The Fire Resistance and Heat Conductivity of Natural Construction Material Based on Straw and Numerical Simulation of Building Energy Demand

Abstract

The motivation for research to help address climate change is a continuous process of searching for eco-friendly materials in the building industry, which will allow minimizing the negative impact of this sector on the environment. The main objective of the paper is to assess the properties of a natural resource such as straw as an eco-friendly material in various variants for use in low-energy demand construction. The research results will fill the knowledge gap in the field of numerical analyses of the energy demand of straw material buildings based on the results of the conducted laboratory tests. A test of a heat transfer coefficient was conducted with different orientation of straw stalks. Then, samples were subject to a fire resistance test to determine material behavior at 1000 °C. During the fire resistance test, the clay-based plaster was ‘burnt out’, which hardened its structure, effectively preventing flames from reaching the insulation layers in the form of straw stalks. As a result of shrinkage (no plaster mesh), the plaster cracked and turned brick-red in color. The insulation layer of straw under the plaster was charred to a depth of 3.0 mm due to the high temperature. However, when the torch was turned off, no fire spread was observed in any layer of the sample. The 3D models of the buildings were created for different eco-friendly materials applied to make external walls. The results of numerical simulation allowed determining the amount of final energy needed to heat the designed building at the level of 26.38 (kWh/m²·year). Conclusions of the above-mentioned tests indicate very good thermal insulating properties determined using the lambda coefficient of 0.069 (W/m·K) and the possible application of straw bales as an alternative for conventional construction. Research has proven that it is possible to construct a building with low energy requirements using natural, easily available waste and completely biodegradable material.

1. Introduction

To have an easily accessible and safe shelter was and still is one of the fundamental needs of humans. For centuries, men have been building structures using materials found in a close vicinity. Even today, the building industry in certain regions of the world is based on natural materials acquired locally in the vicinity of a construction site. In highly developed countries, this tradition has been supplanted by modern, complex, and highly processed construction systems [1,2]. However, in order to follow the idea of sustainable construction, many materials of natural origin are currently applied, as well as waste materials from other branches of industry, which constitute a valuable mineral raw material [3,4,5]. When using industrial waste, its safe application shall be verified due to the possible release of e.g., heavy metals [6,7].

A solution that has attracted more and more interest due to its limited carbon footprint and elimination of a greenhouse effect, which accompanies the production of the cement itself, is alternative materials based on geopolymers, which more frequently contain waste such as fly ash, limestone powder, or brick rubble [8,9].

The continuously growing popularity of environment protection ideas also affects the usual construction of single-family houses, which should be user-friendly and to a minimum extent pollute air and soil as a result of production processes and the disposal of used materials [4,6,10].

The main objective of this paper is to show that it is possible to use the analyzed natural materials as they meet the current standards in Poland regarding the energy intensity of buildings. The authors used the results of laboratory tests to carry out numerical simulations of the energy demand of facilities. This way of construction is based on easily accessible raw materials, i.e., cheap and renewable materials, such as timber [11], straw [12,13], hemp shives [14], clay [15], or cellulose [16]. Depending on the geographical latitude and availability, applications of such materials as rice straw [17] and date fiber [18] are also known. In regions where harvest is lower, the natural housing development is developing toward the utilization of light clay with the addition of straw [15]. This eco-friendly building material assures minimum processing, low emission, and the raw material available locally that is an alternative to conventional building materials and methods [19]. The advantages of these raw materials include the price, easy acquisition, and undoubtedly, a minimum negative impact on the natural environment [20].

For supporters of natural building technologies, an important parameter is the building’s embodied energy, which is the quantity of energy consumed in the material production process [21]. Unlike structures of reinforced concrete, houses built of natural materials, due to their biodegradable character, may be easily processed when their service life ends and their organic parts, after being composted, become a fertilizer in home gardens. Therefore, this idea adapts to a “cradle to grave” architectural trend [22], which focuses on the conscious use of the material covering an entire building cycle from production to demolition [23].

