Efficient Bio-Based Insulation Panels Produced from Eucalyptus Bark Waste

Abstract

Traditional thermal insulation panels consume large amounts of energy during production and emits pollutants into the environment. To mitigate this impact, the development of bio-based materials is an attractive alternative. In this context, the characteristics of the Eucalyptus fiber bark (EGFB) make it a candidate for insulation applications. However, more knowledge about the manufacturing process and in-service performance is needed. The present study characterized the properties that determine the in-service behavior of the EGFB insulation panel. The assessment involved two different manufacturing processes. The results indicated that the hot plates and the saturated steam injection manufacturing system can produce panels with similar target and bulk density. The thermal conductivity fluctuated between 0.064 and 0.077 W/m·K, which indicated good insulation, and the values obtained for thermal diffusivity (0.10–0.37 m mm²/s) and water vapor permeability (0.032–0.055 m kg/GN s) are comparable with other commercially available panels. To guarantee a good in-service performance, the panels need to be treated with flame retardant and antifungal additive. The good performance of the panel is relevant because bio-based Eucalyptus bark panels generate less CO₂ eq and require less energy consumption compared to traditional alternatives, contributing to the sustainability of the forestry and the construction industry.

1. Introduction

The building construction industry significantly contributes to greenhouse gasses, accounting for 37% of global energy-related emissions. About 10% of these emissions are due to the manufacture of building construction materials [1]. Further, conventional thermal insulation materials have the disadvantage of consuming large amounts of energy during production and releasing pollutant compounds into the environment [2,3]. This contributes to the increment of the greenhouse gas emissions of buildings and can lead to further health problems [4,5,6].

Therefore, there is a need to develop materials that mitigate the environmental impacts generated by traditional panels. The search for new strategies to use renewable feedstocks is one of the main current challenges for the development of new insulation materials. In this way, new ecological alternatives would reduce the environmental impact resulting from the excessive use of materials of fossil origin. In this regard, natural fibers are environmentally friendly resources that have several advantages, such as biodegradability, low density, abundance, and low cost, among others. For these and other reasons, the use of natural fibers, biopolymers, biofilms, and biosourced composites has increased significantly in recent years [7,8,9,10,11,12]. The main challenge today is to develop new applications for natural fibers in order to avoid the overexploitation of existing sources and thus provide the means to ensure a long-term sustainable market [11,13]. In addition, to reduce the environmental impact generated by conventional insulating materials, alternatives focusing on bio-based materials are being developed [4,6,11,14,15]. Examples of natural sources for these materials are flax [16,17], hemp [18,19], miscanthus [20,21,22,23], spruce bark fibers [24], wheat [25,26], wood fiber and chip [5,27], cork [28], camphor branches [29], kenaf fibers [30], sugar palm fiber composites [31], corn cub [32], larch bark [33,34], and sunflower stalks [35].

However, low availability and lack of fundamental knowledge regarding feedstock processing continue to hinder the ability to scale up these technologies to commercial scale [36]. Actually, non-conventional insulation materials account for just 1% of the market [1].

Studies have shown that bio-based materials made of natural fibers offer good performance in terms of both thermal insulation and hygrometric comfort. In particular, their thermal phase shift, breathability, and acoustic insulating properties have been reported [15,37,38,39,40]. Studies have been carried out with different sources of natural fibers to develop materials with thermal insulation capabilities as insulating boards, with thermal conductivities ranging from 0.04 to 0.06 W/m·K and good mechanical properties [8,41,42,43,44].

Furthermore, the use of bio-based materials reduces the environmental impact of a product even before its use phase, i.e., environmental benefits are generated immediately from raw material production and material manufacturing. Ranefjärd et al. [45] indicate that compared to conventional building materials, bio-based materials tend to have lower greenhouse gas emissions. This generally makes them a more sustainable choice for building materials and reduces the building sector’s environmental impact, which is extremely important, with more regulations incorporating whole-life carbon, as currently happens in Denmark, the Netherlands, and France; this should be the case in many other countries in order to achieve carbon neutrality on the whole [1].

This characteristic can be enhanced if the origin of the biological material is not a virgin source, but instead the feedstock is waste. In this case, a circular economy dynamic is generated, in which bio-based waste can be valorized as raw material and used to generate new products that can be used locally [46]. In this regard, a bio-based product, generated from waste and used in the local building industry, would also represent more environmental benefits due to the shorter transport distance compared to purchasing materials from other areas.

