A primer on low-carbon design in architectural acoustics using a case study of residential floors^a)

The architecture, engineering, and construction (AEC) sector continues to be shaped by efforts to reduce its impact on climate change. Because the AEC sector contributes around 37% of global carbon emissions,¹ building designers and engineers must participate in decarbonization efforts to reach climate goals. Currently, nearly 28% of the AEC sector's carbon emissions comes from operational carbon (OC), which is defined as the carbon emitted during the use-phase of a building, primarily for electricity and conditioning.² The remaining emissions come in the form of embodied carbon (EC), which is defined as the carbon emissions associated with the extraction, transportation, manufacturing, construction, and end-of-life removal of building products.³ As urban areas transition from electrical grids that rely on fossil fuels to renewable energy sources, and with the development of energy-optimized building systems,⁴ OC emissions are projected to decrease in the coming years.¹ Correspondingly, EC emissions are forecasted to account for an increasingly higher proportion of a building's total carbon footprint.¹ Therefore, design strategies aimed at reducing EC and OC emissions are necessary to curb the carbon footprint of the built environment.⁵ 

Although there has been significant research on pathways to reduce carbon emissions in core building subdisciplines, including mechanical,⁶ structural,⁷ and construction⁸ disciplines, further research is needed for architectural acoustics. Each discipline provides a unique outlook for carbon-influencing design decisions, and architectural acoustics is no exception. Historically, the strongest association between architectural acoustics and sustainability has been in terms of the positive occupant health and wellness outcomes associated with proper acoustic design.⁹ As a result, acoustic design is incorporated in many popular building sustainability rating systems, including Leadership in Energy and Environmental Design, WELL Building Standard, Building Research Establishment Environmental Assessment Methodology, and the International Green Construction Code.

Research on low-carbon architectural acoustic design strategies has been studied less in comparison to other building disciplines.^10,11 However, architectural acoustics is uniquely positioned to aid building decarbonization efforts because of its influence on EC and OC emissions. Acousticians commonly help inform the design and selection of specialty acoustic finish materials, the makeup of floor and wall assemblies, and provide quiet, efficient, mechanical systems in buildings,¹² which all directly affect the carbon footprint of a building. Acoustically driven decisions often require more material layers (e.g., more mass) than is needed to erect a non-acoustically treated building, thereby presenting a design challenge to synergize low-carbon and high-performing acoustic design goals.¹³ 

Despite the opportunity to positively influence carbon-related design decisions, architectural acoustic practitioners are often unaware of how their design decisions can influence a building's carbon footprint. Acousticians may also be unfamiliar with methods to calculate the carbon emissions of building products and systems, limiting their ability to make sustainable design decisions. Furthermore, building codes and city policies are adapting to require the calculation of carbon emissions,¹⁴ potentially putting architectural acoustic practitioners behind-the-curve compared to other building disciplines. A related challenge is that sustainability consultants and general building practitioners can be unaware of the importance of the acoustic environment and are often not educated on strategies to improve building acoustics, despite being informed on sustainable design solutions. As a result, acoustic considerations may be overlooked in the design of sustainable, low-carbon buildings.

A major gap exists at the intersection of low-carbon and high-performing acoustic design strategies in buildings, including industry knowledge and academic research. To begin to address this gap, this paper has four aims:

1. Inform acoustic practitioners on the current methods of quantifying environmental impacts by conducting life cycle assessments (LCAs), knowing the life cycle stages, collecting the life cycle inventory (LCI), and using environmental product declarations (EPDs);

2. discuss how LCAs can be used to inform low-carbon design decisions;

3. demonstrate how LCAs can be used in the selection of a low-carbon floor-ceiling assembly for a residential building while achieving high acoustic performance (i.e., demonstrating aim 2 in a case study); and

4. propose future research directions at the intersection of architectural acoustics and sustainability in buildings.

Overall, this paper contributes to the growing collection of literature^8,15–17 to reduce carbon emissions in the built environment by highlighting the need for harmonizing acoustic engineering principles with low-carbon design and demonstrating a LCA framework on a case study of nine floor-ceiling assemblies to evaluate sustainable and acoustic design goals.

