Practical Use of Materials of Natural Origin as Loose-Fill Insulations in Open-Diffusion Constructions—Observation and Numerical Simulation

Abstract

The increasing requirements concerning the consideration of the environmental impact of building materials, along with the simultaneous preservation and enhancement of building thermal parameters, have led to a surge in interest in insulations based on organic or recycled materials. Despite the growing interest in these materials, there remains a scarcity of scientific studies regarding their hygrothermal properties. Within the scope of the research described in the text, the insulation properties of loose-fill materials (hemp shives, cellulose fibers, loose wood wool, and mineral wool as a reference) in wooden frame walls were analyzed. The authors simulated walls with the same U value filled with these materials using Delphin 6.1 software. The simulation time was 3 years, considering the appropriate climatic conditions of Olsztyn and different microclimatic conditions inside the rooms. Insulations made of natural organic can absorb and reveal moisture to the internal environment, while mineral wool transports the moisture to the outside, which may cause condensation problems. Insulations made of hemp shives or wood wool do not increase the level of accumulated moisture over time, which results in thermal stability. In contrast, cellulose and mineral wool store more moisture, which in wet conditions increases the heat flux by 6.9% and 5.2%, respectively.

1. Introduction

The production of materials, the process of erecting building structures, subsequent operation, and the maintenance of buildings constitute the largest components of the construction sector. According to estimates, they account for approximately 30–40% of the world’s energy demand and are responsible for emitting around 27–30% of greenhouse gases [1,2,3,4,5]. Furthermore, according to the UNEP report of 2022, the sector’s contribution to global CO₂ emissions was 37% in 2021 [6].

Considering the aforementioned indicators, global climate policy is currently heavily focused on reducing them and lowering the carbon footprint of the construction sector. The United Nations Framework Convention on Climate Change, on which the Paris Agreement’s principles are based, is one of the key documents addressing global warming. The document obliges member states to reduce greenhouse gas emissions, significantly impacting the construction sector and promoting energy efficiency in buildings [7]. According to the European Green Deal objectives, EU member states are to achieve climate neutrality by 2050. One of the envisaged strategies is the improvement in the efficiency of existing buildings by implementing appropriate standards for their modernization. The document enforcing these principles within the Union is the European Union Directive on the Energy Performance of Buildings (2002/91/EC) [8], amended by Directive (2023/7191/EU) [9], which imposed an additional obligation to consider the cumulative forecasted carbon footprint. According to estimates, actions related to improving energy efficiency should reduce CO₂ emissions by approximately 30% in the construction sector within the EU [10].

In both the realm of structural and insulating materials, various scientific research endeavors are underway, focusing on the quest for more sustainable materials that could serve as alternatives to those produced through industrial methods.

Among the commonly utilized materials for erecting walls are ceramic materials and silicates, as well as elements based on cement and structural steel. According to data, the production of individual materials emits the following amounts of CO₂ per kilogram of product: for full ceramic bricks, it ranges from 0.12 to 0.271 kg CO₂ [11,12]; for silicate blocks with an average density of 405 kg/m³, it ranges from 0.08 to 0.1 kg CO₂ [13]. The production of one kilogram of cement emits 0.22 kg CO₂ [13], while structural steel emits 0.114 to 0.73 kg CO₂ [14]. One of the alternative, more environmentally sustainable substitutes for industrially produced load-bearing materials is structural timber. According to research conducted by Schenek and Amiri [15], buildings constructed with timber elements, depending on the type of technology used, exhibit a 28–47% lower carbon footprint compared to buildings constructed with traditional materials [15]. Furthermore, the dismantling processes of timber-framed buildings require approximately 50% less energy input compared to buildings made from traditional materials [15].

Achieving proper energy efficiency in a building can be attained through the application of suitable insulation layers in the external elements of the structure. Mineral wool, polystyrene, and polyurethane foam, currently among the most prevalent insulation materials, exhibit favorable thermal resistance coefficients, resulting in reduced energy consumption required for heating the building. However, these materials also feature high emission coefficients during the production stage. According to the literature, per kilogram of produced material, expanded polystyrene (EPS) emits between 5.05 kg and 8.25 kg of CO₂ [16], mineral wool emits between 2.77 and 3.62 kg of CO₂ [16], and polyurethane foam emits approximately 5.31 kg of CO₂ [16]. Considering these indicators and the current direction of global climate policy, research is underway to explore the possibilities of utilizing insulation materials based on natural fibers or processed waste from the timber, paper, or textile industries, which would possess both favorable thermal resistance parameters and low emission coefficients.

