Environmental and Performance Assessment of Protective Outer Layers for Paper-Based Building Envelopes

Although protection against destructive factors is crucial for paper-based building elements, it may also significantly increase its environmental impact, weight and manufacturing costs. Therefore, the use of bio-based and recyclable materials is highly recommended. Two main groups of protective techniques are coatings and impregnation (including paper modification during production) and non-paper finishing materials (including foil lamination).

Due to its porous structure, paper is a material that is easy to coat with coating materials. During spreading, the coating penetrates into the pores of the paper causing its anchorage. Protective coating materials include polymeric resins such as epoxies, polyurethanes, polyacrylates, or formaldehyde resins. They form tight and durable coatings that prevent moisture from penetrating deep into the paper, thus weakening its properties. The coatings can also provide fire protection by blocking access of flames to the paper, or by decomposing into fire-resistant compounds during combustion. To achieve such effects, additives called flame retardants are added to resins, which include such compounds as metal oxides and hydroxides, boron, silicon, and halogen compounds [15, 16]. Another effective way to protect paper is to impregnate it with non-coating materials like waxes and oils, which can improve the hydrophobicity of produced paper elements [17]. A range of coating and impregnates suitable for use on paper-based elements can be found in Table 2.

Polymer coatings made from non-renewable sources are extremely increasing the ecological impact of paper. By using these protective materials layered structures are made, and their recyclability is harder. Year by year materials and techniques with less environmental impact is being investigated as protection of paper elements, which reduces the harm to the environment. As a substitute for polyolefins protection biodegradable polymeric materials are being used to cover the paper surface, for example, poly(lactic acid), PLA, or polyhydroxyalkanoates [18].

The second group of protective techniques are non-paper finishing materials. There are many different types of materials such as metal sheets, plywood, fibre-cement and HPL board, membranes, fibreglass wallpaper, vinyl veneer, etc. Protective materials may be combined with various coating, to obtain a finishing layer with desired properties. Products such as HPL (high-pressure laminate) or fibre-cement board, thanks to their high damage resistance can be used on vertical outdoor surfaces. On the contrary, lightweight materials like fibreglass or polyvinyl chloride veneer can provide sufficient and low-cost protection for partition walls. Significant differences can be observed in the environmental burden caused by each of the finishing materials. A range of materials suitable for use as protective layers in paper-based building envelopes is resented in Table 3.

Protective outer layer is an indispensable part of every paper-based building envelop. With an efficient design, it can provide water tightness, fire resistance and mechanical damage resistance, without compromising the envelopes environmental properties. Despite that, the issue of a protective outer layer had a minor role in most of the previously conducted research, which usually opted for conventional solutions such as wood cladding or foil lamination. In order to facilitate and popularise the use of pro-ecological paper-based building envelopes, the outer layer design should be studied and evaluated it terms of ecological impact.

Therefore, the aim of the research is (i) to review a group of original outer layers designs, suitable for use on indoor, outdoor and roof surfaces, (ii) to evaluate their resistance to various destructive factors, (iii) to assess the environmental impact of proposed designs, and (iv) to select the most appropriate designs for various applications. As a conclusion from the research, guidelines and suggestions for further development of paper-based building envelopes are provided.

The research was conducted on fourteen outer layer designs with various levels of damage resistance. Data about the designs served as input for Life Cycle Assessment analysis, which led to the final evaluation.

A series of outer layers for paper-based cores was designed, based on a literature review, material characteristics and authors’ experience from previous research and works with paper architecture.

Proposals cover seven layers for outdoor surfaces, including three suitable for roofs, and seven for indoor surfaces, for both dry and humid rooms.

A set of principles was followed in the design process. Each design should form a non-combustible finishing layer, to prevent the spread of fire through the surface and slow down fire penetration into the envelope. All the designs suitable for outdoor conditions should feature ventilated air cavities, to avoid water vapour condensation inside the structure. Each design should provide water and humidity protection – water tightness for roof finishes, high water resistance for outdoor and wet room envelopes and moderate resistance for indoor, and dry room finish. All the designs have to be resistant to mechanical damage, however, the outdoor layers should provide high resistance level, while for indoor ones moderate resistance is sufficient. The proposed designs should be environmentally optimised, thus incorporating natural, recycled and recyclable materials and allowing for their separation at the end-of-life phase.

