Technical Performance Overview of Bio-Based Insulation Materials Compared to Expanded Polystyrene

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Article Technical Performance Overview of Bio-Based Insulation Materials Compared to Expanded Polystyrene

Cassandra Lafond and Pierre Blanchet *

Department of Wood and Forest Sciences, Laval University, Québec, QC G1V0A6, Canada; [email protected] * Correspondence: [email protected]

Received: 5 February 2020; Accepted: 22 April 2020; Published: 26 April 2020

Abstract: The energy efficiency of buildings is well documented. However, to improve standards of energy efficiency, the embodied energy of materials included in the envelope is also increasing. Natural fibers like wood and hemp are used to make low environmental impact insulation products. Technical characterizations of five bio-based materials are described and compared to a common, traditional, synthetic-based insulation material, i.e., expanded polystyrene. The study tests the thermal conductivity and the vapor transmission performance, as well as the combustibility of the material. Achieving densities below 60 kg/m3, wood and hemp batt insulation products show thermal conductivity in the same range as expanded polystyrene (0.036 kW/mK). The vapor permeability depends on the geometry of the internal structure of the material. With long fibers are intertwined with interstices, vapor can diffuse and flow through the natural insulation up to three times more than with cellular synthetic (polymer) -based insulation. Having a short ignition times, natural insulation materials are highly combustible. On the other hand, they release a significantly lower amount of smoke and heat during combustion, making them safer than the expanded polystyrene. The behavior of a bio-based building envelopes needs to be assessed to understand the hygrothermal characteristics of these nontraditional materials which are currently being used in building systems.

Keywords: bio-based; ﬁre behavior; hemp; thermal conductivity; thermal insulation; vapor permeability; wood

1. Introduction In industrialized countries, insulation is now an important tool for the improvement of a building’s energy behavior. The degree of insulation achieved in a building is directly linked to the minimal thickness imposed by the national regulations that has been increasing over the years. As Papadopoulos [1] indicates, during the period of 1980 to 2000, the thickness of insulation for walls has doubled in some countries in Northern Europe, although it has remained stable for less prosperous countries like Greece. The choice of insulation materials should be based on multiple factors. A couple of decades ago, other than the building’s energy behavior and the physical properties of the materials, the focus moved on the use of environmentally-friendly products in the construction industry. Indeed, diﬀerent materials could be used to provide similar functions in buildings but the related energy-use and emissions could vary signiﬁcantly. A study by Grand View Research [2] stated that the North American building thermal insulation market size was valued at 7.09 billion USD in 2015. Higher energy costs have led to increases in the demand for insulation, and can explain the slow but stable growth witnessed in the market. In Canada, the National Energy Code of Canada for Building Code (NECB 2015) sets the requirements

Buildings 2020, 10, 81; doi:10.3390/buildings10050081 www.mdpi.com/journal/buildings Buildings 2020, 10, 81 2 of 13 for the energy efficiency of a building. In the U.S., an initiative called The Weatherization Assistance Program is funded by the U.S. federal government and has the goal of promoting more efficient thermal insulation. These programs are key to educating the population and industry about the necessity of achieving high-energy efficiency in buildings. The positive impacts are both economic and environmental in the long term. After glass wool, the most used insulation material in North America is expanded polystyrene (EPS), accounting for 23.5% of share by volume. Made of 98 to 98.5% air, the main component is polystyrol pearls blown with pentane as a propellant gas. Under heating, the pearls expand at 212 ◦F, i.e., the temperature at which the pentane evaporates. To further eliminate of the pentane by diffusion, the expanded pearls are stored for a few days, after which they are poured into molds and melted under controlled steam heating to bind with each other. A counterbalance to the high thermal resistance of this material is its poor fire behavior. Indeed, in the event of a fire, EPS melts and releases toxic fumes. As Grand Review reported [2], current thermal insulation materials in the construction market are generally inorganic, e.g., extruded polystyrene (XPS), expanded polystyrene (EPS), polyisocyanurate and polyurethane foam with high performance in relation to heat transfer, but also with the highest environmental impact. As the International Energy Agency revealed, the building sector is responsible for 40% of energy use, one third of GHG emissions and the consumption of 30% of all resources [3]. There is a need to reduce this impact, and it could be linked with the choice of materials included in the envelope. In a study by Thormark [4], the embodied energy of an energy efficient building reached 40% of the total energy needed for a life expectancy of 50 years. His results showed that through material substitution, the embodied energy can be decreased by 17%. Numerous studies have examined the use of bio-based or natural insulation materials with the aim of replacing conventional inorganic ones [5–9]. During photosynthesis, plants sequester atmospheric carbon dioxide, meaning that the use of crop-based materials in a building can reduce its embodied energy. Their transformation process also involves fewer steps and requires less energy than petrochemical or mineral insulation [10]. Nonetheless, the use of bio-based insulation materials is limited due to their thermal conductivity and density, which are usually higher than those of EPS. Reif et al. [11] showed that natural fiber insulation materials can achieve thermal conductivities of around 0.060 to 0.080 W/mK with densities ranging from 150 to 450 kg/m3; EPS is known to have a thermal conductivity of 0.036 W/mK with an appreciably lower density of around 35 kg/m3. It is also well known that synthetic-based insulation materials such as EPS do not let water vapor evacuate by diffusion in cases of water leakages in walls. The growth of mold which occurs in walls is often due to a mix of air tightness and the use of impermeable-to-vapor insulation materials. Hygroscopic materials like natural fibers can equilibrate indoor humidity through their ability to absorb, store and release water vapor from the air. The movement of vapor in the envelope is essential, as even an efficient design cannot ensure that a leakage will never occur. Vink et al. [12] defined an ideal, environmentally sustainable product as providing the equivalent function of the product it replaces, and which is available at competitive cost. It also needs to be sourced from renewable raw materials and have a minimal negative impact on the environment and on human health. Despite critical embodied energy and a poor end-of-life behavior, organic foams can achieve high thermal resistance. Conventional natural insulations cannot achieve the same thermal performance due to their layered structures, as opposed to porous ones, limiting the transport of heat by trapping air inside. As Steen-Hansen, Mikalsen and Jensen [13] proposed, combustible insulation materials are likely to contribute to fires by their tendency to ignite from smoldering combustion as well as visible flame. Smoldering combustion can increase fire risk due to its starting point, i.e., a lower temperature than traditional flaming fire. The authors’ results showed that with the same level of fire retardant, fire developed at lower temperatures for loose-fill insulation with smaller-sized fibers than with the larger ones. The function of insulation is complex, and the materials from which it is made should not be strictly evaluated as independent entities, but rather, as a part of a complex system, i.e., the building Buildings 2020, 10, 81 3 of 13 envelope. By blocking, limiting and controlling the transport of air, water and heat through the building, the shield needs to be designed according to different physical mechanisms. Furthermore, the quality of an insulation material is not restricted to its technical and environmental performance, but is related to local needs, i.e., adaptability to national regulations, traditional methods of building and the availability and use of the product in a defined area [10]. Our technical characterization included studying the thermal conductivity and vapor transmission, as well as the combustibility of the material. As a building material, insulation should be assessed in view of performance-based fire design. Therefore, this study assesses the technical performance of various bio-based insulation materials which may be able to replace EPS. The focus is on natural fiber materials and excludes other potential bio-based insulations like mycelium composite foam or bio-based polymeric foams.

