturated reed (1040 kg/m 3 ). Since the reed used as a construction material does not receive any treatment, they considered the density equal to 583 kg/m 3 and determined its moisture content to be 8.63%. This value was similar to moisture content value found by Molari et al. [26]. Since the properties of natural materials can be site-dependent [ 29 , 30 ] and no characterisation of reed from Portugal was found in previous studies, this research fills a gap in this area of knowledge, thus assuming an innovative position. In the present paper, the characterisation results of Portuguese reed are presented and discussed. This experimental investigation assessed the physical and thermal behaviour of reed and its durability, aiming to produce knowledge that can be used to design new solutions for more sustainable construction and the conservation of vernacular buildings. 2. Materials and Methods 2.1. Specimens The giant reed (Arundo donax) used in this research was harvested in Serpa, inland Southern Portugal. The location was chosen due to the abundance of the reed [ 7 ] and proximity to areas where the reed has an important presence in vernacular architecture, as mentioned in the previous section. The harvest took place in winter. In winter, the plant has less sap (it is drier), and, therefore, it can regenerate quickly. The cut stems have less moisture, dry faster, and are less susceptible to biological agents. According to Alentejo’s locals, “reed harvested in January lasts all year” [18]. Energies 2021,14, 4276 5 of 19 The harvested reeds were stored in a laboratory environment (20 ± 2 ◦ C and 50 ± 2% RH) in a vertical position, and they remained there for six months in the drying process. After the dying process, the reeds with cracks or other anomalies were discarded, and the others were identified and classified according to their external diameter. To characterise the reeds, they were divided into three groups: G0—diameter less than 11 mm; G1—diameter between 11 and 15 mm (average thickness of 1.6 mm); and G2—diameter between 15 and 22 mm (average thickness of 2.5 mm). G0 was discarded because it showed insufficient mechanical resistance, often breaking during the works. G1 and G2 were characterised in three ways: physical properties, thermal properties, and durability. 2.2. Physical Properties Since there are no normative procedures for the characterisation of reeds, the methods used were adapted from other materials. Due to the differences between the complex reed node structure and the reed stem, it was decided to assess the node influence on the results. Thus, in the physical tests, reed samples with and without nodes were studied. For each test, five samples were used (5-cm-length stems). 2.2.1. Moisture Content The moisture content (MC) test was carried out based on the Portuguese Standard NP—614 [ 31 ] for wood. The test consisted of drying the sample in an oven (103 ± 2 ◦ C) until it reached a constant mass. The moisture content was the difference between the wet mass (m 0 (g)) and the dry mass (m 1 (g)) divided by the dry mass, according to Equation (1). MC =(m0−m1) m1 ×100, (1) 2.2.2. Apparent Density The apparent density ( ρ ) test was carried out based on the Portuguese Standard NP— 616 [ 32 ] for wood. The test consisted of determining the specimen mass and volume at a specific moisture content. The graphic program AutoCAD was used to determine the specimen’s volume. The apparent density was the mass (m (g)) divided by the volume ( ν (m3)), according to Equation (2). ρ=m ν, (2) 2.2.3. Dimensional Stability—Retraction The linear retraction ( ε ) test was carried out based on NP—615 [ 33 ] for wood. The test consisted of determining the variation in studied dimensions after saturation and drying. The linear retraction was the difference between the saturated specimen dimensions (l 1 (mm)) and the air-dried specimen dimensions (l 2 (mm)) divided by the oven-dried specimen dimensions (l3(mm)), according to Equation (3). ε=(l1−l2) l3 ×100, (3) Reed walls’ dimensional variations in length, diameter, and thickness were monitored during the test. Concerning diameter, two directions were monitored: 1–3, 2–4 (Figure 3a). Concerning thickness, four points were monitored: 1, 2, 3, and 4 (Figure 3a). Concerning the length, four directions were monitored: 1–1, 2–2, 3–3, and 4–4 (Figure 3b). Energies 2021,14, 4276 6 of 19 Figure 3. Markings used in the reeds to carry out the retraction test: (a) diameter and (b) length. 