All sorts of organic materials available in given geographic regions are utilized as a main layer of a wall or they are added to plaster mortars, where a small amount of the additive introduced in the form of organic fibers may significantly improve the thermal insulation and strength parameters [24]. Another example for improvement of the heat transfer coefficient and thermal inertia of lime mortars is application of the additive in the form of iron (III) oxide [25]. In contrast, an improvement of strength parameters can be achieved by adding a few per cents of fly ash, which is the waste product from the power generation sector [26]. However, there are still very few publications that collect data and then can be used to standardize regulations for application of materials such as hemp shives [26], straw [27], or light clay [15]. Meanwhile, these materials show a very low impact on the natural environment, and in the case of hemp shives and straw, the CO₂ emission balance in their production process is negative (due to a large quantity of CO₂ absorbed during the growth of these plants) [28].

The utilization of natural resources in the building industry contributes to an enhancement of ecological effects through the following [29]:

• Reducing energy consumption to minimum—during the production of materials, construction, use, and demolition of the structure;
• Reducing the load for natural environment—limited pollution due to the construction process;
• Improving a microclimate and comfort in the building—limited use of materials that are harmful to the health of future dwellers.

A hemp–lime composite is an eco-friendly material, which has the chance to become part of a trend toward renewable energy sources. It has a lot of advantages, such as high insulation performance and heat accumulation, which are close to the parameters of the materials currently produced, such as e.g., polystyrene boards. Due to its structure, it is a good sound-proof material. In addition, it shows high water vapor permeability, and it is 100% biodegradable [30,31].

Another important asset is its high non-flammability, which makes it defined as a safe material. It is also worth noting that it is a non-structural material; i.e., it is used only as the infill of the wall and it does not transfer any loads occurring in the building. Therefore, the use of natural materials still requires a (wooden) framework [32,33].

Comparatively, straw is a material with a higher degree of research and technology development. A technology called “straw bale” has been developed consisting of the formation of straw cubes used as infill of walls and “straw bale loadbearing”, where straw bales are used as a load-bearing structure of the building. Technologies based on the modern technical knowledge that involves a series of testing allow assuring very good parameters of buildings made of straw and confirm that this material has a huge innovative potential (Figure 1) [34,35,36,37].

Studies on the hydrothermal properties and thermal conductivity of straw bales have shown that as the density increases, the thermal conductivity also increases. It was found that the value of thermal conductivity of a straw bale is similar to that of wood fibers and wood chips and lower than that of reeds [38]. Research has revealed that the orientation of straw bale fibers and their relative moisture level influence the thermal conductivity of the bale. McCabes carried out tests that showed that the thermal conductivity of straw bales is 0.048 W/m·K when the fibers are oriented perpendicular to the heat flow and 0.061 W/m·K when they are oriented parallel [39]. The authors of the paper [40] prepared two straw bale samples, one with a density of 75 kg/m³ and the other with a density of 90 kg/m³, and they showed that the thermal conductivity of the first sample was 0.052 W/m·K when the fibers were oriented perpendicular to the heat convection and 0.056 W/m·K when they were parallel to the heat convection. Examination of the second sample confirmed the previous sample alignment relationship. This time, the values increased slightly, such that when the fibers were parallel to the direction of flow, the conductivity was 0.06 W/m·K, and when they were perpendicular to it, it was 0.056 W/m·K [40]. The European Straw Building Association (ESBA) conducted a significant number of studies on the thermal conductivity of bales in 2010, and according to the report, the straw bales with a density of about 126 kg/m³, when the fibers are parallel to the heat flux, have a thermal conductivity of 0.078 W/m·K, and in the direction perpendicular to the flow, it is 0.056 W/m·K. The conclusion from the above results indicates that the proper orientation of the straw stalks can improve the thermal resistance by up to 28% [41].

Low-energy buildings are built all over the world, which are passive and zero-energy buildings, depending on the local conditions adopted, installation equipment, and design assumptions. In China, a prefabricated straw bale construction (PSBC) was developed, which has been recognized as the most efficient construction method. This method allows achieving low-energy buildings with a very limited impact on the environment. The results of tests prove that there is a significant reduction in energy consumption needed for heating and cooling, as well as the intensity of heating in regions, where severe freezing weather occurs compared to residential buildings made in conventional technology [42].

