Combining Energy Performance and Indoor Environmental Quality (IEQ) in Buildings: A Systematic Review on Common IEQ Guidelines and Energy Codes in North America

Abstract

The indoor environmental quality (IEQ) in buildings is vital for health, work efficiency, productivity, and the overall sustainability of buildings. IEQ is governed by four parameters: indoor air quality and thermal, acoustic, and visual comfort. The recent pandemic has compelled people to think beyond energy efficiency and refocus on the health, well-being, and productivity of building occupants. Despite numerous IEQ guidelines and standards, there remains a paucity of systematic research that critically examines the relationship between IEQ and building energy efficiency. This systematic review explores the existing equilibrium and identifies gaps between IEQ standards and building energy codes. Firstly, this review examined the status of the IEQ standards and identified that most of the North American IEQ guidelines cannot achieve energy efficiency targets. Secondly, existing building energy codes were reviewed to determine how well these codes fare with IEQ requirements. It was revealed that the expensive energy certification documents are more focused on IEQ than traditional energy codes. The identified factors indicate that most building energy codes can meet only indoor air quality thresholds (a subset of IEQ), while other parameters are inadequately addressed. This review revealed 19 relationships between IEQs and energy efficiency. Building energy code/IEQ guidelines developers could consider the identified 19 relationships to develop a combined set of guidelines/standards for future building stock. An integration model between IEQ and energy efficiency is proposed as a future research direction to contribute to the better design and construction of modern buildings. The findings will facilitate the construction of healthy and sustainable buildings, and they aim to generate new residential communities that achieve an optimal health–energy–carbon nexus.

1. Introduction

The rapid growth of the human population and increasing levels of activities have resulted in an exponential increase in energy and resource use. Several adverse impacts have been induced due to increased energy consumption. Climate change, potential energy crises, energy poverty, geopolitical issues, and negative impacts on human health and well-being are among the adverse effects [1]. The global building sector is one of the primary consumers of global fossil-fuel-based energy. Buildings generate vast amounts of anthropogenic greenhouse gas (GHG) emissions [2]. Reducing building-related energy consumption and emissions has been a dominant aim in the building construction industry over the past decade [3,4]. Building energy use in North America is predicted to be a significant contributor to fossil-fuel-based carbon emissions by 2025 [5,6]. In Canada, 80% of the GHG emissions are attributed to fossil-fuel-based energy usage, and out of that, the building sector is responsible for 12% of the GHG emissions [7]. There are numerous initiatives to reduce building energy consumption and mitigate building-related emissions. The most widely applied initiatives include building energy codes and green building performance standards. The primary purpose of these initiatives is to act as a policy mechanism to improve energy use while reducing building-related emissions. The current building designs and construction process heavily depend on these methods since building energy efficiency is mandated by regulations and laws in many countries [8]. However, the potential of these initiatives to maintain and improve the occupant-related dimensions of building performance, including occupant health, satisfaction, and productivity, is not fully explored [9].

The design and development of healthy, energy-efficient, and occupant-centric buildings have recently become popular, as indoor living conditions remain unsatisfactory in many buildings [10]. Domestic houses, schools, and office buildings (residential, institutional, and commercial) have garnered major attention here. These are the places where most daily human activities take place. Moreover, the aging population further contributes to increased indoor dwelling time, as they spend around 95% of their time indoors. The COVID-19 pandemic has contributed to significant lifestyle changes and has increased indoor dwelling time among all age groups. Currently, several building standard organizations, such as the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), are introducing response resources to decrease the impact and spread of airborne infections like COVID-19 by improving indoor environmental conditions. However, these response resources are still in the early development stages and do not cover overall human health conditions, including full indoor environmental aspects with energy efficiency as a whole package. Hence, the necessity to develop comprehensive guidelines and standards that maintain building energy efficiency and improve indoor environmental conditions is increasing [11].

The indoor conditions of buildings are determined by indoor environmental quality (IEQ) standards and relevant IEQ parameters. IEQ standards are crucial as the public interest is now shifting towards building indoor conditions and healthy living environments. IEQ standards set the mechanisms for developing indoor environment conditions based on prescriptive or performance levels. Thermal comfort, indoor air quality, acoustic comfort, and visual comfort are the pillars of IEQ. The overall building design, construction method, operation, and maintenance techniques directly impact a building’s indoor environmental conditions [12,13].

The implementation of building energy codes may favor positive or negative impacts on IEQ parameters based on the fundamental design differences. Most building energy codes emphasize reduced infiltration, lower air changes, and higher thermal-mass-based insulation [14]. Previous studies on using high thermal masses in residential buildings have shown that hot summers adversely increase indoor temperatures [15]. Such issues arise from the excessive use of building code details without properly accounting for the combined effect of IEQ and energy efficiency. The recent climate events involving extremely hot temperatures further increase the requirement for proper indoor environmental thermal balance with energy efficiency. The 2023 summer heat wave in North America [16] and the 2021 summer heat dome in the province of British Columbia, Canada [17], are the most recent of such events.

The adverse impacts of poor IEQ are not just tied to the thermal environment but also to the health, well-being, and workplace satisfaction of the occupants through indoor air quality [18]. Reduced infiltration and low fresh air rates can cause poor indoor air quality. Likewise, improved indoor air quality in occupant spaces can enhance work productivity by 0.5–5%. The equivalent monetary value of this productivity increase is estimated at USD 12–125 billion annually [19]. Furthermore, the global annual extra expenses are around USD 100 billion due to poor IAQ issues, including the treatment of SBS and other illnesses, worker absenteeism, and maintenance costs for problematic buildings [19]. Sick building syndrome (SBS) is one of the most common adverse effects residents suffer in energy-efficient buildings related to indoor air [20]. With SBS, occupants may experience adverse health symptoms without a specific illness when indoor climate control methods are in effect under energy code practices. Hence, indoor air quality is also essential for the health of occupants and the overall sustainability of the building through energy, emissions, and economic performance. In North America, adverse impacts on building IAQ due to energy efficiency practices are well documented. The National Institute of Occupational Safety and Health in the United States of America has identified inadequate ventilation rates as the primary contributor to SBS [21,22]. In Canada, PM2.5, a major indoor pollutant, is estimated to cause CAD 5.5 billion worth of healthcare costs associated with premature deaths [23].