To quantify the carbon footprint of buildings, it is essential, first, to know what LCAs are. LCAs are a standardized method for systematically tracking environmental impacts of interest (e.g., carbon emissions) for a product or assembly during its partial or full life cycle.¹⁸ The steps to conduct a LCA follow the International Standards Organization (ISO) standards 14040 (Ref. 19) and 14044 (Ref. 20) to properly assess sustainable performance. Whereas many environmental impact metrics exist,²¹ this paper focuses on carbon emissions in terms of the kilogram of carbon dioxide equivalent (kgCO₂e) or global warming potential (GWP), which is a metric that normalizes the contribution to global warming from any atmospheric heat-trapping gas to the equivalent effect of CO₂. The GWP can be determined for an individual product, assembly (e.g., the combined elements in a floor or wall construction), or a whole building.

Within a LCA, the carbon emissions are subdivided into different stages and modules, each of which produces carbon emissions during the life cycle of a building.¹⁸ These LCA stages, as observed in Fig. 1, include the product stage (modules A1–A3, which are the focus of the case study in Sec. 3), the construction stage (modules A4 and A5), the use stage (modules B1–B7), and the end-of-life stage (modules C1–C4). Module D quantifies the carbon impacts beyond the life cycle of a building and describes circular economy pathways for a building. Circularity within the built environment typically refers to the reuse and/or regeneration of building products and construction materials in comparison to common end-of-life disposal (e.g., landfill).²² Accurate tracking of carbon emissions emitted during each LCA stage is necessary to understand which design decisions are the most carbon-intensive.

A critical step of the LCA is obtaining a LCI. The LCI consists of a bill of materials (BOM)—which describes the material intensities (e.g., thickness and weight) of a product or assembly—and carbon coefficients—which are the GWP emissions of a product, normalized by a declared unit (e.g., per volume, weight, and floor area). Calculating the carbon emissions for a given product is simply the material intensity multiplied by the GWP emissions corresponding to the product. Because of this, accurate BOMs and carbon coefficients are necessary to make informed sustainable design decisions. In a building context, accurate structural and enclosure BOMs can be obtained from building models,²³ and mechanical system BOMs can be obtained from highly detailed models; however, reliable carbon coefficients can be more difficult to determine.

One common source of carbon coefficients is EPDs of building materials or products. These documents detail the environmental impacts (e.g., GWP) from the material extraction, transportation to plant, manufacturing of a product and, sometimes, later life cycle stages with each of the impacts subdivided into the relevant LCA stages. Although certain building products may have limited EPDs (e.g., concrete masonry units),²⁴ other products, such as concrete mixtures, have many thousands of EPDs because of the product composition and functional variations of concrete.²⁵ Therefore, it is important to select the most appropriate EPD to best quantify the environmental impacts in a LCA.^26,27 For a geographically bound specific building project, EPDs should be sought from (in order of decreasing preference):

1. a plant-specific EPD from a manufacturer;

2. a manufacturer-averaged EPD;

3. a regional industry-averaged EPD; and

4. a national industry-averaged EPD for a given product.

Because of the urgent need to reduce building carbon emissions, many manufacturers of building products have publicly available EPDs on their websites, which is becoming increasingly common, or publish them on EPD repositories such as American Society for Testing and Materials (ASTM) and EC3.²⁸ Acoustic practitioners should follow the above list when selecting EPDs to obtain the most accurate carbon coefficients for the products considered in the LCA.

Acousticians have a large influence on the product (A1–A3) stage of a building's life as their design decisions related to material selections need to absorb, disperse, or isolate sound. For example, to provide a suitable interior acoustical environment, practitioners commonly inform the selection of acoustically absorptive (e.g., open-cell foam, mineral wool glass fiber, PET, cellulose, shredded wood fiber, etc.) and diffusive finish materials (e.g., wood, molded thermoplastic, etc.). The material composition and manufacturing processes of these materials directly affect the EC emissions of the product and subsequent carbon footprint of a building. Therefore, implementing LCAs to compare carbon impacts can inform practitioners on what material or product to select.

Acousticians also engage in material design decisions on assemblies, like walls and floors, to achieve the building's desired sound isolation performance. Practitioners often specify additional mass (e.g., a concrete topping) or added material and resilient layers (e.g., resiliently suspended ceilings and floating floors) from a base assembly to achieve target sound transmission class (STC) and impact insulation class (IIC) ratings in buildings. The additional mass and/or other material layers increase EC emissions compared to the base assembly. As a result, acousticians should consider evaluating (possibly through early-stage design collaboration with sustainability consultants) the carbon footprint of multiple assemblies or products to guide low-carbon design decisions, optimized for acoustic performance.

