## Introduction

The design and implementation of residential building interventions that reduce energy costs while protecting indoor environmental health is a high priority in the United States (U.S.) and elsewhere [1]. Approximately 20% of total energy consumption in the U.S. occurs in the residential sector [2], and up to half of this demand is associated with heating and cooling [3]. To reduce these energy costs, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) and the International Energy Conservation Code publish minimum insulation and maximum building envelope leakage requirements to minimize conductive heat transfer and reduce infiltration of unconditioned air across the building envelope, respectively. However, weatherization-related reductions in ventilation can lead to lower air change rates (ACRs) and increased concentrations of indoor-sourced pollutants, such as particulate matter less than or equal to  2.5 μm in aerodynamic diameter (PM2.5) from cooking or smoking [4,5,6] and corresponding health impacts [7,8,9,10]. ASHRAE Standard 62.2 Ventilation for Acceptable Indoor Air Quality in Low-Rise Residential Buildings [11] provides minimum ventilation and filtration requirements, which are intended to minimize these indoor air quality (IAQ) disbenefits, but may also increase energy consumption and upfront installation costs. Several green building standards [12], certification programs (e.g., LEED®, U.S. Green Building Council [13]), and guidelines [14] provide direction on retrofits that meet and exceed the code-minimum requirements of both energy and ventilation standards; however, due to the sometimes complex relationship between energy efficiency and IAQ, it has been previously emphasized that more research is needed to determine the combined impact of building weatherization and ventilation retrofits on energy consumption and IAQ prescribed by these programs [1]. Furthermore, it is important to consider these impacts across variable occupant activities, such as smoking, cooking, and window-opening, which can influence the IAQ and energy performance of building retrofits [15].

Balancing energy savings and healthy IAQ are especially important in multifamily residences, in which ~20% of the U.S. population resides [16]. Most energy retrofitting programs are geared toward the single-family residential sector [17] and may not address the unique retrofitting needs of multifamily buildings. For example, multifamily buildings can include common-use spaces and ventilation systems, and there is often air and contaminant leakage between individual family dwelling units [18]. Multifamily rental units are predominantly occupied by low- and middle-income populations in urban settings [19], where smoking rates [20] and baseline rates of respiratory illness [21] tend to be higher. Furthermore, energy retrofitting programs are underutilized in the multifamily housing sector due to financial and logistical challenges (e.g., split incentives where building owners cover the retrofit implementation costs while residents may experience most of the savings) [19].

Since the implementation of energy retrofits is often driven by financial incentives, a more refined understanding of how to maximize IAQ benefits while reducing energy costs is important for more widespread implementation. Numerous field investigations have demonstrated that multifamily energy retrofit programs can lead to net improvements in IAQ [17, 22,23,24,25,26]; however, it is difficult to parse out the benefits from upgraded ventilation systems and improved ACRs reported in some studies [26, 17, 24] from post-retrofit behavioral changes (e.g., window-opening [23] or indoor smoking cessation [22, 24]) and indoor source reductions (e.g., replacement of gas stoves [22]) in others. Furthermore, most field studies are not able to characterize the separate impacts of weatherization interventions designed to save energy and ventilation retrofits that can increase energy consumption.

Building simulation models provide a means to evaluate multiple types of building interventions, but to date these models have tended to focus on optimizing a single aspect of building performance without comprehensively evaluating tradeoffs between energy and IAQ. One example of such a model is CONTAM, a multizone airflow and pollutant transport analysis program that can account for the complex airflows in buildings, including multifamily units [27]. Studies utilizing this model have demonstrated that building sealing can lead to increased indoor pollutant concentrations due to lower ACRs, while sealing coupled with improved ventilation can mitigate these adverse impacts [4, 6, 8, 15, 28]. However, CONTAM does not perform heat transfer calculations or other related computations needed to predict building energy use. Previous studies that employed loose IAQ-energy coupling used CONTAM outputs (e.g., ventilation rates) as inputs to energy models to estimate energy savings potential [8, 29]; but this approach does not necessarily account for heat loss due to infiltration heat transfer particularly between units in multifamily buildings. In addition, CONTAM relies on user-defined indoor temperature schedules that do not necessarily correlate with changes in occupant activity and ambient meteorological conditions. Recent studies have demonstrated substantial differences in predicted indoor pollutant concentrations between models with dynamic and static indoor temperatures [30, 31].

