Introduction

Several academic studies reported the effect of the huge energy crisis currently underway and heightened after the beginning of Russia-Ukraine War in 2022 (Zhou et al., 2023; Blanchard and Pisani-Ferry, 2022; Benton et al., 2022). The building sector currently is experiencing several challenges for limiting the energy demand, on one hand, and, on the other hand, proving the existence of alternative ways to non-renewable resources.

In recent years, research has demonstrated that the scenario of environmental disaster caused by the climate change is not so far from what we expected (Olivier et al., 2017; Solomon, 2007; Pachauri et al., 2008). Conventional materials request high energy amount in their production processes, contributing to intensify the air and environmental pollution, land and water use (Solomon, 2007).

The framework above described allows to point out, on one side, that the passive strategies represent the best solution to achieve the reduction of energy demand of new and existing buildings and, on the other side, that low-energy manufacturing process materials are efficient alternatives for the environmental issue. In this sense, current literature was filled up of new studies and reviews showing the importance of thermal insulation materials (Aditya et al., 2017; Korjenic et al., 2011; Liuzzi et al., 2020; Brzyski et al., 2019; Mandili et al., 2019) in reducing the building energy consumption.

The Green Deal and the Circular Economy Action Plan (2020), issued by the European Commission, underlined the significance of reaching energy saving without exploiting the natural resources; using urban waste, discards, by-products and giving them new added value has become a new pillar of the international policy.

Paper dates back to 105 AD and consists mainly of cellulose fiber, the most common natural polymer. It is used in several architectural fields as paperboard, corrugated cardboard, honeycomb panels, and paper tubes (Latka, 2017). Various research contributions presented the potentiality of using waste paper components for achieving sustainable buildings (Mandili et al., 2019; Rajput et al., 2012; Raut et al., 2012). Mandili et al. (2019) mixed waste paper with lime developing building components with thermal conductivity ranging between 0.097 and 0.12 W·m−1·K−1.

Rajput et al. (2012) experimented the addition of cotton waste to fired bricks, mixing paper mill waste with cement binder and using components previously tested (Raut et al., 2012). The results demonstrated that thermal conductivity changes between 0.25 to 0.32 W·m−1·K−1.

Liuzzi et al. (2023) investigated three different bio-based by-products using them as additives to the waste paper based mix: exhausted coffee grounds (CG), sawdust powder (SP), and fava bean residues (FB). The thermal conductivity of final components varies from 0.071 to 0.093 W·m−1·K−1; therefore, it was possible to count the tested materials in the category of thermal insulators. Moreover, it was proved that the CG and FB mix had a bulk density and thermal conductivity comparable to plywood.

A consistent literature collection pointed out the advantages reachable when using fibrous components in combination with other based components; various research papers proved the excellent contribution supplied to circular economy by the use of textile discards. Sheep wool (Rubino et al., 2018; Zach et al., 2012), jute (Ricciardi et al., 2014; Samanta et al., 2022), and wood fibers (Lacoste et al., 2018; Del Mar Barbero-Barbera & Pombo, 2016) can generate great eco-insulators, exhibiting a thermal conductivity varying from 0.078 to 0.089 W⋅m−1⋅K1, with a density in the range of 308–333 kg⋅m−3.

Rubino et al. (2021) demonstrated the great thermal and acoustic performances of sustainable insulators manufactured with textiles discards used as new raw materials in combination with different binders.

Taking into account the life cycle of a textile fabric, it can be asserted that from the earliest manufacturing process (pre-consumer waste) to the end of its useful life (post-consumer waste) it represents one of the greatest causes of environmental pollution (Rubino et al., 2018); therefore, founding a proper way to re-use or re-cycle the textile fibers should be evaluated as a strategy to explore. Various attempts to use textile fibres in the construction industry exist; several research works focused on experimenting sustainable insulating panels (Ricciardi et al., 2014; Del Mar Barbero-Barbera & Pombo, 2016).

The present paper shows the hygrothermal and physical performances of building materials made of waste paper-pulp with the addition of textile fibers, supposing an internal use as thermal insulators. In order to compare the properties and find the best solution in terms of thermal insulation, the raw components were used in different percentages.