Likewise, problems of poor visual and acoustic comfort are documented even in energy-efficient office buildings. For instance, a statistical survey of 120 employees who shifted from an old conventional office building to a low-energy building revealed that despite improved thermal comfort, indoor air quality and levels of visual and acoustic comfort decreased. Similarly, a study conducted on two LEED (Leadership in Energy and Environment Design)-certified buildings found that over 50% of the occupants believed that the indoor conditions were negatively influencing their work performance and health [22,24]. Therefore, the above practical cases highlight major conflicts between the design of the building envelope and energy systems to achieve higher energy efficiency and IEQ of occupant spaces. This indicates that the relationship between IEQ and existing building energy codes is poorly understood. Based on the existing body of knowledge, very few studies have attempted to identify these dynamics between IEQ standards and building energy codes in a combined platform. Indoor conditions and occupants’ health are extremely sensitive to design methods and energy-saving practices in occupant-centric building designs. Therefore, the evaluation of all the IEQ parameters needs to be comprehensively explored in relation to building energy efficiency. In particular, the significance of proper IEQ levels and quality of living spaces have been highlighted since the COVID-19 pandemic.

The primary objective of this study is to identify the current state of commonly available IEQ standards and the interrelationships between IEQ standards and building energy codes/standards from a North American perspective. This review focuses on the common IEQ parameters, current guidelines, standards for IEQ maintenance, various assessment models to analyze IEQs, and interrelationships between IEQs and existing building energy codes. Moreover, the limitations and missing aspects of these North American codes are discussed compared to other global standards as to how they should be revised or modified accordingly. Building energy code developers, building-related mandates, and policymakers could greatly benefit from the content of this review. Moreover, future research directions could be derived based on the findings to integrate the IEQ standard assessments into building energy efficiency enhancement methods.

2. Materials and Methods

This review investigates the existing status of the common North-America-based IEQ standards and guidelines and their interaction with building energy codes and green building standards. This work aims to see how well IEQ standards align with existing energy codes and standards and to identify conflicts, synergies, and overall implications for sustainable building policies and practices. To achieve these objectives, the Preferred Reporting Items for Systemic Reviews and Meta-Analysis (PRISMA) 2020 guidelines were followed, and a systematic content analysis was conducted [25]. The PRISMA guidelines were first introduced in 2009 as a technique to perform systematic analysis. The PRISMA 2020 guidelines are the latest revision that considers advancements in data identification, collection, and analysis. Content analysis is a widely used research tool to determine the presence and relationships of certain concepts and words where qualitative data can be presented under certain themes. The integration of the PRISMA guidelines and content in this work ensures that the relevant literature is identified and themes and trends in existing IEQ standards and building energy codes are uncovered.

The following figure, Figure 1, describes the results of the search and selection process, from the number of records identified in the search to the number of studies included in this review, using the PRISMA 2020 flow diagram for systematic reviews. The flow diagram was made with the assistance of the Shinyapp tool available on the Prisma 2020 statement website [26]. A detailed PRISMA 2020 checklist is provided in the Supplementary Materials File S1 that explains further details on the process of data extraction from databases.

Data collection is the first step in the PRISMA 2020 method. The data collection process involved keyword searches in peer-reviewed journals and scientific databases such as Compendex Engineering Village, Google Scholar, and IEEE Xplore [27,28]. These three databases were selected based on their broad coverage of engineering disciplines, ease of access, and global reach. In particular, Compendex Engineering Village and IEEE Xplore contain a vast collection of peer-reviewed journals and book chapters. Moreover, the inbuilt automation features such as sorting, abstract summarization, and key word combinations are available in the Compendex Engineering Village database with full access to Elsevier data. Moreover, the Compendex Engineering Village database narrows down the Elsevier database through the lens of engineering disciplines. Since this review does not include any statistical data and experiments, statistical databases were excluded from the research data extraction process. The content gathered by the selected databases included peer-reviewed research articles, conference publications, and scientific reports. Moreover, government-issued documents, provincial or regional municipality reports, North American standards and guidelines (e.g., ASHRAE, WHO, and ISO), and relevant websites from several organizations were also identified.

Data related to specific guidelines/standards were mostly adopted from direct online sources through online access to the standards (depending on the availability of the access within the research/study institute). This was carried out due to the continuously updating nature of the guidelines and standards.

Screening is the second main step after the report/data search. Some of the primary keywords used for data screening were ‘thermal comfort’, ‘indoor environmental quality’, ‘air quality’, ‘standards and policies for IEQ and energy efficiency’, ‘North American building energy codes’, ‘building energy code and standards’, ‘energy efficiency and human comfort’, ‘building energy codes’, ‘energy certification standards’, and ‘green buildings’. The keywords were used to filter the number of previous studies while searching in the above three databases. This review primarily focused on indoor environmental quality and building energy efficiency. IEQ is fundamentally developed based on thermal comfort, air quality, acoustic comfort, and visual comfort. Therefore, it is a must to include the most important keywords for the indoor environmental quality in the database search. On the energy efficiency aspects, the key words of ‘energy efficiency’, ‘green building’, and ‘energy performance enhance’ were used to narrow the search records where IEQ and energy performance were both considered. Moreover, this review focused on the interrelations and connections between these two aspects. Hence, combined keywords between IEQ and energy efficiency were used in the databases to narrow down specific studies and records that considered both of these aspects. The combined keyword search is a specific feature used in the Compendex Engineering Village database from Elsevier. No AI-based instruments were used to screen and summarize the existing literature prior to the investigation. The inbuilt automation tools, such as sorting and duplicate removal, were used inside some of the databases to screen the vast amounts of records in the keyword search results

While specifying the inclusion and exclusion criteria for the above source selections, any data and sources after the COVID-19 pandemic were given priority. Moreover, since this review is primarily about energy codes and IEQ guides, any relevant revisions and guidelines before 2019 were also considered when the reference was limited. The scope of the content analysis was limited to the codes and guidelines in the North American region. These inclusion criteria were essential, as numerous IEQ guidelines and energy codes are available globally, making a thorough analysis technically and practically non-feasible. In addition, narrowing the geographical location can help to focus and discuss tailoring polices and standards specific to the region.