In particular, forestry waste and by-products can be a rich source of useful raw materials [47,48]. Currently, Eucalyptus plantations cover large areas worldwide (>22.57 million ha in 95 countries) and they are continuously growing; most prominently in Australia, Spain, Portugal, Kenya, Central and South Brazil, Uruguay, and Chile [49,50]. The industrial transformation of Eucalyptus globulus sp. into pulp and solid wood generates high volumes of bark as a by-product.

For instance, in Chile, Eucalyptus is the second largest commercial species in terms of cultivated area. Its industrial consumption is approximately 11.8 million m³ per year, its main use being the production of pulp and timber [51,52]. The main by-product is bark, which accounts for 10% of tree volume, generating 1.5 million m³ [53]. Currently, this bark has little practical application, i.e., there is no valorization of this resource. In fact, E. globulus bark is only used to generate energy. However, this type of bark has a lower calorific value than pine bark and is difficult to transport in the boiler conveyor system, making it less attractive as a fuel. Therefore, the bark is accumulated in landfills, generating a disposal problem.

This situation, combined with its low cost (20–50 USD/ton) and high availability that guarantees a constant supply of the material, has prompted forest industry experts to focus their efforts on developing new applications for E. globulus bark. For example, several studies have focused on isolating and modifying the extractives in the bark, because of their potential uses as chemicals [54,55], metal removers [56,57], and biofuel [58], i.e., their potential chemical valorization. Anther possibility is to explore how to take advantage of the intrinsic physical properties of bark, such as its low thermal conductivity [59]. In this context, E. globulus bark has a particular fibrous morphology, making it an ideal candidate for applications in which such structures would benefit the overall performance.

In addition, Casa-Lédon et al. [60] examined the environmental impacts of the production of insulation boards from E. globulus bark. That study reports that eucalyptus bark panels with densities above 50 kg/m³ have a carbon emission of 1.4–2.8 kg CO₂ eq and energy demands between 16.2 and 33.6 MJ. These values contrast with the 9.8 kg CO₂ eq and 229 MJ reported for the traditional glass wool insulation panel and the 10 kg CO₂ eq and 140 MJ reported for the conventional glass fiber insulation panel.

Therefore, from an environmental perspective, the E. globulus fiber bark (EGFB) insulation panel can be a very attractive alternative with less carbon emissions and embodied energy in comparison to conventional insulation materials. Additionally, it provides benefits to both the forestry and construction industries by aligning with circular economy principles, such as reducing dependence on fossil resources, efficiently using and valorizing waste, and increasing the use of high-quality biomass, as established by Vural Gursel et al. [46].

Despite the promising results in terms of generated emissions and embodied energy, the potential of these insulating panels can only be realized through greater market penetration and massification in practical applications. To achieve this, it is relevant to evaluate the insulating properties of these panels, and to ensure an adequate response in service conditions.

If this happens, the benefits associated with this proposal would be achieved and the EGFB panel could be a real strategy for reusing abundant local waste, to mitigate the forestry waste landfill problem and to decrease the environmental impact of local buildings.

Considering the potential benefit of using E. globulus bark fibers in the circular economy as raw material for insulation panels, the objectives of this study are as follows: (1) to evaluate the feasibility of producing fibers from the bark of Eucalyptus globulus, using a mechanical process; (2) to manufacture insulating panels from these fibers; and (3) to evaluate their performance as cavity wall insulation materials.

2. Materials and Methods

2.1. Eucalyptus globulus Fiber Production

Triturated E. globulus bark obtained from sawmill placed in the Bio-Bio Region, Chile, was used as a raw material (Figure 1A,B). The sample was air-dried for 24 h to adjust its moisture content to 16% (oven dry matter). Afterwards it was mechanically processed in a Peerless hammer mill to obtain EGFB (Figure 1C). Optical imaging of EGFB is presented in Figure 1D, which was captured with an optical microscope system using a 20× and 4× magnifier. The length, diameter, and aspect ratio were obtained.