Moreover, the properties collected were implemented in a BESTEST building located in a Mediterranean area, validating the laboratory tests with a numerical simulation.

Materials

Basic components

In order to realize the experimental materials, paper waste strips and textile fibers were used (Fig. 1). The first step was the preparation of the paper pulp obtained from disposed office and house papers and crumbled by an office paper shredder in thin strips of about 3 × 10 mm.

Fig. 1
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Paper strips after cutting with an office shredder (a); textile waste fibers after frying (b)

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Textile fibers used in this study resulted from discarded shreds from the manufacturing process of an Italian clothing company (Gordon Confezioni, Bari, Italy). They consist in manufacturing waste of autumn–winter collection and they include a small fraction of synthetic fibers. presented the complexity of separating polyester from cotton and wool fibers as one of the main issues of recycling. Currently, at commercially level, any suitable separation mechanism for these kinds of fibers exists (Payne, 2015). As matter of fact, in the present research study, it was impossible to calculate accurately the exact fraction of the different fibers presented in the waste yarn.

In order to avoid clump formation and to achieve a more homogeneous mix, the textile fibers were previously frayed.

As binding agent of the experimental composite, the polyvinyl acetate glue (PVA) was used. Polyvinyl acetate glue has a great biodegradation resistance, and it is mainly adopted for non-load-bearing scope at environmental conditions.

Sample preparation

The manufacturing process of the laboratory samples is shown in Fig. 2. In order to prepare a workable blend, it was necessary to make paper as softer as possible; therefore, the paper strips were kept submerged in water at environmental temperature for 24 h. Then, a mixing machine was used (Fig. 2a) in order to achieve a homogeneous paper pulp. The excess water was removed by firstly wringing out the mixture and, then, by leaving it to drip in a colander for about 15 min (Fig. 2b). The final step consisted in distributing the textile fibers and setting the achieved mix in moulds of different sizes according to the international standard prescriptions considered for the measurement campaign (Fig. 2c).

Fig. 2
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Paper strips after wringing out (a); paper pulp after mixing and removing the excess water in a colander (b); final paper pulp mixed with textile fibers and glue set in the mould (c)

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Square wooden formworks with a 200 mm × 200 mm base and 40 mm thickness for the thermal tests and cylindrical sample of 150 mm of diameter and 10 mm of thickness for the water permeability tests (UNI EN ISO 12572, 2016) were used. The final specimens were cured at 70 °C for 7 days in a ventilated oven and then left in a silica gel desiccator for cooling at environmental temperature.

Three different kinds of mix were prepared: the first one is composed of sole waste paper pulp (P) joined together with the polyvinyl acetate glue (PVA). The second mix (PTX-0.5) was prepared adding 0.5 wt% of textile waste to the paper based mix; the third mix (PTX-1) was obtained with the addition of 1 wt% of textile waste.

The mix percentages of the PVA glue and of the textile fibers, shown in Table 1, were selected after following in laboratory a trial-and-error procedure. The aim of this criteria was reaching a compromise between the best workability of the blend during the manufacturing process avoiding the clumps formation due to the textile fiber presence, the compactness of the final specimens after the curing time, and the best results in terms of physical parameters and hygrothermal performances. As matter of fact, the numerous attempts performed in the laboratory demonstrated that the addition to the paper pulp blend of the textile fibers in percentage greater than 1% can represent a real hang-up in the manufacturing process; on the other hand, the higher the target porosity, corresponding to the smaller thermal conductivity, the more fragile the final specimens can be. Bories et al. (2014) reviewing eco-friendly porous fired bricks confirm that the decrease in density of the material, due to the creation of porosity has important consequences concerning the product’s mechanical resistance. Increasing the percentage of the additives, lightweight products can be achieved with a significant decrease of the mechanical properties. Sadrolodabaee et al. (2021) found that in cement composites the reduction of the cement amounts with the increase of the number of fibers contribute to reduce significantly to the resistant mechanism. Thus, finding a compromise between weight loss, thermal and mechanical resistance is fundamental to avoid fragile materials.