Straw has been used in construction as reinforcement and infill, acting as thermal and sound-proof insulation of external walls and roof coverings [43]. Straw bale construction technology is mainly used to construct detached houses, but there are also constructions of multi-family buildings, public buildings [44], and buildings for agricultural purposes [45]. A research issue frequently undertaken is the compressive strength of the wall filled with straw bales and plastered with clay (Figure 2) [46].

In the Germany, there have been technical standards for straw-bale technology for years [47]. In the UK, there are already approved and patented systems of prefabricates made of straw [48], while in France, branch-specific national standards have been issued [49].

Pressed straw should be absolutely protected against weather conditions—in particular against the harmful impact of moisture. The suitable ventilation of storage spaces shall be provided. In addition, the nesting of birds and rodents shall be avoided [50]. Protection of the straw, both from the inside and outside, after it is built into the wall, should assure good breathability and protect against the impact of water, sunlight, fire, and pests (insects, birds, and rodents).

The following materials are used to make protective layers [50]:

• Clay plaster applied directly on straw;
• Lime plaster applied directly on straw;
• Wood fiber boards covered with plaster;
• Non-fired bricks separated with an air gap (three-layer wall);
• Front panels of planks impregnated with oil, waxed, and/or singed planks (boards).

Rye, wheat, or spelt straw is most frequently used for thermal insulation. Starting from reaping, the straw designed for construction purposes should be subject to quality assessment in terms of its color, structure, and odor. The stage of straw bale pressing is also important. To achieve the proper density and shape of bales, proper adjustment of pressing machines shall be taken care of. Desired straw bales have the following [44,51]:

• A shape close to a cuboid (rounding off contribute to occurrence of thermal bridges);
• A homogeneous structure and good binding with a string or wire;
• Volume density >90 kg/m³ for a framework-type structure;
• Volume density >120 kg/m³ in case of self-supporting walls made of straw;
• Mass moisture content <15%.

The main scientific aim of the paper is to assess properties of straw as an eco-friendly building material in various variants for use in low-energy demand construction. The research results will fill the research gap in the field of numerical analyses of the energy demand of straw material buildings. The authors based their numerical simulations on the laboratory test results. A limitation to a full analysis of the use of straw in building structures was the inability to verify numerical simulations with tests performed on a real object built with this technology. This issue is another research goal to be pursued in the future.

2. Materials and Methods

2.1. Structure and Purpose of Test Samples

To perform tests of the heat transfer coefficient, fire resistance, a series of 20 test samples P1 ÷ P20 was made. Thermal transmittance tests were conducted on a series of 10 test samples P1 ÷ P10 in plywood frames with a wall thickness of 10.0 mm, which were made especially for that purpose, and the frame weight was 0.2255 kg. The frame used did not affect the results obtained, because it was used only to form the sample into desired shapes. The external dimension of the samples was 15 × 15 × 15 cm. The straw fibers in the P1 ÷ P5 test samples were laid perpendicular to the heat transfer direction, and in P6 ÷ P10 samples, they were laid parallel to the heat flow direction (Figure 2). In order to limit the hot air flow through straw stems, specimens were covered with a layer of gypsum 1.0 mm thick. After completion of the non-destructive thermal transmittance test, the gypsum layer was removed and replaced with a 5.0 mm layer of clay plaster to use the specimens again to test their fire resistance. After this treatment, the same samples were named P1a-P10a. A fire resistance test was performed on all created samples (P1a ÷ P10a and P11 ÷ P20). P11 ÷ P16 samples were made of light clay, i.e., a mix of straw and the clay-based binder in a volume ratio 50:50, laid in steel cubic molds 15 × 15 × 15 cm. In these specimens, straw shred with random arrangement of fibers was added to the clay-based binder. P16 ÷ P20 samples were made of clay-based mortar with a sand to clay ratio of 3:1 and placed in cubic steel molds, size 15 × 15 × 15 cm. Samples were taken out of molds after 3 days and conditioned for 28 days to achieve full strength.

The density was determined by measuring the weight and volume of the test material.

