Towards Zero-Carbon Buildings: Challenges and Opportunities from Reversing the Material Pyramid

Abstract

The decarbonization of the built environment, both in new construction and renovation, is crucial to mitigate its relevant impact on climate change and achieve the Paris Agreement goals. This study presents a systematic LCA-based methodology to assess the whole-life carbon emissions of buildings, applied to a proposal for the regeneration of one of Milan, Italy’s, disused railway yards. As an entry for the 2020 Reinventing Cities competition, Scalo Lambrate is a project for a mainly residential neighborhood with a public park. Strategies to reduce carbon emissions deriving both from the operational energy and construction and maintenance were evaluated and their effects compared to a reference scenario over a time horizon of 100 years. The results show that, while the opportunities to reduce carbon emissions during the use phase are somehow limited due to the already stringent performance requirements for new builds, the use of fast-growing biogenic materials for construction materials, even if mixed with more traditional ones, can provide a significant reduction in the global warming potential over the whole life cycle, with a reduction of 70% compared to the baseline. The remaining emissions can be offset with afforestation initiatives, which, however, must be assessed against land use issues.

1. Introduction

1.1. Background

In 2022, the building and construction sector accounted for 34% of the global energy demand and 37% of the energy and process-related carbon emissions; despite a reduction in energy intensity, both those fractions increased by around 1% compared with 2021 because of the sheer increase in the number of buildings worldwide [1]. As a significant part of the problem, the sector is crucial to achieving the decarbonization goals set by several countries and entities for the middle of this century. The European Union pledged to become climate-neutral by 2050 in its Green Deal [2], writing into law the target and the intermediate steps in its European Climate Law [3]. While solutions to achieve both carbon-neutral new buildings and a decarbonized building stock are already available [4], the gap between the current state of emissions and the desired decarbonization path is significant and additional efforts are required to remain on track to meet the 2030 interim milestone and the 2050 goal [1].

In this trajectory towards decarbonization, it is necessary to acknowledge that climate change is due to past emissions generated by developed countries, and that, at the same time, huge quantities of new building materials will be needed in the medium term to compensate for the housing deficit in the Global South, as well as for the construction of new infrastructure, much of which consists of carbon-intensive materials such as cement, steel and aluminum [5]. It is therefore fair to consider that developed countries should not have a carbon budget to spend from now to 2050, and that they should rapidly transition to decarbonized practices, together with additional investments in developing and promoting carbon capture and storage (CCS) technologies. Within this framework, new construction in Global North countries should be designed according to zero carbon standards [6].

Radical and quick actions are required to correct the current trend, acting on the causes of emissions from the construction sector. Whereas in the past emissions from the use, heating and/or cooling of buildings (so-called operational emissions) were dominant in the life cycle, their contribution has progressively become less relevant today, at least in those countries where increasingly stringent energy-saving requirements have been introduced [7]. In contrast, material-related specific emissions, measured in kgCO_2-eq/kg or kgCO₂-eq/m³, have not seen significant reductions over the years, other than those indirectly related to the decarbonization of the energy grid, and they are responsible for the “carbon spike”, a peak of carbon emission which occurs at the early stage of the building process [8]. Reducing emissions due to the production of materials is, in fact, more complex than containing operational emissions, where it is often sufficient to increase the level of thermal insulation of the building envelope and generate thermal energy with energy-efficient systems, possibly powered by renewable energy [9]. At the same time, however, increasing the attention to the careful selection of materials opens opportunities to use buildings as carbon storage [10], as repositories of materials to be reused in the future, or even as catalysts to reactivate a neighborhoods’ social and economic networks [11].

Therefore, to achieve the goal of a decarbonized construction sector, one possible option is the change in the current “materials diet” [6]. The most reasonable option implies that new buildings should be carbon neutral at every stage of the life cycle, i.e., zero net emissions and zero impact on emissions from building materials. A shift towards bio-based materials is needed to reduce the carbon intensity of materials processing, store biogenic CO₂ in the building elements, and quickly offset the initial emission through carbon restoration in soils [12]. Bio-based materials are not only renewable in nature, but they naturally capture carbon dioxide from the atmosphere as they grow; the sequestered CO₂ is then stored as biogenic carbon during their use as a building product, with positive effects on the Earth’s climate. This effect is particularly pronounced if the biogenic material regenerates rapidly in the soil, e.g., through the regrowth of forests or plantations. The delayed emission of carbon previously stored in fast-growing biogenic materials—typically grass, such as straw, hemp, cane, bamboo, etc.—is completely captured by the regrowth of crops within a few years, which makes these materials more effective in mitigating climate change than wood products, which instead require long regrowth periods. Biogenic materials can be used as innovative components in different building elements; their availability and type are related to the specific geographical context. Thanks to their properties, bio-based materials must be part of the composition of carbon-neutral buildings, in a balanced quantity, in order to compensate for the positive emissions from high CO₂-emitting materials, e.g., cement and concrete, which are required in any case for the foundations and basements of buildings [13].

As one of the global initiatives to control the emissions from the construction sector, the C40 global network of ninety-six among the world’s leading cities is committed to sharing best practices and implementing collaborative actions to cut their fair share of emissions in half by 2030 [14]. The Reinventing Cities competition was established in 2017 by the C40 group to stimulate sustainable development and promote replicable models of low- or zero-carbon urban regeneration projects [15], inviting professional, multi-disciplinary teams to submit proposals for underutilized sites in several cities worldwide [16,17]. The 2020 edition of the global competition included, among the other sites, several brownfield areas in Milan, Italy, which the municipality intended to regenerate as part of its city development plan by 2030. The goal of a low- or zero-carbon development, with a significant limitation of the onsite emissions, appears particularly important to a city which, for a combination of reasons, has one of the worst air quality indexes in Europe. This paper presents one of the proposals submitted for the site of the former Lambrate railway yard, now being decommissioned by the public rail network company, where a systematic methodology was applied to assess the impact of construction material selection on the achievement of the life-cycle zero carbon target, comparing the design scenarios to the current construction standards and to existing carbon benchmarks.

1.2. Literature Review

While the need for the decarbonization of the building stock is evident, so far regulations have mostly focused on the limitation of operational energy; for example, the previous European Directive on the Energy Performance of Buildings (EPBD, 2010/31/UE) [18] introduced the concept of Nearly Zero-Energy Buildings, but did not include any provision about the related carbon emissions, nor did it consider those deriving from other phases of the buildings’ life cycle, such as construction and demolition. The current version of the EPBD (2024/1275) [19,20], which has been recently approved, introduces, among other innovations, a new zero-emission standard for new buildings, requiring that all new residential and non-residential buildings have zero on-site emissions from fossil fuels, as of 1st January 2028 for publicly-owned buildings and as of 1st January 2030 for all other new buildings [21].

The mandatory part of the new version of the EPBD, however, still focuses exclusively on operational energy, while the whole life-cycle emissions will have to be calculated only for information purposes; limit values on the total cumulative life-cycle GWP of all new buildings will not be introduced before 2030. A more comprehensive approach to limit carbon emissions from the other phases of a building’s life cycle are typically included in green building rating systems [22]; also, the European Commission’s framework for sustainable buildings, Level(s), includes greenhouse gas emissions along a building’s life cycle among its six macro-objectives [23]. Conceived as an EU-wide system for assessing and reporting on the sustainability of buildings, the Level(s) framework adopts the life-cycle global warming potential indicator (GWP, expressed in kgCO_2-eq) to provide information about the whole-life carbon output as the design progresses.