2.2. Chemical Characterization of Eucalyptus globulus Bark

Particles of Eucalyptus bark were placed in a Soxhlet system with a mixture of ethanol/water to obtain free-extractives bark, in accordance with Sluiter et al. [61]. A two-step acid hydrolysis was carried out—by following Sluiter et al. [62]—to fractionate the bark to cellulose, hemicellulose, and lignin. The ash content was determined according to ISO 18122:2022 [63].

2.3. Morphological Characterization of E. globulus Bark’s Fibers

The morphological characterization of E. globulus bark was carried out after mill hammer defibration. Approximately 150 fibers were analyzed using an optical microscope Leica model DM 500 of Leica Microsystems (Wetzlar, Germany), with a 10× magnifying glass. Several images were taken, which were subsequently processed in a specialized software that sorted them out by length and diameter.

2.4. Production of Thermal Insulation Panel

The panels were manufactured using a commercial phenol–formaldehyde resin (Oxilite 5731^®, manufactured by Oxiquím S.A, city of Coronel, Chile, and with a solid content of 43.01%, viscosity of 1050 cP, and pH 11) as the adhesive. Resin and bark fibers were mixed in a rotatory system. The resin mixer is a self-manufactured piece of equipment comprising a container with a front door, a rotating system, and fins located within the container to facilitate the movement of the material. A nozzle is located at the center of the mixer, which is used to inject the resin through a spray system.

Then, a specific mass of the mixture was moved into a 400 × 400 mm mold to adjust its target density to a predetermined value (see

Table 1. Panel production parameters.

Parameter	Value
Moisture content fibers (%, dry basis)	4
Target density (kg/m3)	80/100/150/200/250/300
Thickness (mm)	50
Resin content (% w/w)	10
Press system—pressing time (min)	Hot plates—13 min Steam injection—3 min
Press (kgf/cm2)	45
Curing temperature (°C)	110

).

Two press systems were tested: (a) hot plates and (b) saturated steam injection at 8 bar. We define the pressing time as the time necessary to reach the curing temperature (110 °C). Our testing revealed that system (b) required a significantly lower pressing time than system (a): 3 min compared with 13 min, respectively. Afterwards, the panels were conditioned at 20 °C and a relative humidity of 65% for one week to reach the equilibrium humidity of the panels.

The parameters used to fabricate a rigid insulation panel are shown in Table 1. The pressure applied is maintained for all the panels. The pressing parameters in Table 1 were selected according to previous trials, equipment capabilities, and restrictions on materials.

The quantity of fibers used for each panel depends on the target density. Regarding the resin, it is incorporated in a proportion of 10% of the total weight of the panel, which agrees with the range usually present in particleboard (8%–12%).

Table 2. Fiber and resin amounts according to target density for a 400 mm × 400 mm × 50 mm panel.

Target Density (kg/m3)	Panel Weight (g)	Fiber Quantity (g)	Resin Quantity (g)
80	640	576	64
100	800	720	80
150	1200	1080	120
200	1600	1440	160
250	200	1800	200
300	2400	2160	240

shows the quantity of fibers and resin defined for the case of a 400 mm × 400 mm × 50 mm panel.

The dimensional stability of a material depends on its intrinsic nature and the prevailing environmental conditions. In our case, bark fiber has an inherent hygroscopic behavior, mainly due to the presence of hemicellulose; however, this was not considered in this study because the application of the panel as a thermal insulation material is not exposed to external weather conditions. However, as an integral part of vegetal organisms, EGFB is quite prone to degradation by fungi and is an easily flammable material, just like wood. Therefore, the final product must be adapted to reduce its susceptibility to fire and increase its fungal resistance. To that end, the flame retardant AF7000^® AP TX AT (manufactured by AF7000 FIRE PROTECTION, Santiago, Chile) and the antifungal solution Nipacide^® CO 5 (manufactured by Clariant, Brasil, Sao Paulo) were incorporated into the panels with a density of 100 kg/m³ to evaluate their flame and fungal resistances according to the methodology presented later.

2.5. Determination of Apparent (Bulk) Density

The apparent (bulk) density was calculated according to standard UNE EN 323 [64] as a ratio between the sample’s mass (with a precision of 0.01 g) and its volume. To that end, the standard defines a necessary number of samples and follows the methodology specified in the UNE EN 325 [65] to determine both the sample’s mass and its dimensions.