Although an acoustician's material-related decisions affect a building's EC emissions, especially during the early LCA stages of a building's life, acousticians also have input on decisions that can affect OC emissions throughout the use stage of a building. For example, acoustic-related material decisions on the building's enclosure can influence OC emissions when the building is in operation as a result of effects of varying thermal performance (i.e., R-values) and corresponding heating and cooling loads while also affecting the EC emissions (because of their affiliated densities and thicknesses). Additionally, acousticians provide input on a building's Heating, Ventilation, and Air Conditioning design to meet suitably quiet background noise targets. Common recommendations include providing appropriately sized (i.e., not over-designed), efficient systems and duct designs that reduce turbulence, providing an example of when quiet design can be compatible with strategies to lower OC emissions over a building's life. Last, acousticians also specify sound controlling technologies (e.g., sound masking), which can contribute to the OC emissions in a building. In summary, common acoustic decisions affect a building's EC and OC emissions and, therefore, influence sustainable design decisions in buildings.

To showcase how acousticians can ensure high acoustic performance while reducing carbon emissions, a LCA framework is demonstrated to aid the selection of a low EC floor-ceiling assembly in Sec. 3.1. Nine floor-ceiling assemblies are evaluated for their A1–A3 EC emissions and the International Building Code (IBC)²⁹ requirements of STC 50 and IIC 50 for multifamily residential sound isolation.

A sustainable design strategy is to compare functionally equivalent (for sound isolation) assemblies with their corresponding carbon footprint. Floor-ceiling assemblies are typically required to provide sufficient air- (STC) and structure-borne (IIC) sound attenuation between vertically adjacent spaces in a building. Because these assemblies commonly encompass multiple material layers or products, the carbon emissions of all components in the assembly must be considered to accurately compare different options.

To demonstrate this strategy, a LCA is conducted for nine different floor-ceiling assemblies, commonly designed in multistory residential buildings, including three cross-laminated timber (CLT) floors, two concrete slabs, and four steel composite systems. These nine systems represent a wide range of common floor-ceiling assemblies in residential buildings. The assemblies are compared for acoustic isolation (STC and IIC ratings) and their A1–A3 EC emissions (reported per floor area in kgCO₂e/m²). The EC emissions are normalized per floor area to provide a fair comparison when selecting a floor-ceiling assembly and make the comparison independent of a specific building footprint. Only the product stage emissions (the A1–A3 LCA modules) are considered because these would be the most carbon-intensive LCA modules, and the later stages (e.g., construction, use, and end-of-life stages) would be influenced by the building- and site-specific considerations. The STC and IIC ratings are provided from test reports from the Pliteq EchoOne database.³⁰ 

Figure 2 shows the nine assemblies, and the LCI data are provided in Table 1, which provides the A1–A3 carbon coefficients (product stage GWP), declared unit, EPD source that reports the carbon coefficients and declared units, the BOM (material intensities), and the calculated A1–A3 EC emissions for each material or layer specified in the floor-ceiling assemblies. For more information on how the A1–A3 EC emissions are calculated, the reader is referred to the supplementary material, detailing the calculations and assumptions. The total A1–A3 EC emissions and acoustic performance of the assemblies are given in Table 2.

The carbon emission breakdown for each assembly's individual materials or layers is depicted in Fig. 3. Each assembly's calculated total carbon emissions and their associated STC and IIC performance ratings are shown in Fig. 4.

Figure 3 shows the wide range of EC emissions associated with each assembly. The largest contribution to the total EC of a floor-ceiling assembly comes from the concrete (typically ranging from 50% to 78%), followed by steel decking (28%–35%), and wood (13%–24%). The acoustic underlayment, channel, furring, and gypsum panel accounted for no greater than 12% of the total EC of an assembly.

Figure 4 reveals that while all but one floor assembly satisfies air-borne sound insulation goals (<STC-50), many assemblies require several material layers to meet design requirements for impact sound and would not be appropriate for IBC minimum requirements for residential applications (<IIC-50).¹⁹ Of the assemblies considered, assemblies C, E, G, and I would be considered acoustically viable to meet building code requirements. Although the three timber-based assemblies have lower EC emissions when compared to the concrete and steel composite assemblies, only floor C (timber and concrete with a resilient layer) has an IIC-50+ rating. However, floor C does have the lowest EC emissions out of the four assemblies that achieve a STC-50+ rating and an IIC-50+ rating.