To capture the interdependencies between airflow and heat transfer, multizone airflow models can be dynamically coupled with energy simulation software [31, 32]. For example, the National Institute of Standards and Technology (NIST) coupled the two public-domain programs, CONTAM and EnergyPlus, using the Functional Mock-up Interface for Co-simulation specification [33, 34]. EnergyPlus is a whole-building energy simulation tool that utilizes a multizone heat balance model to account for conductive, convective, and radiant heat transfer; heating, ventilation, and air-conditioning (HVAC) system dynamics; and user-defined interzone and infiltration airflows. While numerous studies have utilized EnergyPlus to estimate the impacts of residential retrofits on energy, few have considered the corresponding impacts on IAQ. A recent study indirectly coupled EnergyPlus simulation results with a single zone contaminant transport model to perform energy and IAQ analysis for a representative set of U.S. residential buildings [35]. Meanwhile, dynamic co-simulation of CONTAM and EnergyPlus allows run-time data exchange between the two programs, so that interzone airflows and infiltration rates from CONTAM are passed to EnergyPlus while indoor temperatures and HVAC system airflow rates determined by EnergyPlus are passed to CONTAM. Since infiltration and interzone airflows are dependent on temperature differences between zones, accounting for heat transfer and associated changes in interior temperatures can provide for more realistic interzone airflows and resultant contaminant transport.

In this study, the CONTAM-EnergyPlus co-simulation platform was utilized to quantify the influence of weatherization, ventilation retrofits, and resident behavior (e.g., cooking and smoking) on IAQ and energy consumption in a multifamily building model located in Boston, Massachusetts. To the authors’ knowledge, this is the first study to utilize co-simulation to dynamically couple two widely-used and publicly available software tools to predict the impacts of multifamily building retrofits on both IAQ and energy consumption.

## Methods

### Building model

IAQ and energy utilization in a hypothetical mid-rise multifamily apartment building in Boston, MA were modeled using a coupled simulation of CONTAM 3.2.0.3 [27] and EnergyPlus 8.5 [36, 37]. The simulations performed in this study utilized a prototypical mid-rise apartment building selected from a nationally representative set of U.S. Department of Energy Commercial Reference Building EnergyPlus models that were developed by the National Renewable Energy Laboratory [38]. NIST developed a corresponding CONTAM representation of this building to be compatible with the co-simulation, as outlined previously [39]. The model was further modified to include stair and elevator shafts to simulate vertical stack flows that can be particularly important to infiltration, energy use, and contaminant transport in multi-story buildings.

The final model (shown in Figs. 1 and 2) consists of four stories with a total building volume of 9641 m3 and floor area of 3163 m2. Each story contains eight apartments and a hallway having a stair and elevator shaft at each end. Each of the 32 apartment units is 269 m3 and 88 m2 in size and assumed to be occupied by a family of four. Each apartment was modeled as a single well-mixed zone served by its own HVAC system with cooling and heating thermostatic set points of 24 °C and 21 °C, respectively. The HVAC systems each consisted of a unitary system with a direct expansion cooling coil (approximately SEER 12), a natural gas furnace (nominal efficiency of 80%) that was properly vented (i.e., emissions discharged outdoors), and a constant volume supply fan (nominal efficiency of 54%). Each apartment was served by a dedicated exhaust system (nominal fan efficiency of 60%) that could be scheduled depending on the ventilation system type as explained below.

### Ventilation systems

Whole-building ventilation is defined as the continuous provision of outdoor air to dilute indoor air contaminants [40]. ASHRAE Standard 62.2-2016 allows for either mechanical supply or exhaust systems that provide continuous ventilation with outdoor air [11]. The mid-rise apartment building was developed with three alternative ventilation configurations: one infiltration-only and two whole-building mechanical ventilation systems outlined below.

1. 1.

Infiltration-only. Outdoor air enters via unintentional openings (e.g., cracks) in the building envelope due to natural driving forces of wind and buoyancy. No outdoor air is introduced via the HVAC systems; therefore, supply fans only operate intermittently in recirculation mode for heating and cooling purposes. This scenario is meant to represent an older, pre-retrofit building that is not ASHRAE-compliant.

2. 2.