Each outdoor outer layer design uses a general scheme of fire retardant, breather membrane, wooden battens with a ventilated air cavity in between and finishing layer composed of various non-paper boards (see Fig. 1). Materials used for roof-appropriate designs are aluminium plate, EPDM (ethylene propylene diene monomer) membrane and bitumen shingle on wood-based sheathing. For external wall – fibre-cement boards, HPL (high-pressure laminate) boards, coated plywood and carbonised wood were chosen. The characteristic of each design can be found in Table 4.

Figure 1.

Scheme of the outdoor outer layer

On the contrary, part of designs for indoor spaces is based on paperboard (that provides stiffness and certain fire protection [6]) with various types of laminates and coatings. The paperboard is coated with polyvinyl chloride veneer or membrane and painted fibreglass wallpaper. For more durable designs – HPL, plywood, gypsum and magnesium oxide boards were used. The characteristic of each design can be found in Table 5.

Life Cycle Assessment is currently the most recognised and widely used method to evaluate the impact on the natural environment generated by products or services [19]. LCA methodology allows assessment over the entire life cycle, from the extraction of raw materials, through manufacturing and use, to end-of-life disposal or recovery. Hoverer, it may also be used to assess the impact in the cradle-to-gate scenario, which is often beneficial for decision-making during manufacturing. The principle of the LCA methodology is collecting data regarding the exchanges between the natural environment and Technosphere (called elementary flows) i.e. raw materials extracted and substances emitted to water, air or ground. This information is the input data for environmental burden calculations [20].

LCA analysis is regulated by standards ISO 14040 and ISO 14044, which divide the process into four stages: goals and scope definition, life cycle inventory analysis, and life cycle impact assessment and interpretation [21, 22].

The aim of the analysis is to assess the environmental impact of a series of protective outer layers that may be used on paper-based building envelopes, providing protection against humidity, water, fire and mechanical damage. Layers for indoor and outdoor applications are evaluated separately. The functional unit for all of the analysed case studies is 1m² of the protective layer. The study will indicate designs with the lowest environmental burden, that may be recommended for further development and real-life applications in architecture.

The scope of the analysis was defined in accordance with EN 15804 standard, which regulates LCA for building products. The document distinguishes four main stages of the analysis, which are: the products stage (A1-A3), construction process stage (A4-A5), use stage (B1-B7) and end-of-life stage (C1-C4) [23]. Considering limited data availability and accuracy, the discussed analysis was conducted in the ‘cradle-to-gate with options’ approach, covering stages A1-A3 and C2-C4. Furthermore, it should be noted, that additional connecting elements, that may be necessary for the assembly of the designed layer are excluded from the study, as the assembly techniques may vary depending on the envelope core or building design.

The life cycle inventory for each of the outer layers was prepared based on provided designs and technical data from material manufacturers, using the Ecoinvent 3.8 database in the OpenLCA software. Ecoinvent is an extensive database, focused on the European context, providing data regarding industrial processes, including wood and paper industry [24, 25]. The material inventory for analysed designs is presented in Tables 6 and 7.

The following assumptions have been made in the area of recycling and waste treatment:

paperboard used made from 100% recycled fibre and 90% of the material in recycled at the End of Life phase;

To increase the accuracy of the results, two different impact assessment methods were implemented: CLM 2001 and ReCiPe. The CLM 2001, which follows the EN 15804 standard, is a midpoint method of evaluation, i.e. it focuses on quantifiable, direct impact, linkable to specific emissions. The impact was assessed in seven categories: global warming, ozone layer depletion, photochemical oxidation, acidification, eutrophication, fossil fuel abiotic depletion, and elements abiotic depletion. On the contrary, ReCiPe is one of the endpoint methods, also called damage-oriented ones, that translates midpoint impact into endpoint results, e.g. damage to the ecosystem or human health. The ReCiPe results are also presented as a single indicator, allowing direct comparisons between evaluated scenarios [26,27,28].