The definition of numerical benchmarks on the European scale that would support mandatory or voluntary targets about the whole-life carbon emissions of buildings has proven challenging, due to the large variability, among others, of climate (and therefore of operational energy needs), building type, and standard construction practices [24]. Some targets to limit carbon emissions from the non-operational phases of a building’s life cycle have been introduced in recent years in some European countries [25], but apply only in specific local contexts. To overcome, at least in part, these limitations, the “Carbon Heroes benchmark program” was launched a few years ago [26]. The aim of the program is to set uniform embodied carbon emission benchmarks for common building types, based on the anonymized, verified data from thousands of buildings calculated through a standardized life-cycle model. The results are reported in graphical forms, including a performance metric with seven bands (A to G, from best to worst) equally distributed, and with the mean of results falling within band D; this scale can be used to assess the performance of any project against the Carbon Heroes benchmarks. A recent report published by the European Commission provides whole-life carbon emission baselines for EU buildings through the modelling of building archetypes, as well as projections of emissions in 2050 according to three different scenarios [27]. The study confirms the large variability of results; however, it also shows that the current best practice of low-carbon construction is already below the possible 2050 average for upfront embodied carbon. This suggests that even today there is a substantial potential to improve construction practices using available materials and strategies, and that low-carbon construction is already technically possible.

While Life-Cycle Assessment (LCA) is now the standard methodology to estimate GWP values, among other environmental impacts, the assessment of biogenic carbon remains a point of contention [28,29]. Biogenic carbon is emitted to air as CO₂, CO or CH_4, as a result of the oxidation and/or reduction of biomass by means of its transformation or degradation (e.g., combustion, digestion, composting and landfilling). It can also be captured as CO₂ from the atmosphere through photosynthesis during biomass growth, a process commonly known as carbon sequestration [30]. The main reason why it is often not considered in LCA [31] has to do with the complex estimation of the time dependency of the cycles (emission uptake) and their consequence for GWP values within standard “static” LCA analyses. Moreover, information about the real service life of building components, and the treatments of materials at their end-of-life, are difficult to predict at the time of design.

On the other hand, several studies stressed the importance of taking biogenic carbon into account. Bio-based products contain roughly 50% carbon in dry mass, creating an opportunity to store carbon in buildings constructed with biogenic materials (e.g., timber, hemp, straw, etc.) thanks to the uptake of carbon dioxide by its replacement via photosynthesis during forest growth. In principle, this neutralizes the release of carbon from a biogenic product at the end of its life, leading the majority of LCA scholars and practitioners in the early 2000s to use the “carbon neutral approach” when calculating biogenic carbon [32]. The main criticism of this assumption is that it does not consider the timing of the carbon emissions and the influence of the rotation periods related to the biomass growth, i.e., when carbon is actually absorbed by the plants [33]. Timber products (e.g., wood that has been processed into beams or planks) have a longer rotation period due to slow forest-growth periods, so they cannot be considered as carbon neutral in a short time horizon. Conversely, fast-growing bio-based materials, such as straw and hemp, have a short rotation period and can provide an effective mitigation effect on carbon emissions by rapidly removing carbon from the atmosphere [34]. The dynamic approach proposed by Levasseur et al. [35] allows for taking into account the different timing of emissions and uptake, allowing for a transparent assessment of biogenic construction materials, especially when long-rotation species are used for products with an expected long service life [36].

1.3. Scope of the Work

This paper introduces an integrated assessment method to identify a decarbonization strategy for achieving life-cycle zero-carbon buildings, with a particular emphasis on the material selection process. The method is applied to the case study of the Scalo Lambrate district in Milan, Italy, developed as an entry for the 2020 edition of the C40 Reinventing Cities competition, where all carbon emissions, namely embodied, operational, maintenance and end-of-life, over the life cycle of the buildings, are assessed.

2. Materials and Methods

2.1. General Framework

The methodology adopted is based on a comparative carbon footprint assessment applied to the case study of Scalo Lambrate in Milan, presented in Section 2.2, which is aimed at identifying energy solutions and material combinations able to minimize both operational and embodied carbon over the life cycle of the building to achieve the absolute zero carbon target. As shown in Figure 1, after the definition of the main geometrical properties of the building (shape, volume, number of floors, window-to-wall ratio, etc.), and connectivity, two main scenarios are identified for the selection of the construction solutions: (i) Business As Usual (BAU), which considers conventional solutions for construction and energy systems according to the mandatory standard; and (ii) Zero-Carbon Building (ZCB), which is based on the identification of construction solutions and energy systems minimizing the embodied carbon from material processing and installation. The assessment of operational energy was based on dynamic energy models for both alternatives, providing the amount of primary energy required for heating, cooling, domestic hot water and all other electrical services during the use of the building, as well as the renewable energy produced onsite by photovoltaic (PV) panels installed on the roofs.

An expected ordinary service life of 50 years, as required by the Italian regulation for ordinary structures [37], was assumed for the BAU scenario, while for the ZCB alternative an extended service life of 100 years was assumed, with a refurbishment scenario including major renovation and material replacement at year 50. A linear regression method was applied to the carbon emission factor of the energy mix to estimate the sensitivity of future decarbonization strategies to energy generation, in line with similar approaches used by scholars to set a zero-carbon goal by 2050 [34].

On the materials side, the main maintenance activities were estimated according to the expected service life of the selected construction products reported in the EPD Italy dataset [38] and the carbon emissions from replacement and waste processing were assessed through the LCA methodology. An extended description about the assumptions considered in the model is reported in Section 2.5.3. All benefits from urban greening and carbon storage in bio-based materials, implemented in the ZCB design to reduce the carbon spike, were assessed and the dynamic method developed by Guest et al. [39] was adopted to include the contribution of biogenic carbon in the mitigation of the global warming potential (GWP). Finally, the residual CO_2-eq values were defined and compensation instruments for carbon offset identified, to reach the absolute-zero target.

2.2. Case Study: The “Scalo Lambrate” Project

2.2.1. Background

The Scalo Lambrate project in Milan was developed as an entry for the fourth edition of the urban design and redevelopment competition called “Reinventing Cities”, organized by the C40 group. The goal of these competitions is to stimulate cities to build “ambitious” carbon-neutral developments in large urban areas that are currently abandoned or underused, promoting the cooperation between the public and private sectors and stimulating the implementation of design solutions (architecture, materials, construction technologies, etc.) that can be replicated in other projects on a global scale.

In order to achieve these objectives, the Reinventing Cities calls for action, organized in several cities at the same time, explicitly encourage the formation of multidisciplinary teams from the outset, with the capacity to represent the many necessary competences being part of the evaluation. The competition rules also require the appointment of three leading figures, representing the most important roles in the process: the architectural/urban designer, the environmental expert for sustainability and decarbonization, and the financial/real estate promoter.

An essential aspect, common to all the Reinventing Cities calls for action, are the “10 Climate Challenges”, covering different aspects of sustainability, and each measured through specific Key Performance Indicators (KPIs). Among these challenges, those with the most significant impact on carbon emissions are those of the “Green buildings and energy efficiency” and “Clean construction and building life cycle”, and to a lesser extent, or indirectly, those of “High-quality architecture and urban design”, “Low-carbon mobility”, “Climate resilience and adaptation” and “Green space, urban nature and biodiversity”. While the project had to take into account all the challenges and their complex interactions, this article mostly focuses on the first two.

2.2.2. Main Characteristics of the Case Study

The Scalo Lambrate project proposed the urban regeneration of one of Milan’s former railway yards, used in the past for the handling and storage of goods in transit in the city. There are currently seven of these disused areas across the city, with a total surface of around 120 hectares, which, until recently, remained essentially inaccessible to people, and which are now undergoing a complex process of rezoning and redevelopment that started back in 2007. The Lambrate railway yard is a 7-hectare area on Milan’s East side, where the city council stipulated that both private and public housing, student housing and a park should be constructed. Besides the functional program, the design development also had to take into account several constraints related to the following: the distance from live train tracks, height limits due to the nearby airport, and a requirement that 50% of the area should be dedicated to public green space.

To develop the proposal, a multidisciplinary team covering different areas of expertise was assembled. The three main figures requested by the competition were the Benedetta Tagliabue-EMBT Architects studio, as the leader for architecture, the Politecnico di Milano (with Matteo Ruta, of the Department of Architecture, Built Environment and Construction Engineering, leading a team of twenty-five professors and researchers belonging to different departments) as the scientific expert for environmental aspects, and the Co-Inventing group as the developer. Several more experts, with a total of 170 consultants and stakeholders, took part in the design development, or were consulted during the process. To help validate decisions, building material suppliers and construction companies were also part of the group. Part of the team—in particular the architect and the environmental expert—were already familiar with the area, since they took part in a previous workshop to develop visions about the transformation of Milan’s disused railway yards.

