Upscaling bio-based construction: challenges and opportunities

ABSTRACT Construction projects using emerging bio-based materials have been realized over the past ten to fifteen years within Europe. Bio-based buildings utilize properties of natural materials to regulate internal environments, particularly fluctuations in temperature and relative humidity. Despite individual exemplar projects demonstrating functional performance and long-term operational cost savings, there hasn’t been a proliferation of commercial or domestic bio-based projects. With a growing shift towards circular economy construction, bio-based buildings could be readily adopted to meet this development. This study evaluates barriers faced by bio-based materials, making the upscaling of production and a breakthrough into mainstream construction challenging. Evaluation was achieved through senior professionals with experience in bio-based construction participating in semi-structured interviews based on core categories of finance, knowledge, and policy. Challenges include the upscaling of production by manufacturers of emerging materials, inconsistencies in life cycle assessment, material certification and accreditation, vested interests in the construction industry, and concerns regarding initial costs, availability, and knowledge of products. Potential solutions for upscaling bio-based construction are identified and include increased case studies, positive legislation, regional economic regeneration, the wellness agenda, long-term economic sustainability, and engagement with established construction companies. This insight has informed the procurement process, material evaluation, and adoption of policy.


Introduction
A circular economy aims to extend the life of products and components (Zuofa et al., 2022) and eliminate waste products by the reuse, repair, refurbishment, or recycling of source materials upon completion of first use (Shooshtarian et al., 2022). This contrasts with linear economy practice which ultimately involves waste products requiring disposal and new, increased demand for source material (Adabre et al., 2022), thus resulting in a 'take-make-dispose' model which is ultimately unsustainable (Norouzi et al., 2021).
Circular economy in the context of the construction industry represents the aim to construct in a sustainable manner, with buildings being considered dynamic and design being adaptable or reversible (Durmisevic, 2019) and building elements being re-configured/re-used or recycled following the first design life of a building, rather than becoming waste being disposed of by landfill or incineration (Durmisevic, 2001). However, the application of circular principles to the built environment is not considered mainstream or wide-scale in adoption (Adams et al., 2017). Circular construction procurement can be considered an emerging field with barriers existing in the development of circular procurement initiatives (Sönnichsen & Clement, 2020) and the transition to a circular economy requiring engagement with all involved stakeholders as well as technological advances (Adabre et al., 2022). The construction industry uses 30% of raw materials globally (Bilal et al., 2020) and is therefore required to play a major role in reducing the use of resources, reducing the energy required to process resources, and reducing waste.
When considering the construction industry, the term 'bio-based materials' can incorporate materials derived from once-living organisms such as agricultural straws, hemp, flax, cotton stalks, and cork (Liu et al., 2017). Bio-based materials can be modern products which consist of a certain percentage of bio-based material (Sijtsema et al., 2016), recent innovations such as mycelium (Jones et al., 2020) or indeed timber which has been used for construction throughout recorded history (Smith & Snow, 2008). Bio-based materials are potentially suitable for construction in accordance with circular economy principals and can be used for structural systems, thermal performance, sound insulation, thermal mass, indoor air quality, and the prevention or reduction of condensation. Biobased materials are renewable when sustainably and responsibly managed with ecologically-sound agricultural practices (Van Dam et al., 2005). They can be recycled or re-used in the same or a different form, resulting in minimal waste and reduced demand for resource material (Leipold & Petit-Boix, 2018) and bio-waste may be reused in another application, for example, biofuels with emerging waste-to-energy technologies (Rabbat et al., 2022).
The last 10-15 years saw the realization of highprofile industrial-scale bio-based buildings; however, despite on-budget delivery and operational success from environmental, economic, and building physics perspectives, realized projects have not resulted in a significant increase in bio-based projects.
The aim of this study is to evaluate the challenges regarding the upscaling of bio-based materials for use in construction projects and identify potential opportunities and approaches to promote the upscaling of biobased material production, progress the bio-based agenda and help facilitate a transition within the construction industry towards adopting bio-based materials for building in conjunction with circular principals and sustainable land management. The study examines barriers to upscaling bio-based construction materials for both private and public sectors and examines why the successful realization of pioneering case studies has not led to a proliferation of bio-based buildings. To this end, it was decided to interview senior professionals with first-hand experience in the realization of buildings using bio-based materials and the development, certification, and marketing of sustainably sourced and managed bio-based materials. The bio-based materials scope focuses on non-traditional construction industry materials as experienced by the selected interviewees such as hemp-lime, recycled paper, and mycelium, with an emphasis on the thermal insulation and internal environmental regulation properties and performance of the materials, primarily moisture buffering with attenuation of temperature and relative humidity fluctuations.
Areas for discussion are collated into three core categories; finance (concerning the realization of a projectclients, consultants, contractors, materials, and supply chains), knowledge (of construction industry professionals, academia, and client organizations), and policy (the role of national and local governing bodies and policymakers), within which questions for semi-structured interviews were formulated and arranged.

Finance
Finance is a key driving factor in the construction industry. There is a perception that green building inevitably comes with an increase in capital cost (Tsai et al., 2017) and an overriding perception of sustainable construction being an expensive option (Preuss, 2009) in an industry which typically has low levels of innovation (Koebel et al., 2015) and is driven by cost and value for money (Sönnichsen & Clement, 2020). Biobased materials need to be a profitable option for construction industry professionals to have confidence in their use (Markström et al., 2016).
Environmentally certified natural products can attract premiums in price (Lombard, 2017), although this is not universally the case with bio-based construction materials (Krasny et al., 2017). Stakeholders and decision-makers involved with project funding typically focus on initial costs and managing budgets rather than taking a long-term perspective with environmental implications (Sourani & Sohail, 2011) or a financial view to recouping initially higher costs through operational savings on maintenance, energy, and water (Lombard, 2017). Finance can also be dictated by external events and during economically challenging times such as the global financial crisis from 2008 (Nistorescu & Ploscaru, 2010), which saw a reduction in construction activity along with a fall in prices and profits.