2.2. Methods

The following tests were conducted to meet the research objectives (

Table 1. The list of tests performed and the number of tested samples.

Type of Materials	Test of Heat Flow	Fire Resistance Test
Straw along the stems	P1–P5	P1a–P5a
Straw across the stems	P6–P10	P6a–P10a
Light clay with random arrangement of stems	-	P11–P15
Clay-based mortar with a 3:1 sand-to-clay ratio	-	P16–P20

):

-

Heat conduction test;

-

Fire resistance test.

Each test was performed three times with the same material to minimize the possibility of measurement errors.

2.3. Fire Resistance Test of Straw Covered with a Layer of Clay Plaster

P1a ÷ P5a specimens with an average density of 110.50 kg/m³ and P6a ÷ P10a with an average density of 105.56 kg/m³ were covered with clay plaster 5.0 mm thick. The volume density of P11 ÷ P15 specimens amounted to ca. 682.51 kg/m³ and specimens P16 ÷ P20 were 829.81 kg/m³. The density was determined by measuring the weight and volume of the test material. When the plaster dried out, the specimen was exposed to a flame from a roofing torch with the flame temperature above 1000 °C for 15 min (Figure 3).

When that time ended, the sample was subject to visual assessment. During the study, in addition to visual evaluation, a series of images were taken with a TESTO 885-1 thermal imaging camera (Manufacturer name: Testo Sp. z o.o., Pruszków, Poland) to analyze the temperature distribution on the surface of the clay-coated straw sample. The measurement accuracy of the camera is 30 mK, and the image resolution is 320 × 240.

2.4. Test of Time-Controlled Heat Flow through the Sample during Exposure to Torch Flame

A purpose of testing was the simulation of straw samples’ resistance to fire determined in time, depending on the direction of stalk orientation (parallel and perpendicular) to the heat flow direction in the worst fire conditions possible and checking how the material tested would behave. The test consisted of exposure of samples made of straw (P1a ÷ P10a), light clay (P11 ÷ P15), and solid clay mortar (P16 ÷ P20) to fire by means of the torch with a maximum flame temperature above 1000 °C at a 10.0 cm distance (Figure 4).

During the experiment, by means of a thermocouple connected directly to a computer, a course of temperature changes inside samples was monitored in real time (automatic measurement every 5 s) at the depth of 50.0 mm from the surface of the sample exposed to the flame. The time of exposure to flame assumed was 15 min. After that time, the measurement of temperature changes was continued inside the samples.

2.5. Determination of Heat Transfer Coefficient (λ)

Measurements were conducted in steady heat flux conditions using a two-compartment “heat box” with heat flux density sensors, according to PN-EN 12667:2002 standard [52]. The sample to be tested was put between compartments and sealed with multi-element spacer frames (Figure 5). The source of heat in the compartment was a 250 W Helios heat lamp. Temperature was measured with a temperature sensor PT-100 (Manufacturer name: Transfer Multisort Elektronik Sp. z o.o., Łódź, Poland) with a measurement accuracy of 10% and a humidity meter HR-202 (Manufacturer name: Conrad Electronic Sp. z o.o., Cracov, Poland) with a measurement accuracy of 5% RH and measurement response time <10 s.

During testing, measurements of heat flux density, temperature in both compartments, as well as temperature on both surfaces of the specimen were conducted. The measurement was conducted at the specimen average temperature of 25 °C and horizontal heat flow on 10 specimens sized 15 × 15 × 15 cm. The volume density of the material tested in samples filled with straw fluctuated from 86 to 135 kg/m³, and the average density of straw stalk infill was 108 kg/m³.

3. Results

3.1. Fire Resistance Testing of Straw Covered with Clay Plaster Layer

The test was conducted on P1a ÷ P10a, P11 ÷ P15, and P16 ÷ P20 samples following the methodology given in Section 2.3. Based on the fire resistance test results, the following were demonstrated (Figure 6):

• Clay-based plaster had “burned out”, which hardened its structure, effectively preventing the flame from reaching the insulation layers;
• Due to shrinkage (no plaster mesh applied), the plaster cracked and changed its color to brick-red;
• The insulation layer of straw under the plaster got charred to a depth of 30.0 mm, due to high-temperature impact;
• When the torch was off, no fire spreading was observed in any layer of the sample.