The team worked from January 2020 to March 2021 through an integrated design effort based on weekly meetings, where all the contact persons for each discipline discussed the advancement of the project and validated decisions based on technical and scientific aspects. The resulting project has a scale and level of detail that was deemed appropriate for the competition stage, i.e., corresponding to an early-stage design. The functions and internal layouts of the buildings were defined according to the allowable floor areas and the local building laws, while the technical aspects about structures, construction technologies, and mechanical, electrical and plumbing systems were developed at a level of detail sufficient for the calculations required by the competition brief and for early parametric estimations of costs. The design of these systems was in the charge of the engineering firms, in accordance with the rest of the multidisciplinary team; their outputs were used to develop the calculations about carbon emissions presented in this article.

The plot, which is narrow and long, is characterized by a north–south orientation. The project developed from a focus on the public space, predominantly located between the high railway embankment and the new buildings, to create four new semi-circular public squares, with different functions which also gave them their names: “railway”, “entertainment”, “water” and “flowers”. The architecture of the new buildings and their layout aim to reconnect the fabric of the former industrial district of Lambrate, characterized by disused factories and a few dwellings, with the rest of the city, also by connecting new and existing greenery, opening up new paths and viewpoints.

Since the area is empty, the proposed buildings are all new (Figure 2); the only existing building on the site, a former warehouse building, was, however, preserved and integrated into the design in memory of the district’s industrial past, but also to conserve resources.

Overall, the case study includes four buildings with a crescent layout, with different functions. Starting from the north, a first building, integrated into the preserved building, is a hostel, which is particularly useful since the neighborhood has a strong presence of students due to its proximity to two important universities. The second building is a student dormitory, and has an internal courtyard organization, which mitigates the climate and takes up a typically Milanese typology. The third building, also with a courtyard, is social housing. Finally, the fourth and fifth buildings are other apartment buildings for the private market (Figure 3).

Table 1. Total floor areas of the buildings in the district.

	Hostel	Student Housing	Housing 1	Housing 2	Housing 3
Surface (m2)	3298	9537	11,129	4901	4370

shows the total floor area of each building.

All the buildings overlook green spaces and the new four squares or courtyards. The materials and colors used were chosen to integrate into both the neighborhood and the city of Milan. There was a desire to create diversified spaces, in terms of the size of the dwellings and also with different internal heights, to encourage the presence of different users, in terms of age and household composition (Figure 4).

2.3. Solutions for High Energy Performance

To achieve a very good energy performance during the use stage, the guiding principle in the design of Scalo Lambrate was to combine a high efficiency of building envelope solutions and mechanical systems with a significant energy production from renewable energy sources available on-site. The adopted approach followed a consequential order of priority: (i) passive solutions to orientation, massing, and window-to-wall ratio; (ii) appropriate technologies for construction elements and mechanical systems; and (iii) active systems for the production of energy from renewable sources, mainly located on the roof surfaces. The underlying idea was that, thanks to the design choices of “(i)” and “(ii)”, it would be possible to cover most of the limited energy needs of a typical year through the renewable energy available locally, considering the practical limitations deriving from the architectural design (iii).

Since the site was elongated in the north–south direction, it was not possible to rely widely on passive solar gains for the winter period; therefore, the volumes are quite compact and with relatively small articulations to reduce heat losses through the envelope. The overall window-to-wall ratio has also been carefully controlled to limit heat losses and gains on the east and west fronts; moreover, the long western facades, receiving a lot of solar radiation during summer afternoons, present shading devices and loggias to reduce the risk of overheating.

The thermal resistance of the building envelope (walls, windows and roofs) for the ZCB design goes beyond the standard requirements of Italian regulation, assuming instead the BAU reference scenario;

Table 2. Design U-values for the ZCB and the BAU scenarios.

	U-Values ZCB Scenario	U-Values BAU Scenario
Perimeter walls	0.15 W/m2K	0.26 W/m2K
Transparent elements	1.3 W/m2K	1.4 W/m2K
Roofs	0.14 W/m2K	0.22 W/m2K

shows the U-values of the most significant building elements. For transparent elements, the use of low-e insulating glazing was assumed, with a g-value below 0.3, including the shading devices.

The reduced thermal loads deriving from a well-insulated envelope made it possible to use heat pumps for all heating and cooling needs and the production of domestic hot water. While the specific distribution and control systems vary according to the function of each individual building, hot and cold water is always produced by water-to-water heat pumps exploiting groundwater as a heat sink: in this way, it was possible to completely avoid the combustion of fossil fuels across the project site.

Building automation solutions and strategies to engage users in energy saving behaviors were also designed to improve the overall efficiency of the district, which runs entirely on electricity.

As the last step in the design process, PV panels were added on the roof of the buildings, since the east and west orientation of the main facades did not make vertical installations suitable. Also, in this case, the goal was to install, compatible with the available budget, a PV surface that would produce more energy than the standard requirement.

All the energy flows of the district would be managed by a local smart grid, enabling the exchange of energy among buildings in case of surplus production.

2.4. Construction System and Material Balance for Carbon Mitigation

2.4.1. Material Diets for Zero-Carbon Building

The resulting balance of material in a BAU scenario, as shown in Figure 5, generates a clear hierarchy where, at the top, the carbon intensive materials take the major space, while the carbon-negative ones, able to regenerate the climate, take a marginal space. New material diets, where the amount of bio-based solutions is increased in the building, allow for reversing the pyramid, generating a new hierarchy where carbon-intensive materials are limited to components where no alternatives are possible (e.g., foundations, windows, PV panels, waterproofing, etc.) and the bio-based solutions are widespread all over the rest of the building, able to compensate for the positive carbon emissions and ideally achieve a zero-carbon target [13].

Thus, the selection of the construction materials for the low-carbon building alternative was inspired by this material diets concept, which involves the identification of locally available negative carbon solutions, especially fast-growing bio-based materials, capable of removing CO₂ with the rapid regrowth of biomass in plantations [40], being largely adopted in the different building components.

Figure 6 summarizes the solutions adopted for the structural system and the building components, which will be described in the next paragraphs.

2.4.2. Load-Bearing Structure

Usually, the load-bearing structure accounts for a significant share (around 40 percent) of the emissions from a building, because of the production of materials and its construction [41]. Considering the whole structural system, the above-ground load-bearing components, such as floors, columns, and beams, constitute roughly 80–85% of the total material volume [42]. Consequently, considering the floor area of the buildings, the choice of the construction solution for the horizontal slabs is crucial for the reduction in the overall carbon footprint. In the selection process, a multi-criteria decision model was used, considering the emission factor of each structural material, the duration of installation, the fire safety, and five S-KPIs—Structural Key Performance Indexes: (i) HPI-Height Performance Index; (ii) WPI-Weight Performance Index; (iii) SPI-Span Performance Index; (iv) CPI-Cost Performance Index; and (v) OPI: Overall Performance Index. As result, a mixed wood–concrete solution was selected for ZCB, providing the required stiffness to respond to seismic stresses according to the Italian regulation NTC 2008 [37] and EN 1998-1-Eurocode 8 [43]. This choice, compared to an ordinary reinforced Portland concrete slab used for BAU, significantly reduces the carbon footprint of the structure [44]. Moreover, this structural solution allows the avoidance of the installation of suspended ceilings by keeping an exposed wood finish, resulting in additional material and carbon savings and a reduced risk of indoor air pollution [45]. This configuration is possible, according to the national fire safety regulation for residential and hospitality buildings [46,47], and thanks also to the design of appropriate egress routes and fire compartments. Additionally, the mixed-construction solution adopted is suitable for covering conventional spans (5.5 m max for this project) with a regular structural grid, allowing a flexible layout in case of future changes to the configuration of interior spaces.