Hemp shives, due to their high porosity [17,18], exhibit low thermal conductivity ranging from 0.04 to 0.045 (W/mK) [19]. Grain straw, an agricultural by-product, can be used as an insulating material in the form of whole straw bales, straw panels, or blown-in insulation. Research by Salonen et al. demonstrated that blown-in chopped straw has a thermal conductivity coefficient of 0.058 (W/mK) [20]. Additionally, studies have shown no significant differences between new and reused straw [20]. Flax fibers, depending on the density level, have a thermal conductivity coefficient ranging from 0.038 to 0.042 (W/mK) [21]. Wood wool exhibits thermal conductivity ranging from 0.038 to 0.044 (W/mK) [22], while the thermal conductivity coefficient of cellulose fibers ranges from 0.037 to 0.042 (W/mK) [21]. Besides their good thermal properties, insulations based on natural fibers feature significantly lower CO₂ emissions compared to industrial insulations. According to the literature, the production of one kilogram of specific insulations emits the following amounts of CO₂: 0.15 kg for hemp shives and 0.18 kg for straw-based insulation [23]. Due to production processes, wood wool emits 1.56 kg of CO₂ per kilogram of product. Due to production processes, wood wool emits 1.56 kg of CO₂ per kilogram of product, while cellulose fibers exhibit emissions ranging from 0.73 to 3.66 kg of CO₂ [11].

To verify the current state of research, an archival query was conducted. Based on it, it was found that studies on the relationship between thermal conductivity and the moisture level of insulating materials are primarily conducted on homogeneous materials in the form of panels [22,24,25,26,27] and composite materials [28]. The research was conducted by saturating samples in immersion chambers [22], in climatic chambers [24,25], in the form of numerical simulation [26], or based on the hot plate method [27,28]. The conducted studies focused on the direct relationship between the moisture content and thermal conductivity and were carried out over short time intervals. During the execution of the archival query, few studies regarding loose insulation were found. One of the studies compared the thermal properties of cellulose fibers with mineral wool, with both materials having the same thickness [29]. Furthermore, no studies were found regarding the locally occurring temperature and humidity conditions, considering locally required thermal transmittance coefficients for walls as one of the reference points for comparative studies conducted based on numerical data over a period of 3 years. According to the authors, conducting such a simulation is significant due to the possibility of more precisely determining the requirements of insulation materials based on natural fibers in relation to local atmospheric conditions, both temperature and humidity, which may vary depending on the location under consideration. Additionally, a more precise determination of thermal insulation requirements in the context of local conditions may translate into the selection of appropriate, sustainable materials.

This article shows the practical use of materials of organic origin for the construction of diffusion-open walls. Based on many years of laboratory tests, the authors determined the thermal and moisture properties of these materials. This made it possible to carry out comprehensive thermal and moisture calculations of building elements insulated with natural materials. This article presents the possibility of using materials of organic origin as loose-fill thermal insulations of frame elements. These materials can also be used to thermally insulate ceilings, but this aspect is not discussed in detail in this work. A comprehensive hygrothermal simulations, presented in this paper, proved a big advantage of materials of organic origin, which is their ability to store moisture and slowly release it into the environment. This is a great advantage of natural materials over highly processed ones, e.g., EPS or PUR. The passive release of buffered moisture into the interior allows for cost-free regulation of the interior microclimate, contributes to energy savings, and has a positive impact on the health of residents and users of buildings.

2. Possible Application of Natural Origin Materials as Loose-Fill Thermal Insulation

Previous research on the thermal and humidity properties of organic materials has led to the conclusion that these materials have the potential to be used as thermal insulation in diffusion-open building elements. It is most reasonable to use such materials as loose-fill in frame elements, e.g., walls, ceilings, and roof slopes. Frame elements usually require stiffening in the form of plates. The space between the plates can be filled with natural materials as thermal insulation. In most cases, if the wall is filled with loose-fill material, no additional layer of thermal insulation is applied on the outside. Therefore, the role of thermal insulation is taken over by the material that fills the wall. To minimize the impact of thermal bridges in places where wooden elements meet thermal insulation, columns made in the shape of I-beams can be used, where both strips are made of wood and the web is made of fiberboard. Alternatively, it is possible to use trussed columns, and then loose insulating material fills the space between the cross braces. However, it should be borne in mind that in the case of narrow wooden and wood-based elements, the difference in thermal conduction between the structure and thermal insulation is small, and therefore the impact of thermal bridges is not big.