Knowledge
Research into the increased use of bio-based material products, which can be defined as materials being partly or wholly of biological origin, excluding fossilized materials embedded in geological formations (European Commission, 2023), is currently taking place both in the construction industry (Bumanis et al., 2020) and multiple other sectors (Ruiz Sierra et al., 2021) such as product packaging (Reichert et al., 2020), biofuels (Kim et al., 2019), and textiles (Hildebrandt et al., 2021). The advantages of using bio-based materials in a construction project include hygroscopic properties (absorption of moisture from the air) (Mnasri et al., 2020), a renewable supply chain of feedstocks (Álvarez-Chávez et al., 2012) and biogenic carbon sequestration within plant-based material for a period of time (Galimshina et al., 2022), with sequestration being more effective in fast-growing biogenic materials such as straw and hemp which can be used for thermal insulation products, rather than in timber (Pittau et al., 2018). Bio-based materials used as a thermal insulation solution can also provide internal environmental stability, with reduced fluctuations in temperature and relative humidity (Shea et al., 2012).
Hygroscopic materials adsorb and desorb moisture, a property known as 'moisture buffering' (Osanyintola & Simonson, 2006), which attenuates peaks and troughs in humidity leading to improvements in indoor environmental quality and potential for improved energy performance (Kraniotis et al., 2016). For example, hemplime is used for its ability to reduce fluctuations and maintain suitable temperatures for storing produce and artefacts. Bio-based materials can sequester more carbon than is embodied in construction, therefore having a negative carbon footprint (Lawrence et al., 2013). Straw is also known for excellent thermal insulation properties and possesses a negative carbon effect with each kg of straw able to sequester 1.35 kg of CO 2 (D'Alessandro et al., 2017).
However, extensive knowledge of the beneficial properties of emerging bio-based materials may not be widely held by construction industry professionals (Markström et al., 2016). Along with uncertainty in thermal performance, potential durability issues surrounding vulnerability to infestation from insects and rodents (Hoxha et al., 2022), and mildew/ fungalinduced decay (Gortner et al., 2022) which can lead to decisions not to use the materials (Kennett, 2006). Influential and risk-averse clients or developers may hold negative views of natural materials, while contractors and architects hold varying levels of confidence (with contractors being generally higher) regarding the extent to which they could influence decisions on whether bio-based materials were used (Markström et al., 2016). The willingness of a client can be an important factor for a sustainable building to be realized (Häkkinen & Belloni, 2011). Within the construction industry, which may be viewed as conservative and risk averse (Bourdeau, 1999), it can be difficult to implement and adopt new processes and technologies (Häkkinen & Belloni, 2011).
Knowledge concerning the Life Cycle Assessment (LCA) of bio-based construction materials is inconsistent. Traditional LCA according to BS EN 15804 and EN 15978 does not comprehensively assess the effect of carbon sequestration by bio-based building materials on climate change (Peñaloza et al., 2016). The lack of temporal information or an accepted consensus on the timing of carbon emissions is a limitation of conventional LCA (Brandão & Levasseur, 2010). Land transformation, termed 'land use and land use change' in EN 15804 (CEN, 2019) is not consistently included in LCA. Analyses which typically favour bio-based materials may not consider the effects of land transformation, which may result in higher carbon emissions (Searchinger et al., 2018). A further gap in knowledge concerns end-of-life scenarios. EN 15804 considers incineration and landfill, but not composting (CEN, 2019). Traditionally, there has been a reliance on landfilling; a European Union directive prohibits the disposal of organic matter in landfill, though this is not reflected on a global scale (Barlaz, 2006).

Policy
Policymakers have a vital role to play to promote sustainability in construction (Odedra, 2017). Governments attending the United Nations Earth summit (1992) committed to a sustainable development policy (Parkin, 2003), international directives and climate treaties such as the European Union directive (2010/31/ EU), Paris Agreement (2015), and Kyoto Protocol (1997) were signed and national governmental policies required planning applications to meet carbon targets (Sourani & Sohail, 2011). However, government-led strategies and initiatives to promote sustainable construction were insufficient due to inadequate funding, inconsistency as to whether requirements were optional or mandatory, and the need for a stronger policy message. Incentives for investing in sustainable solutions beyond minimal compliance (Sourani & Sohail, 2011) and the importance of engineers working within a sustainable development culture (Parkin, 2003) were identified as potential solutions.
There is international variation in greenbelt planning policy, with countries such as the UK having state-controlled planning restrictions on greenbelt construction to preserve natural environments. This has drawbacks in increasing urban density and development beyond greenbelts leading to increased commuting and pollution, and selective greenbelt building can be permitted by the government at regional levels (Han & Go, 2019). A tax on carbon in the construction industry is one option for punitive legislation (Shi et al., 2019) to promote circular economy principals in construction and has been adopted in numerous countries, though the aim of reducing carbon emissions inevitably impacts the operational costs of the construction industry (Tsai et al., 2017). Bio-based subsidy measures are also in operation (Philippidis et al., 2018), but they can be focused on energy and biofuels, with a subsidy for bio-based construction materials limited to research and development (Philp, 2015).
Additional considerations concern how topical political issues and international events and crises can shift focus away from progressing a circular agenda, examples being adverse effects upon construction industry supply chains due to political events (Malik et al., 2019) and the risk of the renewable energy sector not being able to receive government subsidy in the wake of a global pandemic (Eroğlu, 2020).

Methodology
This study conducted a series of semi-structured interviews for a duration of one hour. It was decided that selecting a select group of senior professionals with known first-hand experience (Häkkinen & Belloni, 2011) in the realization of bio-based construction projects or development and bringing-to-market of emerging bio-based construction materials was a suitable method. Circular bio-based construction projects are unique and although case studies of financially and environmentally successful projects exist, they cannot be considered numerous or mainstream. This inherently resulted in a smaller sample sizesix in the case of this studybut it was considered that focusing upon equivalent specialist senior experience was more appropriate than extending to a larger sample group without the same level of experience, expertize, and authority. It was ensured that interviewees were selected so that insights could be gained from multiple perspectives; private sector client, public sector client, academic, civil servant, consultant, and researcher.
Opinion is divided as to what sample size is deemed to be 'enough' regarding qualitative research (Low, 2019), with quasi-empirical, perceived wisdom of elders and researcher experience-based attempts to answer the question deemed inadequate (Galvin, 2015) and differing fields of research requiring different interview-based approaches. It has been stated that small sample size can be suitable for in-depth interview and analysis work due to the scope for greater contact time between research and interviewee (Marshall et al., 2013) repeated contact (and therefore establishment of rapport with the interviewee) plus emerged material remaining in the researchers' mind throughout the investigation (Crouch & McKenzie, 2006) and leading to more in-depth data (Thomson, 2011). Previous studies have determined that 94% of themes under discussion were identified within the first six interviews and information saturation had occurred by the twelfth (Guest et al., 2006) or eighth (Eynon et al., 2018) interview. It has been stated that saturation could occur after 9-17 interviews or 4-8 focus group discussions (Hennink & Kaiser, 2022), or all themes can be covered after seven interviews (Constantinou et al., 2017). It was not an aim of this study to seek saturation given the absence of binary questioning and that every bio-based building featured in this study was essentially a prototype and experiences and perceptions naturally differ. However, it is posited that literature supports the position that six expert interviewees can provide a broad and comprehensive range of extensive insight and experience, and the comprehensive methodological framework developed by the authors for the interviews ( Figure  1) demonstrates procedural steps and how focus if this study diverts from mainstream qualitative research in the field (Corbin & Strauss, 1990), in this case, the construction industry.
Based upon the work of (Markström et al., 2016) and (Grover et al., 2019), the information contained in Table 1 and the text formulated from responses in the following results section anonymize the interviewees.
The key position(s) held by the interviewee along with the nature of circular bio-based construction project involvement and experience is also indicated in Table 1. All interviewees in this study were based in the United Kingdom at the time of the interview. A semi-structured qualitative interview (Sourani & Sohail, 2011), (Markström et al., 2016) approach was selected, with the core categories of finance, knowledge, and policy forming the basis of adaptable questioning tailored to the interviewees' range of personal experiences and perspectives. Selected quantitative information concerning the operations of constructed bio-based buildings was incorporated as appropriate. Interviews were recorded and verbatim responses were transcribed from the recordings by the authors, with written consent to use results for project output obtained from each interviewee.
Analysis arising from responses to questions extending from the three core categories of finance, knowledge, and policy is subsequently presented. The methodology framework forming the scope of the semi-structured interviews, based upon the core categories, is illustrated in Figure 1. Direct experience and personal involvement in the realization of a particular bio-based project or material commenced the interview process. Questions then progressed in a flexible manner to wider views concerning the present climate within the construction industry and thoughts on furthering the agenda of circular construction and bio-based materials for use and reuse in buildings, along with the barriers, and potential opportunities and strategies, for upscaling. It was deemed important that although commencing with a defined methodological framework, the semi-structured approach of the interviews allowed questions to adapt as material emerged during the flow of the interview.
The presentation of the results is also divided into the three core sections of finance, knowledge, and policy, with a table at the conclusion of each summarizing the challenges and opportunities as identified by the authors during the interview process. It is indicated after each bullet point in the tables which interviewee(s) response played a role in the formulation of the text of that bullet point. If an interviewee has been directly quoted, this is indicated in the main body text.
It should be emphasized that the results and discussion elements will relate to the bio-based materials at the forefront of the experience of the UK-based  Table 1) and challenges and opportunities identified may not be universally applicable to all bio-based materials in all global geographical localities.