Whole-building exhaust ventilation. Continuous ventilation is provided via a separate exhaust fan set to the minimum rate of 0.024 m3/s (51 cfm) for a two-bedroom apartment as determined by equation 4.1a of ASHRAE 62.2-2016 [11]. The HVAC supply fans only operate intermittently in recirculation mode for heating and cooling purposes.

3. 3.

Whole-building balanced supply-exhaust ventilation. Continuous ventilation is provided via an outdoor air intake that allows filtered outdoor air to be brought in via the HVAC supply system. HVAC supply fans operate continuously while providing the same ventilation rate as the exhaust system, as well as providing a constant volume of supply air for heating and cooling purposes. Outdoor air intake to each unit is balanced by exhaust air.

In all cases, the supply fan airflow rates were auto-sized by EnergyPlus, so the supply airflow rates for each apartment and each scenario could be different. However, the outdoor air ventilation rates will remain constant for the exhaust and balanced systems.

### Resident activities, pollutants, and environmental parameters

CONTAM was used to model indoor PM2.5 from indoor sources and ambient air. Indoor sources included cooking (low or high cooking) and smoking (no smoking or six cigarettes per day). Pollutant emission and deposition rates and schedules were selected from published literature [41,42,43,44] (Table 1). Envelope and internal penetration factors were modeled at 100%. Hourly ambient concentrations of PM2.5 were obtained from a local monitor in Boston, MA, as described elsewhere [6]. Hourly weather data (i.e., temperature and relative humidity) was obtained from a typical meteorological year (TMY) dataset (Boston Station 14739, 1961–1990: TMY2, National Solar Radiation Data Base, National Renewable Energy Laboratory) [45]. One-year simulations were run with 5-min time-steps to capture short cooking and smoking events and diurnal and seasonal variations. Ambient environmental inputs, e.g., concentrations and temperatures, were linearly interpolated at the simulation time step by the simulation programs.

### Building interventions

A baseline scenario, a standard intervention package, and a high-performance intervention package were modeled across the 12 combinations of whole-building ventilation (infiltration only, exhaust, and balanced) and indoor PM2.5 source scenarios (non-smoking, smoking, low cooking, and high cooking). Each intervention package included weatherization retrofits (i.e., increased insulation and reduced envelope leakage via sealing), improved HVAC filtration, and the installation of local kitchen exhaust (Table 2). Ventilation parameters were based on the ASHRAE Standard 62.2-2016, which requires a filter with a minimum efficiency reporting value (MERV) of 8, a local kitchen exhaust, and the provision of continuous ventilation with an outdoor air exhaust system, supply system, or combination of both [11]. Local kitchen exhaust was modeled by reducing the PM2.5 emission rate for cooking by 30% based on typical capture efficiencies reported previously [46,47,48]. The energy use of local kitchen exhaust was not modeled. Insulation and envelope leakage parameters were based on ASHRAE 90.1-2016 (Energy Standard for Buildings Except Low-Rise Residential Buildings) [49] and ASHRAE 189.1-2014 (Standard for the Design of High-Performance Green Buildings) [12], respectively. Interior, interzone leakage rates were not changed during the retrofit, and an effective leakage area of 5.27 cm2/m2 at 4 Pa was equally distributed across all interior surfaces.

### Simulation logistics and data analysis

Indoor environmental conditions and energy consumption were simulated using co-simulation between CONTAM version 3.2.0.3 (NIST, Gaithersburg, MD, USA) and EnergyPlus 8.5 (U.S. Department of Energy) [39]. Factorial building model generation software developed by NIST was used to produce paired sets of CONTAM and EnergyPlus input files, representing 648 building combinations (3 whole-building ventilation systems × 4 indoor source scenarios × 3 leakage rates × 3 insulation levels × 3 MERV filter efficiency ratings × 2 cooking exhaust settings: on/off). The Boston University Shared Computing Cluster (a heterogeneous Linux cluster) was used to minimize simulation time by running multiple cases in parallel. Each component of the intervention packages was modeled separately and in a step-wise fashion to examine the incremental impacts on PM2.5 concentrations, electricity utilization, and gas consumption.