Below-ground spaces were limited in the early-stage design, to minimize the mass of concrete used in the basement. For this reason, shallow foundations are used in both scenarios, BAU and ZCB, and the soil was modeled with slopes and appropriate landscaping to limit the extension of below-grade perimeter walls. Similarly, the adoption of a system of basins and connecting channels for the collection and reuse of the water for greening and firefighting allowed the avoidance of the use of conventional underground concrete tanks for water storage. In case of ZCB, for all below-ground load-bearing elements, a large use of low-carbon concrete was assumed, based on the addition to the mixture of blended-cement binders with low clinker content, which can decrease the carbon footprint by almost 40% compared to an ordinary Portland cement used for BAU [48]. A general mix design with a cement replacement of about 40% with fly ash (FA) is considered, as reported in

Table A2. List of processes and GWP-100 per FU adopted for LCA modelling of materials used for BAU configuration.

Material	Name of the Process	Source	Functional Unit	GWP-100 Production	GWP 100 End-of-Life
			(FU)	kgCO2-eq/FU	kgCO2-eq/FU
Steel profile	Steel, low-alloyed {RER}|steel production, converter, low-alloyed|Cut-off, U	ecoinvent	kg	0.734	0.000
Reinforcing steel	Reinforcing steel {RER}|production|Cut-off, U	ecoinvent	kg	0.682	0.000
Reinforced soil	85% Sand {CH}|gravel and quarry operation|Cut-off, U; 10% Polypropylene, fibers {RER}|production|Cut-off, U; 5% Cement, unspecified {CH}|cement, unspecified, import from Europe|Cut-off, U	ecoinvent	kg	0.089	0.011
Autoclaved aerated-concrete block	Autoclaved aerated-concrete block {CH}|production|Cut-off, U	ecoinvent	kg	0.408	0.009
Concrete	Concrete, normal {CH}|unreinforced-concrete production, with cement CEM II/A|Cut-off, U	ecoinvent	kg	0.089	0.010
Low-carbon concrete	15% Cement, pozzolana and fly ash 36–55%,non-US {Europe without Switzerland}|cement production, pozzolana and fly ash 36–55%, non-US|Cut-off, U; 37.7% Gravel, crushed {CH}|production|Cut-off, U; 37.7% Sand {CH}|gravel and quarry operation|Cut-off, U; 9.5% Tap water {CH}|market for|Cut-off, U; 0.1% Plasticizer, for concrete, based on sulfonated melamine formaldehyde {GLO}|market for|Cut-off, U	ecoinvent	kg	0.053	0.010
Double glazing	Glazing, double, U < 1.1 W/m2K {RER}|production|Cut-off, U	ecoinvent	m2	44.900	2.980
Drainage	Polyethylene, high-density, granulate {RER}|production|Cut-off, U	ecoinvent	kg	2.380	3.030
Glued laminated timber	Glued laminated timber, for indoor use {RER}|production|Cut-off, U	ecoinvent	kg	0.365	0.121
Wooden geotextile	Horticultural fleece {CH}|horticultural fleece production|Cut-off, U	ecoinvent	kg	3.397	3.030
Gravel	Gravel, round {CH}|gravel and sand quarry operation|Cut-off, S	ecoinvent	kg	0.004	0.009
Floor ventilation	Polypropylene, granulate {RER}|production|Cut-off, U	ecoinvent	kg	1.570	3.030
Plaster	Cover plaster, mineral {CH}|production|Cut-off, U	ecoinvent	kg	0.138	0.009
Wood wool	Wood wool {RER}|production|Cut-off, U	ecoinvent	kg	0.404	0.041
Glass wool	Glass-wool mat {CH}|production|Cut-off, U	ecoinvent	kg	1.120	0.010
Gypsum plasterboard	Gypsum plasterboard {CH}|production|Cut-off, U	ecoinvent	kg	0.527	0.009
Lightweight screed	Lightweight concrete, expanded perlite {CH}|production|Cut-off, U	ecoinvent	kg	0.223	0.010
Kenaf textile	Textile, kenaf {GLO}|market for|Cut-off, U	ecoinvent	kg	1.120	0.221
Waterproof membrane	Fleece, polyethylene {RER}|production|Cut-off, U	ecoinvent	kg	2.380	3.030
PV panel	Photovoltaic panel, single-Si wafer {RER}|production|Cut-off, U	ecoinvent	kWp	2134.400	185.600
Sawnwood	Sawnwood, softwood, dried (u = 10%), planed {RER}|production|Cut-off, U	ecoinvent	kg	0.133	0.010
OSB	Oriented strand board {RER}|production|Cut-off, U	ecoinvent	kg	0.487	0.127
Laminated bamboo parquet	Flattened bamboo (3 ply) (MOSO Bamboo Forest)	INBAR	kg	0.620	0.077
Ceramic tile	Ceramic tile {CH}|production|Cut-off, U	ecoinvent	kg	0.768	0.009
Synthetic turf	Polypropylene, granulate {RER}|production|Cut-off, U	ecoinvent	kg	1.570	3.030
Cement tile	Cement tile {CH}|production|Cut-off, U	ecoinvent	kg	0.050	0.009
Cork insulation	Cork slab {RER}|production|Cut-off, U	ecoinvent	kg	1.120	0.221
Glass fiber-reinforced mash	Glass fiber-reinforced plastic, polyester resin, hand lay-up {RER}|production|Cut-off, U	ecoinvent	kg	0.682	0.000
Bamboo cladding	Decking and Cladding (MOSO Bamboo N-finity)	INBAR	kg	1.193	0.121
Sand	Sand {CH}|gravel and quarry operation|Cut-off, U	ecoinvent	kg	0.003	0.011
Lean concrete screed	Lean concrete {CH}|production, with cement CEM II/A|Cut-off, U	ecoinvent	kg	0.050	0.009
Window timber frame	Window frame, wood, U = 1.5 W/m2K {RER}|production|Cut-off, U	ecoinvent	m2	109.000	19.600
Alkyd paint	Alkyd paint, white, without solvent, in 60% solution state {RER}|Cut-off, U	ecoinvent	m2	0.644	0.719

of Appendix A. The external elements, such as balconies, could not instead be built out of the mixed wood–concrete system described above; therefore, a slab solution with lightening elements was adopted for ZCB, reducing the amount of material required by 18% compared to a traditional concrete slab, which is used instead in BAU. The use of this solution also ensures the durability and reliability of mechanical performance over time, especially considering the outdoor exposure of those elements for an extended service life of up to 100 years.

Table 3. Building composition and materials used for BAU and ZCB scenarios.

Building Element	Component	BAU	ZCB
Structure	Foundations	Reinforced Portland concrete	Low-carbon reinforced concrete
	Below-ground structure	Reinforced Portland concrete for walls	Low-carbon reinforced concrete for walls and columns
	Above-ground structure	Reinforced Portland concrete for beams and columns + concrete slab for interior floors and roof	Low-carbon reinforced concrete + timber-concrete composite slab for interior floors and roof
	Balconies	Reinforced Portland concrete slab	Hollow clay-block floor with reinforced concrete
Envelope	Exterior walls	Hollow concrete blocks + 10 cm EPS panels, mineral finishing or solid larch boards (in places)	Autoclaved aerated concrete blocks + 14 cm wood-fiber panels, mineral finishing or laminated bamboo boards (in places)
	Flat roof	Green roof + 12 cm EPS panels	Green roof + 12 cm glass-wool panels
	Doors and Windows	Aluminum frame with double glazing	Wood frame with double glazing
Partitions	Interior walls	Hollow clay blocks + mineral plaster	Timber frame + recycled paper boards
	Floors	Mineral plaster for the underside + 8 cm cement screed	Structural wood (no finishing cover for the underside) + 15 cm bulk hemp fibers
	Doors	Aluminum frame	Wood frame
Technical systems	Photovoltaic	Single-Si wafer (1475 m2)	Single-Si wafer (2110 m2)
	Water drainage	HDPE + PP	HDPE + PP
	Plumbing	Steel pipes	Steel pipes

summaries the composition of the building according to the two scenarios, BAU and ZCB, and shows the main material used according to the assumptions made during the early-stage design.