2.2.4. Capillary Water Absorption The principle of the capillary water absorption (A) test consisted of determining the amount of water absorbed by porous materials over a certain time. A dry specimen was placed in an oven at a controlled temperature, and the amount of water absorbed through only one surface of a non-saturated specimen immersed in a water film of 5 ± 1 mm was measured. The absorption of water through capillarity was the difference between wet mass (m i (g)) that had one surface in contact with water during a time (t i ) and the dry mass (m (g)) divided by the superficial area that was in contact with water (a (mm 2 )), according to Equation (4). Ai=mi−m a, (4) The samples were dried over 24 h in an oven at 100 ± 5 ◦ C. Afterwards, the reed base (external and internal wall) was isolated with silicone to ensure a unidimensional penetration of the water (Figure 4a). A steel wire support was built to prevent the reeds from floating during the test (Figure 4b). The samples were immersed in a water film of 5 ± 1 mm, and the water level was monitored (Figure 4c). The amount of absorbed water was measured after 15, 30, 60, 90, 120, 150, 180, 210, 240, 300, 360, 1440, and 2880 min (Figure 4d). Figure 4. Cont. Energies 2021,14, 4276 7 of 19 Figure 4. Capillary water absorption test steps: ( a ) sample preparation, ( b ) steel wire support, ( c ) samples immersed in water, and (d) weighing the sample on a precision balance. 2.3. Thermal Properties The characterisation of the thermal properties of the reed was based on the analysis of its thermal transmittance, thermal resistance, and thermal conductivity. Reed panels were built to analyse these properties using the hotbox test. 2.3.1. Reed Panels Reed panels of 15 cm × 15 cm (Figure 5a) were built to carry out the calibrated hotbox tests. The reed stems were overlapped to create a 10-cm-thick panel (Figure 5b). In order to contain the reed stems, the panels were tied with steel wire. A steel wire was applied in the borders of the panel to decrease the influence on the heat flux during the hotbox test. Two types of panels were built: type 1 (only G1 reeds stalks) and type 2 (only G2 reeds stem). Their main characteristics are presented in Table 1. Figure 5. Reed panels used in hotbox test: (a) horizontal dimensions and (b) thickness. Table 1. Characteristics of the panels used in the hotbox test. ID Quantity of Reed Stems Reed Stems with Nodes (%) Quantity of Nodes Average Diameter (mm) SD 1 Average Thickness (mm) SD Steel Wire Mass (g) Panel Mass: Reed and Wire (g) Density (kg/m3) Type 1 84 92.31% 103 13.25 0.93 1.70 0.29 19.50 489.70 208.98 Type 2 56 100% 65 16.83 1.57 2.29 0.52 19.50 479.34 204.37 1Standard deviation (SD). Energies 2021,14, 4276 8 of 19 2.3.2. Hotbox Test The thermal properties of the reed were evaluated in a calibrated hotbox [ 34 ] designed and built at the Department of Civil Engineering of the University of Minho, following the recommendations of ASTM C1363-11:2014 [ 35 ]. The hotbox comprised two five-sided chambers (external dimensions: 2.0 m × 1.4 m × 1.6 m)—a cold one and a hot one—and one mounting ring placed between the two chambers. The envelope was well insulated and made of extruded polystyrene (thickness: 0.20 m; U = 0.21 W/(m2·◦C)). The reed panel was placed in the mounting ring set between the two chambers. It was enclosed between two medium density fibreboards (MDFs) in order to provide a flat surface for installing the flux meter and thermocouples (Figure 6). The reed panels were tested in a horizontal position because the reed accommodation in this position achieved a lower thermal transmittance than the vertical position [21]. Figure 6. Set up of reed panel in hotbox test: (a) horizontal view and (b) vertical view. The temperature difference between the hot and cold chambers was maintained at around 10 ◦ C. The tests were carried out by the thermal flux meter methodology, according to ISO 9869-1:2014 [ 36 ], where the heat flux was measured through a heat flux sensor installed in the central part of the reed panel and the temperature was measured by four thermocouples (two in each chamber: one in the middle of the chamber and the other on the surface of the reed panel). With the values of the heat flux (q) and the surface temperatures (T) while using Equation (5), it was possible to determine the thermal resistance (Re set ) of the set (reed panel and MDF), where ∆ T is the difference between the surface MDF temperature in the hot and cold chambers. Resethm2◦C/Wi=∆T q, (5) The thermal resistance of the reed panel (Re reed ) was determined using Equation (6), where ReMDF is the thermal resistance of the MDF used. Rereedhm2◦C/Wi=Reset −(2∗ReMDF), (6) The thermal transmittance (U reed ) and thermal conductivity ( λreed ) of the reed panel were determined using Equations (7) and (8), where e is the thickness of the panel. UreedhW/m2◦Ci=1 Rereed , (7) Energies 2021,14, 4276 9 of 19 λreed[W/m◦C]=e Rereed , (8) 2.4. Durability—The Mould Development Test Reed is an organic material. As a natural building material, reed becomes mouldy like wood or wood-based products in favourable conditions [ 37 ]. Mould is often