Table 1 Mix design
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Test method

Physical tests

Bulk density and bulk porosity

Bulk density, ρbulk (kg·m−3), is defined as the mass of particles in the material divided by the total volume occupied including particle volume, inter-particle void volume and internal pore volume. Bulk density changes according to the components, the composition, and the voids; thus, it cannot be taken into account as an intrinsic property of materials as it.

The bulk density of the composites was determined in accordance with ISO 12570 (2000). In order to estimate the bulk density, the dry mass and the total volume of each sample after oven drying were measured. In this work, the mass of the samples was weighed using an electronic balance (0.01 g accuracy); the total volume was calculated by multiplying average depth and the average bottom area and by reading the linear dimensions with a digital caliper (accuracy 0.1 mm).

For each mix, six different samples were considered; the final value of the bulk density was obtained as the average value.

The ASTM D 4892-14 (2014) was taken into account to measure the true density ρtrue (kg·m−3) of the specimens. A helium gas pycnometer (ULTRAPYC 1200-e, Quantachrome, Boynton Beach, FL, USA) was used to measure the bulk porosity, assessed according to Archimedes’ principles of gas displacement and the Boyle law of volume–pressure relationship. The measurement is based on the estimation of the pressure drop achieve when the pressurized cell containing the sample is connected to the cell of reference volume. The true density ρtrue (kg·m−3) of the tested sample is defined as the ratio of the sample weight m (kg), previously measured by a laboratory balance (0.01 g accuracy) to the true volume Vtrue (m3) of the sample as measured by the pycnometer. Vtrue (m3) is the difference between the total volume Vtot (m3) and the pore volume Vpores (m3).

$$\varepsilon =\frac{{V}_{{\text{tot}}}-{V}_{{\text{true}}}}{{V}_{{\text{tot}}}}$$
(1)
$${\rho }_{{\text{true}}}=\frac{m}{{V}_{{\text{tot}}}-{V}_{{\text{pores}}}}=\frac{m}{{V}_{{\text{true}}}}$$
(2)

Thermal tests and water vapour permeability measurement

The experimental boundary conditions were settled according to a previous work on waste paper based materials (Liuzzi et al., 2023) and taking into account the UNI EN ISO standard 12,572 (2016); thus, before starting the thermal and hygric tests, all the experimental samples were conditioned in a laboratory air-oven at 50 °C until achieving a change of mass less than 0.5%. Furthermore, for the thermal tests, the specimens were previously prepared polishing the contact surface by a FORCIPOL 300 – 1 V to achieve a suitable adhesion of the circular base of the sample to the thermal probe.

The evaluation of the thermal properties of the experimental materials was performed measuring the dry-state thermal conductivity λ (W·m−1·K−1), the thermal diffusivity α (m2 ·s −1), and the volumetric heat capacity ρ⋅C (J·m−3·K−1) by a transient plane source device ISOMET 2104 (Applied Precision Ltd., Bratislava, Slovakia). For all the mix, the test was replicated on six representative samples of 200 mm × 200 mm and 40 mm thickness, and the final values shown in Table 2 are the mean of three measurements (Fig. 3).

Table 2 Thermal performances (the value in round brackets represents the standard deviation)
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Fig. 3
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Thermal performance laboratory set-up (a) and Helium pycnometry (b)

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The water vapour permeability δp (kg·m−1·s−1·Pa−1) of the three mix types was estimated after measuring daily in a wet cup the rate of water vapor transmission per unit area per unit of vapor pressure (UNI EN ISO 12572, 2016). Three cylindrical samples of 150 mm of diameter and 10 mm of thickness were taken into account. Each sample was sealed, using wax, on the top of a transparent vessel containing water (Fig. 4) leaving an air void of 15 ± 5 mm between the water and the sample. The climatic chamber Angelantoni DY340 (Angelantoni Test Technologies Srl, Massa Martana PG, Italy) was used for monitoring the daily weight of each assembly after setting the internal conditions as 23 °C and 50% RH (Fig. 4). The measurement was interrupted after achieving a constant mass value.

Fig. 4
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Water vapour permeability test set up: climatic chamber (a); sealed specimen (b)

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Results and discussion

Starting from the manufacturing process, the water drop trend of the different mix was evaluated during the curing time (7 days). Figure 5 shows that a significant variation in water loss can be appreciated when considering the specimen of sole paper (P); the addition of the textile waste (PTX-1, PTX-0.5) reduced the initial content of liquid water due to the presence of textile fibers that fill the mix. The time taken to reach the equilibrium was the same in the three cases (approximately 5 days).