Results and discussion
Insights gained from interviewees and discussions of opportunities are arranged in the three core categories, with sub-categories adhering to the interview framework methodology ( Figure 1) and a tabular summary of key challenges and opportunities identified and discussed concluding each category. Direct quotations from interviewees in the text are presented in italics with the interviewee number indicated.

Bio-based material costs and building operational cost-savings
Interviewees were unanimous in the view that a barrier to bio-based construction is the initial higher material cost. For a construction project, materials are typically costed from primarily an economic perspective, rather than by performance, environmental, aesthetic or human health, and well-being factors. 'We need to get there on price, otherwise we're never going to prove this can be done commercially.' (Interviewee 4) It was also unanimous among interviewees that a proposal to construct a circular, bio-based building ignites discussion in the boardroom of a client organization. From the experience of the interviewees, initial client costs ranged from 10% to 25% higher to construct using bio-based materials in comparison to a conventional steel build, although it should be noted that not all of this increase is due solely to materials, but also to additional design considerations. However, it was also unanimous that there are financial savings to be made over the course of the design life of a bio-based building due to lower operational running costs arising from material performance. Interviewee 1 confirmed that the client organization saves six figure-sums a year in energy costs. Finance requires a long-term perspective.
'In one example, shareholders had been persuaded to increase initial financial costs in order to reap longterm benefits.' (Interviewee 3).
In a public sector demonstration building, Interviewee 3 stated that it was shown that a social housing construction built with a bio-based insulation solution used 70% less energy than conventional synthetic material. Interviewee 5 confirmed that a building which utilized biobased materials costs two-thirds less to run than an equivalent building using conventional materials situated on the same site. Other studies in literature support operational cost savings with bio-based materials. Operational running cost savings reported by interviewees exceed an environmental impact study which compared a bio-based building to a concrete building, which Table 1. Professional position and experience of interviewees. All interviewees were based in the United Kingdom at the time of interview. The table also lists the bio-based materials at the forefront of the interviewees' experiences.

Interviewee
Position of interviewee Circular, Bio-based project perspective and experience Interviewee 1 Chief Executive Officer of a mid-cap public limited company which seeks sustainable solutions in all aspects of construction, manufacturing, and distribution and has won numerous awards for environmentally friendly practice. The interviewee has been with the company for over twenty years Private sector client; oversaw the conception, planning, and realization of an industrial-scale circular bio-based building which could be extended from a corporate perspective. Materials used in the build included glulam, sedum-roof and hemp-lime blocks.
Interviewee 2 Founder and director of a company specializing in bio-based research, manufacture, and consultancy and dedicated to a bio-mimetic circular future for the built environment. The company has been active for a decade.
Consultant, academic researcher, and entrepreneur; developing and marketing innovative bio-based construction materials and designing circular construction systems. Specializing in mycelium and use of biological waste materials. Interviewee 3 European Commission external evaluator, bio-based organizations ambassador, and diversity consultant. Multiple decades of experience include positions within UK government departments such as Energy and Climate Change (DECC) and Environment, Food and Rural Affairs (DEFRA).
Civil servant, consultant, and decision maker; Evaluating proposals relating to multiple aspects of bio-based projects and material construction and development.
Interviewee 4 Executive Director and professorial fellow within a University faculty, with a background in biology. Active in environmental policies for over thirty years.
Academic; project-led the realization of an industrial-scale bio-based project on the campus of a university and currently continues leading research and development of circular, bio-based construction projects on campuses. Materials used include glulam, timber, thatch and reed, and recycled paper. Interviewee 5 Senior manager within a British national museum group facility, overseeing the conservation of historic artefacts. The interviewee held the position for seventeen years.
Public sector client; championed and oversaw the use of bio-based materials, principally hempcrete panels, in a completed industrialscale building. Interviewee 6 Senior manager of a division within a large private retail company, a position held for over five years.
Private sector client looking to build upon the success of previous sustainable building projects with a programme of circular, biobased construction. Materials used by the company include hemplime and earth.
found that the bio-based building costs one-third less to run (Krasny et al., 2017). Another study found that biobased insulation materials (and a low-carbon energy system) reduce considerably the heating demand, benefiting the users' operational costs (Galimshina et al., 2022). If the structural, thermal, or hygric properties of a bio-based construction material provide a particular solution for a client company, this can transform the commercial relationships of client companies in addition to forging new networking opportunities and relationships. The operational solution provided by a particular bio-based material may be considered the primary decision-making factor: 'The solution was considered more important; a slightly elevated cost was accepted.' (Interviewee 5).
Additionally, Interviewee 6 explained that the experience of customers is a primary concern for private client organizations; financial considerations and the benefits of material properties must be balanced with the client organizations' ability to continue meeting customer needs and the well-being of those customers.
A major consideration is whether client organizations are financially and structurally in a position to think long term with regards to recovering initial higher financial outlays through reduced energy costs. There are clear long-term financial benefits to the use of bio-based materials, especially if adaptability and modification are considered in the design process to accommodate future company needs and expansion. However, Interviewee 1 pointed out that these may not be enough to satisfy the balance sheets of client companies structurally having to think and work on shorter time frames with high senior personnel turnover and the need to satisfy the interests of multiple stakeholderswho may not be prepared to wait five, ten or twenty years before seeing a return upon initial increased investment.
It is reasoned by this study that measured financial incentives such as low-rate interest or interest-free loans or grants at international, national, or regional decision-making levels may be a route to enable client organizations to plan for construction projects over a longer timescale than would otherwise be possible due to company structure. This route would facilitate greater initial expenditure on emerging bio-based technologies and circular design by client organizations, with the organizations subsequently repaying while benefiting from long-term energy savings over the course of the design life of the building. Initiatives involving private and public partnerships such as the European Union-based Bio-based Industries Joint Undertaking (Mengal et al., 2018) or the developing European Bioeconomy Strategy (Bell et al., 2018) could be examined to inform whether encompassing the construction industry within the scope of such an initiative may potentially financially facilitate the upscaling of biobased construction projects.
A further financial opportunity for clients to benefit from using bio-based materials would be the experience of occupiers and the wellness of both staff and customers. It has been reported that bio-based materials are beneficial to the well-being of human occupants (Sandak et al., 2019) with natural materials emitting less harmful volatile organic compounds (VOC) being positive for well-being (Adamová et al., 2019). The contribution of bio-based materials towards occupant wellness, particularly the staff in a client organization would require further studies and surveys for quantification, but on a qualitative level, it is reasoned that the aesthetic, environmental, and health benefits (lower volatile organic compounds) of using bio-based materials in a building may contribute towards higher staff wellness, morale, and productivity, with lower turnover and absenteeism; this, in turn, would financially benefit the client organization.