Apart from the visual examination, a series of pictures were taken during testing by means of a thermal imaging camera (Figure 7). The following were found:

• At depth of 30.0 mm, the sample temperature did not exceed the straw char point despite the flame temperature of ca. 1000 °C;
• The surface of the sample on the side opposite to the one exposed to the flame, after 25 min from the start of experiment, reached the temperature not exceeding 15 °C (if we do not count a slightly higher temperature in the place not filled with insulation), which allows stating that the sample tested showed perfect thermal insulating properties;
• When the heat source was off, the temperature slowly grew and neither fire spreading nor further glowing of the material was observed, which was recorded by means of the thermal imaging camera (Figure 7).

The standard deviation value of the fire resistance test of specimens with straw blades orientation perpendicular and parallel to the heat flow is equal to 3.0 mm.

The experiments did not dispel doubts on how the straw infill behaved during exposure to fire. Therefore, it was decided to repeat it and monitor the course of temperature changes inside the sample tested (Section 3.2).

3.2. Temperature Measurement When the Sample Surface Was Exposed to Fire

The test was conducted on samples P1a ÷ P10a, P11 ÷ P15, and P16 ÷ P20 following the methodology given in Section 2.1. For all samples, the time and distance from the heat source, ambient temperature, and depth of temperature measurement were constant. The analysis of values obtained allowed differentiating three characteristic phases of the measurement:

• Phase I—(0 ÷15 min)—fire impact;
• Phase II—(15 min ÷ 50 min)—temperature trend without external factors;
• Phase III—(50 min ÷ ∞)—opening of samples.

When analyzing results for the last series of samples made of clay mortar (P16 ÷ P20), a slow growth of temperature in phase I was found, and there was a trend of temperature increase inside the specimens at the beginning of phase II, which could be directly related to the heat flow from the heated face to the inner part (Figure 8).

A similar trend of temperature changes was observed in a P11 sample made of light clay (a blend of clay binder with straw). However, the start of the temperature growth process in phase I was a bit delayed. It happened due to the insulating properties of air gaps in places filled with straw. As in P16, the temperature rise inside the sample, even after the torch was off, was also observed in the P6a sample. What is interesting is that the temperature reduction process was a bit slower compared to the sample of solid clay mortar. It might be associated with the slow burning of organic fractions inside the sample made of light clay (Figure 9).

The P6a sample had straw stalk orientation perpendicular to the direction of the torch action. Due to the good thermal-insulating properties of straw stalks arranged that way, the temperature at a depth of 50.0 mm did not change for the first 200 s of testing. Later, a linear growth of temperature to ca. 80 °C was noted as well as its gentle trend up to ca. 90 °C (Figure 8). It might be associated with the evaporation of moisture included in the straw structure (a similar relation was observed for the P1a straw sample). For a short moment, after the torch had been off, the temperature rise trend remained.

In a later period, instead of the drop expected, the temperature started to grow faster than during the impact of the flame. It most likely happened because a rapid shrinkage during clay burning meant that air rich in oxygen could freely get to the inside, and such oxygen was previously consumed by the torch flame that swept the plastered face of the sample. Then, during phase III, the temperature growth stopped at first, after the wooden box had been opened. A reason for that was probably the convection of warm air from the sample inside up through stalks arranged vertically. After approximately 2 min from the box opening, the temperature started to grow rapidly again thanks to the access of oxygen from the surroundings, until the straw caught fire in the last phase of the test.