Simulation results were summarized to obtain monthly and annual mean outdoor concentrations of PM2.5, outdoor temperature, source-specific (e.g., outdoor air, smoking, and cooking) indoor concentrations of PM2.5, indoor temperature, and ACRs using R statistical software version 3.4.0 [50]. Indoor pollutant concentrations and temperature were reported from a single 3rd-floor corner apartment and do not consider exposures in hallways and other shared spaces. Energy usage is reported in standard cubic feet (SCF) for gas and kilowatt-hours (kWh) for electricity. The fuel-specific and total energy impacts of each intervention scenario were monetized using natural gas and electricity prices for Massachusetts in 2016: $0.19 per kWh of electricity and$12.46 per 1000 SCF of natural gas [51].

## Results

### Baseline building characteristics

Tables 3 and 4 present the total annual energy consumption and yearly average indoor pollutant concentrations per apartment, respectively, for the 12 baseline scenarios. Building scenarios without mechanical ventilation had the lowest heating gas consumption, followed by scenarios with exhaust and balanced ventilation systems (Table 3). This pattern reflects the annual average ACRs, which were 0.40 h−1 in buildings without mechanical whole-building ventilation (i.e., infiltration-only) and 0.55 h−1 and 0.69 h−1 in buildings with exhaust and balanced ventilation, respectively. While the exhaust and balanced systems were set to the same ventilation rate, the balanced system ACR was greater because the associated air intake had a neutral impact on building pressurization and therefore allowed for additional infiltration (as outlined in Chapter 16 of the 2017 ASHRAE Handbook [52].

Meanwhile, electricity utilization associated with space cooling and fan operation was lowest in buildings with exhaust ventilation and highest in buildings with balanced ventilation (Table 3). Across each indoor source scenario, indoor PM2.5 concentrations were highest in buildings lacking mechanical ventilation (range: 18 to 43 μg/m3) and lowest in buildings with balanced ventilation (range: 15 to 29 μg/m3), with the magnitude dependent on the level of the indoor sources (Table 4). For comparison, an annual hourly mean outdoor PM2.5 concentration of 12 μg/m3 (sd = 2.7) was simulated.

### Intervention impacts on electricity and gas consumption

Figures 3 and 4 display the predicted changes in gas consumption and electricity utilization, respectively, attributable to standard and high-performance interventions for each of the three ventilation systems. Across all models, the combination of standard insulation and sealing reduced natural gas consumption for heating by 6.2 to 7.0 thousand SCF per apartment per year (Fig. 3), corresponding to 18–20% reductions in energy consumption compared with baseline. High-performance insulation and sealing resulted in gas consumption savings of 13–15 thousand SCF per apartment per year, representing a 41–45% reduction compared with baseline. While energy benefits were most substantial from a combination of insulation and sealing, reduced leakage from sealing was the driver of these benefits, especially in the high-performance intervention (Fig. 3).

Compared with gas consumption benefits, relative changes in electricity utilization associated with cooling and fan operation were lower (< 15% compared with baseline across all ventilation systems and interventions) and displayed greater variability across ventilation systems (Fig. 4). Overall, the combination of standard insulation and sealing led to electricity decreases of 60, 71, and 136 kWh per apartment per year in buildings without mechanical ventilation, with exhaust ventilation, and with balanced ventilation, respectively. The corresponding decreases for high-performance interventions were 105, 160, and 147 kWh. The differences relate to the relative impact of the insulation and sealing interventions on space cooling and supply fan operation. Across all ventilation scenarios, insulation led to electricity savings due to reduced heating and cooling demands and lower supply fan flow rates. The greatest absolute savings were observed in the balanced ventilation scenario due to the fact that the central supply fans ran intermittently in the exhaust scenario and continuously in the balanced ventilation scenario. Meanwhile, sealing led to increases in cooling electricity as a result of the loss of “free cooling” during the spring and fall months when the infiltration of cooler air can reduce indoor heat gains (results not shown). In buildings without mechanical ventilation or with exhaust ventilation, these increases were offset by large reductions in supply fan electricity use associated with less heating. However, the increases in cooling electricity were not offset for buildings with balanced ventilation due to the fact that system fans would be running continuously to provide outdoor air regardless of heating demands. The effects of higher efficiency MERV filtration on energy consumption were not modeled assuming filtration systems were installed and maintained to achieve similar resistance to airflow.