Fig. 5
figure 5

Water drop trend

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Figure 6 shows the measured thermal conductivity versus dry bulk density, reporting the resulted values of all the six specimens examined for each mix.

Fig. 6
figure 6

Thermal conductivity versus dry bulk density

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Thermal conductivity was in accordance with the bulk density trend. The mix without any textile fiber (P) had the highest bulk density and the highest thermal conductivity. An addition of textile fibers up to 1% caused a significant enhancement of the insulation capacity moving from 0.0864 W·m−1·K−1 (P) to 0.0690 W·m−1·K−1 (PTX-1).

Table 2 reports the thermal performances and the results of the physical tests. It can be asserted that bulk density is in accordance with the trend of bulk porosity; it can be observed that since the bulk porosity decreases, the bulk density grows, as well due to the reduced amount of pores in the specimen. The thermal conductivity confirms that the mix P, PTX-01, and PTX-0.5 are in accordance with the results of thermal tests performed on paper based specimens mixed with biomass by-products (exhausted coffee power, sawdust powder, fava bean residues) studied in a previous work (Fig. 7) (Liuzzi et al., 2023).

Fig. 7
figure 7

Comparison of thermal conductivity with other paper based specimens (SP, sawdust powder; CG, coffee grains; FB, fava beans) (Liuzzi et al., 2023)

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Specifically, the PTX-1 has the same thermal conductivity of the SP which has sawdust powder. The evaluations above described lead to assert that the paper waste allows to develop good thermal insulators; the results are in accordance with the studies of Al-Homoud (2005).

After curing time, the trend of sample daily weight variation was used to calculate the water vapour permeability δp and the water vapour resistance factor µ for the three mixes; the results are expressed in Table 3. It can be note that all the specimens have high water vapour permeability due to the high bulk porosity strictly correlated to the reduction of bulk density. It is clear that the addition of the textile fibers contributes to increase the void spaces in the specimen, thus increasing the bulk porosity and reducing the water vapour resistance coefficient.

Table 3 Hygric performances (the value in round brackets represents the standard deviation)
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Numerical simulation

The test building

The experimental data collected in laboratory were further used as inputs for a numerical simulation, developed by WUFI® Plus (Kunzel, 2005), of the hygrothermal behaviour of a building envelope.

The case study used for the simulation is based on the well-known BESTEST building (Fig. 8) (Neymark & Judkoff, 2002). It occupies a total volume of 129.6 m3, on the east–west axis, it is 8 m long and, on the north–south side, is 6 m long. The total floor area is 48 m2, the wall height is 2.7 m. The building is supposed to consist in one thermal zone. The numerical simulation was performed on an annual basis, run for 10 years to allow the materials reaching hygrothermal equilibrium with the environment regardless of the temperature and humidity conditions assigned to the materials at the beginning of the simulation.

Fig. 8
figure 8

Simulated BESTEST building

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The results of the last year of the simulation are considered valid for evaluating the hygrothermal performance of the simulated building.

On the south wall two windows (1.5 m × 1.5 m) are located, with overhangs 1 m deep.

Two different cases were compared; the first one (reference wall) considers the vertical walls with an internal cover of gypsum plasterboard insulated with an EPS layer (Table 4). In the second case, it was supposed a unique internal panel made with PTX-1 mix (Table 5). The thickness of this panel was chosen in such a way as the whole assembly has the same thermal transmittance (0.46 W·m−2·K−1) in the two cases. The basement floor slab was designed to have a thermal transmittance of 0.776 W·m−2·K−1, and the roof assembly was supposed to have a thermal transmittance of 0.305 W·m−2·K−1.

Table 4 Wall assembly case 1 (reference wall)
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Table 5 Wall assembly case 2 (waste-paper wall with PTX-1 mix)
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The daily inner loads are shown in Fig. 9. They were taken from the WUFI® archive for “Single Household.”