2. Results The density, thermal conductivity, vapor permeability and ﬁre behavior for the studied insulation materials are presented in this section. The means and the coeﬃcients of variation are included to illustrate the interval of distribution.

2.1. Density vs. Thermal Conductivity Figure1 establishes the relationship between the thermal conductivity and density of the six studied materials; the trend seems to be linear. In ascending order of density, the insulation materials are ranked as follows: EPS (21 kg/m3), FHB (34 kg/m3), FWBF (55 kg/m3), FWB (55 kg/m3), WF (143 kg/m3) and RWF (231 kg/m3). The expanded foam structure of EPS makes it less dense than the fiber composites, with an impressive ability to trap air inside its cavities, thereby limiting the transport of heat. It is difficult to make lightweight materials with natural fibers as the density of wood is typically higher than 300 kg/m3. With the longest hemp fibers, it is possible to create a lighter, tridimensional network, and hence, to obtain a lower density and thermal conductivity. Also, here, due to its relatively low density, obtained through an innovative process, the thermal resistance of FWBF (0.039 W/mK) is in the same range as that of EPS (0.036 W/mK). With a gas injection step in the making of the FWBF, the formation of bubbles ensures better thermal resistance properties than conventional wood fiberboard, even at the same density. By changing the parameters in the process, it is possible to obtain a density ranging from 40 kg/m3 to 100 kg/m3 [14]. Here, the density is more than three time higher than that of EPS; the expected end-use is in the form of rigid insulation boards which could replace conventional oriented strand board (OSB) panels. This recycled fiberboard (0.057 W/mK) competes with OSB in terms of its insulation properties; the thermal conductivity of OSB is around 0.13 W/mK [15]. With an average density and thermal conductivity compared to the other materials being studied, the conventional WF manufactured via a dry process is ideal for renovations, as it will provide additional insulation when laid directly onto rafters without significantly increasing the dead load.

2.2. Water Vapor Permeability Figure2 shows the vapor permeability for each of the insulation materials being studied. There is a distinct gap between the vapor permeability of EPS and those of natural fiber composites. The vapor permeability for EPS is only 4 perm-inch, while it increases to 21 perm-inch for RWF, and ranges from 32 to 41 perm-inch for the other wood fiber materials. The hemp fibers, as mentioned earlier, are longer and interlock differently than wood fibers. This offers a pathway of air movement, allowing vapor to flow not only by diffusion, but also by convection. As observed, the vapor permeability of FHB is largely superior (83 perm-inch) to that of wood-based panels. As proposed in Supplement 7 of the International Building Code, materials can be divided in four class of water vapor permeability (Table1)[16]. Buildings 2020, 10, x FOR PEER REVIEW 3 of 14

96 the quality of an insulation material is not restricted to its technical and environmental performance, 97 but is related to local needs, i.e., adaptability to national regulations, traditional methods of building 98 and the availability and use of the product in a defined area [10]. 99 Our technical characterization included studying the thermal conductivity and vapor 100 transmission, as well as the combustibility of the material. As a building material, insulation should 101 be assessed in view of performance-based fire design. Therefore, this study assesses the technical 102 performance of various bio-based insulation materials which may be able to replace EPS. The focus 103 is on natural fiber materials and excludes other potential bio-based insulations like mycelium 104 composite foam or bio-based polymeric foams.

105 2. Results 106 The density, thermal conductivity, vapor permeability and fire behavior for the studied 107 insulation materials are presented in this section. The means and the coefficients of variation are 108 included to illustrate the interval of distribution.