When the torch was on again, the temperature rise for the P1a sample occurred by ca. 1 min. faster than for the P6a sample. It may be explained with the fact that the straw stems inside the P1a sample were parallel oriented to the heat flow, and thus, they did not make such a good thermal barrier. For the same reason, the temperature at a depth of 50.0 mm rapidly increased in the first phase from ca. 20 °C to ca. 90 °C, which was more dynamically than in any other samples tested. In addition, during this test, characteristic maintaining of the constant temperature was observed at approximately 90 °C in case of samples P1a and P6a. When moisture evaporated from the material structure, the rapid growth of temperature remained. When the torch was off at the beginning of phase II, the temperature rise trend remained for the next 2 min, and then, the temperature started to go down from 260 °C to approximately 195 °C. Most likely, however, embers stayed in a part of the samples, as smoke was coming out from the inside through cracks in the clay plaster (Figure 10). After approximately 30 min from the start of testing (i.e., after ca. 15 min from starting the torch), the temperature of the P1a sample at a depth of 50.0 mm started to grow again with a rate of ca. 30 °C/min, such as in case of P6a. When the compartment was opened in phase III, a further growth of temperature was observed. Due to the unlimited access of air to glowing straw, its ignition took place, which was observed as a sudden temperature rise at the end of the graph. A further decrease in temperature visible in the graph was associated with extinguishing of the sample (Figure 8).

3.3. Measurement of Heat Transfer Coefficient (λ)

3.3.1. Test Results of Heat Conductivity by Straw Bales with Transverse Orientation of Stalks

Based on tests conducted with the method described in Section 2.3, values of the heat transfer coefficient λ, were determined for P1 ÷ P5 samples with straw stems orientation perpendicular to the heat flow (

Table 2. Comparison of density and heat transfer (λ) values for straw test pieces with stem orientation perpendicular to heat flow direction.

Sample No	Volume Density kg/m3	The Value of λ W/m·K
P1	109.93	0.070
P2	118.25	0.068
P3	86.42	0.066
P4	135.09	0.071
P5	102.80	0.069
Average:	110.50	0.069

).

Obtained results were compared to volume density values of individual samples (Figure 11). The density was determined by measuring the weight and volume of the test material.

Based on test results of the heat transfer coefficient for P1 ÷ P5 samples, a correlation was found between the volume density and the heat transfer coefficient (λ) (Figure 11). As the density of specimens increased, a deterioration of insulating parameters was observed. It was related with a smaller number of air gaps in the material structure. However, if we compare the average value of the calculated heat transfer coefficient (λ) for the heat flow across the stems in case of P1 ÷ P5 specimens with other tests of that material contained in official German standards [47], we can notice that the entire series of results obtained shows worse parameters than expected. While the expected λ value should be equal to 0.052 W/m·K, the average value calculated for heat flow across the fibers is 0.069 W/m·K. It was probably associated with thermal bridges formed in a place where straw was tied with a string, keeping it inside the frame. The mentioned thermal bridge is visible in the thermal image taken during experiment with exposure to the flame (Figure 7). The obtained results of the heat transfer coefficient may indicate technological problems that may arise when real structures are built of the material tested. Apart from excellent thermal properties, the material also shows a wide diversity of stems size, which is a characteristic feature of natural materials. It may influence the density of the tested sample and at the same time translate into changes of heat transfer coefficient. The value of standard deviation of heat transfer coefficient for samples with straw stalks orientation perpendicular to the heat flow is equal to 0.002 W/m·K.

The obtained results allow concluding that straw may be used as a structural element of a passive house not requiring any additional heat insulation, and its thermal insulating properties are better than the properties of the wall made of autoclaved aerated concrete [53,54].

3.3.2. Test Results of Heat Conductivity along the Stems

The results of heat conductivity for P6 ÷ P10 samples that had stalk orientation parallel to the heat flow direction are presented in

Table 3. Comparison of density and heat transfer (λ) values for straw test pieces with stalk orientation parallel to heat flow direction.

Sample No	Volume Density kg/m3	The Value of λ W/m·K
P6	114.46	0.084
P7	111.03	0.083
P8	106.71	0.078
P9	103.79	0.082
P10	91.82	0.077
Average:	105.56	0.081

.