### Intervention impacts on indoor PM2.5

Figure 5 displays mean changes in total indoor PM2.5 concentrations due to the standard and high-performance interventions across the 12 combinations of whole building ventilation systems and indoor source activity. The interventions are implemented step-wise and include weatherization (i.e., insulation and sealing) and ventilation improvements (i.e., HVAC filtration and local kitchen exhaust). Across all 12 combinations, the combined standard weatherization intervention led to increases in indoor PM2.5 concentrations ranging from 2 to 12 µg/m3, while the high-performance weatherization intervention led to increases of 3 to 75 µg/m3 (Fig. 5). Accordingly, reduced annual average ACRs were observed as a result of sealing: the standard and high-performance interventions reduced annual ACR averages by 0.13 h−1 and 0.27 h−1, respectively, in the exhaust ventilation scenario and by 0.12 h−1 and 0.29 h−1, respectively, in the infiltration-only and balanced ventilation scenarios. Meanwhile, insulation had a negligible impact on ACRs (< 0.01%) and increased indoor concentrations by < 1 µg/m3 in all cases. This increase is also the result of reduced heating and cooling demands from insulation leading to less air recirculation and hence filtration (results not shown). Overall, concentration increases were highest in magnitude in the scenarios without mechanical ventilation, followed by exhaust ventilation, and lowest in buildings with balanced ventilation.

Under the standard intervention, the addition of MERV 8 filters led to net reductions in PM2.5 for all building types and indoor source activity levels. For the high-performance intervention, higher efficiency MERV 12 filters combined with sealing and insulation only provided PM2.5 reductions for buildings with balanced ventilation (Fig. 5). For buildings without mechanical ventilation or with exhaust-only ventilation, the combination of higher efficiency HVAC filtration plus local kitchen exhaust led to PM2.5 reductions in non-smoking apartments, but concentrations remained elevated relative to baseline in apartments with smokers (Fig. 5).

### IAQ and energy cost tradeoffs

Figure 6 highlights the potential trade-offs between changes in IAQ and energy costs associated with implementing standard and high-performance interventions across three different whole-building ventilation systems (infiltration-only, exhaust, and balanced) and two occupant activity scenarios (low cooking plus no smoking and heavy cooking plus smoking). Following the application of the interventions, the exhaust and balanced ventilation models (Fig. 6b, c) would be ASHRAE compliant while the infiltration-only model (Fig. 6a) would remain non-compliant due to the lack of whole-building ventilation. Therefore, Fig. 7 compares intervention impacts in the infiltration-only model with and without the addition of ASHRAE-compliant whole-building exhaust or balanced ventilation. While all points in the lower quadrants (i.e., negative values indicating reduced energy costs) represent energy cost saving interventions, all points in the left quadrants represent interventions with IAQ benefits (i.e., decreased annual average PM2.5 concentrations). Accordingly, any scenario in the lower left quadrant represents a “win–win” scenario (based on the modeling assumptions of this study) in which both energy savings and PM2.5 improvements are achieved.

These models predict that standard weatherization retrofits will reduce energy costs by $89,$101, and $111 per apartment per year in the simulated infiltration-only, exhaust, and balanced ventilation scenarios, respectively (Fig. 6); however, across all ventilation and indoor source scenarios, these weatherization retrofits will also lead to increases in PM2.5 unless local kitchen exhaust is installed (and operated) and HVAC filtration is upgraded. Therefore, under the standard intervention, IAQ-energy “win–win” outcomes require the application of a full intervention (i.e., weatherization plus local kitchen exhaust and filtration improvements). The high-performance intervention led to greater total energy savings than the standard intervention, reducing energy costs by$186, $219, and$225 per apartment per year in the infiltration-only, balanced, and exhaust ventilated buildings, respectively, but with fewer “win–win” outcomes (Fig. 6). In the infiltration-only or exhaust ventilation scenarios, only non-smoking apartments receiving the full high-performance intervention achieved both energy savings and lower indoor PM2.5. Meanwhile, in scenarios with balanced ventilation, the full high-performance intervention led to energy savings and lower levels of PM2.5 in both smoking and non-smoking apartments. Across all ventilation scenarios, weatherization alone provided energy savings, but also led to PM2.5 increases, which were greatest in smoking homes.