109 2.1. Density vs Thermal Conductivity 110 Figure 1 establishes the relationship between the thermal conductivity and density of the six 111 studied materials; the trend seems to be linear. In ascending order of density, the insulation materials 112 are ranked as follows: EPS (21 kg/m3), FHB (34 kg/m3), FWBF (55 kg/m3), FWB (55 kg/m3), WF (143 113 kg/m3) and RWF (231 kg/m3). The expanded foam structure of EPS makes it less dense than the fiber 114 composites, with an impressive ability to trap air inside its cavities, thereby limiting the transport of 115 heat. It is difficult to make lightweight materials with natural fibers as the density of wood is typically 116 higher than 300 kg/m3. With the longest hemp fibers, it is possible to create a lighter, tridimensional 117 network, and hence, to obtain a lower density and thermal conductivity. Also, here, due to its 118 relatively low density, obtained through an innovative process, the thermal resistance of FWBF (0.039 119 W/mK) is in the same range as that of EPS (0.036 W/mK). With a gas injection step in the making of 120 the FWBF, the formation of bubbles ensures better thermal resistance properties than conventional 121 wood fiberboard, even at the same density. By changing the parameters in the process, it is possible 122 to obtain a density ranging from 40 kg/m3 to 100 kg/m3 [14]. Here, the density is more than three time 123 higher than that of EPS; the expected end-use is in the form of rigid insulation boards which could 124 replace conventional oriented strand board (OSB) panels. This recycled fiberboard (0.057 W/mK) 125 competes with OSB in terms of its insulation properties; the thermal conductivity of OSB is around 126 0.13 W/mK [15]. With an average density and thermal conductivity compared to the other materials 127 Buildingsbeing stu2020died, 10, 81the conventional WF manufactured via a dry process is ideal for renovations, as it4 ofwill 13 128 provide additional insulation when laid directly onto rafters without significantly increasing the 129 dead load. 0.06

0.055

0.05

0.045

0.04

0.035 Thermal conductivity Thermal conductivity (W/mK) 0.03 0 100 200 300 Density (kg/m3)

EPS WF RWF FWB FWBF FHB 130

BuildingsFigure 2020, 1. 10Relationship, x FOR PEER betweenREVIEW the density and thermal conductivity of bio-based insulations and EPS.5 of 14

100

90 83 80

inch) 70 - 60

50 41

40 38 32

30 21 20

10 4

0 Water vapour permeability Watervapour permeability (perm 1 EPS WF RWF FWB FWBF FHB 160 Figure 2. Water vapor permeability of bio-based insulations and EPS. 161 Figure 2. Water vapor permeability of bio-based insulations and EPS. Table 1. Building materials permeance classiﬁcation. 162 2.3. Fire Behavior Vapor Impermeable 0.1 Perm or Less 163 2.3.1. Heat Release Vaporand Ignition semi-impermeable 1.0 perm or less and greater than 0.1 perm 164 Fire behavior wasVapor studied semi-permeable using the cone10perms calorimeter or less andmethod. greater The than curves 1.0 perm for the heat release 165 rate (HRR) of each sampleVapor are permeable presented in Figure 3 greater. Other than fire 10 behavior perms aspects, such as time to 166 ignition, peak of heat release rate, carbon monoxide yield and smoke production are presented in 167 FigureMaterials 4. The withrapidity, a permeance intensity inferior and length to 0.1 permof burning are classified may be as impermeable.observed through The semi-impermeable the shape of the 168 HRRcategory curve includes. EPS shows materials the longest with a permeancetime before between ignition 0.1(28.0 and s), 1 represented perm. Below by 10a high perm peak and of greater heat 2 169 releasethan 1 perm,rate (338 EPS kW/m is in the), followed category shortly semi-permeable. by a second With uptake values before greater the heat than release 10 perm, rate the continually wood and 170 decreasehemp-baseds until panels the are material in the classis completely of permeable burned materials.. These Because values of are the representatives hygroscopic nature of Hidalgo’s of wood 2 171 thesis.and hemp For fibers,a sample these of materials EPS under can an react incident with the radiative environment heat flux to reach of 45 an kW/m equilibrium, the ignition moisture time content. was 172 27 seconds and the peak of HRR between 364 to 373 kW/m2 [18]. As for the wood and hemp fiber 173 products, the time to ignition and the peak of heat release rate were almost synchronous. 174 Furthermore, for natural materials, the peak of heat release rate was not as high as EPS, but the 175 tendency was that the denser the material, the higher the energy released. A lower density implies a 176 faster propagation of heat inside the material. Emitting less energy while burning means that the fire 177 load is reduced, and that it would be less likely to contribute to the early stage of a fire when 178 compared to EPS. While the HRR of EPS rose to high levels, its contribution to time to fire was limited 179 due to its fast consumption, as observed by an intense decrease after the second peak. For all natural 180 fiber materials, the HRR reached its peak after ignition and dropped to almost a constant level until 181 the entire combustion of the specimen had occurred. This behavior is associated with the thermal 182 degradation of the material. When wood is burning, the dehydration of cellulose and the 183 repolymerization of levoglucosan lead to the formation of aromatic structures and transform wood 184 into graphitic carbon, commonly known as char. The three polymeric constituents of wood, i.e., 185 cellulose, hemicellulose and lignin, are transformed into a mixture of volatile gases, tar 186 (levoglucosan) and carbonaceous char with a distinct thermal decomposition level. With the lowest 187 temperature of decomposition, hemicellulose starts to degrade at 180 °C to 350 °C. Cellulose follows 188 when temperatures reach 275 °C to 350 °C, and then lignin at 250 °C to 500 °C [19]. With the aid of a 189 pilot flame, like in the cone calorimeter test, the flaming combustion associated with pyrolysis is 190 initiated at a temperature between 225 °C and 275 °C. While lower than with EPS, all samples 191 presented a second HRR peak. Consistent with their respective order of decomposition, the HRR 192 curve for natural fibers presents first the degradation of hemicellulose, with a shoulder before the Buildings 2020, 10, 81 5 of 13