Regional growth and supply chains
Extending economic viability throughout the supply chain is an important factor. The growth of non-food crops or crops with by-product potential for bio-based construction needs to be financially viable and ideally needs to take place locally and be part of a coordinated strategy to boost local and regional economies, using local businesses, materials, trades, and crafts.
'An important point was that farmers and people in rural areas could make a profit.' (Interviewee 3) There are cost-related issues around material supply chains. A typical construction project will involve the purchase of materials from the open market, driven by Quantity Surveyors who may be inclined to buy globally based on price, rather than the embodied carbon within the production or transport of the materials. 'We need to drive the local economy, local businesses, use local suppliers to address the carbon impact of transportation.' (Interviewee 4) Again, company structure will play a role in whether a client organization is in a position to think locally rather than globally and be prepared to accept initial higher costs for a material grown locally to the construction site, with a view to subsequently benefiting from improvements in local supply chains and a regenerated, thriving local economy.

Risk and contractual methods
Contractual issues include methods of tendering and procurement, costs which may result in circular or bio-based elements being reduced within projects, and the important question of risk -which party within a construction project shoulders the burden, and how much risk they are willing to accept?
Cost inevitably dictates methods of procurement. The experience of interviewees suggested that in order to reduce risk, construction companies may seek to value-engineer environmental and sustainability elements out of a project. 'We were fearful that the sustainability aspects would be value-engineered out of the project. So we kept it as a traditional Joint Contracts Tribunal (JCT) contract, where we had control.' (Interviewee 1) Careful management of, and liaison with, contractors by clients can promote the development of a close relationship focused on productivity, sustainability, and a rapport of trust. Interviewee 4 elaborated that a singlepoint delivery team method and pre-qualification questionnaire were used for a bio-based academic building as it was deemed challenging, if not impossible, to realize the non-conventional low-carbon building using a traditional procurement method. Contractors may be unsure of, or lack confidence in, bio-based materials which can be considered to be emerging and subsequently cost risk in every aspect, putting circular and bio-based outputs at risk of not being achieved as designed. Interviewee 5 stated that a tender for a large, public sector industrial-scale storage building using emerging bio-based materials attracted no bids, whereas multiple bids were received for a revised tender using conventional materials. Commercially available risktracking software can be used to help manage projects and regularly monitor progress to ensure original sustainability ambitions are maintained.

Bio-based material development
A major issue for emerging bio-based materials is upscaling and whether materials can be supplied in the quantities required for construction on an industrial scale, and effectively marketed to increase awareness of the long-term positive aspects of specifying bio-based materials. It was identified that there is a problem in turning world-class funded research with an emerging material into a commercially available product, even though considerable finance has been invested in research and development. Promoting a product and increasing awareness is extremely challenging if it is not available in large quantities. Interviewee responses indicated that the construction industry is conservative, risk-averse, and typically uncomfortable with using emerging bio-based materials. Client organizations and contractors can be concerned whether bio-based materials would indeed be available for a proposed construction project in the required quantity.
An important step towards upscaling an emerging product is gaining certification and accreditation, plus dealing with the potentially large costs involved. Products based on emerging bio-based materials currently sitting outside of accreditation will not be specified due to concerns with material properties, especially conformity to fire resistance regulations. Key to addressing this issue is gaining data from research and experimentation such as mechanical properties, fire, acoustic properties, and durability of the new materials in line with existing standards, allowing as close a comparison to conventional materials as can be made with emerging bio-based materials. Certification based on data is crucial for the confidence of clients, contractors, and insurers to work with bio-based materials and finance projects.
'Too many products are sitting outside accreditation. A contractor won't use them because of the risk.' (Interviewee 4) Previous bio-based material studies have highlighted the importance of product certification for the development of a bio-based economy (Ladu & Blind, 2017), (Morone et al., 2021). It was confirmed by interviewees that there are potentially prohibitive costs associated with gaining accreditation for newly developed materials and certification schemes can be viewed as excessively complex by material developers. Interviewee 2 explained that it is challenging to test an emerging bio-based material using a standard developed by a large multi-national company for conventional construction material. The creation of a standard for an emerging material can cost £200,000; the use of a suitable existing standard which may be applied to a new material can reduce the cost of accreditation by an order of magnitude.
The WELL buildings and the Living Building Challenge (LBC) standards have both raised awareness about the health, well-being, and productivity impact of buildings. However, it can be challenging for new products to meet these standards. 'The LBC standard is difficult to achieve with currently available materials, so it's limited to high-end projects. The WELL buildings standard had thirty-seven prerequisites that made it prohibitive to implement. We need a gentler approach for implementing standards.' (Interviewee 2) To help with the considerable amount of finance it requires to develop, test, and certify an emerging product, a possible option here is also the development of sustainable construction loans or grants with involvement from a bioeconomy initiative or decision-making authorities. This would assist innovators and material developers with the finance to test, certify and market their products along with obtaining permits for waste streams.