After the keyword search, most relevant articles were selected for a thorough reading based on the abstract details of the studies. However, since this review mainly focuses on existing energy codes and IEQ guidelines, a review of the reports, papers, and relayed guidelines was conducted to support the findings. Peer-reviewed publications focusing on building energy efficiency, IEQ, or building sustainability from a North American perspective were prioritized. All the data sources and references are cited throughout the review and can be found as a separate bibliography list at the end of the manuscript and Supplementary Materials.

A content analysis was performed on the final list of studies, reports, guidelines, and standards. In the context of the research objectives, a graphical overview of basic steps for data analysis is provided in Figure 2. Please note that under the data source graphics, the figure only presents a limited number of sources for the purpose of illustration. The first step involved the development of a conceptual framework based on how different IEQ parameters were defined in the literature. In the second step, the IEQ standards and guidelines were reviewed to identify the scope, limitations, and assumptions. In the third stage, building energy codes and standards were compared with the IEQ standards to assess how well these two codes and standards aligned with each other. In the last step, particular emphasis was given regarding the future directions for building energy policies and standards in light of the current standards. Recommendations were formulated for designers, engineers, and policymakers to facilitate the development of a more sustainable building sector.

3. IEQ: Parameters and Indicators for the Codes and Guidelines

The four main IEQ parameters are indoor air quality and thermal, acoustic, and visual comfort [32]. Apart from these main four, several other parameters are defined in the literature as a subcategory of these four parameters (e.g., indoor air velocity, relative humidity, PM levels, etc.) [32,33]. These primary IEQ parameters are common for any building archetype. However, their relative importance can vary, with the highest importance associated with residential buildings. In order to identify their connection to energy efficiency, it is important to establish the basic definitions and concepts related to these fundamental IEQ parameters. The third section of this review is dedicated to identifying the primary IEQ parameters and their basics. A clear definition of these IEQ parameters is required, as this review investigates the IEQ guidelines and energy codes on the basis of these parameters. Hence, the fundamental parameters and their relevant indicators are identified here.

3.1. Thermal Comfort (TC)

Thermal comfort is defined as a “Condition of mind which expresses satisfaction with thermal conditions” according to the ISO 7730 [34] and ASHRAE 55 standards [29,35,36]. Defining an exact thermal comfort level is difficult, since personal thermal sensation plays a major role. According to ASHRAE, a building can be labelled as thermally comfortable if 80% of the occupants are satisfied with the thermal conditions [35]. Thermal comfort can be defined by the following:

• Air temperature and velocity;
• Operative temperature (a combination of indoor air temperature and mean radiant temperature);
• Relative humidity;
• Occupant clothing values (Clo value);
• Human metabolic rate (based on activity).

Several methods are defied in the literature to evaluate the thermal comfort of a building. The Fanger analysis method is one of the most commonly used methods. The Fanger method uses thermal sensation and thermal dissatisfaction experienced by occupants to assess thermal comfort limits and balance. The predicted mean vote (PMV) and predicted percentage of dissatisfaction (PPD) are used here. The PMV index aims to predict the mean value of votes from a set of occupants using a seven-point thermal sensation scale (+5: extreme hot, 0: neutral, and −5: extreme cold). The PMV index can be calculated with the mathematical calculation procedures given in ISO 7730 or through building modelling software such as Design Builder (v7.3.0.043) and SIMSCALE (v2021-July) [37]. Several user inputs are needed when using simulation-software-based PMV calculation models. Simulated temperature (as the mean radiant temperature), air velocity, and clothing insulation are provided by the software after the energy simulations and as user inputs to calculate the PMV index. However, the accuracy of thermal comfort PMV data from the software can be different from real-life conditions. Only a generic idea about the thermal comfort of a potential new building design can be obtained from these software calculations. Real-life surveys and data collection should be carried out to validate software-based thermal comfort data. Overall, the final PMV value for a space only delivers the thermal sensation of the general population rather than individual thermal satisfaction. However, the PPD index can be used further to express the level of satisfaction through the percentage of dissatisfaction. The acceptable PMV range is from −0.5 to +0.5 with a PPD value of less than 10%, according to ASHRAE 55 [38,39]. Other popular methods to evaluate thermal comfort are the ASHRAE 55 simplified static graph method and the adaptive-model-based method. The simplified static graph method is based on a graphical method based on psychometric charts. The adaptive method is based on naturally ventilated buildings and elevated air velocities, where two thermal comfort acceptability ranges are defined. The influence on the above-mentioned thermal comfort and its relevant parameters from the energy efficiency measures are evaluated in the sections below.

3.2. Indoor Air Quality (IAQ)

The indoor air quality is defined as the “Interaction between the climate, location, construction system, building materials, furnishing, building occupants, and all other processes within the building” [40]. Indoor air quality (IAQ) plays a crucial role in occupant health and well-being. A wide range of illnesses and human-health-related issues are associated with poor IAQ. Both short-term and long-term exposure to improper IAQ can lead to adverse health effects, including headaches, allergies, skin irritation, asthma, dizziness, cancer, and death [19]. According to the United States Environmental Protection Agency (EPA), poor IAQ is one of the top five environmental health risks to the public. Furthermore, IAQ is crucial for the overall energy efficiency and energy consumption of a building [19]. The EPA has categorized pollutants of concern into seven categories [41]. Some common pollutants include the following:

• Combustion by-products (e.g., CO₂, CO, particulate matter);
• Natural origin substances (e.g., radon, mould, pollen, pet dander);
• Biological agents;
• Chemical contaminants;
• Pesticides, asbestos;
• Ozone;
• Volatile organic compounds.