The concrete assembly floor E has the next lowest EC emissions, followed by the two steel composite assemblies G and I. However, if higher STC and IIC ratings are required to suit higher occupant expectations, only floor I would be appropriate for luxury-quality (STC/IIC-60+) residential applications, which has the highest EC emissions of the nine assemblies. This is expected as impact sound isolation is commonly the limiting factor in acoustical design decisions.^41,42

This demonstration exhibits that although the lowest-carbon assemblies can be attractive to architects, sustainability consultants, and other building stakeholders, project teams must consider building performance factors (i.e., sound isolation) beyond carbon emissions. Additionally, acoustic engineers who are familiar with LCA can quantify the percentage increase in EC of acoustically viable mass timber (+33.6%), concrete (+30.1%), and steel composite (+25.8%) assemblies compared to their base assembly counterpart, reinforcing the need for acousticians to become familiar with building-related carbon emissions.

The case study exhibits a real-life sustainability problem that acoustic engineers will likely face with growing environmental goals, especially as urban policies and building codes begin requiring LCAs and carbon emission reporting. By using LCA, acoustic engineers can consider pathways to minimize EC emissions for different building assemblies as low-carbon systems can sometimes (depending on the design goals) be achieved without sacrificing acoustic performance. The assembly example shows how acousticians can consider different materials and layers to satisfy sound isolation requirements while minimizing carbon-intensive materials.

One limitation of this paper is that carbon emissions are the only sustainability measure evaluated. Whereas the focus on EC emissions reflects a primary consideration in current sustainable design practice, designers in the future may need to factor other environmental impacts (e.g., nonrenewable resource consumption and freshwater consumption) and non-environmental factors (e.g., cost) when making sustainable design decisions. A second limitation is that carbon offsets from the use of biogenic materials (i.e., plant-based materials, such as CLT, which absorb atmospheric carbon) were not considered in the case study. Third, the results of the LCA are subject to the quality of the EPD for each material, implying that more specific data could result in slightly different EC emissions.

There are many opportunities for future research at the intersection of architectural acoustics and sustainability. First, research on how to better educate acoustical consultants and the broader design community must improve to ensure that all building subdisciplines can participate in the reduction of building carbon emissions. The educational training will need to be dynamic and updated frequently because of the rapidly changing building codes and design guidance to achieve sustainable solutions, including net-zero carbon buildings.⁴³ Second, research should concentrate on improving the availability and quality of acoustical and environmental data of acoustic materials, products, and systems. Currently, no architectural acoustics focused database exists that informs sustainable design decisions in addition to acoustic performance. This lack of data can severely limit the thoroughness of LCAs, potentially discouraging practitioners from incorporating LCA frameworks in their design decision-making. Therefore, the development of a future acoustic material database can advise practitioners to make informed sustainable design decisions while achieving the desired acoustic performance. A final future research direction is that the AEC industry is advancing with nontraditional structural elements, building metamaterials, and mechanical systems that aid in building decarbonization.^44–49 The acoustic benefits and consequences of these design decisions are still in development, complicating how acousticians address and respond to these advancements.

To achieve low-carbon emission buildings, architectural acousticians must participate in efforts to decarbonize the built environment. In response, this paper motivates and educates architectural acousticians on building decarbonization concepts and methods which can support low-carbon and acoustically viable design decisions. Specifically, a practical strategy to implement a LCA to identify low-carbon floor-ceiling assemblies in residential buildings while meeting acoustic isolation requirements was demonstrated. The example found that additional material layers are needed to satisfy building code requirements for sound isolation, where mass timber assemblies typically have the lowest EC emissions. Additionally, impact sound isolation requirements were found to be the controlling criterion in selecting a low-carbon assembly. This paper also spurs the need for more research at the intersection of acoustics and decarbonization as there are design trade-offs that acoustic engineers need to be aware of in efforts to reduce the carbon footprint of the built environment.

See the supplementary material for the BOM, LCI, and the calculations and assumptions used in the LCA for the nine floor-ceiling assemblies.

This work was supported by Danté Christian and Jonah Sacks at Acentech. The authors would also like to thank Aedan Callaghan and Wil Byrick at Pliteq for sharing their GenieMat Reduced Sound Transmission Environmental Product Declaration.

The authors have no conflicts to disclose.

The data that support the findings of this study are available within the article and its supplementary material.