Truly, even with a similar density and thermal conductivity, FWBF outperforms EPS due to its different core structure. While EPS is characterized by closed porosity with quasi-nonexistent diffusion between the cell walls, the combination of porosity and interlaying wood fibers in FWBF is ideal for good vapor permeance and thermal resistance. Made with only wood fibers interconnected with each other through heating and pressing with a wax emulsion, the great vapor permeance for RWF indicates its ability to transfer water vapor from a humid environment to a dryer one. Compared to the OSB vapor permeability of 30 perm [15], RWF, with a vapor permeability of 26, is not significantly different. Hence, as it was supposed, natural fiber insulation performs differently than EPS in cases of water leakage by allowing vapor transport to occur in the envelope. The presence of vapor can be critical when reaching the dew point, causing condensation to occur within the wall. Mold, decay and corrosion are signs of envelope failure [17].

2.3. Fire Behavior

2.3.1. Heat Release and Ignition Fire behavior was studied using the cone calorimeter method. The curves for the heat release rate (HRR) of each sample are presented in Figure3. Other fire behavior aspects, such as time to ignition, peak of heat release rate, carbon monoxide yield and smoke production are presented in Figure4. The rapidity, intensity and length of burning may be observed through the shape of the HRR curve. EPS shows the longest time before ignition (28.0 s), represented by a high peak of heat release rate (338 kW/m2), followed shortly by a second uptake before the heat release rate continually decreases until the material is completely burned. These values are representatives of Hidalgo’s thesis. For a sample of EPS under an incident radiative heat flux of 45 kW/m2, the ignition time was 27 s and the peak of HRR between 364 to 373 kW/m2 [18]. As for the wood and hemp fiber products, the time to ignition and the peak of heat release rate were almost synchronous. Furthermore, for natural materials, the peak of heat release rate was not as high as EPS, but the tendency was that the denser the material, the higher the energy released. A lower density implies a faster propagation of heat inside the material. Emitting less energy while burning means that the fire load is reduced, and that it would be less likely to contribute to the early stage of a fire when compared to EPS. While the HRR of EPS rose to high levels, its contribution to time to fire was limited due to its fast consumption, as observed by an intense decrease after the second peak. For all natural fiber materials, the HRR reached its peak after ignition and dropped to almost a constant level until the entire combustion of the specimen had occurred. This behavior is associated with the thermal degradation of the material. When wood is burning, the dehydration of cellulose and the repolymerization of levoglucosan lead to the formation of aromatic structures and transform wood into graphitic carbon, commonly known as char. The three polymeric constituents of wood, i.e., cellulose, hemicellulose and lignin, are transformed into a mixture of volatile gases, tar (levoglucosan) and carbonaceous char with a distinct thermal decomposition level. With the lowest temperature of decomposition, hemicellulose starts to degrade at 180 ◦C to 350 ◦C. Cellulose follows when temperatures reach 275 ◦C to 350 ◦C, and then lignin at 250 ◦C to 500 ◦C[19]. With the aid of a pilot flame, like in the cone calorimeter test, the flaming combustion associated with pyrolysis is initiated at a temperature between 225 ◦C and 275 ◦C. While lower than with EPS, all samples presented a second HRR peak. Consistent with their respective order of decomposition, the HRR curve for natural fibers presents first the degradation of hemicellulose, with a shoulder before the highest peak of HRR due to cellulose, and then a lower tail related to the degradation of the lignin [20]. This quantity of released energy occurs when the entire sample consumes itself. The RWF demonstrates distinctive behavior by showing three major peaks after the initial peak of ignition. The aspect of the curve indicates that each of the three layers in the panel burns successively. Buildings 2020, 10, 81 6 of 13

2.3.2. Smoke Obscuration One of the most characteristic differences between samples is smoke production. As the units dependedBuildings upon 2020, 10 the, x FOR burned PEER areaREVIEW and not the mass of the sample, there was an important gap6 of between 14 wood-based fiberboards of different densities. Having more substance to burn, with longer combustion 193 highest peak of HRR due to cellulose, and then a lower tail related to the degradation of the lignin times, the two rigid wood fiberboards released more smoke than the low density batt insulation 194 [20]. This quantity of released energy occurs when the entire sample consumes itself. The RWF samples. On the other hand, despite having the same density, EPS (845 m2/m2) and the wood batt 195 demonstrates distinctive behavior by showing three major peaks after the initial peak of ignition. The 196insulations aspect of demonstrated the curve indicates the biggest that each di offferences the three of layers smoke in the production, panel burns with successively. a 32 times higher value for the expanded polystyrene. Ostman [21] noted that during combustion, wood releases smoke at a quantity of 25–100 m2/kg, which is far less than synthetic polymers that can produce hundreds of 400 thousands of m2/kg. Regarding the safety of the occupants in a building, smoke release is a particularly important aspect. Indeed, the gases that are released during combustion are not only toxic and irritants, 350 but they also considerably reduce the visibility needed to escape. On this fact, EPS is the poorest choice of insulation here, if only looking at smoke emissions. 300 ) 2.3.3. Toxic2 Gases The products250 of wood combustion are carbon monoxide, carbon dioxide and water. The use of a RWF fire retardant can add toxic components to the smoke and increase the yield. However, it can lower the EPS rate of burning200 and, therefore, decrease the smoke production rate, providing a longer evacuation time. Affecting the cognitive functions of occupants and influencing their ability to take effectiveFWBF actions to find safety,150 carbon monoxide is a toxic gas which needs to be taken into account whenWF anticipating fires. The yield factor of CO (g/g) is the mass of gas generated divided by the mass of theFWB specimen Heatreleaserate (kW/m consumed.100 FWB (0.099 g/g) presents the highest carbon monoxide yield, followed by EPS,FHB albeit the latter is far behind, with two times fewer emissions (0.051 g/g). The presence of polyolefin fibers in the FWB could50 explain this result. According to their difference of density, the air concentration of CO would be greater in the case of burning the same volume of material for RWF compared to EPS. While looking0 at bio-based materials, additives for binding or to protect from mold, insects or fire have 0