2.4.3. Building Envelope

For the envelope, a solid construction solution was selected in order to provide thermal storage, and therefore an effective dynamic control of the inbound energy flow during summer, in addition to a higher thermal resistance than the standard requires, as explained above. Thus, in the case of ZCB, the exterior walls consist of 24 cm autoclaved aerated concrete blocks with the addition of a 14 cm wood-fiber insulation layer [49], able to storing carbon during the service life of the building, while for BAU ordinary 25 cm hollow concrete blocks with an additional 10 cm EPS as thermal insulation are used. All exterior finishings for both scenarios are mineral, except for the ventilated façade of the student residence, where laminated bamboo boards are used as cladding for ZCB [50] and a wooden façade made of solid larch boards is assumed for BAU.

2.4.4. Internal Partitions

Lightweight construction solutions were preferred instead for the vertical internal partition walls, composed of a multi-layer drywall structure with timber frames and rigid finishing panels made of recycled paper [51]. Besides limiting the dead loads on structures, this construction system implements the principles of Design for Deconstruction (DfD), allowing an easy reconfiguration of the internal space layout with limited disturbance to users. A rearrangement of the internal partition walls is inevitable, over an extended lifespan of 100 years, to meet changing living and working needs. Thus, dynamic scenarios were assumed for the calculations, with the reconfiguration of the interior partitions while keeping the existing load-bearing structure intact. The embedded carbon in the load-bearing structure is therefore preserved, avoiding the substantial emissions that would be associated with demolition and reconstruction [52]. The use of a dry-assembled system employing industrialized components, such as the vertical studs and the finishing boards, provides several advantages. Waste is reduced during the production and installation of the walls, thanks also to the standardized size of components. Installation is fast, thanks to the dry-assembly process, and it is possible to easily integrate mechanical, electrical, and plumbing systems into the wall cavities. At the end of the service life, the walls can be easily decommissioned by disassembling their parts, avoiding invasive demolition operations and with a consequential better management of waste, while dismantled elements can be potentially reused [53]. In the case of ZCB, all leveling screeds used for the interior floors are made of 150 mm bulk recycled-hemp fibers [54], providing additional carbon storage, while for BAU 80 mm cement-based screed is adopted.

2.5. Method of Carbon Footprint Modelling

The mitigation of carbon emissions throughout the whole life cycle of the building is one of the central goals of this work. To measure the effectiveness of the selected design solutions in limiting the contribution of building materials and energy systems to climate change, the whole design phase was supported by a life-cycle assessment (LCA) model, suitable for measuring the impact category “global warming potential” over a 100-year time horizon (GWP 100a), according to the IPCC 2021 method [55]. A precise material inventory was extracted from a specific BIM model of the building for both BAU and ZCB scenarios, based on the mass and volume of materials required for the structures, building elements and equipment.

However, traditional LCA is a static approach, based on the aggregation of past, present and future GHG emissions at time zero, without a time factor being applied to the characterization factors of the different greenhouse gasses [56]. When specifically examining products containing biogenic materials, this can be problematic. Studies such as Cherubini et al. [57] demonstrate that not all biogenic products are equal, and that two temporal parameters, namely, storage period (S) and rotation period (R), influence the capacity of those materials to mitigate climate change. In this work, the GWP_bio index method developed by Guest et al. [58] for a time horizon of 100 years was adopted, as it was demonstrated to be one of the most reliable and robust methods in the literature for biogenic carbon modeling [59]. The method allows for the measurement of the mitigation of the GWP, due to the biogenic carbon stored in the different building components and speed of CO₂ uptake during plant regrowth. With the GWP_bio concept, a semi-static approach can be adopted in ordinary attributional LCA for biogenic construction products. The advantage for LCA practitioners is that, through this method, a standard attributional approach can be maintained without dealing with different dynamic characterization factors, and the biogenic CO₂ contribution is added to the calculation through normalized indexes [39]. However, large uncertainties can influence the results due to difficulties in estimating the expected storage period in building as well as in tracking the effective carbon regeneration in the land [60]. In fact, carbon storage in products can be estimated according to the carbon content, which depends on the type of biomass and water content. Then, the calculated index for each biogenic material used in the design is added to the fossil contribution to obtain a net-GWP value as the sum of the two contribution, fossil + biogenic, as described by the following Equation (1):

net-GWP = GWP_fos + GWP_bio

(1)

where

• GWP_fos is the contribution to climate change from fossil greenhouse gasses;
• GWP_bio is the contribution to climate change from biogenic CO₂.

2.5.1. System Boundaries Considered for the Carbon Footprint Assessment

For the calculation of the carbon footprint of the building under the two analyzed scenarios, a quantitative analysis was carried out, based on the measurement of the environmental impacts affecting climate change over the life cycle using a life-cycle assessment (LCA) approach in line with ISO Standards 14067 and EN 15978 [61,62]. The boundaries of the system include all the major phases characterizing the life cycle of the building: from the “cradle”, with the extraction of raw materials, to the “grave”, with the end-of-life of the buildings and the valorization of the waste produced. The modules included in the calculation model are highlighted in green in Figure 7, below. In module A1–A3 (the product stage), the phases of raw material extraction, transportation to the production center and industrial processing were included, while in module A4 (part of the construction process), transportation from the production center to the construction site was calculated. Module A5 is excluded from the system boundaries because of lack of data in the EPD database used for the assessment and the uncertainties revealed in the literature [63]. However, evidence suggested that the influence on the overall carbon emissions is minimal [64]. In module B (use stage), the following items were included: B1, related to building use; B4, replacement of deteriorated components; B5, deep renovation, planned only for the ZCB configuration at mid-life (50 years); B6, energy use for building heating/cooling and ventilation; and B7, domestic hot water (DHW) use. In module C (end-of-life), on the other hand, the following items were considered: C2, transportation of removed components from the site to the waste treatment center; C3, waste treatment; and C4, end-of-life. Additionally, benefits and impacts beyond the system boundaries were accounted for in module D, particularly the benefits from the mass of CO₂ sequestered in the biogenic products and in the vegetation planted in the new urban park of the project area, in addition to the benefits from metal recycling (steel and aluminum).

2.5.2. Material Processing and Transport

In the production phase (A1–A3), the main processes in producing the main building materials used in the two configurations, BAU and ZCB, were modeled. Secondary production data contained in the ecoinvent 3.8 database [65] were considered for both scenarios and modeled within the LCA software SimaPro 9 [66]. All processes used are reported in Appendix A,

Table A1. List of processes and GWP-100 values per FU adopted for LCA modelling of materials used for BAU configuration.