an early indication of increased moisture levels in buildings. Problems caused by mould are mainly discolouration, odours, and health problems. In order to evaluate the resistance of the reed for mould growth, an exploratory overstress test was carried out for the samples. The assessment of the emergence and development of mould in the reed samples was made visually with images collected using binocular materials microscope LEICA DM 750 M from Leica Microsystems, a 5 MP HD Microscope Camera Leica MC170 HD (sourced by Leica Microsistemas Lda., Microscopia e Histologia, Carnaxide Portugal) and Leica Application Suite (version 4.12.0). Four samples in each reed group (G1 and G2) were placed in a Petri dish, visually evaluated, and laid in ARALAB FitoClima 1000EC45 climatic chamber (22 ± 2 ◦ C and 90 ± 5% HR) for 42 days. The chamber conditions were appropriate for mould development in reed [ 38 ]. One reed dried in an oven (103 ± 2 ◦ C) until reaching a constant weight (G1— Dry and G2—Dry), was studied too. The samples were evaluated weekly. To reduce the time the samples were outside the chamber and to avoid changes in the final results, the visual analysis was performed in the shortest possible time of less than 20 min. During the visual analysis, the quantification of mould growth was based on the mould index used in the experiments for visual inspection (Table 2). To minimise the margin of error, the visual inspection was made independently by two persons. Table 2. Mould indexing classifications (adapted from [39]). Mould Index Coverage Description of Classification 0 0 No growth 1 0 Some growth detected only with microscopy 2 >10% Moderated growth detected with microscopy 3 0–10% Some growth detected visually 4 10–50% Visually detected 5 50–80% Visually detected 6 80–100% Visually detected 3. Results and Discussion 3.1. Physical Properties 3.1.1. Moisture Content Figure 7shows the mass loss as a time function. Similar behaviour was observed in all studied samples, regardless of node presence and group. The samples reached a constant mass after around 240 min. There was a marked mass loss during the first 30 min of the test, followed by a residual loss up to 24 h. The verified mass loss was related to the water evaporating during the drying process. Energies 2021,14, 4276 16 of 19 Table 6. Development of surface moulds in G2 reed. Test Period Mould Index Bare Eyes Microscope (10×) 0 day 0 14 days 5 28 days 5.5 42 days 6 The general aspects of the samples presented in Tables 5and 6are quite different. In the photographs corresponding to the bare-eyes analysis, it is possible to observe intense dark areas in the G2 sample, while in the G1—Dry sample, a generalised brown colour was observed. The microscope images show the evolution of the mould in the sample at the same point of the test. In the G2 sample, the fifth stage of growth (index 5) was reached after 14 days of testing. The bare-eyes analysis showed that almost 80% of the sample was covered with grey and black mould. After 14 days, the mould growth was moderate, and the last stage of mould growth (index 6) was reached at the end of the test. In this stage, it was possible to see, in the microscope image, a high density of mould in the sample. In the G1—Dry sample, the third stage of growth (index 3) was reached after 14 days of testing, and the maximum growth stage did not exceed index 4. Grey and black mould was observed in the microscope image only after 28 days, and the sample showed a low density of mould growth until the end of the test. These results show the trend in the development of mould in the studied reeds. The interior surfaces of the reeds were studied individually, but the results were in accordance with the study of [ 37 ]. In this research, [ 37 ] reeds tied with steel wire were subjected to the overstress test (22 ◦ C and 90% HR), and at the end of 42 days, the external surfaces of the reeds showed a mould development index between 5 and 6. The results of this study also showed the important role played by moisture content in the durability context. Drier reeds lead to less intense and slower mould growth. Energies 2021,14, 4276 17 of 19 These results confirmed the trend in the development of mould in the studied reed. However, these results must be carefully analysed. There are two main aspects to consider. First, the chamber conditions were the ones that maximise mould growth in reed [ 38 ]. Similar conditions could occur in particular natural environments. In Portugal, for example, only the coastal region reaches temperature and humidity conditions similar to those studied. This situation can happen during the summer for a short time, usually at night [ 10 ]. Second, the reed was studied while considering the resistance to the mould of the inner face, i.e., the most vulnerable one. This area is the less dense side of the reed [ 41 ] and is therefore more vulnerable to aggressive agents. 