Evaluating indoor air quality is complex and involves the consideration and evaluation of several factors. It depends on a wide range of environmental and building-related parameters, including indoor emissions levels, outside air pollution and emissions, chemical reactions, building air exchange cycles, ventilation and HVAC design, building envelope construction material, occupant activities, indoor and outdoor air temperatures, and relative humidity. These factors can be divided into three pillars: building characteristics, indoor conditions and activities, and outdoor conditions of the ambient environment.

A building’s characteristics can be changed early to improve the material selection and HVAC designs. However, this is a field that is continuously evolving and adapting to new building materials. Also, associated adverse health effects are continuously being identified and categorized over time [19]. The indoor conditions governing IAQ can be changed and altered based on the occupants’ daily activities. The factors affecting IAQ in this category have the flexibility to change and can be controlled the most [42]. Outside environmental emissions and pollutants depend on exterior parameters and are out of the control of the building occupants. Indoor and outdoor pollutants can be measured using several indices. Most IAQ indices are based on pollutant substances and gases [19]. If a particular substance or gas is beyond the allowable maximum level, then the IAQ is not acceptable, and immediate actions need to be taken based on the risk level regarding human health [43,44,45].

Table 1. Common indicators/substance levels to indicate IAQ levels [41].

CO2 (ppm)—carbon dioxide level	TVOC—Total volatile organic compound levels
CO—carbon monoxide level	VOC—volatile organic compound levels
Benzene	NO2—Nitrogen dioxide level
PM 10 (µg/m3)—particulate matter (10 µm or less)	O3—ozone
PM 2.5 (µg/m3)—particulate matter (2.5 µm or less)	Radon level
RH—relative humidity	SO2—sulfur dioxide level
Airspeed	NH3—ammonia level
Temperature	NO—nitric oxide
Dust and pollen, fungi	Odor, bacteria

presents some of the common pollutants used to define IAQ levels.

Experimental and numerical techniques are frequently used in combination to evaluate IAQ [46,47]. Continuous monitoring of IAQ is often impractical due to higher associated costs. However, in recent years, the development of low-cost digital gas sensors, based on nanomaterials, and better computational tools and techniques has enabled IAQ measurements on a larger scale [48]. Numerical techniques have the advantage of being able to be scaled to a desired time and geometrical scale. Computational fluid dynamics (CFD) is one of the most common techniques and is based on partial differential equations [49]. Experimental techniques, on the other hand, involve scaled-down and full-scale models in laboratories or real environments.

The relevance of the above-mentioned IAQ aspects and its indicators in terms of energy efficiency are evaluated in the latter sections of this paper. In particular, the identified IAQ sources and indicators are closely related to the building envelope materials that are commonly used when implementing higher energy efficiency measures.

3.3. Visual Comfort (VC)

Visual comfort is defined as a “Subjective condition of visual well-being induced by the visual surroundings” [50]. In other words, it is the subjective reaction of humans to the quality and quantity of the lighting available within the space considered. Lighting, measured as the luminance level, can directly impact human health conditions (eye strain, soreness, headache) and work efficiency. Poor lighting conditions can influence attention and concentration levels, resulting in safety hazards. The connections between lighting conditions and work efficiency in office buildings were investigated in some existing studies [51,52]. It was revealed that both the light source and overall lighting conditions directly affect office workers’ efficiency and mood, which are closely tied to indoor visual comfort. Sub-aspects such as luminosity, glare, light quality, daylight, outside views, and glazing must also be considered to determine the overall lighting conditions [53]. The International Commission on Illumination (CIE) categorizes the natural light quantity, distribution of light, and illuminance as the most critical parameters affecting visual comfort (Table 2).

In general, there are two standard practices for evaluating visual comfort. One is the ‘non-annoyance approach’, where the qualitative and quantitative measures of visual discomfort are discussed. Parameters of the non-annoyance approach are easier to measure and identify, such as glare, watering eyes, etc. [54]. The second approach is to consider the well-being of occupants, where the positive effects generated by lighting on the well-being of occupants are considered. The Visual Comfort Probability (VCP) system, introduced by the Illuminating Engineering Society of North America (IESNA), is used to define-glare related discomfort and is defined on a scale of 0–100 [55].

Table 2. Common parameters for visual comfort [56].

Parameter	Descriptions
Natural light quantity	Natural light quantity is associated with useful daylight illuminance (UDI) or daylight autonomy (DA). DA reflects the number of hours that a defined daylight level is above throughout the year. Also, there are other indices involving natural daylight, such as the daylight factor (DF), continuous daylight autonomy (DAcon), and spatial daylight autonomy (sDA).
Distribution of light perceived by the eye	The change in the light intensity of space is considered here with the time that the human eye takes to adjust to it. Under dimmed-light conditions, the eye’s perceived intensity of the light is different than the actual intensity (dimmed light intensity). Illuminance uniformity (UO) can be used to evaluate the light distribution.
Illuminance, assessing the quantity of light	Illuminance is the measure of how much light illuminates a surface, which is measured in lux. Also, 1 lux is defined as the total luminous flux on a unit surface area (1 lumen per 1 m2). The lighting-related standards specify the minimum illuminance levels required for daily tasks. When defining the task lux level, most standards consider the task area, immediate surroundings, and background area (e.g., the Illuminating Engineering Society of North America lighting handbook).

Table 2. Common parameters for visual comfort [56].

Parameter	Descriptions
Natural light quantity	Natural light quantity is associated with useful daylight illuminance (UDI) or daylight autonomy (DA). DA reflects the number of hours that a defined daylight level is above throughout the year. Also, there are other indices involving natural daylight, such as the daylight factor (DF), continuous daylight autonomy (DAcon), and spatial daylight autonomy (sDA).
Distribution of light perceived by the eye	The change in the light intensity of space is considered here with the time that the human eye takes to adjust to it. Under dimmed-light conditions, the eye’s perceived intensity of the light is different than the actual intensity (dimmed light intensity). Illuminance uniformity (UO) can be used to evaluate the light distribution.
Illuminance, assessing the quantity of light	Illuminance is the measure of how much light illuminates a surface, which is measured in lux. Also, 1 lux is defined as the total luminous flux on a unit surface area (1 lumen per 1 m2). The lighting-related standards specify the minimum illuminance levels required for daily tasks. When defining the task lux level, most standards consider the task area, immediate surroundings, and background area (e.g., the Illuminating Engineering Society of North America lighting handbook).