Vested interests
There was general agreement among interviewees that the construction industry can be described as conservative and risk averse. Large, powerful companies working in conventional materials may have financial motivations to maintain the status quo and not welcome emerging bio-based technologies developed by new, and typically smaller, companies. Emerging material developers can obtain finance for the front end of Technology Readiness Levels (TRL), but when they reach TRL level six, progress can become challenging because of the vested interests of larger established companies. It is possible for larger companies to purchase emerging ideas, materials, and technologies, but subsequently cease development and effectively shelve them.
'Plastics, concrete and steel industries all have big lobbying groups. Bio-based industries do not; they are disparate and small.' (Interviewee 3) It was further identified that money should be spent on having a trade association for small companies, assisting new innovators to upscale their products. An option for emerging material developers desiring to upscale is rather than attempting to compete with existing large companies, instead engaging with and collaborating with large companies which exert an influence on the construction industry with standards and regulations. 'We're constantly in conversation with large multinationals; you have to collaborate instead of trying to compete.' (Interviewee 2) Engagement from an early stage is undertaken with a view to sharing intellectual property, collaborating on accreditation and marketing strategy, and ultimately share in long-term financial rewards. Such an approach identified in the interviews has compatibility with the concept of Project Network Organisations, which have been identified as relevant to the construction industry (Manning, 2017) with the scope for potentially repeated inter-disciplinary cooperation between multiple organizations. Table 2 summarizes the key finance-based challenges and the opportunities identified by responses and discussion to further the circular bio-based construction agenda.

Material properties and performance
It should be stressed that with the clear exception of natural timber, which has been used for hundreds of years in construction, the use of bio-based materials in the construction industry is a developing and evolving field. Gathering data from experimentation carried out on newly developed materials, and knowledge of where the constituents within the material have globally Table 2. Summary of key finance-based challenges and opportunities for circular bio-based construction. Where interviewee(s) responses informed a bullet point, the interviewee number is indicated.

Finance: Challenges
Finance: Opportunities . High initial costs of emerging materials (1-6) . Higher design costs of materials (5) . Company structure: potential opposition amongst multiple shareholders or board members within a client organization on grounds of initial costs (1,6) . Temptation to buy globally rather than locally on grounds of cost (4) . Potential of circular bio-based elements being value-engineered out of projects on grounds of cost (1,4) . Reluctance to shoulder the financial burden of risk (4) . Establishing a contract and working arrangement all stakeholders on a project are prepared to accept (1,4) . High costs involved in upscaling a product to be available in constructionscale quantities (2,3) . Costs of gaining certification and accreditation for emerging products (2,3,4) . Costs of developing a new standard if an existing standard cannot be used (2) . Clients being able to continue to provide maximum service levels to customers (6) . Established companies not welcoming financial competition from emerging materials (2,3) . Long-term savings to be made on building operational and energy costs due to environment-regulating properties of materials (1,5) . Publicity and dissemination can lead to networking opportunities for clients and the formation of new commercial relationships . Standards such as the wellness agenda and equivalent promoting occupant health and well-being (2,3) . Increased occupant wellnesscustomers prolonging being in the building and potentially higher productivity and lower absenteeism amongst staff (1,4) . Development of low-interest or interest-free loans from authorities for circular, bio-based construction use and development . Regeneration of local and regional economieslabour, trades, skills, crafts, farmers (3,4) . Opportunities for smaller companies with emerging materials to partner with larger companies for development and accreditation, ultimately sharing in financial rewards gained (2) originated will aid the growth of information databases and assist in achieving certification and accreditation. In addition to sustainability and environmental considerations, bio-based materials chosen for realized projects were there to perform a function; the regulation of the internal environment, reducing fluctuations in temperature and Relative Humidity (RH), and maintaining conditions which do not spoil stored goods or valuable artefacts. Without exception, interviewees were happy with the performance of materials and from a building physics perspective, the use of biobased materials can be judged to be a success. Materials, in conjunction with glazing design, have helped to maintain desired temperature levels, leading to clients not having mechanical winter heating and summer cooling costs typically associated with an industrial building realized with conventional materials. 'We wanted a hygroscopic material controlling RH for stored objects and materials that were susceptible to changes; we wanted to reduce extreme fluctuations.' (Interviewee 5) 'It absolutely is performing as we hoped it would perform. Heating in winter and cooling in summer, we have none of those expenses and we've never had a problem with the building.' (Interviewee 1) An opportunity for the uptake and promotion of biobased materials is the concept of wellness and materials creating a natural and appealing environment in which to work and live. Circular buildings using bio-based materials have positive characteristics around air freshness and quality. In addition to energy savings, materials create a fresh, aesthetic, and aromatic environment conducive to occupant wellness. Clients can seek WELL and BREAMM certifications to demonstrate the wellness and sustainability credentials of the organization. Additionally, a circular bio-based building can install a sense of pride in occupants leading to positive changes in behaviour; it was observed by interviewees that company personnel extended sustainability and recycling practices from their professional lives into their personal lives.
'You're creating a better workplace and people are more productive according to wellness.' (Interviewee 4) 'It was a landmark project and it gave members of staff a sense of pride.' (Interviewee 1) Dissemination of case studies, data, academic research interests, bio-based product certification, and courses such as the PassivHaus standard can be promoted and marketed to potential client organizations in addition to professional construction practices, with dialogue between client organizations and academic institutions, particularly useful. It was identified by interviewees that an absence of knowledge and product certification can potentially lead to a lack of confidence and ultimately, opposition within client organizations and industry professionals to using emerging bio-based materials. The importance of certification to upscaling the bio-based material economy was highlighted in an EUwide study along with issues regarding assessment frameworks as a tool towards achieving certification (Majer et al., 2018).

Compatibility with circular design principles
It is important to consider bio-based materials in all applications in compatibility with circular economy principles to mitigate bio-based materials going to landfill and contributing to greenhouse gases (Gilbert & Siebert, 2022). Within a construction industry context, the ability to reuse materials in an adapted configuration or a new design life is desirable to reduce landfill waste. The bio-based projects of the interviewees considered circularity, with a view to future-proofing buildings and engineering in the capability to expand, modify, and reuse bio-based elements and even potentially relocate buildings by disassembling and reassembling.
Examples of circular design in the interviewees' biobased projects include: . Modular design with hempcrete panels suitable for disassembly and reassembly along with mechanical and electrical services not being integral to the building, enabling the whole building to be moved to a new location if required in the future (Interviewee 5) . Space on a plot allocated at one end of a building to allow for future horizontal expansion utilizing the same technologies, solutions, and bio-based materials such as hemp blocks, sedum roof, and glulam beams as the existing building (Interviewee 1) Interviewee 4 raised a conflicting issue with Passiv-Haus certification and using newly developed biobased materials with a circular, layered construction. It may be desirable to change an element in an existing building over the course of a design life as new biobased materials and technologies are developed. However, a PassivHaus building has to maintain airtightness, which is not naturally compatible with the potentially repeated replacing of existing panels or elements with new bio-based materials and technologies as they develop and become available in the future.
'Because of PassivHaus, it has to maintain integrityyou can't cut around the structure all the time.' (Interviewee 4)

