Keywords

1 Introduction

The construction sector is responsible for the largest individual share of greenhouse gas (GHG) emissions, accounting for 37% of all emissions [1], due to energy-intensive activities of material extraction, transportation, construction, and energy to operate buildings. Building construction activities alone represent 10% of all emissions [1]. Hence, the high emissions in the sector worsen the continual warming of the planet and contribute to the climate crisis. Moreover, the construction industry consumes 40% of the global resources [2] and is one of the leading producers of solid waste generated during the production of materials, construction, and demolition of buildings [3]. That means a large share of emission-intensive building materials produced from precious finite resources end up in landfills as a result of manufacturing, construction or demolition activities.

Given the remarkable influence of the construction sector and aiming at a better understanding and evaluation of the full impact of building materials and buildings over their service time, life cycle assessment (LCA) studies have been a field of interest and growing body of knowledge over the past three decade, finding significant differences between common building materials such as steel, concrete or wood [4] and highlighted the crucial role of wood products’ energy recovery at the end-of-life (EoL) to mitigate impacts. Previous studies [5,6,7] by the author of this paper also support the critical role of EoL to mitigate the environmental impact of construction.

In particular, understanding the embodied emissions of materials in a building is paramount to reducing the impacts of the construction sector [8], as it can contribute from 10% to 50% of the total lifecycle GHG emissions, depending on how energy-efficient a building is [9]. However, despite its relevance, Cai [10] warns of a still limited number of studies reporting material-level embodied impacts, which can lead to underestimation. Moreover, recent papers stress the central relevance of materials’ EoL scenario and its further potential for impact mitigation by extending the lifespan of buildings and service times of materials through circular strategies such as design for adaptability, disassembly, and reuse [11, 12].

However, after a broad review of available LCA literature over the past 20 years, analyzing more than 230 published papers, Bahramian [13] showed a tendency for LCA to focus on low-rise commercial buildings in the USA, with only one study about a low-rise house. It was also noteworthy in the literature review the almost complete absence of studies about affordable/social housing options, with only two papers on the topic originating from Latin America. Despite this noticeable scientific gap, there is a pressing and undeniable need for more affordable houses, also in North America.

Hence, understanding the embodied environmental impact of building materials and methods suitable for affordable housing must also be an integral topic of scientific investigation. This study addresses the identified scientific gap related to the lifecycle impact investigation of a single-family home for the affordable market in the southern U.S. This study builds on the initial findings of [7], which identified lightweight wood framing as the most promising option for reducing the environmental impact of single-family homes in the South, by increasing the sample size of such a construction system to seek new insights and verify the results. Thus, it aims to contribute to the field with original information on the environmental performance of affordable housing building options that facilitates decision-making processes, leading to new knowledge that can support the design and construction of lower-embodied-impact affordable single-family homes in the south of the USA.

2 Methods

2.1 LCA Scope

This study conducted an LCA as per EN-15804 (Table 1) of one affordable single-family house (91.69 m2) built in 2020 using light wood framing in the southern U.S. (Fig. 1). The study calculated the global warming potential (GWP) of the as-built house and developed hypothetical scenarios including increased circular strategies and regenerative materials use (item 2.2) to evaluate further possibilities and limitations for impact mitigation.

Fig. 1.
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Case study: massing strategy (left); floor plan (right).

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Table 1. LCA Phases and modules assessed per EN-15804 standard.
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Using the as-built construction documentation of the case-study house, a detailed bill of materials (BOM) was defined (Table 2). Then, lifecycle inventory data for each specified material was gathered from the Athena Sustainable Materials Institute database [14], which provides ISO 14040/14044-compliant data. The database is geographically adjusted to reflect the industry-wide national average values in the USA and regularly updated to account for the latest manufacturing technology, transportation requirements, and energy mix context, with data less than ten years old. At the time of this writing, three building materials were still unavailable in the database (item 2.2). Nonetheless, the impact data on these three materials was also retrieved from ISO-compliant Type III EPDs. After gathering all LCI data, the Impact Estimator for Buildings software (IE4B v.5.4.0101) was used to obtain the lifecycle impact assessment (LCIA) values for a functional unit of 1 m2 of construction and a 60-year lifespan.

Table 2. Bill of materials (BOM) in kg/m2.
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2.2 LCA Scenarios

This study assessed six scenarios to understand the possibilities and limitations for further impact mitigation, described in the sequence. Table 3 presents an outline of each scenario and the assessed variable within each one of them.

Standard Scenarios.

S1: This scenario represents the standard LCA of the as-built case-study. Therefore, it provides a benchmark for the study, with a conventional optimistic view of material recovering and recycling at the end of life. S2: This scenario investigates a conceivable pessimist end-of-life fate for sawn wood and metals (aluminum, and galvalume sheathing), typically assumed to be recycled or incinerated for energy. This scenario considers that only half of these materials would be soundly recycled or incinerated due to the likely challenges of sorting after a conventional demolition process of such buildings [15, 16]. Hence, it estimates the influence of the standard end-of-life fate on the overall LCA results by reducing the benefits obtained in module D. To account for the uncertain effect of different recycling ratios within our scenario, the author performed a sensitivity analysis (±50%).

Circular Strategies.

C1: This scenario investigates the effect of reducing construction waste below the national average through off-site prefabrication. According to previous studies [17, 18], off-site prefabrication can reduce construction waste by up to 40% of the on-site values due to a more controlled manufacturing environment. Therefore, the author accounted for the potential benefits of construction waste reduction by decreasing the waste amount by a factor of 2.5. To account for the uncertain effect of different waste ratios within our scenario, the author performed a sensitivity analysis (±50%). C2: This scenario investigates the effect of reusing wood and metal building materials in construction. The author assumed a conservative 50% reuse ratio for sawn wood [16, 19] and metals (aluminum, and galvalume sheathing). That allows this scenario to measure the benefits of reuse compared to the conventional EoL fate of energy recovery and recycling in S1. The benefits of reuse were allocated to Module D as the avoided burden of producing new materials from the cradle, as per closed material loop allocation methodology (ISO 14044:2006 section 4.3.4.3.3). To account for the uncertain effect of different reuse ratios within our scenario, the author performed a sensitivity analysis (±50%).