an impact on the amount60 of toxic gases released. There is a common goal among scientists to develop 120 180 240 300 360 420 480 540 600 660 720 780 840 900 960 complete bio-based solutions for building materials. The1020 1080 use1140 1200 of1260 tannin-based1320 1380 1440 1500 1560 1620 glues makes up part of these eﬀorts. Time (s) 197

EPS WF

400 400 ) ) 2 350 2 350 EPS-1 WF-1 300 300 EPS-2 WF-2 250 250 WF-3 200 EPS-3 200 150 150 100 100 50 Heatreleaserate (kW/m Heatreleaserate (kW/m 50 0 0 0 200 400 600 0 500 1000 1500 Time (s) Time (s) 198

Figure 3. Cont. Buildings 2020, 10, x FOR PEER REVIEW 7 of 14

Buildings 2020, 10, 81 7 of 13 Buildings 2020, 10, x FOR PEER REVIEW 7 of 14 RWF FWB 400 400

) RWF ) FWB 2 350 2 350 400 RWF-1 400 FWB-1 300 300 ) ) 2 350 RWF-2 2 350 FWB-2 250 RWF-1 250 FWB-1 300 RWF-3 300 FWB-3 200 RWF-2 200 FWB-2 250 250 150 RWF-3 150 200 200 FWB-3 100 100 150 150

Heatreleaserate (kW/m 50100 Heatreleaserate (kW/m 10050 0 0

Heatreleaserate (kW/m 50 Heatreleaserate (kW/m 50 0 1000 2000 0 500 1000 1500 0 0 0 Time1000 (s) 2000 0 500 Time (s)1000 1500 199 Time (s) Time (s) 199 FWBF FHB 400 FWBF 400 FHB ) ) 2 2 350400 400350 )

) FHB-1 2 2 350 FWBF-1 350 300 300 FHB-1 FWBF-2FWBF-1 FHB-2 250300 300250 FWBF-2 FHB-2 250 FWBF-3 250 FHB-3 200 200 FHB-3 200 FWBF-3 200 150 150 150 150 100 100 100 100 Heatreleaserate (kW/m

Heatreleaserate (kW/m 50 50 Heatreleaserate (kW/m

Heatreleaserate (kW/m 50 50 0 0 00 200 400 600 0 0 100 200 300 0 200 400 600 0 100 200 300 Time (s) Time (s) Time (s) Time (s) 200200

201201 FigureFigureFigure 3. 3. Heat3. Heat Heat releaserelease release raterate rate of of EPSEPS and and five ﬁve bio bio-based--basedbased insulation insulation insulation materials materials materials.. .

3535 400 ) ) 2 2 3030 350

2525 300 250 2020 200 1515 150 10 10Time to ignition(s) Time to ignition(s) 100 5 5 5050 Peakof heatrelease rate (kW/m 0 Peakof heatrelease rate (kW/m 0 0 0 EPS WF RWF EPS WF RWF EPS WF RWF EPS WF RWF FWB FWBW FHB FWB FWBF FHB 202 FWB FWBW FHB FWB FWBF FHB 202

Figure 4. Cont. Buildings 2020, 10, 81 8 of 13 Buildings 2020, 10, x FOR PEER REVIEW 8 of 14

0.12 1000 ) 2 900 /m

0.1 2 800 700 0.08 600 0.06 500 400 0.04 300 Yield of CO of CO (g/g) Yield 200 0.02 100 Total Total Smoke (m production 0 0 EPS WF RWF EPS WF RWF FWB FWBF FHB FWB FWBF FHB 203

204 FigureFigure 4. 4.Time Time to to ignition, ignition peak, peak of of heat heat release release rate, rate, carbon carbon monoxide monoxide yield yield and and total total smoke smoke release release for 205 EPSfor and EPS ﬁve and bio-based five bio-based insulation insulation materials, materials determined, determined via via the the cone cone calorimeter calorimeter test. test.