Figure 1 shows an example of a wooden frame wall filled with fibrous insulating material. The thermal insulation layer is placed entirely between the columns. From the inside, the wall is stiffed with a clay board, while from the outside, it is closed with a chipboard, e.g., OSB, covered with a reinforced layer of lime plaster. The use of vapor retarders and wind barriers is optional. If the wall casing is made of rigidly connected OSB boards covered with plaster, wind insulation under the boards is not necessary. If the wall would also serve as a passive moisture buffer, then a vapor retarder should not be used.

Due to the loose state of the analyzed insulations, the material can be blown into the wall structure. It is also possible to build the walls step-by-step by adding narrow strips of slabs and compacting the poured material. It is also possible to erect prefabricated elements in which the insulation is placed in a horizontal position. In the case of ceilings, it is necessary to make a full slab from the bottom, which will support the thermal insulation. In the case of a roof slope, it is possible to blow in or partially raise the panels and fill the empty spaces with thermal insulation.

3. Materials and Methods

3.1. Materials

3.1.1. General Information

Three organic materials were included in this research: wood wool, hemp shives, and cellulose fibers. For comparison, the research also included mineral wool, the most popular fibrous thermal insulation in Poland. All the materials were analyzed as loose-fill thermal insulations of frame walls. Most of the properties were laboratory-investigated by the authors [30,31]. Figure 2 presents SEM pictures of the analyzed materials. Detailed analyses of the materials shown in Figure 2 are provided in their dedicated sections, Section 3.1.2, Section 3.1.3, Section 3.1.4 and Section 3.1.5.

3.1.2. Wood Wool

Wood wool is produced from lignocellulosic mass obtained in the process of pulping wood and wood waste. A flame retardant is applied to the wet fibers and then dried in a tube dryer. The lignocellulosic raw material prepared in this way is fed to the homogenizing device, where it is combined with thermoplastic fibers constituting the mass’s binder. Subsequent processing and shaping of the material then take place. It is also possible to produce products in loose form [32].

The structure of wood wool is constructed of many fibers arranged in multiple directions (Figure 2). The fiber surfaces are rough, with numerous protrusions, constrictions, and holes. The individual fibers are hollow, thus creating a capillary system. Between fibers with diameters of 20–35 µm, there are spaces of 10–45 µm [31]. The dependence of the thermal conductivity coefficient of loose wood wool on density is shown in Figure 3.

3.1.3. Cellulose Fibers

Cellulose insulation is made from cellulose fibers by reusing used newsprint. During production, newsprint is shredded and impregnated, among others, with boron compounds and aluminum hydroxide, as a result of which the material becomes resistant to the formation of fungi and molds. Cellulose granules can be used to insulate hard-to-reach places, insulate external and internal sandwich walls, and insulate ceilings and roofs using special pneumatic devices [31]. Long-grain papers, such as daily newspapers or other high-circulation newspapers, are best to use. In recent years, due to the lower consumption of paper (the role of mass media has been taken over by electronic media) and the increasing ecological awareness of consumers, less paper is milled, and for this reason, cardboard or leaflets printed on slippery paper are often used to produce cellulose fibers. Thus, it is more difficult to obtain the highest-quality material.

A characteristic feature of cellulose fibers is their capillary internal structure (Figure 2). These fibers split into hairlike structures in many directions, creating larger structures that resemble plant fibers in structure. The diameters of the fibers are in the range of 1–50 µm, and the spacing between the fibers is in the range of 1–150 µm [31]. The dependence of the thermal conductivity coefficient of cellulose fibers on density is shown in Figure 4.