2.6. Determination of Thermal Conductivity

The thermal conductivity of the insulation panels was determined through a transient heat method based on ASTM D5334-08 [66]. A KD2-Pro Thermal Properties Analyzer with a thermal sensor KS-1 in the range of 0.02 to 2.00 W/m·K was used as the measuring device. This sensor incorporates a heater and a thermocouple inside, as seen in Figure 2.

2.7. Thermal Diffusivity Determination

The specific heat of bark fibers and polymerized phenolic resin were determined by DSC (Differential Scanning Calorimetry) according to Morintale et al. [67]. A NETZSCH DSC analyzer model 204 F1 Phoenix was used. The heating ratio was 5 °C/min within a temperature range of 25–250 °C, with an inert atmosphere. The specific heat was calculated using Equation (1), where variables are defined as follows: c: specific heat (J/kg·K); DSC: Differential Scanning Calorimetry (mW/mg); $\dot{T}$: heating rate (K/min). The specific heat was calculated as shown in Equation (1).

$$c={\displaystyle \frac{DSC}{\dot{T}}}·60·1000$$

(1)

The specific heat of each constituent of the panel was determined separately, and it was calculated as the average of each value obtained within the temperature range of 25 to 250 °C. Then, the global specific heat of the panel was determined as the sum of the contribution of each constituent in the actual formulation (90% w/w of fibrous material, and 10% w/w of curated phenolic resin).

Thermal diffusivity (α, m²/s) was calculated using Equation (2), where r is density in kg/m³; λ is thermal conductivity in W/m·K; and c is the specific heat in J/kg·K.

$$\alpha ={\displaystyle \frac{\lambda }{\rho ·c}}$$

(2)

2.8. The Determination of the Water Vapor Transmission

Water vapor transmission (WVT) was determined in rigid panels with a target density between 80 and 300 kg/m³ using the dry cup method described in ASTM E 96-95 [68]. A glass wool insulation panel was used as the control value, and each measurement was carried out four times. The sample was placed in a recipient, with silica gel sealing the contact edges. The system shown in Figure 3 was kept in a controlled environment at 23 ± 0.5 °C and a relative humidity of 50 ± 3% for 3 weeks.

The weight of each sample was registered daily to determine the vapor transmission rate across the panel (G, kg/s). With this information, several properties were calculated as follows:

Permeability (δ_MAT, kg m/GN s) is the flow of water vapor (G) that goes through a material of thickness d (m) per unit area (A, m²) when the difference in vapor pressure between the two faces is ∆p (Pa). It is calculated using Equation (3). Vapor resistivity (GN s/kg m) is the reciprocal value of permeability.

$${\delta }_{MAT}={\displaystyle \frac{G·d}{A·∆p}}$$

(3)

Vapor resistance (R_V, GN s/kg) is calculated by multiplying the value of vapor resistivity by the thickness of the material (d), according to Equation (4).

$$R_V={\displaystyle \frac{d}{{\delta }_{MAT}}}$$

(4)

Vapor resistance factor (or µ-value) is a dimensionless parameter used to assess vapor permeability relative to air (Equation (5). The higher the µ-value, the lower the permeability. For calculation purposes, the permeability of the air used was δ_AIR = 0.2 kg m/GN s [69].

$$\mu ={\displaystyle \frac{{\delta }_{AIR}}{{\delta }_{MAT}}}$$

(5)

Equivalent air thickness (s_d, m) is the equivalent air thickness that produces the same water vapor diffusion strength as a material of thickness (d, m). It is calculated using Equation (6).

$$s_d=\mu ·d$$

(6)

2.9. Determination of Fire-Test Response

Six samples of 50 mm × 50 mm × 150 mm were extracted from panels with a target density of 100 kg/m³, soaked with the commercial flame retardant (AF7000^® AP TX AT) and oven-dried at 40 °C for 8 h. Unsoaked samples of the same density were also tested to compare the results. The fire-test response of the samples was measured following the standard ASTM D 4986-03 [70], which describes a small-scale horizontally oriented burning test procedure for comparing the relative rate of burning and the extent and time of the burning of cellular polymeric materials with a density less than 250 kg/m³.