206 2.4.2.3.2. Relative Smoke Assessment Obscuration 207 ToOne illustrate of the themost relationships characteristic between differences the between attributes samples of the is insulation smoke production. materials As studied the units here, 208 a relativedepended assessment upon the burned is presented area and in not the the form mass of of a the radar sample, chart there in Figure was an5 .important The extremes gap between of each 209 categorywood-based can easily fiberboard be detected.s of different Thermal densities conductivity. Having was more inversed substance to obtain to burn, thermal with resistancelonger 210 in ordercombustion to better times, represent the two the rigid overall wood performance. fiberboards released The same more process smoke wasthan applied the low todensity total smokebatt 211 release,insulation the peak samples. of HRR On andthe theother carbon hand, monoxide despite having yield. the With same this density, modification, EPS (845 the m largest2/m2) and surface the is 212 consideredwood batt as insulations the favorable demonstrated case. The overallthe biggest layout differences of EPS isof distinct smoke fromproduction, the bio-based with a 32 materials, times 213 withhigher a maximal value scorefor the for expanded time to ignition polystyrene. and thermal Ostman resistance. [21] noted Looking that during at the thermalcombustion, performance, wood 214 therereleases is no significantsmoke at a gapquantity between of 25 the–100 studied m2/kg, samples; which is this far wasless expected,than synthetic given polymers that these that materials can 2 215 wereproduce chosen hundreds with the of aim thousands of fulfilling of m /kg. the functionRegarding of the limiting safety of heat the transfer occupants in in a building.a building, However, smoke 216 theirrelease behavior is a particularly of mass transfer important differs aspect. more Indeed, significantly. the gases Water that are vapor released can di duringffuse through combustion materials, are 217 ornot can only simply toxic be and transported irritants, but by they air in also the considerably case of deficient reduce airtightness. the visibility The needed use of to vapor-permeableescape. On this 218 fact, EPS is the poorest choice of insulation here, if only looking at smoke emissions. materials in the envelope would ensure that the drying of the envelope would be achieved in case of humidity infiltration. Having a geometry of loosely-intertwined tapered fibers, the hemp fiber 219 2.3.3. Toxic Gases batt demonstrated twice the ability of other fiber-based materials to let water vapor flow. On the 220 fire safetyThe side,products the principalof wood combustion point of interest are carbon is smoke monoxide, release, carbon due dioxide to its impact and water. on theThe ability use of to 221 breathe.a fire retardant The amount can add ofsmoke toxic components release for to natural the smoke insulation and increase is less the than yield. thrice However, that of it the can synthetic lower 222 basedthe rate one. of Otherburning than and smoke,, therefore, combustion decrease the leads smoke to theproduction release rate, of toxic providing gases. a Thislonger is evacuation a concern to time. Affecting the cognitive functions of occupants and influencing their ability to take effective 223 consider when choosing adhesives to include in bio-based materials. Here, the polyolefins added 224 actions to find safety, carbon monoxide is a toxic gas which needs to be taken into account when to the wood batt insulation led to a significant amount of carbon monoxide being emitted. Globally, 225 anticipating fires. The yield factor of CO (g/g) is the mass of gas generated divided by the mass of the the technical behavior of insulation products made from different natural fibers present the same trend. 226 specimen consumed. FWB (0.099 g/g) presents the highest carbon monoxide yield, followed by EPS, They provide proper thermal resistance with advantageous water vapor permeability, and although 227 albeit the latter is far behind, with two times fewer emissions (0.051 g/g). The presence of polyolefin 228 theyfibers are combustible,in the FWB demonstratecould explain safer this combustionresult. According behavior to thantheir polystyrene. difference of density, the air 229 concentration of CO would be greater in the case of burning the same volume of material for RWF 230 compared to EPS. While looking at bio-based materials, additives for binding or to protect from mold, 231 insects or fire have an impact on the amount of toxic gases released. There is a common goal among 232 scientists to develop complete bio-based solutions for building materials. The use of tannin-based 233 glues makes up part of these efforts.

234 2.4. Relative Assessment Buildings 2020, 10, x FOR PEER REVIEW 9 of 14

235 To illustrate the relationships between the attributes of the insulation materials studied here, a 236 relative assessment is presented in the form of a radar chart in Figure 5. The extremes of each category 237 can easily be detected. Thermal conductivity was inversed to obtain thermal resistance in order to 238 better represent the overall performance. The same process was applied to total smoke release, the 239 peak of HRR and the carbon monoxide yield. With this modification, the largest surface is considered 240 as the favorable case. The overall layout of EPS is distinct from the bio-based materials, with a 241 maximal score for time to ignition and thermal resistance. Looking at the thermal performance, there 242 is no significant gap between the studied samples; this was expected, given that these materials were 243 chosen with the aim of fulfilling the function of limiting heat transfer in a building. However, their 244 behavior of mass transfer differs more significantly. Water vapor can diffuse through materials, or 245 can simply be transported by air in the case of deficient airtightness. The use of vapor-permeable 246 materials in the envelope would ensure that the drying of the envelope would be achieved in case of 247 humidity infiltration. Having a geometry of loosely-intertwined tapered fibers, the hemp fiber batt 248 demonstrated twice the ability of other fiber-based materials to let water vapor flow. On the fire safety 249 side, the principal point of interest is smoke release, due to its impact on the ability to breathe. The 250 amount of smoke release for natural insulation is less than thrice that of the synthetic based one. 251 Other than smoke, combustion leads to the release of toxic gases. This is a concern to consider when 252 choosing adhesives to include in bio-based materials. Here, the polyolefins added to the wood batt 253 insulation led to a significant amount of carbon monoxide being emitted. Globally, the technical 254 behavior of insulation products made from different natural fibers present the same trend. They Buildings 2020 10 255 provide, , proper 81 thermal resistance with advantageous water vapor permeability, and although they 9 of 13 256 are combustible, demonstrate safer combustion behavior than polystyrene.