Life cycle assessment
It is challenging to perform life cycle assessments (LCA) on bio-based technologies; products may be composite, combine materials from different sources, and may incorporate non-organic elements.
'LCA at the moment is based on databases and if the materials or approaches you use have not been logged, it is difficult to carry out an accurate LCA.' (Interviewee 2).
There have been issues with the LCA of bio-based materials resulting from the geographical origin of the material, with large quantities of carbon embodied in transportation.
'Quantity surveyors will buy globally, wherever cheapest and will have no regard for carbon; it's based on, and driven by, price.' (Interviewee 4) Interviewee 2 raised the issue of unsustainable land practices concerned with growing non-food crops on an industrial scale such as the clearance of natural land, deforestation, use of chemical pesticides, and intensive mono-crop farming, which increases the risk of crop diseases. Interviewee 3 raised a prime example of bad practice being deforestation in South America to grow crops for biofuels, which resulted in very poor LCA results. Therefore, growing crops locally for the construction site and reducing a culture of buying globally may be beneficial for bio-based material LCA.
Interviewees identified issues with how to deal with end-of-life scenarios of material, an issue also confirmed by other studies identifying gaps in knowledge and uncertainty regarding end-of-life scenarios (Hawkins et al., 2021), (Kayaçetin et al., 2023). A bio-based material can sequester carbon, but carbon can be released over time as the material degrades and the remaining carbon can be released at the end of life if the material is not reused or repurposed. To address end-of-life scenarios and carbon sequestration, studies have discussed that a more dynamic approach to LCA for bio-based materials needs to be developed (Peñaloza et al., 2016), (Pittau et al., 2018). It was broadly agreed by interviewees that the reuse of elements at the end of first design life was preferable. 'If you're able to reuse material at end of life, or re-purpose it into another use or application, that is always preferred to landfill.' (Interviewee 2)

Research and case studies
Acquiring existing knowledge on emerging bio-based technologies for construction industry professionals can be difficulty. There are several methods by which information can be effectively disseminated. Case studies of bio-based material use and circular construction are deemed to be very effective and it was widely agreed that there are currently insufficient case studies in existence to demonstrate the value of circular, bio-based construction in terms of both the extent of materials available to use, along with how they work and the benefits provided. Academic institutions, official bodies, such as national building research establishments, and organizations, such as the Ellen MacArthur foundation, were all identified by interviewees as having a role to play in disseminating knowledge arising from conducted research and accreditation. Online courses were considered to have some value, but inherently not as effective as having case studies.
'There's nothing more effective than a real life case study or taking part in a physical visit or workshop. It's important that case studies aren't limited to highend projects.' (Interviewee 2) Databases of information on bio-based materials have been established, for example, the bio-based directory of the Nova Institute in Germany (Carus et al., 2015) and the EU project Bio-voices (Wageningen Research, 2018). However, they require finance to be promoted and kept up to date. It was proposed that the merging of disparate databases and information sources into an accessible linked or body of knowledge which can be effectively promoted and kept current would be useful.
'Finance should be spent on keeping directories current; we should be getting old databases together and keep them up to date.' (Interviewee 3) It is submitted by the authors personnel data from case studies could also be gathered and assessed (subject to clients and ethics approvals); if it is found that biobased buildings are conducive to high levels of staff performance and productivity, or customers are spending a greater period of time within a certain client building, this information could be collated and disseminated along with material performance and building physics aspects.

Professional knowledge and reluctance
Knowledge of bio-based materials and their accredited properties by professionals involved in construction projects is crucial, but the question of who should be knowledgeableclient or construction industry professionalsand thus influence the bio-based circular agenda in design, was a subject of debate. 'We need informed clients who know what it is they're asking for.' (Interviewee 4) 'This drive has to come from architects, structural engineersrather than the vision from the companies.' (Interviewee 1) 'I think it is the responsibility of the professionals. Assuming clients are aware is a dangerous gamethere aren't enough case studies.' (Interviewee 2) 'I think it's a bit of both.' (Interviewee 6) It is posited by the authors that senior figures within client organizations being aware of the properties and potential long-term benefits of bio-based materials and circular construction can only be positive, even more so when a sustainability culture is embedded in personnel throughout a client organization. One approach is professionals tendering for a construction project as a pre-aligned integrated team (architect/ engineer/contractor), sharing knowledge and pooling resources to discuss circular, bio-based options with the client organization. This can be challenging for professionals to achieve though, as it requires time, finance, and resource prior to submitting a bid for a project.
High-profile circular bio-based case studies have not led to an acceleration in bio-based construction. Interviewees broadly agree that rather than progress being made, the bio-based agenda has, if anything, slipped slightly backwards over the course of the past five years in the UK. Interviewee 5 found it impossible to find a contractor willing to work with bio-based materials on a large industrial building, with existing data being insufficient to convince a contractor to take on the project. Interviewee 4 explained that there can be a problem with technology translation; world-class research can be conducted, but results are not implemented or translated into industrial practice.
Dissemination of data and certifications to construction industry professionals will aid the promotion of the products' use and reassure clients of properties and performance. Certifications such as PassivHaus may need refinement to be entirely compatible with circularity in construction. Refresher courses or follow-on modules in periodic timescales would facilitate certified professionals being informed of continuing developments in emerging materials as the knowledge base advances. Frameworks developed with circular construction in mind such as 'ReSOLVE' (Regenerate, Share, Optimize, Loop, Virtualise, and Exchange), developed by Arup and the Ellen MacArthur Foundation are further tools to assist professionals.

Leadership and opposition within client organizations
Issues can be faced within client companies, with personnel promoting the use of circular bio-based construction potentially conflicting with reluctant colleagues who are perhaps unaware of the benefits of using bio-based materials or are primarily concerned with initial costs. The term 'champion' was used by multiple intervieweesthe idea that a circular bio-based project requires a champion, an advocate at a very senior level within a client organization to inform and convince unsure colleagues in the boardroom of the long-term benefits of choosing circular bio-based construction.
The structure of a client organization will influence the time scales over which it may plan. A company with majority shareholders and a stable board who are prepared to think, and can plan, over generational time scales will be more inclined and able to realize a circular bio-based project than a company with multiple shareholders who are looking for a return on investments on shorter time scales; indeed, senior management personnel may turn over at a relatively quick rate within client organizations and a decision-making potential project champion in a senior position may not be in that position long enough in order to realize and drive a circular bio-based project through to completion. Even with incentives to plan long-term, client organizations still require leadership from senior personnel who are informed and enthused about using bio-based materials and designing innovative structures which are designed for adaptability and can be modified to reflect the future changing requirements of the business. A project still has to be ultimately financially beneficial for a client organization to function as a business.
'Every project needs a champion in the boardroom, who has credibility and can argue the long term case.' (Interviewee 1) 'There was quite a lot of opposition, we must have been very persuasive.' (Interviewee 5) 'You need to be an ambitious leaderand never take 'no' for an answer; otherwise, intellectual value you put in will be value-engineered out on the grounds of cost.' (Interviewee 4). Table 3 summarizes the knowledge-based challenges and opportunities identified by responses and discussion to progress circular bio-based construction.