Each sample was placed on a metallic support and exposed to a blue flame with energy equivalent to 37 MJ/m³ for 60 s (Figure 4). Then, the flame was removed, and the time required to consume 125 mm of the panel was recorded.

2.10. Determination of Mold Resistance

Ten samples of 80 mm × 120 mm × 50 mm were obtained from an EGFB-based panel with a target density of 100 kg/m³. Then, they were treated with an antifungal (Nipacide^® CO5) and oven-dried at 40 °C for 8 h. In addition, untreated samples and samples without resin were also studied for comparison. A commercial fiberboard (MDF) was used as a control unit.

Table 3. Panels with additive incorporation.

Sample	PF-Resin (% w/w)	Additive	Additive (% w/w)
EBFP-100-1	0	-	0
EBFP-100-2	10	-	0
EBFP-100-3	10	Antifungal	1

summarizes the composition of the EGFB-based panel samples.

The mold resistance of the EGFB-based panel was determined according to ASTM D 3273-12 [71]. The samples were inoculated with a mold colony arranged in two chambers conditioned at a temperature of 34 °C and a relative humidity of 95%–98%, according to the standard. All samples were removed at weeks 1, 3, and 5 and photographed with a digital camera. The images were analyzed and classified visually, according to the size of the surface covered by mycelium molds. A scale of 0 to 10 was used to quantify the visual assessment. A total absence of mold had a score of 10 and the presence of mold on all of the surface (equivalent to 91%–100% of the surface area) had a score of 0.

3. Results

3.1. Characterization of Eucalyptus Bark Fibers

Pictures of the fibers (EGFB) obtained by milling the E. globulus bark (Figure 1A,B) are given in Figure 1C,D. EGFB composition in terms of extractives, cellulose, hemicellulose, lignin, and ash is shown in

Table 4. Chemical composition of some natural fibers compared to E. globulus bark fiber.

Component	Content (%, Dry Solid Basis)
	EGFB	Bark E. globulus	Wood E. globulus	Hemp Fibers
		Pereira, 1988 [72]
Ethanol/water extractives	7.43 ± 0.03	2.11 ± 0.52	2.8 ± 0.3	4.6
Cellulose	49.91 ± 2.56	51.5 ± 1.4	52.3 ± 0.9	79
Hemicelluloses	18.12 ± 4.16	14.6 ± 0.5	17.8 ± 1.0	9
Klason lignin	17.60 ± 0.49	18.3 ± 0.8	21.4 ± 0.6	6.5
Ash	7.62 ± 0.32	2.11 ± 0.52	0.76 ± 0.19	n.d.

in comparison with other raw materials [72]. The fibrous nature of E. globulus bark is related to its particularly high cellulose content (≈50%).

Figure 5 shows the length and diameter distribution of EGFB. The fibers have an average length of 6.40 mm and an average diameter of 0.26 mm, giving an average aspect ratio of 25.

3.2. Characterization of EGFB-Based Panels

Thermal insulation panels have been prepared using phenol–formaldehyde resin and two press systems: (a) hot plates and (b) saturated steam injection at different densities (see the Section 2). The parameters of the pressing conditions, including the pressing time and temperature, are given in Table 1. It should be noted that the saturated steam pressing method leads to a significant reduction in the pressing time required to ensure a panel has the required specifications: 3 min compared to the 13 min required when using the hot plate system (based on a 50 mm wide panel).

A comparison of the target density and bulk density between the two pressing methods was performed, and the results are presented in Figure 6. It appeared that both pressing technologies allow one to obtain a panel with similar densities.

The influence of the final density on the thermal conductivity of the two panels is shown in Figure 7. It was observed that the thermal conductivity fluctuated between 0.064 and 0.077 W/m·K. According to DIN 4108-2 [73], materials with a λ value lower than 0.10 W/m K can be classified as thermal insulating materials. This fact shows the good insulating properties of the materials generated from EGFB. As a reference, the thermal conductivity of commercial glass wool with a real density of 20 kg/m³, measured with the same method, was 0.042 W/m·K.

As expected, the thermal conductivity increases with the increment of the final density of the panel, regardless of the pressing system used (Figure 7). This is consistent with the fact that lower density panels have a higher volume of stagnant air (porosity) in their structure, which has a rather low thermal conductivity value of 0.026 W/m·K [74]. Therefore, the density plays a main role in the response of the thermal conductivity.