3.1.4. Hemp Shives

Hemp shives are the woody parts of the stalk of industrial hemp. These are fiber plants grown for various industries, including textile, food, pharmaceutical, and construction. Hemp shives are currently used (since the 1980s) in construction as a light aggregate in composites based on lime, clay, or magnesia binder. Dry shives, without the need to strictly adhere to a specific fraction and fiber content, can also be used as loose insulation in elements in wooden frame structures (insulation of walls, floors, ceilings, and roofs). The effectiveness of the insulation depends on the degree of shive density [30]. The shives analyzed in this study came from the Polish Białobrzeskie hemp variety, with a particle size up to 20 mm.

The outer surfaces of the shives are porous (Figure 2), with pores ranging in size from a dozen to 80 μm. The smaller pores are interspersed, with pores with sizes ranging from 50 μm and above. The structure away from the large pores is ordered, and the walls separating the internal pores are parallel to each other in one of the directions. The walls separating the pores are approximately 1 μm thick, regardless of the material. The pores are usually oblong, forming an ellipse. The results of testing the thermal conductivity of hemp shives are shown in Figure 5.

3.1.5. Mineral Wool

Mineral wool is made from basalt rock by smelting at a temperature of over 1400 °C. The term glass wool is often used alternatively, but identifying both names with the same material is not correct. Glass wool is made from quartz sand and glass cullet melted at a temperature of approximately 1000 °C. Additionally, other raw materials are added in the production process, such as gabbro, dolomite, or limestone. The result is a material with a density of 15 to 140 kg/m³.

The surfaces of mineral wool fibers are usually smooth, and changes in diameter along the length of the fibers are insignificant (Figure 2). Single fibers, unconnected, are arranged in a thick yet multidirectional manner. Observation of the fibers reveals a spatial mesh formed by the fibers in multiple layers. Between fibers with diameters of 2–10 µm, there are spaces of 1–50 µm [31]. The dependence of thermal conductivity on the density of loose mineral wool is shown in Figure 6.

All the analyzed materials are characterized by a low thermal conductivity coefficient, which does not exceed 0.052 (W/(m·K)) for the recommended material densities. Mineral wool has the lowest conductivity, but wood wool is not significantly different. Importantly, when performing thermal calculations for fibrous materials in bulk form, their density should always be taken into account.

Table 1. Properties of the materials used in simulations.

Properties	Lime-Cement Plaster	OSB	Clay Board	Mineral Wool	Cellulose Fibers	Hemp Shives	Wood Wool
Thickness (m)	0.010	0.025	0.016	0.200	0.250	0.260	0.210
Density (kg/m3)	1023.8	595.0	1699.9	100.0	60.0	115.0	50.0
Porosity (m3/m3)	0.61	0.90	0.36	0.92	0.78	0.79	0.82
Specific heat (J/(kg⋅K))	1090.0	1500.0	833.8	840.0	2500	1600.0	1660
Thermal conductivity (W/(m⋅K))	0.220	0.130	0.91	0.040	0.0495	0.051	0.039
Diffusion resistance (-)	6.1	165.0	14.2	1.00	3.5	3.5	1.2

lists the thermal and moisture properties of the materials used to build the simulation models.

3.2. Method

The authors conducted comprehensive hygrothermal simulations to demonstrate the suitability of organic materials as thermal insulation of frame walls. Parametric tests were performed comparing walls insulated with loose wood wool (WW), cellulose fibers (CF), hemp shives (HS), and mineral wool (MW). Simulations were performed for models in which the thermal insulation was entirely placed between the columns. Figure 7 and Figure 8 present two models of walls filled with fibrous insulating material, corresponding to Figure 1. The model shown in Figure 7 is built on wooden beams with a rectangular cross-section, and the model in Figure 8 contains I-beams as an alternative to solid wood. In both cases, vapor retarders and wind barriers are not used. The wall is covered with clay boards from the inside, OSB boards from the outside, and finished with lime plaster. Top and bottom flanges of I-beams are constructed from spruce wood, while the web is made from the beaverboard. The dimensions of the flanges are 40 × 70 mm, and the web thickness is 20 mm.

Since the used thermal insulation materials differ in thermal conductivity, the authors modeled walls with different thicknesses but a constant thermal transmittance coefficient, U = 0.20 (W/(m²·K)). The U value corresponds to the current thermal requirements in Poland [33]. The coefficient is calculated for non-uniform elements by ISO 6946 [34]. The following thicknesses of thermal insulations were assumed: mineral wool (0.20 m), cellulose fibers (0.25 m), hemp shives (0.26 m), and wood wool (0.21 m). Rounding to whole centimeters resulted in negligible discrepancies in achieving a constant U value. The properties of the materials used for simulation are listed in Table 1. The properties were adopted based on the built-in database of the Delphin 6.1 program or based on [30,31,35,36].