Material	Name of the Process	Source	Functional Unit	GWP-100 Production	GWP 100 End-of-Life
			(FU)	kgCO2-eq/FU	kgCO2-eq/FU
Steel profile	Steel, low-alloyed {RER}|steel production, converter, low-alloyed|Cut-off, U	ecoinvent	kg	0.734	0.000
Asphalt	Mastic asphalt {CH}|production|Cut-off, U	ecoinvent	kg	25.800	2.340
Reinforcing steel	Reinforcing steel {RER}|production|Cut-off, U	ecoinvent	kg	0.682	0.000
Concrete	Concrete, normal {CH}|unreinforced concrete production, with cement CEM II/A|Cut-off, U	ecoinvent	kg	0.089	0.010
Double glazing	Glazing, double, U < 1.1 W/m2K {RER}|production|Cut-off, U	ecoinvent	m2	44.900	2.980
Drainage	Polyethylene, high-density, granulate {RER}|production|Cut-off, U	ecoinvent	kg	2.380	3.030
EPS	Polystyrene foam slab for perimeter insulation {CH}|processing|Cut-off, U	ecoinvent	kg	4.460	3.190
Gypsum fiberboard	Gypsum fiberboard {CH}|production|Cut-off, U	ecoinvent	kg	0.527	0.009
Woven geotextile	Horticultural fleece {CH}|horticultural fleece production|Cut-off, U	ecoinvent	kg	3.397	3.030
Floor ventilation	Polypropylene, granulate {RER}|production|Cut-off, U	ecoinvent	kg	2.400	3.030
Plaster	Base plaster {CH}|production|Cut-off, U	ecoinvent	kg	0.138	0.009
Mineral wool	Stone wool {CH}|stone wool production|Cut-off, U	ecoinvent	kg	1.120	0.010
Clay brick	Clay brick {RER}|production|Cut-off, U	ecoinvent	kg	0.249	0.009
Waterproof membrane	Fleece, polyethylene {RER}|production|Cut-off, U	ecoinvent	kg	2.380	3.030
PV panel	Photovoltaic panel, single-Si wafer {RER}|production|Cut-off, U	ecoinvent	kWp	2134.400	185.600
Ceramic tile	Ceramic tile {CH}|production|Cut-off, U	ecoinvent	kg	0.768	0.009
Light clay brick	Light clay brick {DE}|production|Cut-off, U	ecoinvent	kg	0.161	0.009
Synthetic turf	Polypropylene, granulate {RER}|production|Cut-off, U	ecoinvent	kg	2.400	3.030
Cement tile	Cement tile {CH}|production|Cut-off, U	ecoinvent	kg	0.050	0.009
Glass fiber reinforced plastic	Glass fiber reinforced plastic, polyester resin, hand lay-up {RER}|production|Cut-off, S	ecoinvent	kg	0.682	0.000
Screed	Lean concrete {CH}|production, with cement CEM II/A|Cut-off, U	ecoinvent	kg	0.050	0.009
Polyethylene fleece	Fleece, polyethylene {RER}|production|Cut-off, U	ecoinvent	kg	2.760	2.580
Aluminum frame	Window frame, aluminum, U = 1.6 W/m2K {RER}|production|Cut-off, U	ecoinvent	m2	191.000	25.700
Sand	Sand {CH}|gravel and quarry operation|Cut-off, U	ecoinvent	kg	0.003	0.011
Alkyd paint	Alkyd paint, white, without water, in 60% solution state {RER}|Cut-off, S	ecoinvent	m2	0.877	0.719

for the BAU and Table A2 for the ZCB scenario. The processes related to the innovative materials used in ZCB configuration, namely the laminated bamboo used as external cladding, the low-emission concrete, and the low-carbon stabilized earthen wattle used for the external pavement of the park [67] are specifically modeled in SimaPro or obtained from EPDs or other sources, as in the case of the bamboo façade, for which LCI data are taken from INBAR [68].

The transport phase was modeled considering a short-haul supply of materials, with a maximum distance traveled of 50 km, an assumption supported by the site’s proximity to the main distribution centers for building materials in the Milan hinterland. The transport mode was considered to be entirely by road, according to the “Transport, freight, lorry 16–32 metric ton, EURO4” process of ecoinvent, except for bamboo-based products used in decking and cladding, where a mixed transportation of 18,000 km by sea freight and 150 km by lorry was assumed. Sea freight is processed in Simapro, according to the process “Transport, freight, sea, transoceanic ship (GLO)”. All processes were considered according to the cut-off by classification allocation, and the classification adopted in the model is unit process (U).

2.5.3. Use of the Building and Maintenance

To model the use and maintenance phase, the operational energy needs, calculated by the specialists of the design team, were considered. Specifically, both the estimated annual electricity consumption to cover heat (B6), DHW (B7) and use (B1) needs, and the estimated annual production of the PV system installed on the roof, and operational during the first 30 years of the district’s life, were taken into account. Dynamic energy simulation software was used for the calculations; the BAU scenario was based on the same floor area and arrangement of volumes as the ZCB design, but adopting standard values for the thermal performances of building components, the efficiency of mechanical systems and the contribution from renewable energy sources. These values are based on the national implementation of the EPBD 2010/31/EU as part of the Italian definition of the Nearly Zero-Energy Building. Since, as explained above, all energy-related services use electricity, an initial conversion factor (f_c,p) of 0.432 kgCO_2-eq/kWh for primary energy consumption from electrical sources and a conversion coefficient (f_c,PV) of 0.081 kgCO_2-eq/kWh for on-site electricity generation from solar sources were considered for estimating the related operational emissions. A linear decreasing function of the coefficient f_c,p was also implemented to model a progressive and constant decarbonization of the electrical grid, consistent with the target of carbon-free power by 2050.

Modeling of module B4, on the other hand, was based on the assumption of replacement cycles of worn components to restore the expected performance. These included all interior wall and ceiling finishes, repainting all surfaces with two coats of acrylic paint every 10 years; interior/exterior flooring and façade cladding, laminated bamboo boards for the ZCB scenario and timber boards for ventilated façades for BAU, with replacement cycles of 25 years for both scenarios; and fixtures, assumed to have a replacement cycle of 30 years. The PV system installed in the roofs, on the other hand, is removed 30 years after installation and no longer reinstalled, since complete decarbonization of electricity from the grid is assumed. Finally, for the ZCB scenario only, a deep retrofit is considered after 50 years from the construction, in order to extend the useful life of the building by an additional 50 years. This involves the replacement of the ETICS system, i.e., the insulation and exterior finish, and the ventilated facades, as well as the replacement and rehabilitation of all plumbing components, interior finishes, and vertical partition elements, with the possible reconfiguration of the interior room layout, thanks to the use of lightweight drywall systems.

2.5.4. End-of-Life and Final Disposal

The end-of-life of the buildings was assumed to be extended to 100 years for the ZCB configuration, with recycling of the waste produced as a result of selective demolition. In contrast, a partially selective demolition was assumed for the BAU configuration after 50 years of service life, with landfilling of all mineral components and recycling of reinforcement steel only. For all wood-based components, and for general biogenic material in the ZCB configuration, incineration with energy recovery was considered as a reference scenario, while for all metal parts (steel rebars for concrete reinforcement and cold-formed aluminum profiles for the interior partition substructures) a recycling process with 100% efficiency was assumed. All GWP 100-year end-of-life values for each process are shown in Appendix A, Table A1 for the BAU and Table A2 for the ZCB scenario.

2.5.5. Biogenic Carbon Accounting from Carbon Uptake and Storage

In Module D, all environmental benefits related to waste treatment and CO₂ sequestration related to carbon storage in building components and to vegetation planted in the urban park were accounted for. Specifically, for storage calculations in wood-based and bamboo products, a carbon content of 50% of the dry biomass was assumed, with 20% moisture content for structural components and interior finishes, 25% for exterior finishes, and 10% for insulation fibers. All in-place densities were assumed from the values reported in the ecoinvent 3.8 database, EPDs, and INBAR report [68].

3. Results

3.1. Operational Carbon

Thanks to the passive design choices and the high-performance building envelopes, the calculated thermal energy needs for the ZCB design are on average around 50% lower than the BAU reference scenario (

Table 4. Annual thermal energy needs of the buildings, in MWh/y, divided by function: comparison of baseline (BAU) and ZCB scenarios.

	Hostel			Student Housing			Housing 1, 2 and 3
	ZCB	BAU	Saving	ZCB	BAU	Saving	ZCB	BAU	Saving
Heating	217	378	−43%	135	270	−50%	156	338	−54%
Cooling	362	683	−47%	445	843	−47%	879	1318	−33%

), which is compliant with the Italian definition of Nearly Zero-Energy Building, as explained above.

Once the use of heat pumps is taken into account, the total calculated electrical energy need for all the buildings in the ZCB scenario, including heating, cooling, domestic hot water, and other electrical end-uses (e.g., artificial lighting and plug loads), is 3500 MWh per year;

Table 5. Annual electrical energy needs of the buildings, divided by block and by service, ZCB scenario.