Similar to indoor air quality, visual comfort is closely tied to the building envelope factors in terms of its relation to energy efficiency. The latter sections of this review evaluate the connection between the above-mentioned visual comfort aspects and building energy efficiency enhancement through the lens of building energy code/standards practices.

3.4. Acoustic Comfort (AC)

Acoustic comfort is defined as “a state of contentment with acoustic conditions” [57]. Acoustic comfort may not always directly affect human health other than through discomfort and exhaustion. However, long-term exposure can cause irreversible damage. There can be indirect impacts due to a lack of acoustic comfort, such as a reduction in work efficiency, not having proper sleep time, and sleep disturbances [58,59].

The relative importance of acoustic comfort is higher in offices, classrooms, and other commercial-type buildings than in generic residential spaces. Occupants have more control over sound levels in residential spaces, and acoustic comfort is a direct function of occupant actions. Similar to other IEQ parameters, residential constructions prioritize energy efficiency compared to acoustic comfort levels [60].

Acoustic comfort is evaluated by considering several sound-related measurements, including sound pressure (measured in decibels/dB), sound power (Watts), and sound intensity (dB.Watts/m²). Based on these measurements, various indicators have been developed to assess acoustic comfort performance. These indicators can be classified into different categories based on the perspective of acoustic quality and acoustic comfort.

Table 3. Categories of acoustic comfort and quality indicators [61,62].

Category 1	Category 2	Category 3
Acoustic performance index (AP)	Balanced noise criterion (NCB)	Articulation index (AI)
Weighted normalized impact sound pressure level of floor (L’nw)	Combined noise index (CNI)	Late arrival sound/strength of the late arriving (Glate)
Weighted sound reduction index (Rw)	Noise criterion curves (NC)	Speech clarity (C50)
The noise level produced by discontinuous service equipment (Lic)	Room criterion (RC)	Speech intelligibility index (SII)
The noise level produced by continuous service equipment (Lid)	Noise rating curves (NR)	Speech transmission index (STI)

includes some common acoustic indicator categories observed in the literature. Category 1 includes acoustic indicators reflecting the acoustic insulation performance of buildings. These indicators are commonly used in building construction codes. Category 2 includes indicators of human perceptions against undesired noises and disturbances, while the overall acoustic quality within an environment is considered under the indicators in category 3 (e.g., sound absorption and reflection indicators from inside surfaces are noted here). These indicators are crucial for conference or meeting rooms, movie theatres, auditoriums, or classrooms. On-site measurements, laboratory experiments, and simulations are common for assessing acoustic comfort.

The above-mentioned acoustic comfort parameters and their subcategories are influenced by various methods used to enhance energy efficiency. In particular, building envelope materials and envelope design criteria can affect acoustic comfort levels significantly. The followings sections discuss how energy efficiency codes/guidelines discuss acoustic comfort and what are the specific relationships.

4. IEQ Guidelines and Standards

IEQ encompasses the indoor environmental factors of IAQ and visual, acoustic, and thermal comfort. A set of government or non-governmental guidelines and standards provide limits for the different indoor parameters identified above to ensure the health and comfort of occupants [57]. In broad terms, IEQ guidelines can be defined as informative documents that provide practices and procedures needed to achieve acceptable levels of the above-defined IEQ parameters, ensuring the well-being and health of occupants. On the other hand, a standard is defined as a “document, established by consensus and approved by a recognized body, that provides, for common and repeated use, rules, guidelines, or characteristics for activities or their results, aimed at the achievement of the optimum degree of order in a given context” [63]. Compared to standards, guidelines are not legally binding.

There are many guidelines and standards that exist to govern the IEQ of buildings around the world. The focus area of IEQ guidelines and standards can vary based on the geographical location and social and economic conditions. For instance, more priority may be given to indoor air quality and acoustic comfort in a heavily air-polluted city, while more focus will be given to thermal comfort in regions with extremely cold or hot climate conditions. Several organizations have developed national or regional-level guidelines and regulations based on such conditions.

Table 4. Common indoor environmental quality standards and guidelines in North America.