The simulations were performed using the Control Volume Method in Delphin 6.1 software. The external boundary conditions were adopted based on the Typical Meteorological Year in Olsztyn, Poland [37]. Internal conditions were set as sinusoidal temperature and relative humidity. Temperature was in the range of 20–22 °C, with a peak at the turn of June/July, while relative humidity had two variants: dry with a minimum of 40% at the turn of February/March, with a peak of 60% in September, and wet with a minimum of 60% at the turn of February/March, with a peak of 80% in September. The boundary climatic conditions are shown in Figure 9.

The wall models were divided into approximately a 7800-mesh calculation grid. Next, the boundary conditions were assigned, and the result files were implemented. The simulation resulted in the temperature and relative humidity distribution maps in the walls. In addition, heat fluxes over the internal surface of walls and the moisture content in the thermal insulation layer were calculated. To facilitate a comparison of results, when recording the moisture content of the thermal insulation, the small area of insulation lying at the position formed as the difference between the full wooden section and the I-beam was not taken into account.

The simulation time was set to 3 years; the time step was one hour for curves and one day for maps.

4. Results and Discussion

4.1. Temperature Distribution and Heat Flux

The temperature distribution in the models is shown for the winter period in Figure 10 and for the summer period in Figure 11, both for dry conditions. In the winter period, a slight impact of replacing the wooden beam with an I-beam on reducing the impact of the thermal bridge is visible. In all the tested walls, the temperature distribution is almost uniform. For wet conditions, at a distance of 35 cm from the beam, the temperature on the surface of the tested model on 17 February (the greatest cooling of the walls) is from 19.1 °C for WW to 19.5 °C for CF. At the level of the middle of the beam, it is from 18.3 °C for WW to 18.6 °C for CF. For dry conditions, the temperature at the edge of the models ranges from 19.3 °C for WW to 19.5 °C for CF. At the level of the middle of the beam, it is from 18.8 °C for WW to 19.1 °C for CF. The edge temperature in dry conditions is higher than in wet conditions, and it is visible at the level of the beams.

There are differences between the results for individual materials due to their thermal conductivity and thickness rounding to full centimeters.

In the case of an I-beam for dry conditions, the temperature at the edge increases slightly compared to models with a wooden beam, from 19.2 °C for HS to 19.6 °C for WM and CF. At the level of the middle of the thickness of the I-beam, the temperature increases from 18.8 °C for WW to 19.2 °C for CF. For wet conditions, the temperature increases on the edge from 19.1 °C for WW and HS to 19.4 °C for CF. In the middle of the beam thickness, the temperature increases from 18.8 °C for WW to 19.1 for CF. Similarly to the model with wooden beams, the edge temperature decreases in wet conditions.

There is a visible difference in the distribution of isotherms depending on the construction element used. When using I-beams, in the area where the beam connects with the cladding slabs, the impact of thermal bridges is smaller and there are fewer disturbances there. The ideal case would be parallel isotherms, so getting close to this is already a big step forward.

In summer, the temperature distribution on the internal surface is practically constant for all tested models and humidity conditions and amounts to 22.0 °C. With strong heating of walls, it is visible that wood wool has a lower ability to absorb excess external heat compared to other materials. Thus, a bigger part of the wall insulated with WW achieves a smaller temperature than in all other cases.

Figure 12 presents the heat flux values calculated at the internal surface of analyzed walls in the case of I-beams and dry conditions. The analysis of heat fluxes (HF) indicates that during the 3-year simulation period, the HF values are sinusoidal, depending on the temperature difference. In winter, they reach their maximum values. For CF and MW, an increase in heat flux of approximately 3% year to year for CF and 8% for WM is visible. There is no such increase for HS and WW.

In wet conditions, heat fluxes show higher values than in dry conditions for all cases. For HS-insulated walls, this is a constant difference from 11% in summer to 16% in winter. For WW-insulated walls, there is a constant difference of 28% in summer to 28% in winter. For CF-insulated walls, the difference is greater each year; in the third year of the simulation, there is a 25% difference in summer and 22% in winter. Similarly, for WM-insulated walls, in the summer of the third year of simulation, it is as much as 121%, and in the winter, 27%. It is visible that in the case of mineral wool, moisture causes the greatest difference in heat flux.