	Heating		Cooling		Domestic Hot Water		Other Electrical Uses
	kWh m−2y−1	MWhy−1	kWh m−2y−1	MWhy−1	kWh m−2y−1	MWhy−1	kWh m−2y−1	MWhy−1
Hostel	14	110.4	16	126.8	22	176.5	113	900.0
Student housing	11	105.6	16	156.6	16	157.8	35	350.0
Housing 1 and 2	10	150.6	21	317.7	12	176.7	33	495.0
Housing 3	13	57.1	27	117.6	13	56.5	33	142.0

shows a breakdown divided into end-use and function.

The estimated energy production from the PV system on the roofs of the ZCB design scenario is 1.5 MWh per year, i.e., 42% of the total electricity need as described above. The energy from PV panels covers 87% of the thermal energy need for heating and cooling and domestic hot water, well above the 60% fraction mandated by national regulations (Figure 8).

To calculate the carbon emissions during the operational life of the buildings, the electricity needs for heating and cooling (phase B6), domestic hot water (phase B7) and other end-uses (phase B1) were converted into carbon emissions via a conversion coefficient for the use of electricity from the grid and another coefficient for energy produced on-site by PV panels, as explained in Section 2.5.3, above. The results show a significant decrease in yearly carbon emissions for the ZCB design scenario compared to the BAU scenario (

Table 6. Annual carbon emissions deriving from the operational energy for all the buildings: comparison of baseline (BAU) and ZCB scenarios.

	Emissions from Heating and Cooling [kgCO2-eq/m2y]	Emissions from Domestic hot Water [kgCO2-eq/m2y]	Emissions from Other Electrical Loads [kgCO2-eq/m2y]
BAU	5.6	2.7	5.7
ZCB	1.4	0.7	2.3
Saving	−74%	−73%	−60%

).

These annual values were calculated considering the current energy mix in the national grid. In terms of total carbon emissions due to operational energy over a period of 30 years, the project achieved a 38% reduction, equal to 10.6 MtCO_2-eq, compared to the BAU scenario, taking into account a constant decrease in the carbon intensity of electricity taken from the grid, as explained above.

3.2. Embodied Carbon

The results of the carbon footprint assessment show the influence of the material selection under the two design options and the effect of the carbon compensation from a large share of bio-based materials in the material diet adopted for the ZCB design. In particular, the choice of the mixed wood–concrete horizontal structures contributes to the sequestration of an amount of biogenic CO₂ equal to 2500 tons. The adoption of bamboo as cladding for the ventilated façade contributes to reducing the carbon footprint for the production and installation phases by almost 75%. As shown in

Table 7. Comparison of the annual global warming potential (GWP) between baseline (BAU) and zero-carbon building (ZCB), calculated over a 100-year time horizon over the entire life cycle, broken down by building components. The lifecycle includes the following modules, defined according to ISO EN 15978 [62]: A1–A3 (production), A4 (transport to site), B4–B5 (maintenance/replacements and refurbishment), C2 (transport demolition works), C3–C4 (waste treatment and end-of-life), D (benefits beyond system boundaries). The benefits allocated in module D include both the biogenic GWP from carbon storage in construction products and the CO₂ uptake by project site plantings during the service life of the building.

		ST	EW	BR	IF	IW	DW	PV	HS	PL	UF	UG
		kgCO2-eq/m−2
BAU	A1-4; B4-5; C2-4	166.7	44.6	29.4	100.1	59.8	38.2	40.1	19.2	9.1	9.12	-
	D	-	-	-	-	-	-	-	-	-	-	−0.9
ZCB	A1-4; B4-5; C2-4	105.3	74.3	21.2	58.7	41.6	48.2	57.3	38.5	18.2	6.9	-
	D	−45.5	−26.5	−7.0	−49.3	−51.2	−2.2	-	-	-	-	−3.6

ST = structure; EW = exterior walls; BR = basement and roof; IF = interior floors; IW = interior walls; DW = doors and windows; PV = photovoltaic; HS = heating system; PL = plumbing; UF = urban furniture; UG = urban greening.

, considering the carbon emissions from material processing (A1–3), transport (A4), maintenance/replacement and renovation (B4-5) and end-of-life (C2–4), the structure (ST) is the part of the building contributing the most to climate change, representing 32% of emissions for the BAU scenario and 22% for the ZCB one. However, when the contribution to GWP of the stored carbon in the hybrid wood–concrete structure for 100 years is taken into account, the net GWP of ST drops to nearly 60 kgCO_2-eq/m², with 64% of net saving. Similarly, the use of fast-growing bio-based fibers as lightning material for screed in the interior floors (IF) contributes to providing a nearly zero net GWP, with a net saving of more than 90%. The lightweight multilayered interior walls, using a timber frame and recycled paper panels, further contribute to providing a negative net GWP, saving nearly 70 kgCO_2-eq/m². The advanced energy systems adopted for the ZCB option, as well as the additional PV installed on the roofs, instead double the carbon emissions, compared to the BAU scenario. However, these extra burdens are fully compensated by the contribution of bio-based materials in building components, which in the end contribute to dropping the overall life cycle of GWP by 45%. If the biogenic CO₂ is excluded from the calculation and only the fossil contribution is taken into account, the carbon saving between BAU and ZCB is only 9% in favor of the latter.

In Figure 9, the annualized carbon emissions of the BAU solution are compared with the zero-carbon building (ZCB) configuration. The effect of the material diet shows a reduction of about 50% of the carbon emissions compared to BAU. If the contribution of CO₂ sequestered in the biogenic materials is considered, this percentage is increased by an additional 20%, bringing the total carbon savings up to 70% over 100 years.

3.3. Life Cycle of Zero-Carbon Building

To estimate the carbon mitigation effect deriving from the material selection, the time-dependent emissions from embodied and operational carbon were evaluated over the whole life cycle of the building. As shown in Figure 10, nearly 400 kgCO_2-eq are emitted per square meter under the BAU scenario. On the other hand, the initial carbon spike in the case of the ZCB option is reduced by nearly 80%, mainly due to the fast carbon regeneration in the fast-growing materials used in the envelope and interior floors. After the construction, the annual emissions due to the operational energy need of the BAU more than double its GWP after 25 years. In this period, the transition to zero-carbon energy, which is assumed to linearly decrease the emission factor of the electricity mix, is not fully completed and the low on-site production of solar energy only partially balances the positive emissions from the use of the buildings. On the other hand, the extra PV panels installed in the roof, as well as the low-carbon energy system adopted for the ZCB, limit the operational emissions, reducing the peak by 56%. From year 30, the operational energy is supposed to be carbon-free, and no additional contribution is expected till the end of the building’s service life. Consequently, the PV panels, which are assumed to have a lifespan equal to 30 years, are not replaced, and a landfill scenario is assumed for both BAU and ZCB. While at year 50 the BAU scenario assumes a final disposal of the buildings, with most of the material demolished and sent to landfill, with the only exception being the metal components, in the ZCB option a refurbishment with a deep retrofit is assumed, with the replacement of all exhausted materials (i.e., finishing, external insulation, mechanical, electrical and plumbing systems (MEPs), doors and windows, etc.) and 10% of the concrete structure repaired with the replacement of heavily cracked parts.

The avoided demolition at year 50 and the extension of the buildings’ service life allows for a saving of more than 50 kgCO_2-eq/m², with a relative saving of nearly 60% compared to BAU. Finally, the extended end-of-life of the ZCB scenario accounts for an additional 67 kgCO_2-eq/m², with a resulting GWP of 483 kgCO_2-eq/m² which compared to 945 kgCO_2-eq/m² estimated for BAU, contributes to a total saving of nearly 50% of carbon emissions.

At the building scale, the total carbon emissions (operational + embodied carbon) for BAU are equal to nearly 50 ktCO_2-eq. As shown in Figure 11, the mitigation from material selection and energy efficiency allows for a saving of more than 23 ktCO_2-eq, while the contribution of biogenic carbon storage and uptake in biomass regrowth and urban greening additionally saves nearly 10 ktCO_2-eq. The residual carbon emission, equal to nearly 20 ktCO_2-eq, which was not possible to avoid through additional onsite mitigation strategies, can be offset with the plantation of about 68 ha of coniferous forest.