EPS WF RWF FWB FWBF FHB

Time of ignition 1 0.9 0.8 0.7 0.6 Thermal resistance 0.5 Peak HRR inv 0.4 0.3 0.2 0.1 0

Vapour permeance Carbon monoxide yield inv

Total smoke release inv

257 258 Figure 5. Relative assessment of the attributes of the studied insulation materials. Figure 5. Relative assessment of the attributes of the studied insulation materials. 259 3. Materials and Methods 3. Materials and Methods 260 3.1. Materials 3.1. Materials Five bio-based materials (Table2) were compared with expanded graphite polystyrene (EPS), a common insulation material in SIP. EPS was manufactured by Isolofoam (Canada) [22]. Using a dry process, STEICO (Poland) manufactures wood fiberboards (WF) for external insulation with airtightness and weatherproofing properties. Polyurethane resin is the bonding agent in this process. Incorporating locally-sourced recycled wood fibers, the SONOclimat ECO4 (35 1220 2440 mm) × × is a recycled wood fiberboard (RWF) manufactured through a wet process and supplied by MSL in Louiseville (Canada) [23]. To represent low density flexible wood batt insulation (FWB), STEICO flex 036 was chosen. The ingredients entering the production were wood fibers, polyolefin fibers and paraffin wax. Still in the low density fiberboard product group, the ultra-low density composite was provided by FPInnovations (Canada). The boards received in dimension of (39 560 1270 mm) × × were produced according to the patent WO2015066806A1 Method of producing ultra-low density fiber composite materials [14]. The product was labelled in this study as flexible wood batt with foaming process (FWBF). Hemp was considered by adding a new product offered by NatureFibres, a company based in Asbestos (Canada). Their flexible hemp batt (FHB) insulation competes directly with wood batt insulation. Consistent with the different testing methods used in this study, the panels were cut to sample size and conditioned in an environmental chamber at 20 ◦C and 65% HR.

Table 2. Insulation products included for characterization.

Sample Product Commercial Name Manufacturer Abbreviation EPS Expanded graphite polystyrene Iso R plus Premium Isolofoam (Canada) WF Wood fiberboard STEICO internal STEICO (Poland) RWF Recycled wood fiberboard SONO climat ECO4 MSL (Canada) FWB Flexible wood batt Steico flex036 STEICO (Poland) FWBF Flexible wood batt with foaming process Ultra-low density composite (R&D state) FPInnovations (Canada) FHB Flexible hemp batt Naturechanvre Naturefibres (Canada) Buildings 2020, 10, 81 10 of 13

3.2. Thermal Conductivity The thermal conductivity was determined by means of the heat flow meter method described in ASTM C518 [24]. The Lasercomp FOX 314 Heat Flow Meter (TA Instruments, New Castle, DE, USA) allowed us to test samples of thickness up to 102 mm with an area of 300 mm2. The specimen (35 150 150 mm) was inserted between two plates of user-defined temperature. To achieve a × × temperature gap of 25 ◦C, the lower plate was set to 15 ◦C and the upper one to 40 ◦C. Four in-built encoders detected the actual sample thickness used in the data calculations. The heat flux from a steady state heat transfer through the specimen was determined by a heat flux transducer located in the center area of each plate. Using the thermal conductivity of a calibration standard, the thermal conductivity of the studied material could be evaluated.

3.3. Water Vapor Transmission The water vapor transmission properties of the materials were determined using the water method described in ASTM E96 [25]. Glass jars were ﬁlled with water to a level of 38 mm from the mouth. The round samples of 60 mm diameter and 12.7 mm thickness were then sealed with silicone at the upper limit of a glass jar’s aperture. The screw lid with an opening of 61 mm was attached. The dish assembly was weighed before being placed in a conditioned chamber at 50% HR and 21 ◦C. The weight and time of measurement were recorded periodically. The test ﬁnished when the change in mass was constant over six weighings. The slope of the straight line on the graph of weight change over time was used to calculate the water vapor transmission (WVT) properties as follows: Water Vapor Transmission: WVT = (G/t)/A (1)

G = weight change (g) t = time (h) G/t = slope of straight line (g/h) A = test area (m2) WVT = rate of water vapor transmission (g/hm2) Permeance: Permeance = WVT/∆p = WVT/S(R R ) (2) 1 − 2 ∆p = vapor pressure diﬀerence in mm Hg S = saturation vapor R1 = relative humidity in the dish (100%HR) R2 = relative humidity in the controlled chamber (50%HR)

3.4. Fire Behaviour The cone calorimeter (Figure6) was chosen to evaluate the fire behavior of the insulation materials in the spirit of the ISO 5660 method, as suggested by Dagenais [26]. Some parameters pf fire behavior like ignitability, flammability, and smoke and gas release could be evaluated with this apparatus. A specimen was placed in a sample holder oriented horizontally with a spark-igniter just above it. A highly insulating material such as mineral fibers was lay down under the sample. The truncated cone was heated to produce a uniform and common testing radiant heat flux of 50 kW/m2 across the surface of the 10 cm 10 cm specimen. On top of the truncated cone, the exhaust hood collected the × smoke and hot gases released after ignition. The concentration of various gases was analyzed in the exhaust system. To determine the heat release rate (HRR), the calculation was based on the general hypothesis developed by Huggett [27] that the amount of oxygen required for complete combustion is proportional to the net heat of combustion, or more precisely, that 13.1 103 kJ of heat is released per × kilogram of oxygen consumed. With the addition of a calibration constant determined by burning methane of at least 99.5% purity, the combustibility parameters can be calculated [28]. These fire tests Buildings 2020, 10, x FOR PEER REVIEW 11 of 14

299 t = time (h) 300 G/t = slope of straight line (g/h) 301 A = test area (m2) 302 WVT = rate of water vapor transmission (g/hm2) 303 Permeance: Permeance = WVT/∆p = WVT/S(R1-R2) (2) 304 ∆p = vapor pressure difference in mm Hg 305 S = saturation vapor 306 R1 = relative humidity in the dish (100%HR) 307 R2 = relative humidity in the controlled chamber (50%HR)