A comparison of intervention impacts in the infiltration-only model with and without the addition of ASHRAE-compliant whole-building ventilation (Fig. 7) demonstrates that both the exhaust system and balanced ventilation system were associated with increased energy use; however, they also led to greater reductions of indoor PM2.5 (Fig. 7b, c vs. 7a). When infiltration-only models were upgraded with whole-building exhaust ventilation, all full intervention packages resulted in “win–win” outcomes (Fig. 7b), despite the increased energy use associated with added whole-building ventilation. The infiltration-only buildings that were upgraded with balanced ventilation (Fig. 7c) resulted in indoor concentrations that were reduced even further; however, the energy penalty associated with balanced ventilation was so large that all interventions resulted in net increases in energy costs. Overall, while infiltration-only and added continuous exhaust ventilation led to more “win–win” outcomes, they were less resilient to the negative impacts of strong indoor sources (e.g., heavy cooking and smoking) and high performance weatherization on IAQ compared with the balanced ventilation.

## Discussion

In this study, a recently developed, freely-available IAQ-energy co-simulation model of a mid-rise apartment building was used to demonstrate that combined investments in weatherization and improved ventilation sometimes, but not always, lead to both energy savings and IAQ-related benefits. The magnitude of these impacts varied by the presence and type of mechanical ventilation, the extent of retrofits, and indoor occupant activity (i.e., contaminant generation rates). Given this variation and the fact that energy and ventilation standards are often developed and implemented without consideration of their combined impacts, the IAQ-energy model presented here stands as a powerful tool for identifying retrofits that provide both economic and indoor environmental benefits across diverse housing environments, such as low-income multifamily buildings.

An important observation of this study is that more stringent weatherization measures, as part of the high-performance interventions, can lead to greater increases in indoor PM2.5 compared with those modeled in the standard interventions. Furthermore, ventilation measures that meet or even exceed minimum requirements may not be sufficient to avoid these IAQ disbenefits in the presence of strong indoor sources, such as heavy cooking and smoking. These results are in line with previous modeling evidence of substantial increases in concentrations of indoor-sourced pollutants for high levels of airtightness [53]. They also reinforce the fact that advanced building sealing measures should be implemented with caution because even small incremental increases in building tightness could lead to substantial IAQ disbenefits for some building populations. For example, in smoking apartments without mechanical ventilation, additional source control policies, such as no-smoking interventions, would be required to implement high-performance interventions without IAQ disbenefits. While green building guidelines and standards (e.g., EPA’s Energy Savings Plus Health, 2016 [14]) often suggest source control policies, IAQ benefits ultimately rely on compliance from occupants and neighboring apartments.

This study also provides insight into maximizing IAQ-energy “win–win” outcomes by minimizing both ventilation-related energy consumption and weatherization-related IAQ disbenefits. Modeling energy savings by fuel and end use revealed that while weatherization retrofits provided heating gas consumption benefits across all ventilation scenarios, their impacts on electricity utilization were more nuanced due to multiple end uses. In particular, sealing led to slight increases in cooling electricity across all ventilation systems due to a loss in “free cooling.” In addition, the installation of a continuous supply fan to meet the ventilation requirement of the ASHRAE Standard 62.2 resulted in net increases in energy costs despite weatherization-related energy benefits. Potential design choices exist to alleviate these effects, including heat and energy recovery ventilation (HRV and ERV), economizers, thermostatic control, and intermittent ventilation. In addition, resizing the supply fan to account for the increased heating efficiency of the building following sealing measures can greatly reduce energy impacts. The coupled CONTAM-EnergyPlus model is flexible and could accommodate these potential design modifications to address their respective energy and IAQ costs and benefits.

Although this study has a number of strengths given the novel, publically-available modeling platform and physically interpretable outputs, some limitations should be acknowledged. First, each apartment was modeled as a single, well-mixed zone, limiting the ability to account for spatial variations in concentrations across zones or directly model local kitchen exhaust. To avoid underestimating the effect of kitchen exhaust, the cooking emission rate was reduced to reflect previously observed capture efficiencies [46,47,48]. Future CONTAM-EnergyPlus co-simulation models could utilize the multizone modeling and exhaust system capabilities of each program to directly account for the energy impacts associated with fan operation, conditioning of replacement air, and loss of heat generated by the stove (previously reviewed by Logue and Singer [54]). In addition, energy increases due to flow resistance associated with high-efficiency MERV filters were not modeled. While the potential for energy impacts due to higher efficiency HVAC filtration is supported by previous research, results have varied by MERV efficiency, ventilation configuration, and building typology (e.g., large commercial office buildings [55] versus smaller residential homes [56, 57]); therefore, the evidence was considered to be too limited to incorporate into this site-specific model with certainty, and neither CONTAM nor EnergyPlus currently has a built-in mechanism to model this effect.