The conducted Life Cycle Assessment analysis revealed significant differences in the scenarios' environmental burden. As was to be expected, there is a visible positive correlation between the layer weight and its impact on the natural environment. In the case of roof-appropriate layers, the most favourable result was obtained in variant E5, using aluminium sheet. On the contrary, the impact of layer E7, with bitumen shingle doubled the results of all the analysed scenarios. Considering all the outer layers, the lowest impact can be linked to carbonised wood and HPL variants (E4 and E1). In ReCiPe analysis, for most of the cases, the largest share of total impact is located in the area of human health, and human toxicity in particular. However, the category of agricultural land occupation (ecosystem quality area) also plays an important role, mostly due to timber consumption (see Fig. 2). As presented by CLM analysis (see Fig. 3), the energy consumption from fossil fuels varies between 31 MJ for E4 to 474 MJ for E7 variant, and the carbon dioxide emission from 2.8 kg (E4) to 38.4 kg (E7).

Figure 2.

Result of ReCiPe method endpoint LCA analysis for exterior layers E1-E7

Figure 3.

Result of CLM 2001 method midpoint LCA analysis for exterior layers E1-E7

A large discrepancy in results was observed in the interior layer analysis – from 0.42 (I3 variant) to 2.52 (I7) in the ReCiPe single indicator results. The highest burden was associated with designs containing the largest amounts of petroleum-derived polymers, which are I5 with PVC membrane and I7 with HPL board. On the other hand, the most lightweight variants I3 (PVC veneer) and I4 (fibreglass) can be linked with the lowest impact. As with exterior layers, the most significant impact category in the endpoint analysis was human health – human toxicity (see Fig. 4). In the categories of midpoint analysis, a bigger variety was observed (see Fig. 5). The energy from fossil fuels consumption varies from 21 MJ (I2 and I3) to 182 MJ (I7), and the carbon dioxide emission – from 2.0 kg (I3) to 10.9 kg (I2).

Figure 4.

Result of ReCiPe method endpoint LCA analysis for interior layers I1-I7

Outer layers evaluation

Based on materials characteristics the design performance was assessed, with the highest score among roof layers for E5, among external walls – for E2, among interior dry room – I6 and wet room – I3 and I7 (see Table 8). Secondly, the performance score was juxtaposed with single-indicator LCA results. The designs with high-performance scores and low environmental impact (indicated by the yellowish quarter in the plots in Fig. 6) can be recommended as the most optimal solutions.

Figure 5.

Result of CLM 2001 method midpoint LCA analysis for interior layers I1-I7

Figure 6.

Outer layers evaluation metric

Table 8.

Outer layers performance assessment

Layer No.	E1	E2	E3	E4	E5	E6	E7	I1	I2	I3	I4	I5	I6	I7
application	EW	EW	EW	EW	ER	ER	ER	ID	ID	IW	ID	IW	ID	IW
Fire resistance	1	2	1	1	2	1	1	1	2	0	1	1	2	1
Water resistance	1	1	1	1	2	2	2	1	0	1	0	1	0	1
Mechanic. resistance	2	2	1	1	2	2	2	1	1	0	0	0	1	1
Price and availability	0	0	1	0	1	1	1	1	0	2	2	0	2	0
Performance score	4	5	4	3	7	6	6	4	3	3	3	2	5	3

EW - exterior wall, ER - exterior roof, ID - indoor.

Considering outdoor-appropriate layers, designs E5 (aluminium sheet) and E2 (fibre-cement board) can be recommended. The E6 design also performed highly, however, the use of EDM membrane, which must be welded at high temperatures, raises questions about the safety and risk of spontaneous combustion of paper-based core that is meant to be protected. The choice of layers E1 (HPL) and E3 (plywood) may also be considered. In the case of indoor-only layers, design I6 (gypsum board) proved to be the most favourable. Nevertheless, variants I1 (plywood), I3 (PVC veneer) and I4 (fibreglass) should not be neglected.

The general performance and environmental burden are a good starting point for the selection of the outer layer for paper-based cores. However, for each application individual characteristics should be taken into account and factors such as type of paper-based core, type of building, life-span, assembly method, user requirements and local law regulations need to be considered. Some of the designs offer other benefits that have not been discussed in the presented analysis. For example, plywood or fibre-cement board may be used for aesthetic reasons, to showcase the beauty of the natural material, while painted variants (e.g. fibreglass) offer unlimited colour pallets. Furthermore, the use of heavy indoor layers, such as plywood, gypsum or MgO board, may noticeably increase the acoustic properties of lightweight partition walls, while lightweight proposals (e.g. fibreglass of PCV veneer) should be considered when low weight is a key factor, for example in temporary structures.