4. Conclusions To reduce the environmental impact of the construction sector, preserve and renovate vernacular buildings, and create commercial value for locally available natural materials such as the giant reed (Arundo donax), it is necessary to know a material’s physical and thermal proprieties. Reed is considered to a carbon-neutral raw material and a carbon dioxide sink, and it has been used for centuries for diverse uses. Its high availability and properties made it a popular building material, including in the construction of Portuguese vernacular buildings. In the present paper, an experimental study was carried out to characterise the giant reed (Arundo donax) that is most common in Portugal. Considering the uses of reed in Portuguese vernacular architecture (as for thermal insulation) and the absence of data on the physical properties of the reed, its thermal and physical properties were assessed in this study. The durability of natural materials plays an important role in this context, so it was also evaluated in this study. The results were presented and discussed in the previous sections, and the main conclusions are as follows: (1) Based on the studied thermal parameters (thermal resistance, thermal transmittance, and thermal conductivity), it is possible to conclude that reed from Portugal, under the studied conditions, has an adequate thermal performance. Furthermore, its thermal resistance (1.8 m 2·◦ C/W) and thermal conductivity (0.06 W/m ·◦ C) were found to be in accordance with the requirements defined by Portuguese law for thermal insulation materials. The thermal resistance of reed is almost 60% of the insulation materials used in Portugal (e.g., rock wool, XPS, and cork). The geometric characteristics (diameter) and node presence were found to have no influence on the studied thermal properties. (2) The physical characteristics of reed from Portugal are compatible with its use as a construction material. The reed’s density and water content were found to be similar to the organic materials conventionally used in the construction sector. The reed has satisfactory dimensional stability, making it compatible with rigid connection accessories. However, attention should be paid to aspects such as retraction in its thickness. The hygroscopic nature of reed could be related to the high water absorption reached in the tests. (3) There is a trend for mould growth under favourable conditions (22 ± 2 ◦ C and 90 ± 5% HR). Nevertheless, these specific temperature and humidity conditions are uncommon in Portuguese climatic conditions. Therefore, the durability of this material will not hinder its use as a building material. The presented characterisation provides valuable data to be considered in the renovation of vernacular buildings. In addition, regarding the studied samples, the reed found in Portugal has characteristics suitable for use as a building material, especially as a thermal insulation material. Additionally, considering the abundance of reed throughout the Portuguese territory, this is a sustainable, eco-friendly, and low-cost option. Energies 2021,14, 4276 18 of 19 Author Contributions: R.M. (Raphaele Malheiro) and A.A. undertook the main part of the research that was the base of this article. They developed the research method and analysed the results with the contribution of C.G. and J.F. R.M. (Raphaele Malheiro) wrote the document with the input of S.M.S. and R.M. (Ricardo Mateus). S.M.S. and R.M. (Ricardo Mateus) helped to develop the discussion sections of the paper and provided critical judgment on the undertaken research. Additionally, they supervised all the works and revised the document. M.T.A. helped in the durability tests (development and data analyse). All authors have read and agreed to the published version of the manuscript. Funding: The authors would like to acknowledge the support granted by the FEDER funds through the Competitively and Internationalization Operational Programme (POCI) and by national funds through FCT (the Foundation for Science and Technology) within the scope of the project with the reference POCI-01-0145-FEDER-029328, reVer+. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Acknowledgments: The authors would like to acknowledge the support granted by DANOSA “Derivados asfálticos normalizados, S.A.” industry for the hotbox construction by providing all the necessary insulation material. Conflicts of Interest: The authors declare no conflict of interest. References 1. Allirand, J.-M.; Gosse, G. An above-ground biomass production model for a common reed (Phragmites communis Trin.) stand. Biomass Bioenergy 1995,9, 441–448. [CrossRef] 2. 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