IEQ Guideline/Standard and Policy	Key Focus Area/Strengths	Limitations	Assessment Criteria	Consider Energy Efficiency/Performance	Ref.
ANSI/ASHRAE Standard 62.2-2022, Ventilation and Acceptable Indoor Air Quality in Residential Buildings ANSI/ASHRAE Standard 62.1-2022, Ventilation and Acceptable Indoor Air Quality in Commercial Buildings	ASHRAE has been providing various standards for almost all types of buildings related to IEQ. Standards 62.1 and 62.2 provide minimum requirements to achieve acceptable IAQ via ventilation, mechanical exhaust, and source control. Regular updates and proper guidelines are provided to constructors, designers, and other stakeholders.	Details could overlap due to the vast number of available standards and guidelines. The building designs are restricted to several conditions and limits. The public or specific stakeholders may find it hard to identify or comprehend the needed information. Most of these guidelines and procedures are voluntary-based standards. Governments could adopt these and make their mandates based on the requirements.	IAQ	Energy efficiency is considered to a certain limit through HVAC design.	[30,31,64]
ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy	Provides detailed guidelines for achieving occupant thermal comfort under various conditions. Easy-to-understand tables and graphical methods are presented. Thermal comfort assessment surveys are provided.	A thorough understating of TC assessment methods is needed with comprehensive knowledge from the ASHRAE handbook fundamentals. Each TC assessment method is limited to certain criteria and limitations.	TC	No	[29]
OSHA IAQ Guidelines, Occupational Safety and Health Administration guidelines on IAQ	OSHA air quality guidelines provide ventilation requirements for workers based on ASHRAE 62.1. Discusses mold and moisture, VOCs, and CO2 levels.	Specific IAQ standard levels for most pollutants are not defined.	IAQ	No	[65,66]
EPA Indoor Air Quality (IAQ) Guidelines, U.S. Environmental Protection Agency guidance for IAQ in buildings	Compliance with ASHRAE 62.1 and 62.2. Indoor pollution buildup is mainly tackled through HVAC designs. A comprehensive set of IAQ guidelines. Most building types are considered when defining IAQ.	These are mainly guidelines and voluntary-based adaptations. The detailed, comprehensive nature is useless since it is not a mandate.	IAQ	No	[41,67]
WELL Building Standard V2	Covers air, temperature, sound, and lighting for occupant health and well-being. A holistic set of standards that actively looks into human health. Adaptable for several building archetypes.	The certification cost is known to be very high in the Well V2 standard. Requires significant instrumentation, testing, and data collection.	IAQ TC AC VC	Not directly, but lighting optimization can save energy.	[68]
IES-ANSI/IES RP 1, National Standard Practice for Office Lighting	Comprehensive guidelines for indoor lighting conditions. Covers all types of indoor spaces. ANSI/IES RP 1 emphasizes lighting uniformity, glare control, and daylighting.	Focus is only oriented towards lighting environment and illuminance. Energy-efficiency-oriented lighting practices are not explored much. Minimum lighting needs are based on the average sample population and not age-based categories.	VC	No	[69,70]
BC Building Code, 2024 Canada (Provincial Regulations)	A general building energy construction code focusing on BC, Canada. Heating, ventilating, and air-conditioning are discussed.	Recommends using the basics of ASHRAE 62.1 for ventilation and not specific IAQ assessment.	IAQ—but not specific to health	Yes	[71,72]
Health Canada IAQ Guidelines, Indoor air quality recommendations for various pollutants and exposure limits	Mainly focuses on indoor air pollutant threshold limits.	Only the pollutant threshold limits are defined. Not a comprehensive guideline on IAQ.	IAQ	No	[73,74]
CSA Z317.2, Ventilation and thermal comfort: Canada	Outlines the heating, ventilation, and air-conditioning aspects. Provides adequate guidelines for designing and maintaining IAQ and TC in healthcare buildings.	Limited only to healthcare facilities. Highly specific for Canadian regions, and others may not be able to adopt its comprehensive guidelines.	TC IAQ	Yes	[75]
NOM-015-STPS-2001, Mexico	A Mexican-based thermal comfort standard for workplaces. Human health and workplace safety are focused on here.	A basic upper- and lower-bound temperature limits are provided. Only suitable for dry, semi-arid hot climate	TC	No	[76]
CSA Z412, standards for ergonomic performance: Canada	Designed for office ergonomics. Multiple aspects of lighting (artificial, natural) are discussed.	Only lighting-based ergonomics are discussed.	VC	No	[77]
NOM-013-ENER-2013, Energy efficiency and lighting comfort standards	Mexican-based standards on lighting and energy efficiency. Covers a wide range of indoor spaces in many building archetypes. Heavily emphasize energy savings while maintaining proper lighting levels.	Not directly human-health- and well-being-oriented; mainly for energy efficiency.	VC	Yes	[78]
ELI’s (Environmental Law Institute) Database of State: Indoor Air Quality Laws	A comprehensive collection of laws reflecting a range of USA’s policy strategies to enhance the indoor air quality. The database of policies is freely available to the public.	Mostly it contains laws and regulations, which could be too technical for the general public. Only focuses on indoor air quality under overall IEQ parameters.	IAQ	No	[79]
ISO 7730:2005 Ergonomics of the thermal environment	Local thermal comfort parameters/criteria are considered here. Analytical determination and interpretation of thermal comfort using the calculation of the PMV and PPD indices and local thermal comfort criteria. ISO standard mentions cultural, national, and geographical differences, unlike ASHRAE 55.	PMV and PPD methods are used to predict/calculate general thermal sensations rather than adaptive techniques. Only devoted to mechanical ventilation in buildings. The standard does not receive frequent updates compared to ASHRAE 55. Users may need to integrate other ISO standards for the missing IEQ aspects.	TC	Yes	[34]
ISO 353 Acoustics, Measurement of sound absorption in a reverberation room	The standards for measuring the sound absorption coefficient of materials.	Only focuses on sound absorption levels. Not suitable for multiple types of indoor spaces.	AC	No	[80]
ISO 717-1:2020 Acoustics, Rating of sound insulation in buildings and of building elements	Airborne sound insulation values are defined for common envelope elements (walls, roofs, doors, windows). Various noise sources are considered with a good range in terms of sound level spectra.	Only the building element sound insulation level is considered as the index. Not freely available to the general public.	AC	No	[81,82]
REHVA: The Federation of European Heating, Ventilation, and Air Conditioning Associations: Ventilation and indoor air quality guidelines	The primary focus is building ventilation, and several ventilation-related aspects are discussed in the guidelines. Health-based ventilation guidelines for the EU. Clean ventilation system design and maintenance guidelines (GB8). Hygiene requirement guidelines for ventilation and air-conditioning (GB9). REHVA COVID-19 updated guidance directory.	Most of the available guidelines focus on building ventilation and indoor air quality. Focuses on the thermal comfort guidelines, and standards are not present at the time of writing this paper.	IAQ	No	[83]
Illuminating Engineering Society (IES) IES RP-1-12, American National Standard Practice for Office Lighting IES RP-29-06, Lighting for Hospitals and Health Care Facilities IES Lighting Handbook guidelines	These IES standards define the lighting requirements for the considered space types. Important design aspects are covered, such as light uniformity, daylighting, and glare.	Visual comfort is the only IEQ parameter considered here. Not all the types of spaces are considered in the IES standards.	VC	No	[70,84]
International Commission on Illumination (CIE) ISO/CIE 8995, Lighting of Workplace CIE S 015/E 2005, Lighting of outdoor workplaces ISO 30061:2007 CIE S 020, Emergency lighting	CIE has developed several guidelines, best practices, and standards for building lighting and outdoor lighting). A vast scope of lighting applications is considered here, with various aspects.	Only the visual comfort aspect of IEQ is considered. Due to the vast number of standards and guidelines available, it might not be easy to select the required user guidelines. Some guidelines and standards are combined with ISO standards.	VC	No	[85,86]
Residential Indoor Air Quality Guidelines (RIAQG): Canada	Pollutant sources and identification guidelines. Health effects and recommended exposure levels in Canadian homes.	Only a guideline and basic information about air pollutants rather than an actual mandate. Limited details relating to building construction and operation. However, important information is presented for pollutant identification and their maintenance to accepted levels.	IAQ	No	[74]
ISO 16000-1, Indoor air, General aspects of IAQ and measurement strategy ISO 16000-6, VOC (Volatile Organic Compounds) measurement in indoor air	Main focus is IAQ measurement techniques. Uses several reference parameters/air quality pollutant types.	Limited practical guidance for residents/occupants. More oriented towards design/construction aspects. Requires regular monitoring and significant instrumentation.	IAQ	No	[87,88]
World Health Organization (WHO) guidelines for indoor air quality: Selected Pollutants	WHO guidelines focus on health risks from commonly available chemicals in indoor air. Health risks, exposure pathways, and recommended pollutant levels are discussed here.	Only selected chemicals are presented here. (e.g., benzene, carbon monoxide, radon, etc.). Countries may or may not consider this guideline, as the WHO does not mandate it for human health.	IAQ	No	[89]