As in the case of P1 ÷ P5 samples with stalks orientation across the heat transfer direction, also for P6 ÷ P10 specimens with stalk orientation parallel to its direction, a relation between volume density and the heat transfer coefficient (λ) can be noticed (Figure 11). The obtained test results allowed stating that the higher the density, the worse the insulating parameters. However, this relation is not linear for both series. The value of standard deviation of heat transfer coefficient for samples with straw stalks orientation across the heat flow is equal to 0.003 W/m·K. However, the average compression value of samples with stalks orientation along the heat penetration direction was lower by 4%, so due to the larger quantity of air gaps inside the sample structure, the thermal insulation was still weaker by ca. 30%. It is confirmed by the result (Figure 11) proving the better thermal insulating properties of straw built-in that way, so that stalk orientation perpendicular to the heat flow through the barrier is maintained.

The calculated values of heat penetration coefficients for straw correspond to results available in the literature [12] and confirm a statement on the very good thermal insulating properties of straw in walls. These results also confirm that the wall thermal insulation depends on the arrangement (orientation) of the straw stalks inside [43,55,56].

4. Practical Application of Test Results

For further analysis of whether the application of straw bales in the building industry is reasonable, a building was modeled to compare selected materials. The ArCADia-Architektura software of Intersoft company (Manufacturer name: INTERsoft sp z o.o., Łódź, Poland) was selected, which allowed creating professional architectural documentation, and at the same time, the model was created in BIM technology (Building Information Modeling). A model of a two-story building was created with the total surface area of 47.97 m² (Figure 12). It was equipped with the same mechanical ventilation system capable of recovering approximately 70% of heat as well as the central heating system supplied by an air source heat pump. The roof was covered with ceramic roof tiles and insulated with plates of wooden wool by STEICO company with the total thickness 30 cm and λ = 0.038 W/m·K.

By changing the structure of the external walls, four variants with natural materials were created. The remaining models were exported to another Intersoft software, i.e., ArCADia TERMOCAD (Manufacturer name: INTERsoft sp z o.o., Łódź, Poland), which was used to prepare energy audits or energy performance certificates for new buildings. Using ArCADia TERMOCAD, the demand for energy was calculated that was needed to heat the created structures A list included straw bales with fibers arranged parallel and perpendicularly to the heat flow, hempcrete, and a ceramic hollow brick with heat insulation made of polystyrene boards (Figure 13).

The above tests allowed adopting an average heat transfer coefficient (λ) for straw cubes with stalk orientation along the heat flow on a level of 0.081 W/m·K. The barrier was designed of straw cubes 35 cm thick, which were finished on both sides with light clay mortar with λ = 0.22 W/m·K and a thickness of 5 cm. The total heat transfer coefficient amounted to U = 0.20 W/m²·K. When straw cubes were turned and stalks were directed perpendicularly to the heat flow, a lower heat transfer coefficient was obtained equal to 0.069 W/m·K, which translated to a reduction of the total heat transfer coefficient (U) to the value of 0.18 W/m²·K.

The next barrier is composed of hempcrete, i.e., hemp shives, mixed with a lime binder 20 cm thick, with the adopted value λ = 0.044 W/m·K, taken from the paper [31]. Due to its natural origin, that material was also protected with the light clay plaster with λ = 0.22 W/m·K, 5 cm thick, and the total heat transfer coefficient amounted to U = 0.19 W/m²·K [26].

The third barrier was composed of two layers. The first layer is a Porotherm ceramic block, 25 cm thick, with the coefficient λ = 0.313 W/m·K, whereas the second layer is insulation in the form of polystyrene boards15 cm thick and λ = 0.036 W/m·K. In this case, an acrylic plaster outside was adopted, which was 1 cm thick and λ = 1.0 W/m·K. While inside, there was the cement–lime plaster, which was 1.5 cm thick and λ = 0.820 W/m·K. The total heat transfer coefficient of the barrier made of the ceramic block and polystyrene boards amounted to U = 0.19 W/m²·K.

The results of calculations and tests proved that natural materials could meet technical requirements [57] in terms of thermal insulation and be successfully used in the construction of residential and service buildings. An important aspect that shall be emphasized is the thickness of barriers in each variant. Based on the knowledge gathered as well as observations, testing, and experiments conducted, straw shall be classified as a building material that meets the needs of a developing trend in the ecological building industry. In this context, it could also be used in the marketplace for the energy-efficient renovation of detached houses. In the UE population, based on statistics from 2019 [58], only about 46% lived in flats, while the majority lived in houses, of which the market of single-family houses is over 34%. Almost one-fifth (18.5%) are semi-detached or terraced houses. The potential of this housing group indicates the possible market success of the proposed solutions, as most of these houses are located in rural areas, where the natural materials used in construction perfectly match the environment.