Within the framework of our study aiming at the design of insulating materials, the manufacture of materials with a low density is preferred. However, a balance must be maintained because less homogeneous fiber bonding is generally observed in low-density panels. As it is presented in Figure 7, and like other studies dealing with insulating natural fiber-based panels, the same relationship between thermal conductivity and density can be observed. Korjenic et al. [75] reported the thermal conductivity of jute, flax, and technical hemp panels with a density between 26.1 and 82.1 kg/m³, obtaining values between 0.0393 and 0.0486 W/m·K. Similarly, Khedari et al. [76] developed insulating panels from durian peel and coconut coir particles with a density between 288 and 234 kg/m³, showing thermal conductivities between 0.0728 and 0.0788 W/mK.

Another important property of the EGFB-based panel measured was the thermal diffusivity, which describes how fast the material reacts to a temperature change, with the thermal diffusivity dependent on the thermal conductivity, density, and specific heat of the material. As it was mentioned in the methodology, the specific heat of the EGFB-based panel results from the sum of each specific heat value of the constituents of the panel and their contribution (90% w/w of fibrous material and 10% w/w of curated phenolic resin).

Table 5. Specific heat of rigid insulation panel.

Material	Specific Heat (J/kg K)
EGFB	2290	This study
Phenolic resin (cured state)	1928	This study
EGFB-based panel	2254	This study
Wood fiber a (commercial)	1600–2100 J	a
Glass wool b (commercial)	840	b

^a Actis [77]; Hauser, Otto, and Ringeler [78]; ^b Neroth [79].

presents the specific heat of EGFB at 12% moisture content, the cured phenolic resin, and the EGFB-based panel. The values for commercial wood fiber products and glass wool from the literature are presented in Table 4 as well.

It can be seen that EGFB showed a relatively high specific heat. Compared to wood, the bark has greater thermal insulation and specific heat properties, as it fulfills a protective function for the tree against fire and the weather, among other things [59]. Therefore, compared to commercial products, the EGFB panel has a higher heat storage capacity, which ensures thermal inertia and effective protection against extreme temperature [80].

Figure 8 shows the thermal diffusivity obtained for all EGFB- based panels as a function of density. As is expected, there is a direct relationship between density and thermal diffusivity [81,82]. The higher the density, the less empty spaces there are in the material, with the thermal diffusivity approaching that of the wood (

Table 6. Thermal diffusivity of some categories of insulation materials.

	Thermal Diffusivity (mm2/s)
EGFB-based panel 50–300 kg/m3, both press systems	0.10–0.37	This study
Bio-based insulation panel	0.1–1	Johra [81]
Mineral insulation	0.1–5
Polymer insulation	0.1–8
Wood	0.08–0.13	Božiková et al. [83]

). In general, bio-based insulation material shows low thermal diffusivity and it is expected that it responds slower to changes in the thermal environment compared to other materials with high thermal diffusivity.

The water vapor permeability of rigid panels with a target density between 80 and 300 kg/m³ was studied using the dry chamber method.

Table 7. Water vapor transmission properties of EGBF-based panels (EBFP) and glass wool (GW).

Parameter	EBFP-80	EBFP-100	EBFP-300	GW
Density (kg/m3)	78 ± 1	110 ± 1	307 ± 7	20 ± 1
δ (kg m/GN s)	0.055 ± 0.008	0.047 ± 0.001	0.032 ± 0.001	0.054 ± 0.001
W (kg/GN s)	1.1 ± 0.1	0.946 ± 0.001	0.634 ± 0.002	1.08 ± 0.02
Resistivity (GN s/kg m)	18 ± 3	21.15 ± 0.03	31.54 ± 0.09	18.4 ± 0.4
WVR (GN s/kg)	0.92 ± 0.02	1.058 ± 0.001	1.577 ± 0.004	0.92 ± 0.01
µ (adim.)	3.6 ± 0.5	4.23 ± 0.01	6.31 ± 0.02	3.69 ± 0.07
sd (m)	0.18 ± 0.03	0.21 ± 0.001	0.32 ± 0.001	0.185 ± 0.004

δ: water vapor permeability; W: water vapor permeance; WVR: water vapor resistance; µ: water vapor diffusion resistance factor; s_d: equivalent air layer thickness.

shows the water vapor resistance properties of the panels.