lists the reviewed IEQ mandates, guidelines, and standards in the North America and the identified strengths and limitations. This section of the review identifies the existing status of North American IEQ guidelines and their connections with building energy efficiency. Figure 3 gives an overview of the focus of these standards and guidelines in terms of the IEQ parameters outlined in Section 3 and building energy efficiency.

This review of 23 North American guidelines and standards indicated that the majority are focused on only a specific parameter and do not provide impacts regarding other IEQ dimensions. For instance, ASHRAE standard 55 is focused on thermal comfort [29]. A thermally comfortable indoor environment does not always indicate that other indoor parameters are within acceptable limits or that they are performing equally or better. For instance, a field study on a dormitory building ventilated by split air-conditioning showed a high thermal comfort (80%) rating indicated by the occupants [90]. However, when compared against a similar naturally ventilated building with only 60% acceptance, the air contained a 1.6 times higher concentration of CO₂ or poor air quality. Likewise, a building can achieve acceptable levels of thermal comfort but have poor acoustics. For example, a survey study in the United States on occupants of office buildings with radiant heating and cooling systems indicated that majority of the participants had low satisfaction with the office acoustics, primarily related to sound privacy [91]. Hence, for a building to be comfortable and healthy for occupants, different guidelines and standards need to be implemented to ensure that acceptable thresholds are being met. Moreover, the application and validation of different guidelines and standards to meet IEQ parameter thresholds are neither practical nor economical.

Figure 3 shows the practicality of this problem through the real IEQ guidelines in North America. ‘Bar A’ in Figure 3 shows the percentage coverage of each IEQ parameter in the reviewed guidelines. A total of 48% of the guidelines only address indoor air quality, 17% of the guidelines only address thermal comfort, and the percentages for visual comfort and acoustic comfort are 22% and 9%. Only 4% of the guidelines consider a combination of IEQ parameters simultaneously. This provides a clear picture of the issue that needs to be addressed when implementing IEQ guidelines. Either designers and constructors should consider multiple guidelines to avoid the problems mentioned previously or there should be more holistic solution where a combination of IEQ guidelines must be developed. Among the reviewed standards, only one, the WELL Building Standard V2, addressed different dimensions of IEQ [68]. However, the WELL Building Standard itself has a set of limitations. Firstly, it is an international standard that cannot be representative of the specific local IEQ requirements in North America. Secondly, the standard is complex and may only be feasible for large projects, as small organizations have a limited budget for its application [92].

From ‘Bar B’ in Figure 3, it can be seen that only about 21% of the guidelines and standards considered or were combined with energy efficiency. This is a concern, since IEQ parameters often compete with energy efficiency. There are a plethora of studies showing that more energy-efficient buildings come at the cost of poor acoustics, poor visual comfort, and/or poor IAQ and vice versa. For example, in order to maintain thermal comfort in mixed-ventilated tall buildings, an additional 38% of energy is required [93]. Likewise, low ventilation rates are associated with low energy use but have been shown to trap indoor pollutants such as radon, VOCs, and carbon dioxide [94]. Increased levels of indoor air pollutants are especially common in retrofitted houses where only the envelope is improved and the HVAC system remains the same [95].

In short, this review of North American IEQ standards and guidelines indicates the following limitations. Most IEQ guidelines have been developed independently and can be adopted by any form of building construction process regardless of energy efficiency. This is a significant gap in terms of the combined IEQ and energy efficiency target in modern buildings. IEQ guidelines are generally based on voluntary adaptation. Therefore, pursuing energy-efficient buildings through existing IEQ guidelines seems almost impossible in the context of North America.

5. Building Energy Codes and Standards: Energy Efficiency and IEQ

Building energy codes and standards are policy instruments that aim to improve the energy efficiency of buildings. This is achieved by reducing energy loads and by using more energy-efficient systems and equipment. They can broadly be categorized into prescriptive and performance-based codes. Prescriptive codes define the minimum and maximum requirements to be followed by building envelopes and systems [96]. Performance-based codes, on the other hand, focus on the overall performance of buildings and are neutral regarding the building system and equipment used to achieve the target rather than focusing individual systems and components and defining energy targets to be achieved by the building [97]. Performance-based codes are considered more flexible than prescriptive codes. This section of the review focuses on North American building energy codes and energy efficiency standards in relation to IEQ. The energy codes are evaluated for their primary focus and capacity to tackle the IEQ parameters defined in Section 3. In Section 4, the accountability of the existing North American IEQ guidelines in terms of energy efficiency was analyzed in-depth. This section provides a vice versa approach, where the accountability of the existing North American building energy codes in terms of IEQ parameters is investigated.

A full investigation table of the building energy codes is provided in File S1 in the Supplementary Materials. Table S1 contains 18 common building energy codes available in North America. The sign ‘✗’ in front of a standard/code indicates that the relevant IEQ parameter is not addressed or is below a ‘satisfactory level’.