Regenerative Materials.

M1: This scenario evaluated the effect of replacing the most impactful envelope material in S1 with a functionally equivalent regenerative material. The GWP results per material showed the highest impacts in S1 arise from glass followed by insulation. The latter material was replaced with wood fiber insulation because there is no regenerative option for glass on the market yet. M2: This scenario evaluated the effect of replacing the most impactful finishing material in S1 with functionally equivalent regenerative materials. The GWP results per material showed the highest impacts arise from the galvalume roof sheathing followed by paints. The latter material was replaced with clay plaster applied on the gypsum plasterboard, following the specifications of the manufacturer [20], because there is no functionally equivalent regenerative option for galvalume sheathing on the market yet.

Table 3. Description of modified parameters assessed in each scenario.
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3 Results and Discussion

3.1 Aggregated LCA Results

The aggregated results (Fig. 2) showed that S2 has the highest GWP value of all scenarios, while S1came in third. That attests to the high relevance of a pessimistic yet plausible EoL scenario on the environmental performance of light wood framing affordable houses. Contrarily, C2 is the second most impactful scenario, showing a more limited mitigation potential of reusing materials. However, for reuse (C2), it is worth noting that the reduced mitigation potential from reuse derives from a smaller amount of wood available at the end of life, thus reducing the benefits at module D and increasing GWP values. That is a methodological issue within the cradle-to-grave approach of a static LCA. S1, C1, and M1 scenarios display minor variation with a mean of 1.5%. C1 was the least impactful of the three, indicating a preference for reducing material waste to mitigate impacts, compared to replacing rock wool insulation with wood fiber. Finally, M2 achieved the lowest GWP value, demonstrating the high environmental costs that a conventional painted finish can have on light wood framing houses and the potential of clay plaster as a mitigation alternative.

Fig. 2.
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Total embodied Global Warming Potential (GWP) by scenario.

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3.2 LCA Results by Construction Role

Figure 3 shows that the finishing materials account for the highest share of GWP regardless of the scenario. For most scenarios, the foundations were the second-most impactful part of construction, followed by the envelope in third place. The impact of finishing materials was virtually the same for S1, S2, M1, and C2, with a mean variation of 0.25%, indicating a minor mitigation potential of reusing the finishing materials of the case study. The GWP of finishing materials in C1 was slightly below the mentioned scenarios (−2.5%) due to a lower construction waste rate, which shows the relevance, although limited, of the implementing measures that increase material efficiency during the construction phase. The GWP of finishing materials in M2 was the lowest, circa 71% lower than S1, because of the replacement of latex and acrylic paint finishing with clay plaster. As for envelope materials, S2 showed the highest GWP, followed by C2 with 200% and 143% more impact than S1, respectively. That reiterates the critical need for a suitable EoL of materials and limited mitigation potential of reusing materials and measuring its benefits in a cradle-to-grave LCA model. S1, C1, and M2 showed the lowest GWP values, demonstrating the effectiveness of guaranteeing a suitable EoL for materials, reducing construction waste below the average, and using regenerative finishing materials, respectively. M1 presented the lowest GWP value for the envelope, 10.3% lower than S1, attesting the benefits of using wood fiber insulation as a replacement for rock wool.

Fig. 3.
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Embodied Global Warming Potential by construction role.

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4 Conclusions

This study conducted an LCA study of one affordable single-family house built in 2020 using light wood framing in the southern U.S. The study calculated the GWP of the as-built house and developed five hypothetical scenarios, including increased circular strategies and regenerative materials use.

The Standard Scenarios result showed suitable EoL handling of materials, namely recycling of metals and recovery of wood-based materials, has a significant mitigation impact on GWP compared to the pessimistic scenario where only half of these materials would be recycled or recovered due to the possible challenges of material sorting after a conventional demolition process of such construction system, consisting predominantly of nailed connections. The first circular scenario of reducing construction waste below the national average showed a small but sure potential to lower the GWP; the second circular scenario of reusing metals and wood-based materials increased the impact compared to the standard scenario due to the reduced availability of wood to be recovered and converted into energy at the end of life, thus reducing the benefits at module D. Moreover, replacing mineral wool with wood fiber insulation in the first regenerative scenario showed a limited mitigation potential. However, replacing latex and acrylic paints with clay plaster reduced the GWP of the case study by a large margin, demonstrating the high environmental costs that a conventional painted finish can have on light wood framing houses.

To sum up, the results indicate that increased use of circular strategies and regenerative materials offered further mitigation and decarbonization potential for the light wood frame affordable single-family house case study. Throughout this study, results pointed out that priorities should be to (1) guarantee proper end-of-life of metals and wood-based materials. (2) To replace non-renewable materials with regenerative wood or earth-based ones. Lastly, (3) to improve construction waste diversion rates. This study assessed one case of a light wood framed affordable house. Therefore, more case studies under the same circumstances should be assessed to validate the results as general design guidelines. Additional topics that were out of scope but can tackled in further research could include 1) investigating the real-life challenges and possibilities of design for the disassembly, recovery, and reuse of lightweight construction methods. 2) Developing LCA studies that assess a broader range of regenerative materials for structural, insulating, and finishing roles in an affordable house. 3) Evaluating the financial impacts of incorporating novel regenerative materials in affordable single-family homes built with lightweight construction methods.