It was observed that water permeability decreased with panel density due to the resistance offered by the mass of the material. The lower density panels showed similar behavior to commercial glass wool.

Both glass wool and eucalyptus fiber insulators have a low equivalent air thickness, S_d < 0.5 m, and therefore allow water vapor to pass. These values are related to high breathability, a highly desirable property in many building systems, as it allows conditions that prevent condensation, while improving thermal comfort [84,85]. Thus, like glass wool, the panels developed in this study will require a vapor barrier in their application. Bark panels of higher density (>100 kg/m³) have a greater resistance than those of lower density, which is confirmed by the permeability values obtained. The higher the µ value, the lower the water vapor permeability. The results obtained are in accordance with those reported for wood fiber insulating materials, which are in the range of 3 to 7, and are higher than those reported for glass wool [77,80]. However, compared to expanded polystyrene (µ = 58), E. globulus bark panels are much more permeable [86].

Figure 9 shows the ratio of mass gained per panel area over time. We observed that samples of panels with a density of 78 and 110 kg/m³ displayed a similar behavior, unlike the sample of density 307 kg/m³ that showed greater resistance to the passage of vapor per unit of area. This behavior can be attributed to the morphology of the fiber and the nature of the resin. In this case, the fibers, due to their elongated shape, are randomly distributed in the mat, creating more obstacles for the water vapor and increasing the difficulty of going through the material to the other side. Additionally, a higher density implies more weight and, consequently, a greater resin content. This can result in the formation of a hydrophobic barrier due to the chemical nature of the resin (phenol formaldehyde). The results also show that the bark panels have a lower permeability to the passage of steam than the glass wool sample.

3.3. Fire-Test Response of Eucalyptus Bark Panel

The fire behavior of EGFB-based panels has been examined without and with the addition of 1% w/w of a commercial flame-retardant base on ammonium salts (FR). A rigid panel of the target density 100 kg/m³ without adhesive was made, resulting in an easily burnt material, given its fully organic nature. However, after removing the flame, we observed that no flame is generated during ignition, but passive combustion in which combustion gasses are released (smoke generation). The untreated rigid panel has a burning rate of 2.6 mm/min, independent of the presence of phenolic resin. The incorporation of 1% flame-retardant product grants the material with complete protection against the testing conditions. This result is comparable with commercial insulators from mineral sources, such as glass wool.

3.4. Mold Resistance

The mold resistance of an EGFB-based panel has been studied using a standardized method [71]. The resistance with and without 10% phenolic resin and with a commercial antifungal additive were compared. In Figure 10, the samples were ranked in terms of resistance against mold.

The commercial antifungal additive incorporated into the EGFB-based panel allowed for the protection of the material during the 5 weeks of study, ending with a rank of 9.4 in this period. In contrast, untreated panels, regardless of the phenolic resin content, were affected from the first week by the mold colony, reaching the fifth test week with a rank of 1.9 for samples with resin, and 1.8 for plain samples. This may be a result of its sugar-rich chemical composition and the favorable environmental conditions of experimentation. Although the bark plays a protective role against the attack of pathogens in the tree, this protection is limited when temperature and humidity conditions favor the growth of these molds. It is, therefore, necessary to incorporate commercial additives to maximize the durability of the insulating material.

4. Discussion

The results indicate that EGFB-based panels are a viable alternative as a thermal insulation solution. Indeed, the values obtained for thermal diffusivity and water vapor transmission properties are comparable with other commercially available insulation materials [81,83]. Still, for good in-service performance, the EGFB-based panels need be treated with flame retardant and antifungal additive.

The good performance of the panel is relevant because bio-based eucalyptus bark panels generate less CO₂ eq and require less energy consumption compared to traditional alternatives [60]. In effect, E. globulus panels have an immediate reduction in their environmental impact, generating benefits from raw material production and product manufacturing [60], contributing to a more sustainable construction industry and building sustainability. This agrees with the basis of a circular economy, defined as a model of production and consumption where materials are reused and recycled as often as possible with the objective of increasing resource efficiency and a lower demand for virgin raw materials [46,47]. In this regard, the bio-based resources and waste streams can be converted into value-added products such as food, feed, bio-based products, bioenergy, and construction materials. Therefore, bio-based materials, especially the ones from waste streams (such as the E. globulus bark), have a direct role in an economic model with less environmental impacts. In fact, in relation to low-emission insulation panel alternatives, the growing interest in bio-based options as a main raw material has been observed in other studies [18,19,23,24,33,34,44,87].