Policy challenges
The construction industry and bio-based material use within it have not necessarily been at the top of governmental agendas in looking at societal needs and the issues of energy and climate change. Interviewee 3 revealed that energy industries such as nuclear, solar, and wind, have been primary priority areas for legislation, while biomass and bioeconomy were secondary. Aeronautic, automobile, and chemical industries have taken priority over the construction industry.
Major topical issues of the day can form an obstacle or promote the restructuring of governmental policyand clientpriorities. Financial crises, pandemics, politics, and international relations were identified by interviewees as factors that can all affect decision-making policies and governmental agendas.
Policy can affect the growing of crops which may be used for construction purposes and ultimately whether material products can be supplied on an industrial scale. With high embodied carbon associated with the transportation of crops sourced internationally, a clear way forward is to encourage the growth of more crops for construction use within the country where a construction project is taking place. 'We are going to have to grow more crops in the UK to be biobased; it doesn't make sense to bring it from somewhere else.' (Interviewee 3) Where currently available land makes this challenging, develop the use of food crop by-products for construction use and examine ways forward to grow enough crops locally to construction by evaluating existing legislation and brownfield regeneration possibilities. The ease of growing for construction use also varies from product to productfor example, in Europe, it has been found that land availability is sufficient for timber and straw (which is a by-product) but it is more challenging for hemp (Göswein et al., 2021). Subsidies to increase growth would be required to stimulate supply and create regional regeneration, boosting regional economies and local employment, businesses, and supply chains. The authors submit that in order for countries to become more self-sufficient in growing crops that can be used for construction materials (either wholly or as a by-product of food-related growth), landowners and farmers can be financially encouraged and rewarded for donating or setting aside unused land for the use of such crop growth; state financial intervention may be required for this and would vary globallyfor example, a Chinese study concluded that generating extra income from non-food crop growth is challenging for farmers (Yang et al., 2022). Appropriate regulation to protect the virgin forest and prevent natural land use changes that would cause harm to the environment. Another potential option is to further explore the possibility of using land classed as 'contaminated', and therefore unsuitable crops destined to enter into food chains, for the growth of non-food crops for various industrial sectors in conjunction with developing land remediation methods (Khan, 2020), thereby creating jobs and boosting local economies (Papazoglou, 2020).

Government legislation
Governmental and regional policies can be identified as being key to the future growth and upscaling of biobased construction, along with the need to provide guidance for companies and individuals to create a Table 3. Summary of key finance-based challenges and opportunities for circular bio-based construction. Where interviewee(s) responses informed a bullet point, the interviewee number is indicated.

Knowledge: Challenges
Knowledge: Opportunities . Many bio-based materials are emerging products and knowledge may not be readily available or widely known (2,3,4) . Construction industry professionals and clients are reluctant to work with emerging materials with which there are low levels of knowledge (2,3,4,5) . Circular construction can conflict with existing certifications, for example PassivHaus which requires airtightness (4) . LCA for bio-based materials is challenging with regards to how to assess the end of life scenario and the gradual emission of carbon over one or multiple design lives (2,3) . LCA for bio-based materials can result in a poor score if unsustainable land practices such as the clearing of natural land or mono-crop farming are practiced to profit from crops intended for construction (2,3) . Crops for construction purposes being grown geographically far away from construction sites can further impact on LCA with transport considerations (2,3,4) . Case studies are very important and there are currently not enough (2,4) . It is a source of disagreement as to which stakeholders should be knowledgeable; construction industry professionals, clients, or a mix of both resulting in an exchange of ideas . A project requires a 'champion', senior within a client company, to counter internal opposition and realize a circular bio-based project (1,6) . Existing databases and information sources to be linked/merged and maintained/updated with new developments (3) . Disseminate that materials have performed successfully in terms of regulating internal environmentsmaintaining temperatures and reducing fluctuations in RH (3,5) . Clients can demonstrate and publicize commitment to sustainable/ environmental values and wellness among staff/customers/building occupants (1,4,6) . Staff and customers from client companies can translate sustainable behaviours and practice into their personal lives (1,6) . Knowledge of modular and circular design can promote designing for disassembly and/or adaptability, extending the service life of a building as use requirements evolve . Smaller scale case studies, as well as industrial scale, can demonstrate and educate that circular bio-based construction is possible on domestic as well as industrial scales (2,3) . Construction industry professionals to be encouraged to become certified on sustainable design (for example PassivHaus and refresher courses) reflecting new developments can be attended to update knowledge (2,4) . Project champions and sustainability champion employees within a client can promote a sustainable culture within an organization which can inform construction decisions (6) pathway towards sustainability. Legislation at the national level to guide and direct change may be punitive; banning, taxing or penalizing, or positive; incentives and subsidies. All interviewees were of the opinion that government legislation was needed to drive a bio-based, circular agenda in the construction industry.
'I don't know if we need a revolution, but we do need legislationbanning certain things.' (Interviewee 3) 'It has to be legislated; commercial builders are never going to move to bio-based materials until they are made to.' (Interviewee 5) 'We need quite strict legislation to drive this through the supply chain; otherwise, it won't happen.' (Interviewee 4) There are pros and cons attached to both punitive and positive legislation approaches. Banning products have been effective at changing behaviour and practice in other industries, such as pesticides and incinerators to minimize noxious emissions escaping into the atmosphere, along with lead petrol for the automobile industry. Interviewee 2 submitted that typically about 15% of resources or materials delivered to construction sites in the United Kingdom goes to landfill; this cannot continue to exist and it is certainly not circular. But punitive legislation was not favoured by interviewees in isolation. The authors posit that while it may be intuitive to surmise that a considered combination of punitive and positive measures would be a correct approach, it must be noted that increases in circular bio-based construction would be a gradual evolution over long timescales. Biobased materials are certainly not ready to entirely replace conventional materials and materials such as concrete and steel will realistically continue to be required to meet the construction needs of society. Therefore, any punitive measures for an industry already in a difficult position as a result of austerity and a pandemic during the previous decade would be divisive and may not be conducive to gaining wider support.
Positive legislation such as grants was unanimously viewed as a suitable approach to increase the market share for circular bio-based construction.
'I'm always in favour of positive legislation. Try to level the playing fieldrather than put whole industries out of business.' (Interviewee 1).
However, there are risks associated with positive legislation. Interviewee 3 emphasized that standards and governmental enforcement are required to prevent financial incentives being taken advantage of, with unscrupulous and unsustainable land practices or by tradespeople with inadequate skills and knowledge, in order to financially benefit from crops for construction material purposes.
Based upon comments from interviewees 2 and 3, the authors submit that positive legislation can be used to help create trade associations for small disparate biobased companies to promote upscaling and stimulate demand. Large established companies known for dealing with conventional materials and smaller companies developing bio-based materials can be encouraged to engage with each other with joint partnerships and mergers to share and mutually benefit from the evolution and development of bio-based material in construction, rather than allow a culture of separatism, threat, and competition to exist.
A dynamic and holistic approach to the legislation was proposed.
'Building regulations and standards are very static; to change one bit of a standard can take up to five years. A more dynamic or semi-permanent approach is required.' (Interviewee 2) A semi-permanent approach to change can be used; for example, a change can be implemented quickly for a trial period and if the outcome is positive, it can be rolled out over the long term.
An opportunity identified for sustainably sourced bio-based materials in construction is the potential for building design to be sympathetic to the surrounding natural environment, especially in greenbelt planning and areas of natural beauty. A bio-based exterior and a circular, sustainable building can be an incentive for decision-makers to grant planning permission for a building that may otherwise be rejected if conventional structural and façade materials are specified.
'To get planning permission in an area of outstanding natural beauty we would need to do something special consistent with our stated values.' (Interviewee 1) The authors further submit that green planning policies can be implemented to promote bio-based circular construction, with local planning authorities being encouraged to look favourably upon planning applications that are designed for adaptable, modular construction, future expansion, and aesthetically blend into the environment with the use of bio-based materials as cladding (including green walls and green roofs). This may be a solution for greenbelt development with both industrial and residential buildings, ranging from high-profile projects to social housing. If one punitive measure may be considered, it is that of consistently placing conditions upon the granting of planning applications; in order for permission to be granted on a project, the application must demonstrate a degree of circularity in the design, with modular features capable of future adaptability or expansion and satisfy a certain level or quota of bio-based material use. Legislation may be introduced on a trial basis and kept under continuous review while performance and levels of positive effect are monitored. An approach of promoting bio-based construction would be compatible with wider green development planning policies. Examples of developing initiatives to promote green planning policies for sustainable development include the European Union's Green Infrastructure project (Slätmo et al., 2019) and the associated Green Surge project (Pauleit et al., 2019) plus bluegreen urban design and infrastructure projects (Suleiman, 2021), (Puchol-Salort et al., 2021). Table 4 summarizes policy-based challenges and opportunities identified by responses and discussion to further the agenda for circular bio-based construction.