308 3.4. Fire Behaviour 309 The cone calorimeter (Figure 6) was chosen to evaluate the fire behavior of the insulation 310 materials in the spirit of the ISO 5660 method, as suggested by Dagenais [26]. Some parameters pf 311 fire behavior like ignitability, flammability, and smoke and gas release could be evaluated with this 312 apparatus. A specimen was placed in a sample holder oriented horizontally with a spark-igniter just 313 above it. A highly insulating material such as mineral fibers was lay down under the sample. The 314 truncated cone was heated to produce a uniform and common testing radiant heat flux of 50 kW/m2 315 across the surface of the 10 cm × 10 cm specimen. On top of the truncated cone, the exhaust hood 316 collected the smoke and hot gases released after ignition. The concentration of various gases was 317 analyzed in the exhaust system. To determine the heat release rate (HRR), the calculation was based 318 on the general hypothesis developed by Huggett [27] that the amount of oxygen required for 319 complete combustion is proportional to the net heat of combustion, or more precisely, that 13.1 x 103 320 BuildingskJ of heat2020 ,is10 released, 81 per kilogram of oxygen consumed. With the addition of a calibration constant11 of 13 321 determined by burning methane of at least 99.5% purity, the combustibility parameters can be 322 calculated [28]. These fire tests were realized at FPInnovations’ Materials Evaluation Laboratory in 323 wereQuebec realized City at(Quebec). FPInnovations’ The heat Materials release rate Evaluation curve over Laboratory time can inbe Quebectraced. The City time (Quebec). to ignition The heatis an 324 releaseimportant rate variable curve over as it time indicates can be the traced. flammability The time of to the ignition material. is an Other important parameters variable such as itas indicates the mass 325 theloss ﬂammability rate, smoke ofproduction the material. and Other carbon parameters monoxide such release as thehelp mass to predict loss rate, the smoke hazard production behavior of and the 326 carbonproduct monoxide in case of release a building help fire. to predict the hazard behavior of the product in case of a building ﬁre.

327 328 FigureFigure 6. 6.Small-scale Small-scale cone cone calorimeter calorimeter used used in in this this study study (Hurley). (Hurley).

329 4.4. Conclusions Conclusions The aim of this study was to evaluate and compare the technical performance of five bio-based insulation materials with a conventional, synthetic-based insulation, i.e., expanded polystyrene. The low density wood batts manufactured through a dry process (FWB) or a wet one with gas injection (FWBF) demonstrated densities and thermal conductivities in the same range as EPS. The wood fiberboard (WF) and recycled wood fiberboard (RWF) were designed as external rigid insulation boards with superior density. While comparing well to OSB in its vapor permeability ability, the thermal resistance was far greater than that of the traditional sheeting product. However, the best product in terms of the transport of water vapor was the flexible hemp batt (FHB) insulation, with a permeability almost twenty-five times higher than that of EPS. This attribute is significant for evacuating moisture in case of water leakage, and limiting the development of mold and decay. The intertwined pattern of longer hemp fibers allows the transport to occur of humidity, not only by vapor diffusion, but also by convection through the interstices. Regarding fire behavior, low density batt insulations were consumed faster than EPS, but release a significantly lower amount of energy and would not be an enhancing component in the early stages of a fire. All wood and hemp-based insulations have the advantage of releasing smaller amounts of smoke than EPS. Smoke production is problematic in many respects, among which is the possibility that it will prevent occupants from safely finding an exit. The carbon monoxide yield was also lower for most of natural fibers materials compared to the EPS. In the end, the properties of the bio-based insulations were not similar to the conventional expanded polystyrene on every studied aspect. Consequently, it is not possible to replace synthetic-based insulation with natural insulation without adjusting the end-use. As for the hemp and wood batt insulations, they could be used as the main core insulation in a box insulated panel bearing a heavy load, or simply in the cavities between wood frames, as mineral wool is used. Therefore, mastering the properties of bio-based insulation may allow architects and building designers to properly design Buildings 2020, 10, 81 12 of 13 building envelopes in low environmental impact buildings. There is not an extensive range available of sustainable insulation products with densities and thermal resistances similar to those of EPS. Low density bio-based materials will provide an environmentally friendly solution to replace organic foams as building insulation. Eliminating synthetic-based materials with significant embodied energy will help to reduce the environmental impact of the construction industry, even more so in areas where energy is produced at a low environmental cost. There is a need to assess the overall hygrothermal behavior of bio-based material in building envelopes. Validation of the safety of these natural fiber materials regarding the risk of mold development will be performed in a future study. The finite element method will be used to simulate different levels of air leakages inside the envelope with numerical models.

Author Contributions: Methodology, C.L.; formal analysis, C.L.; writing—original draft preparation, C.L.; writing—review and editing, P.B.; supervision, P.B.; project administration, P.B.; funding acquisition, P.B. All authors have read and agree to the published version of the manuscript. Funding: The authors are grateful to Natural Sciences and Engineering Research Council of Canada for the financial support through its IRC and CRD programs (IRCPJ 461745-18 and RDCPJ 524504-18) as well as the industrial partners of the NSERC industrial chair on eco-responsible wood construction (CIRCERB). More specifically, the authors thank Art Massif, FPInnovations, MSL, NatureFibres and STEICO for the generous donation of materials. Conflicts of Interest: The authors declare no conflict of interest.

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