For CF, in the case of walls made of I-beams, in dry conditions, heat fluxes in summer are lower than those for wooden beams by approximately 5%, and in winter conditions, they are lower by approximately 8%. For wet conditions in summer, they are smaller by approximately 6%, and for winter conditions, by approximately 9%.

For HS, in the case of walls made of I-beams, heat fluxes in dry conditions in summer and winter are lower than in the corresponding periods by approximately 4%. For humid conditions in summer, they are smaller by approximately 6%, and for winter conditions, by approximately 3%.

For MW, in the case of walls made of I-beams, in dry conditions, heat fluxes in the summer period are lower than in the corresponding periods by approximately 9%, and in winter conditions, they are lower by approximately 15%. For humid conditions in summer, they are lower by approximately 4%, and for winter conditions, by approximately 5%.

For WW, in the case of walls made of I-beams, in dry conditions, heat fluxes in the summer period are lower than in the corresponding periods by approximately 5%, and in winter conditions, they are lower by approximately 8%. For humid conditions in summer, they are smaller by approximately 2%, and for winter conditions, by approximately 4%.

This confirms the beneficial effect of changing the cross-section of the columns to an I-shaped one in order to reduce heat losses or overheating.

The results presented in Figure 12 almost overlap. However, differences occur in characteristic moments of the year. In the coldest and warmest periods, the appearance of peaks indicates that the materials differ from each other. In the summer, the highest heat gains come from MW-insulated walls, and the smallest come from HS. In the winter, the smallest peaks appear in the case of the CF insulated wall, but in the case of other materials, their nature is similar.

4.2. Relative Humidity Distribution

Figure 13, Figure 14, Figure 15 and Figure 16 show the detailed relative humidity (RH) distribution in the analyzed walls. Figure 13 and Figure 14 present the RH distribution in the winter time (end of February) both for dry and wet conditions. Figure 13 presents the results of the walls constructed with wooden beams, while Figure 14 presents the results of the walls constructed with I-beams.

The maps (Figure 13 and Figure 14) of the distribution of RH in wintertime show that the highest humidity occurs for WW and the lowest for MW cases.

In the case of CF, MW, and WW, a gradient distribution of moisture is visible, from the highest on the cold side to the lowest on the warm side. For HS, the distribution is almost uniform, with the highest RH on the external side; inside the model, there is no clear transition from high to low moisture.

In the case of wooden beams, for dry conditions, the RH of thermal insulation is in the range of 38–97% in the case of MW, 41–99% in the case of CF, 47–66% in the case of HS, and 49–93% in the case of WW. For wet conditions, RH of thermal insulation is in the range of 51–98% in the case of MW, 62–99% in the case of CF, 74–77% in the case of HS, and 72–94% in the case of WW.

In the case of MW and CF, in all analyzed cases, a zone of lower moisture is visible immediately next to the surface of the internal clay board. During the dry period, less moisture in all analyzed walls occurs. Disturbances in the distribution of RH are mostly visible near the structural elements. Wooden beams and I-beams near the inner clay board have less moisture than the adjacent thermal insulation, but the other end of the beams has more moisture. This influences the distribution of RH isotherms.

Figure 15 and Figure 16 show the RH distribution in the summertime (early August), both for dry and wet conditions. Figure 15 presents the results of the walls constructed with wooden beams, while Figure 16 presents the results of the walls constructed with I-beams.

The maps of the distribution of RH in summer (Figure 15 and Figure 16) show that the highest RH values occur in thermal insulation, close to the external board in the case of MW, then CF, and then WW. In the case of the HS example, the whole distribution of the RH in the model is almost equal and lower than in the other cases. The RH values in the external board are lower than in the insulation in the case of MW, CF, and WW. In the case of HS, it is slightly higher. In the case of wooden beams, for dry conditions, the RH of thermal insulation is in the range of 69–97% in the case of MW, 61–82% in the case of CF, 55–61% in the case of HS, and 64–77% in the case of WW. For wet conditions, the RH of thermal insulation is in the range of 78–98% in the case of MW, 79–99% in the case of CF, 72–73% in the case of HS, and 77–92% in the case of WW.