While variable indoor source activity was modeled, the full extent of variability in human activity was not accounted for, including flexible cooking schedules and the use of windows for natural ventilation. Since the building models in this study were controlled by a thermostat year-round, the assumption of closed windows is reasonable since one of the major drivers of window-opening is temperature [58]. However, the assumption of closed windows does not account for window-opening in response to reduced IAQ (e.g., due to cooking or smoking), other physical parameters (e.g., odor, humidity, noise, illumination), or seasonal behaviors (e.g., increased window-opening in temperate seasons) [58]. Window-opening in response to these factors could mitigate some of the higher modeled concentrations as well as substantially impact energy utilization. In terms of cost, the annual savings due to various interventions was estimated, but the installation costs were not taken into account, which would be required in order to reach more definitive conclusions regarding the cost-effectiveness of the interventions. Lastly, the mid-rise apartment building and associated set of intervention scenarios in a single apartment modeled in Boston, MA may not be generalizable to other apartment locations, buildings, ventilation systems, climates, and outdoor pollution profiles. In future work, the CONTAM-EnergyPlus modeling platform could be utilized to build a nationally representative sample of U.S. homes, similar to previous simulations of energy utilization [3, 59, 60] and indirect co-simulations of energy and IAQ [35]. The novel contribution of this modeling framework would be the ability to dynamically co-simulate both energy and IAQ.

In spite of these limitations, results were comparable to previously reported data. According to the 2009 Residential Energy Consumption Survey, households in multifamily buildings with five or more units in the northeastern U.S. used, on average, 457 kWh of electricity for air conditioning and 38,000 SCF of natural gas for space heating [61] (vs. 580 kWh–640 kWh of electricity and 28,000–37,000 SCF of natural gas, respectively, in this baseline model). As the building template used in this study was not necessarily representative of the full distribution of multifamily buildings in the northeastern U.S., some differences would be expected, and these values support the validity of the simulated energy outputs. Simulated indoor PM2.5 results similarly align with previous field-based literature in spite of structural differences. In a cohort of Boston metropolitan area homes that were primarily non-smoking multifamily units, Baxter et al. measured a mean indoor PM2.5 concentration of 20 μg/m3 that ranged from 8 to 75 μg/m3 [62], which falls within the 15–29 μg/m3 range of simulated indoor PM2.5 concentrations in the baseline, non-smoking models used in this study. Meanwhile, Van Deusen et al. measured average PM2.5 concentrations of 63 and 84 μg/m3, ranging from 14 to 194 μg/m3 and 23 to 285 μg/m3, respectively, in rooms close to and far from residents smoking 44 cigarettes/day [63]. Wallace et al. reported average concentrations of 133 and 66 μg/m3 in homes with smoking rates of seven and four cigarettes per day, respectively [64]. The smoking models in this study included indoor PM2.5 concentrations ranging from 23 to 43 μg/m3, which are lower than the previously cited literature, but reasonable considering a smoking rate of 6 cigarettes/day and year-round HVAC filtration in all models. More generally, the findings in this study align with trends from previous simulation studies of nationally and regionally representative sets of single and multifamily homes in the U.S. and U.K [3, 8, 59, 65]. Results of this study also highlight the interactions of weatherization components on energy and indoor pollutant levels, which are expected based on first principles of heat transfer and pollutant transport, respectively, and current consensus in the literature [4, 28].

## Conclusions

Overall, this co-simulation capability provides valuable insights regarding the economic and IAQ implications of energy and ventilation retrofits recommended by current building standards across diverse indoor occupant activity scenarios. Coupling two validated, public domain building physics models enables the accounting of the complex and interdependent relationship between energy, airflow, and pollutant transport modeling in multifamily buildings to more accurately predict IAQ-energy “win–win” intervention outcomes compared with previous energy- or IAQ-only models. This modeling capability can accommodate alternative assumptions regarding building configurations, intervention types, occupant activity, and weather; therefore, it is well-poised to inform building standards development and policy across diverse populations, housing stock, and geographic areas.