Performed testing contributes to explaining a relation between the physical, mechanical, and ecological aspects, indicating economic benefits and the possibility for the common application of natural structural materials in ecological construction. The authors hope that this study will become the foundation for the future standardization of construction of straw bales, which can be both effective and much safer for health than modern conventional construction.

5. Discussion

The fire resistance test results of straw blocks presented in this paper indicate the non-combustibility of this material resulting from high relative compression (average 108.03 kg/m³). In addition, the demonstrated ability to increase the fire resistance of straw is due to the fact that the samples were protected with only a 5.0 mm layer of clay plaster, while in reality, the plaster layer applied is at least 50.0 mm. Nevertheless, the material withstood temperatures above 1000 °C for more than 15 min, depending on the orientation of the stem, which is an additional argument for its use in green building.

The comparison of the materials in terms of fire resistance showed that lightweight clay is the best material because of taking the lowest temperature, 70 °C, from the flame at a given time (Figure 8). The results support the claim that lightweight clay is the best material for finishing a wall made with the straw bale technique.

The coefficient of heat transfer for straw with the stem orientation perpendicular to the direction of heat flow was determined to be 0.069 W/m·K. The value of this coefficient for straw orientation along the direction of heat flow was determined to be 0.081 W/m·K. As shown in the study, the thermal insulation properties of straw, regardless of the orientation of the straw stalks, are much better than cellular concrete popular in recent years (λ = 0.130 W/m·K at a density of 500 kg/m³).

Using computational modeling of architecture, a demand for final energy in the building was determined in four variants, which differed in the type of material used for the construction of external walls. Each variant based on natural materials meets the technical requirements in force in Poland since 2021 [57]. The building constructed in the straw bale technology with straw stalks arranged parallel to the heat flow reached the result of Ek = 27.77 kWh/m²·year. Turning the straw stalks and changing their direction to perpendicular to the heat flow caused a reduction of the final energy to a level of 26.38 kWh/m²·year. The lowest value of energy needed to heat the designed building was achieved by the building made of hemp–lime composite, i.e., Ek = 25.44 kWh/m²·year. Each of these variants may be successfully used as the alternative for conventional construction, which obtained the result of Ek = 26.83 kWh/m²·year.

6. Conclusions and Observations

Summing up tests conducted, the following conclusions can be formulated:

• Samples made of straw covered with a 5.0 mm layer of clay plaster withstood the impact of fire for 15 min. Under the temperature, the clay plaster hardened, and the flame got in only through cracks in its surface formed due to shrinkage. After 15 min, the straw inside the samples became charred to the depth of 55.0 mm. Due to the proper compression of straw inside the specimen of ca. 108 kg/m³, the flame did not spread despite charring. That test confirmed that the clay binder serves as protection for the straw infill and prevents burning the straw shreds out inside the sample.
• Based on test results of the heat transfer coefficient, the relation was found between the volume density and the heat transfer coefficient (λ). The higher the density of samples, the lower were their thermal-insulating properties, as shown in Figure 11. This is related to the lower number of air gaps in the material structure.
• Based on the tests conducted, the heat transfer coefficient was determined for straw with stalks orientation perpendicular to the heat flow direction equal to 0.069 W/m·K. The value of that coefficient for straw stalks orientation along the direction of heat flow was determined as 0.081 W/m·K.
• Numerical calculations indicated that each of the analyzed variants of buildings built with natural materials meets final energy EK values below 40.0 kWh/m²·year, which allows them to be classified as low-energy buildings.

The results obtained provide a basis for further research, which should focus on environmental studies to verify the resistance to biological corrosion as well as ways to improve selected properties and develop technology to protect the external surface of such a wall. Subsequent studies will be based on measurements on an existing building.