Changing the construction elements to the I-beams does not influence the RH distribution in the models in general. The differences are rather visible in the contact areas between the beam and thermal insulation.

Figure 17 presents the moisture mass in the insulation materials in the case of the walls constructed with wooden beams. The presented values were converted to mass in the volume of the material used. The accumulation of moisture in materials depends on the ambient humidity in the room. In dry conditions, the curves for all materials are sinusoidal. After one year of observation, changes in the moisture content in the material become repeatable. In the autumn and winter, thermal insulation absorbs moisture and releases it in the summer. WM releases almost all the moisture accumulated in it. On a year-to-year basis, an increase in moisture content in the materials can be observed at the level of 0.6% for CF, 12.4% for WM, and 0.0% for HS and WW.

In wet conditions, plant-based materials retain a distinct sinusoidal shape in moisture content. These values are higher than in dry conditions. On a year-to-year basis, an increase in moisture content in materials of 7.5% for CF and 0.0% for HS and WW can be observed. In the case of mineral wool, there is a constant increase in the moisture content. A slight release of moisture in MW is visible in the warmest period, and another increase occurs. On a year-to-year basis, it is at the level of 53.0%. This observation is consistent with observations showing the distribution of RH in the walls. The moisture mass in thermal insulations in the case of construction with I-beams is almost the same, as presented in Figure 17.

The observations presented in this chapter indicate the high ability of materials of natural origin to accumulate and release moisture. There is also a strong influence of the ambient RH on the ability to accumulate moisture in materials. In the case of WM, which has practically no ability to store moisture in the fibers, the increase in moisture content occurs very quickly and affects the thermal condition of the elements. Moisture may condense at contact with the OSB board if the thermal insulation made of WM is not protected from water vapor on the inside. There is less risk in the case of CF insulation, and importantly, CF insulation manufacturers often advise against using vapor retarders from the inside. The lowest risk of condensation between the thermal insulation and external boards occurs in the case of HS and WW. It should be borne in mind that among the analyzed materials, HS is the only one that is created completely naturally, without chemical processing. Research conducted so far (under review) indicates that high ambient humidity (more than 90%) may contribute to the development of mold in raw shives. As shown by numerical analysis, the RH in HS was below 77%.

The presented results of the moisture mass in thermal insulations (Figure 17) indicate that insulations made of HS and WW behave stably in both dry and humid conditions, and there was no increase in moisture in them year to year. On the other hand, CF has a bigger ability to accumulate moisture in the structure of the material, which probably increases the moisture level, below which it does not fall.

In the dry conditions, the increase in moisture in insulation made of CF or MW has no practical impact on heat transfer (less than 0.8% in both cases). In wet conditions, the impact is greater and is equal to 6.9% for CF and 5.2% for MW. No increase in heat flux was found for HS- and WW-insulated walls. This may mean that CF-insulated walls require a larger moisture buffer on the interior side, e.g., a thicker layer of clay plaster.

The presented results correspond to buildings located in a moderate climate, e.g., Olsztyn. Placing such buildings in a different climate will result in different calculated values, but the characteristics of the tested materials will turn out to be similar to those in this analysis.

5. Comparison of the Environmental Impact of Organic Insulation with Traditional Insulation in a Single-Family House

The energy efficiency of a building is one of the factors allowing for a reduction in the overall carbon footprint of the building. Another crucial factor may be the selection of the construction and insulation materials, considering the amount of CO₂ generated during the production phase of each material. In the context of the issue addressed in this article, the emissions of individual insulation materials were calculated for a sample building, as presented in Table 1 in the subsequent part of the article. For calculations, a representative standalone single-story building (Figure 18) with an attic, constructed with a wooden frame, was assumed. The building rests on a foundation slab and is covered by a gable roof with a slope of 30 degrees. The internal surface area of the building is approximately 110 m², with internal dimensions of 14.00 m in length and 7.90 m in width. The height from the wall to the underside of the roof overhang is 3.92 m, while the height from the wall to the underside of the ridge is 2.41 m. The calculations excluded window and door openings, as well as wooden wall elements. Considering that the subject of the text is vertical partitions, roof structures were omitted from the calculations.