4. Discussion

The results show that a significant reduction in carbon emissions, compared to the business-as-usual situation, is indeed possible thanks to a combination of passive design strategies, efficient mechanical systems, production of on-site energy from renewable sources, a lean choice of construction systems, and an appropriate selection of building materials, including bio-based ones. In terms of embodied carbon, the reversal of the material pyramid, prioritizing low-carbon materials whenever possible, which covers nearly 65% of the total built volume for the ZCB scenario, allowed for the cutting of annualized carbon emissions by around 50% compared to the BAU scenario, while the use of fast-growing bio-based materials for secondary building components over large surface elements, such as horizontal floors and external facades, provides an additional 20% reduction, thanks to carbon sequestration. A more widespread use of timber components, in particular for the structural elements, would have increased the benefits deriving from bio-based materials; however, budget constraints deriving from the low margins associated with the public housing function of the district prevented the use of structural timber, which is currently more expensive than on-site concrete. Nevertheless, the final GWP value of 4.7 kgCO_2-eq/m² for embodied carbon ranks the project in the B class according to the Carbon Heroes Benchmark [26] and is in line with the best practices for the embodied carbon of new builds in the EU [27].

While the use of bio-based construction materials delivers significant benefits in terms of embodied carbon emissions, there are some concerns revolving around their aesthetic appeal, aging characteristics, durability, hydrothermal performance—particularly in humid conditions—and fire resistance [69]. Furthermore, their integration into high-rise constructions remains restricted in various countries, owing to regulatory limitations concerning fire safety. The utilization and incorporation of bio-based materials into architectural design necessitate specialized expertise, and enhancing their reaction to fire and their fire resistance requires the application of tailored coatings [70]. In general, fire accidents drastically affect the carbon footprint when bio-based products are used as construction materials. When the fire affects a building and the biomass in the bio-products is partially or totally incinerated, the effect on climate change might be relevant, as the total carbon goes massively into the atmosphere as CO₂, erasing the benefit from long-term storage. However, even if the DLCA method used to model the biogenic carbon were able to take into account different storage periods, the vulnerability to fire hazard, both in the technosphere (building) and in the biosphere (forests and plantations), is an unpredictable factor which generates large uncertainties. For this reason, fire scenarios were not considered, and an ordinary use of the building, as well as an ordinary management of the forests, were assumed in this study.

The results of this study also show that, because of, in particular, the budget limitations for the adoption of bio-based materials for the structures, the ZCB scenario still has residual carbon emissions over the project’s life cycle which the on-site mitigation strategies, relating in particular to the park and its trees, are not sufficient to offset. The plantation area required to compensate these residual emissions corresponds to an area about 10-fold larger than the Scalo Lambrate site itself, which raises questions about land use issues and the potential risk of transferring burdens elsewhere. The transition towards low-carbon materials and processes, and the necessity of using land to offset hard-to-abate emissions, may inadvertently exacerbate pressures on environmental fronts other than the presence of greenhouse gases in the atmosphere, such as biodiversity loss, water scarcity and land use [71,72,73]. Certain resources may, in fact, emerge as critical bottlenecks in the endeavor to achieve a low-carbon economy [74,75]. Realizing the widespread adoption of such materials while concurrently ensuring sustainable forest and crop management requires a comprehensive political framework and incentivization [76].

Once the embodied carbon emissions are combined with those deriving from operational energy over the life cycle of buildings, the influence of the energy mix in the electrical grid also becomes apparent. The results show how the energy efficiency of the buildings and their system strongly differentiate the total carbon emissions of the BAU and ZCB scenarios in the first 30 years, during the transition to a decarbonized grid by 2050. Assuming this target will be achieved, greenhouse gas emissions associated with energy generation will be drastically reduced or eliminated; this entails a shift away from fossil fuels towards renewable energy sources such as solar, wind, hydroelectric, and nuclear power, coupled with advancements in energy storage technologies and energy efficiency measures [77]. After this transition, predictably, the operational emissions in both scenarios flatten out and the difference in total carbon emissions remains almost constant.

On the other hand, another significant consequence of decarbonizing the electrical grid will be a significant reduction in the emission factors associated with material manufacturing [78,79], which currently means that the construction sector is a major contributor to global carbon emissions due to energy-intensive processes and reliance on fossil fuels [80]. However, as the energy grid becomes increasingly decarbonized, the emission intensity of electricity used in manufacturing processes decreases, thanks to a wider use of renewable energy sources with lower carbon footprints, compared to fossil fuels, for electricity generation [81]. This means that industries relying on electricity from the grid will inherently have lower emission factors associated with their manufacturing processes. For instance, the production of steel, cement, and chemicals—industries known for their high carbon emissions—can significantly reduce their environmental impact by transitioning to electricity derived from renewable sources [82]. Moreover, the decarbonization of the energy grid enables the electrification of various industrial processes that were previously reliant on fossil fuels, further reducing emissions factors. However, it is essential to consider the broader implications and challenges associated with decarbonizing the energy grid [83]. This includes addressing intermittency issues of renewable energy sources, ensuring grid reliability and resilience, and managing the transition for industries heavily reliant on fossil fuels. Additionally, the adoption of low-carbon technologies in material manufacturing, such as carbon capture and utilization, will play a crucial role in further reducing emissions in sectors where complete electrification may be challenging, such as the cement industry [84]. When this scenario is achieved, the embodied carbon emissions of construction materials will be significantly lower than today, potentially making bio-based materials less crucial for achieving zero carbon targets over the life cycle of buildings, and therefore limiting the issues related to their use that were highlighted above.

However, negative consequences might be generated by the increasing pressure of electricity demand by the construction industry. In particular, energy prices can be affected by large fluctuations based on different energy management modalities. As found by Petrichenko et al. [85], the energy community is a more profitable framework than an individual distributed prosumer, since it can lead to savings of up to 20% of the energy costs. However, several uncertainties affect the estimation, and a reliable prediction of the future variation in energy costs after the energy transition is hard to achieve.

5. Conclusions

This article presents a systematic methodology for assessing the global warming potential (GWP) of the project for the regeneration of one of Milan, Italy’s, former railway yards into a neighborhood with several residential buildings and a public park. The project was submitted as an entry to the 2020 edition of the Reinventing Cities competition, organized by the C40 group. The methodology allowed for the estimation of the whole life-cycle carbon emissions of the neighborhood over a time scale of 100 years, comparing the design scenario, based on the zero-carbon target, with a “business as usual” reference scenario, based on the current Italian regulations for Nearly Zero-Energy Buildings.

The results show that an appropriate combination of strategies to limit the emissions deriving from operational energy, together with the reversal of the material pyramid, using, whenever possible, fast-growing bio-based materials with negative carbon footprint, leads to a 70% reduction in carbon emissions over the 100-year period; the remaining emissions can be offset with the afforestation of a 68 ha area.

These results are interesting, because they prove the actual feasibility of zero-carbon neighborhoods also in situations with limited budget, such as developments mostly dedicated to affordable housing, where margins for the developer are low and it is not possible to use timber for structures and walls, due to budget limitations. This work shows the opportunities for carbon storage offered by fast-growing biogenic products that can be used in secondary building components, such as finishings and fillers spread over wide surfaces.

At the same time, however, the results demonstrate that, with these constraints, the neighborhood still has residual carbon emissions, and that the on-site mitigation strategies such as the large public park are not sufficient to offset them. Only with the adoption of off-site mitigation measures, such as the afforestation of an area 10 times larger than the site itself, is it possible to achieve the zero-carbon target; this strategy, however, would open up significant issues about land use that may question its replicability on a larger scale.

Since the remaining opportunities to reduce carbon emissions deriving from operational energy appear limited, this work also clarifies the urgent need to implement policies and incentives for the introduction of bio-based materials in buildings, thanks to the carbon storage potential they offer. Moreover, new standards should be developed to include dynamic methods for biogenic carbon accounting in building, matching more closely the peaks of emissions at the time of construction and maintenance and the carbon actually absorbed by vegetation during its growth.