Study limitations
It should be reiterated that the approach of the study towards conducting interviews with senior professionals with direct experience in realizing circular bio-based construction projects results in a relatively small sample and is not representative of a larger sample of construction industry professionals. However, it was desired to have a level of expertize and experience which naturally is selected in what is an emerging, perhaps niche, corner of the wider construction sector. Views have been represented by the varied stakeholder experiences of what may be considered elite interviewees (Marshall & Rossman, 2014). The results obtained can be considered as effective and insightful and efficiently gathered from the expert interviewees (Bogner et al., 2009). A future study concerning perceptions of circular bio-based construction targeting a larger sample of construction industry professionals who are aware of circular construction with bio-based materials but do not have direct experience with it at a senior level would form a viable continuation of research in this emerging field.

Conclusions and recommendations
Multiple perspectives and extensive insights were provided by the interviewees, facilitated by the chosen selective, semi-structured interview methodology. It is clear from the interviews that bio-based materials face significant difficulties in breaking through into mainstream construction practice. The contribution of this study is the identification of opportunities and potential ways to progress the bio-based agenda, in line with circular economy principals, and upscale production within the construction industry aimed at informing construction industry professionals, assisting policymakers, and enthusing clients.
Potential solutions and recommended courses of action for policymakers and professionals can be found in promoting an economical case to client organizations with the regulation of indoor environments and associated low running costs and energy savings, facilitating long-term payback of initial higher financial investment; regenerating and boosting local economies with farms locally growing crops which can be partly or wholly used for construction material purposes, and local businesses, skills and labour to transform crops into products; promotion of the wellness agenda providing clients with high productivity and low absenteeism in their staff along with enhancing the experience Table 4. Summary of key finance-based challenges and opportunities for circular bio-based construction. Where interviewee(s) responses informed a bullet point, the interviewee number is indicated.

Policy: Challenges
Policy: Opportunities . Policy in other industries suggests legislation at national and regional levels is required to promote circular bio-based construction (1-6) . The construction industry has not been at the top of government agendas, with energy (nuclear, solar, wind) and alternative sectors such as automobile taking priority (3) . Challenging national and international circumstances and crises such as economic downturn, pandemic, and geopolitical events can impact policy and the capability to prioritize circularity and sustainability in decisionmaking (1,3,6) . Construction is a global industry and policy is required to drive a change towards localizing construction (3,4) . Punitive legislation has been implemented in other industries (automobile, chemical)would punitive measures be appropriate for the construction industry? (2,3,5) . Positive legislation to promote circular bio-based construction requires care and regulation to prevent profiting from subsidies by unqualified and unscrupulous parties (3) . Greenbelt planning applications can be an opportunity for circular biobased construction, with bio-based materials sympathetic to a natural environment and landscape (1) . Subsidies to increase crop growth and associated trades, skills, and crafts would promote supply and create regional regeneration, boosting regional economies, local employment and business, with crops grown, and by-product materials made, locally to construction projects (1,3,4) . Positive legislation to price in new technologies and emerging products would help diversify construction, with the aim of traditional (concrete, steel) and emerging bio-based materials sharing the market, removing reliance on one or a few solutions (1,2) . A dynamic approach to legislation would help to trial policies for a time period to evaluate worth and impact (2) . Further research into the use of 'contaminated' land unsuitable for food crops of customers; emphasizing sustainable and aesthetic qualities of bio-based materials as an asset in greenbelt planning applications thus encouraging successful decisions; bio-based practitioners seeking to upscale a product engaging and partnering with larger companies to develop and ultimately mutually benefit from increased market representation. Policy both at national and regional levels can be viewed as crucial to drive the growth of bio-based circular construction; though caution should be exercised, positive legislation can encourage circular bio-based construction. Bio-based materials, partnered with circular economy principals have the potential to complement conventional materials in the construction industry. In order to ultimately be considered conventional materials in construction, bio-based material case studies of successfully realized projects would help to overcome challenges and stakeholder reservations and demonstrate the feasibility and potential benefits of bio-based materials. The ultimate aim of upscaling circular bio-based construction is for the industry to reduce carbon emissions and achieve a lower environmental impact. While there remain technical issues, these should not be considered in isolation but in a broader context and a holistic view of financial, knowledge-based, and policy drivers.