Referring to the data collected in Table 1, the calculations adopted the quantity in kg/m³ derived from various specified insulation thicknesses. According to the calculations, the volume of mineral wool insulation amounts to 29.84 m³, that of cellulose fiber insulation amounts to 37.55 m³, that of hemp shives insulation amounts to 39.18 m³, and that of wood wool insulation amounts to 31.43 m³. The volume was calculated using the following Formula (1):

$$V=\left(A_1·H_1-W\right)·I+\left(A_1·H_2·0.5-W\right)·I+\left(A_2·H_1-\left(W+D\right)\right)·I$$

(1)

where:

V—the volume of insulation in the walls (m³).

A₁—the length of insulation along the entire end wall to the outer edges of A₂ (m).

A₂—the length of insulation for side walls between the inner edges of the end walls (m).

H₁—the height to the underside of the eaves (m).

H₂—the height between the underside of the eaves and the underside of the ridge (m).

W—the surface area of window openings (m²).

D—the surface area of door openings (m²).

I—the insulation thickness (m).

The calculated CO₂ emissions, according to Formula (2) and the above-presented emission values, generated by the respective insulation materials were as follows: for mineral wool, 8265.7–10,802.1 kg of CO₂; cellulose fibers, 1644.69–8245.98 kg of CO₂; hemp hurds, 675.85 kg of CO₂; wood wool, 2685.54 kg of CO₂.

$$Em=V·D·E$$

(2)

where:

V—the volume of insulation in walls (m³).

D—the density of insulation according to Table 1 (kg/m³).

E—the emission coefficient based on [11,23] (kg/kg).

As can be observed from the obtained results, despite its smaller volume, mineral wool exhibits the highest amount of emitted CO₂. Cellulose fibers, with approximately 20% greater volume compared to mineral wool, depending on the method of product extraction, generate a carbon footprint ranging from eight times smaller to close to the lower values of the carbon footprint generated by mineral wool. Wood wool, despite a slight difference in volume, accounts for four times smaller CO₂ emissions than mineral wool. Meanwhile, hemp shives, despite a roughly 20% larger volume compared to mineral wool, produce over twelve times less CO₂. In terms of the necessity to consider the carbon footprint of building materials, this suggests a favorable outlook for materials based on natural fibers.

6. Conclusions

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Organic materials, presented in the paper, can be used as loose-fill thermal insulations in wooden frame walls. They are characterized by low thermal conductivity, not higher than 0.052 (W/(m·K)).

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A complex hygrothermal analysis of diffusion-open frame walls indicates the ability of these materials to buffer moisture and release it into the interior. However, diffusion openness in both directions is required.

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In the case of MW, the moisture transport is clearly toward the outside, which has its advantages but also carries the risk of condensation at contact with the external cladding boards. Therefore, walls insulated with mineral wool require a water vapor retarder.

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A comparative analysis of internal humidity conditions, dry and wet, indicates that in the case of increased humidity inside the room, the moisture content in the insulating materials increases significantly. MW tends to increase the moisture accumulation in the material in subsequent years. This will result in condensation in the material. Organic materials, except CF, do not have this feature. HS and WW remain stable and in the summer release the moisture, accumulated in the autumn and winter, into the interior. Thanks to this feature, organic materials have the ability to regulate the humidity level in the room.

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Among the analyzed organic materials, HS is characterized by the highest moisture capacity and, at the same time, next to WW, the highest stability.

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The analysis showed little influence of the shape of the structural elements (rectangular wooden or I-beams) on the moisture condition of the walls. Wooden, rectangular elements, having bigger dimensions and high hygroscopicity, store more moisture than the I-beams. Thus, this will cause disturbances in the adjacent thermal insulation materials.

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The influence of the shape of construction elements is visible on the heat flow; limiting thermal bridges by narrowing the wooden cross-section slightly increases the temperature on the inner surface of the wall.

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In terms of the carbon footprint of building materials, the usage of natural-based materials as thermal insulation accounts for a huge reduction in CO₂ emissions when compared to mineral wool.

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Currently, the authors are working on practical aspects of other organic materials as thermal insulations. These mainly include basic research on raw materials, but we are also testing ready-made modules and building elements, checking their behavior in changing climatic conditions. With the examined material properties, it will be possible to perform further numerical calculations. Attention is now focused on the behavior of thermal insulation made of organic materials during periods of elevated temperatures. In parallel, research on the possibilities of using organic materials to isolate log houses from the inside is being conducted.