In the case of the EGFB panel, by taking advantage of a resource generally considered as waste, these panels also contribute to reducing the impact of disposing eucalyptus bark in landfills, which is a worldwide problem, considering that Eucalyptus plantations have a strong global presence, covering up to 22.57 million hectares in countries such Australia, Brazil, Chile, Kenya, Portugal, Spain, and Uruguay [49,50]. Considering that the utilization of this residue from its fibrous potential is not the approach usually adopted, this solution contributes to the development of a sustainable and bio-based thermal insulation product using a low-cost abundant raw material.

Particularly in the case of Chile, approximately 14.4 million m³ of Eucalyptus is generated per year, generating 1.5 million m³ of bark. Therefore, the E. globulus bark is a biodegradable and locally abundant natural waste. In this regard, the EGFB-based panel can be an alternative to solve the bark disposal problem generated by the forestry industry.

According to the Camara Chilena de la Construcción [88], approximately 12.2 million m² of houses and buildings are constructed per year. Assuming that all dwellings use insulation materials, as required by the Chilean standards [89], the demand for insulation would be approximately 12 million m². In a conservative scenario, where the demand for a new product represents 5% of the market, 600,000 m³ of EGFB-based insulation material will be required. For the case of a 50 mm thick E. globulus bark panel with a density of 100 kg/m³, the demand would be 3000 tonnes/year of insulation material.

As E. globulus bark is a local waste used to solve the insulation challenges of local buildings, the EGFB-based insulation panel is also an important alternative to mitigate the impact generated by the construction industry. In fact, the utilization of this industrial waste represents a potential reduction in excessive transport and the intense demand for non-renewable insulation materials. With this approach, EGFB-based panels present environmental benefits from the beginning, i.e., as early as the procurement of the raw material, the production of the panels, and the transportation to the buildings.

The study of non-conventional natural fibers opens an important path in biomaterials research. In this sense, the evaluation methodology applied in this study can be replicated for the analysis of other potential alternatives of bio-based insulation materials, especially the ones including the valorization of the waste of one industry (with its preparation and panel manufacturing) to expand the use of high-quality biomass for the generation of materials that respond to the identified environmental challenges of other industries, especially construction ones, that need effective solutions to reduce their enormous environmental impact.

5. Conclusions

The present article studied the feasibility of producing E. globulus bark fibers suitable to manufacturing insulating panels, and the performance of panels produced by the hot plate method and by the saturated steam injection method.

Both production methods result in panels with similar densities and good insulation. The thermal conductivity values range from 0.064 to 0.077 W/m·K. Furthermore, the EGFB-based panels have low equivalent air thickness (Sd < 0.5 m), allowing water vapor passage and enhancing breathability, which reduces condensation and improves thermal comfort. The higher density panels (>100 kg/m³) showed higher resistivity, which is confirmed by the permeability values obtained. In this respect, increasing the water vapor diffusion resistance factor (µ) decreases the water vapor permeability (δ).

Adding 1% of a commercial flame retardant based on ammonium salts significantly improves the fire resistance of the panels. In terms of mold resistance, panels treated with an antifungal additive maintained good protection over a 5-week period; in this case the rank obtained for the samples was 9.4. Compared to wood fiber and glass wool commercial panels, the EGFB-based panel has a higher heat storage capacity. This fact ensures protection against extreme temperature changes. Therefore, EGFB-based panels are suitable for cavity wall insulation; however, they need to be treated with flame retardants and antifungal additive to ensure long-term performance.

EGFB-based panels offer a sustainable alternative by reusing renewable, biodegradable forestry waste, addressing bark disposal issues, and reducing environmental impacts. In this sense, EGFB panels contribute to the improvement of the circular economy and bio-based waste valorization. Considering that insulation panels are a key material to ensure indoor living comfort, the application and benefits of EGFB-based insulation panels provide a significant sustainable solution to be evaluated in a real-life scenario.