Fig. 9
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Inner daily loads

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The test building was simulated supposing the Mediterranean context of Bari, characterized by a temperate climate with dry and hot summer according to Koppen-Geiger Climate Classification (Köppen, 1900; Geiger & Pohl, 1954). In Fig. 10, seasonal relative humidity and temperature range variation are shown.

Fig. 10
figure 10

Weather data for Bari

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The building was studied in two different scenarios: supposing the presence and the absence of HVAC system. According to the national standards (DPR 74, 2013), the heating system was assumed to be on from November 15th to March 31st. The air change rate was set to be 0.5 vol/h during the heating period; during the rest of the year, it was set an air change rate of 0.5 vol/h from 7.00 a.m to 7.00 p.m and 1.5 vol/h from 7 p.m. to 7 a.m.

The air conditioning system is an ideal HVAC system for heating–cooling-humidification-dehumidification capable of instantly supplying all the power necessary to reach the fixed temperature and relative humidity set points.

The system starts working as soon as the internal temperature falls below 20 °C and when it is higher than 26 °C. The control of the inner air relative humidity proceeds when it is higher than 45% and lower than 55%.

Results achieved

In absence of HVAC, the temperature trend in both cases is the same due to the same thermal transmittance value of the vertical opaque envelope of 0.46 W·m−2·K−1. A great difference in terms of relative humidity fluctuations can be noted when using the PTX-1 panel (Fig. 11) that causes a significant damping if compared to the reference wall. The most significant variation can be appreciated in winter and autumn; the use of PTX-1 panel settles the relative humidity value around 60%. In the summer season, the relative humidity ranges between 50 and 60%.

Fig. 11
figure 11

Temperature and relative humidity of inner air, annual variation without HVAC

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Figures 12 and 13 show the rate of absolute humidity along with the moisture flux exchange between the room air and walls, focusing on 2 days of July. It can be observed that the walls absorb and desorb moisture in response to moisture generation. In the PTX-1 wall, unlike reference wall, the presence of the experimental panel in paper waste has significantly reduced the value of absolute humidity due to the higher moisture exchange with partitions; this is strictly correlated to a consequence significant reduction in the amplitude of indoor RH fluctuations.

Fig. 12
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Absolute humidity and moisture exchange with partitions (reference wall)

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Fig. 13
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Absolute humidity and moisture exchange with partition (PTX-1 wall)

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In presence of HVAC system, considering the yearly sensible energy demand for heating and cooling, it can be observed that there was no significant variation between the two wall assemblies, due to the same thermal transmittance value of the walls in the two cases (Fig. 14).

Fig. 14
figure 14

Yearly moisture exchange with partition and energy demand in presence of HVAC

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There is a significant increase in terms of water vapor entering and leaving the walls in the presence of the PTX-1 cladding, which results into a reduction of the load for humidification and dehumidification.

Conclusions

The present paper investigated the hygrothermal performances of waste paper based panels, developed with the addition of textile fibers derived from industry discards. The conclusions can be pointed out as follows:

  1. 1.

    The thermal tests performed showed good results in terms of thermal conductivity, allowing to assert that the materials under study could be considered great insulators. On the other hand, also the hygric measurements highlighted that the experimental materials demonstrate excellent water vapour permeability performance, consequently allowing to deduce that the presence of the textile fibers, compared to the specimens with sole paper pulp, and contribute to reduce the water vapour resistance.

  2. 2.

    The results, collected by the measurement campaign, demonstrated that the mix with the highest percentage of textile fibers (PTX-1) has the best hygrothermal performances.

  3. 3.

    Furthermore, a numerical simulation of a TESTROOM in a Mediterranean climate using the WUFI Plus software was carried out in order to compare the hygrothermal behaviour in absence and in presence of PTX-1 panel, considering a conditioned and unconditioned scenario.

  4. 4.

    It is clear that the PTX-1 internal coating could cause a significant improvement in the indoor comfort level combined with operational energy savings resulting into a reduction of the load for humidification and dehumidification.

Further tests are underway to assess the moisture buffer value, the mechanical features, and the durability of the experimental materials supposing to use them also as external insulators. Moreover, LCA (Life Cycle Analysis) is ongoing to get a comprehensive knowledge of sustainability impacts of all the processes considered.