Energy codes mainly provide a pathway to achieve minimum energy performance goals in practical constructions. On the other hand, energy certification programs provide a holistic pathway to achieve overall building sustainability. Figure 4 visually demonstrates the IEQ coverage on the reviewed energy codes and standards. Overall, 50% of them do not consider IEQ at all. The rest of the 50% consider IEQ in two paths. Six (17%) energy codes and standards consider all the IEQ parameters parallel to the energy efficiency of buildings (ASHRAE 90.1/90.2, California Title 24, BREEAM, CASBEE, LEED, the WELL Building Standard [98,99,100,101]), while 33% consider one or two (not all) IEQ parameters in addition to energy efficiency.

Out of the six that consider all the IEQ parameters, four of the documents covering IEQ aspects are building energy performance certifications. Some codes, such as ASHRAE 90.1/90.2, discuss energy performance as a standalone guide. However, supplementary standards such as ASHRAE 55 and ASHRAE 60.1/60.2 can be combined with 90.1 and 90.2 to determine energy efficiency and IEQ. It was also observed that most energy codes and standards are vague about indoor environmental parameters and only define one parameter. For instance, indoor temperature limits are often part of building standards; however, the impact on thermal comfort is not ensured. For example, the National Energy Code for Buildings in Canada defines heating (21 °C) and cooling (19 °C) temperature set points [102]. On the other hand, the BC Energy Code addresses the concerns of overheating, but other factors remain unaddressed [103]. Likewise, the complexity of the IEQ parameters and their high reliance on how occupants interact with the building systems and internal and external conditions of the building can result in poor IEQ. Post-occupancy evaluation (POE) and the use of monitored IEQ parameters is not a common requirement in the majority of the building codes and standards. Numerous studies have explored and shown the advantages of performing POE for attaining improved IEQ [104]. Thus, there is need to have post-occupancy surveys be a requirement in codes to check the effectiveness of the codes and standards for different types of buildings. Another associated factor is the changing lifestyle and occupancy patterns of occupants that affect how buildings operate and the associated IEQ maintenance needs. For example, COVID-19 resulted in significant changes in occupant patterns and increased energy needs for residential buildings [105]. These changing patterns need to be incorporated into codes, and the associated IEQ needs to be investigated for different codes and standards.

The development of building energy codes is often slow, with most codes revised every 3 years. However, regional codes can be extremely outdated; for example, the states of Oklahoma and South Carolina follow older versions of the International Energy Conservation Code (2006 and 2009, respectively) [106]. Hence, the progress of codes can be out of line with actual IEQ requirements and the latest technology developments. Moreover, the revision of building energy codes and standards needs keep pace with changing climate demands. For instance, the recent wildfires in British Columbia (Canada) in 2022 showed increased levels of air pollutants (PM 2.5), which are especially harmful to children [107]. Extreme weather events such as higher rainfall, heat domes, and cold snaps are predicted to increase, thus further affecting the effectiveness of the existing standards in terms of ensuring a safer environment for occupants. Increasing temperatures and outdoor CO₂ levels increase the level of allergens infiltrating building ventilation systems [108]. In British Columbia alone, the heat dome of 2021 caused 691 deaths, the majority of whom were elderly, and these deaths were associated with overheating. Some codes already consider the changing climate and keep their occupants relatively safe under extreme climate events. A study conducted by the Pacific Northwest National Laboratory on building resiliency enhancement showed that people residing in buildings complying with the IECC 2021 update will be able to cope with heating (140%) and cold snaps (120%) more than people complying with prior codes [109]. Hence, in addition to new buildings that need to keep pace with rising demands, existing buildings need to be renovated to ensure acceptable IEQ levels under the building energy code implementation process.

6. Discussion

The two primary aspects that need to be considered in today’s sustainable building construction industry are building energy performance and indoor environmental quality. Many measures have been taken to enhance building energy efficiency due to global climate change and energy poverty. The level of the pursuit of building energy efficiency has become too significant during the last few decades. Hence, IEQ aspects have been pushed off to a neglected level in many cases. Therefore, the investigation of existing IEQ guidelines and building energy codes in terms of practical industry is vital for future code development and building stock construction. IEQ and energy efficiency cannot perform well individually, and these two aspects need to be considered together, as identified by the above investigations.

6.1. Interrelationships

Section 4 and Section 5 show the strengths and weaknesses of the existing IEQ guidelines and standards and building energy codes and standards, respectively. Some IEQ standards and energy codes can show synergy, where an improvement in one aspect improves the other parameter. For example, an improved HVAC system under an energy performance standard can result in better indoor air quality. Likewise, codes can encourage dependence on daylight-responsive systems that decrease energy use and improve visual and thermal comfort for occupants. On the other hand, conflicting parameters are also prevalent, where improved energy efficiency is determinantal to IEQ. Hence, it is important to consider and prioritize IEQ parameters when designing and developing energy efficiency guidelines and practices. In terms of considering these aspects and the identified limitations of the existing guidelines, codes, and standards, the first important question that needs to be answered is an understanding of the relationship between IEQ parameters and building energy efficiency. The interrelationships and dependability between these two aspects are demonstrated in Figure 5 based on the previous investigations into IEQ standards and energy efficiency standards.

Table 5. The connections between IEQ parameters and energy efficiency enhancement methods.

1	HVAC system characteristics heavily change the thermal conditions and indoor temperature distribution. Therefore, building energy codes must consider thermal comfort when defining HVAC guidelines. On the other hand, IEQ guidelines on thermal comfort must be able to guide a range of HVAC systems.
2	Heat recovery ventilation can cause an imbalance in the indoor air temperature, which may result in changes in thermal comfort conditions.
3	Hot water systems may or may not impact thermal comfort indirectly. Based on the location of heat tanks, pipes, and other equipment, the localized thermal comfort levels can be influenced.
4	The indoor conditioning rate (heating/cooling) may vary based on the energy source and demand if alternative energy sources are used (e.g., if solar PV or geothermal heating are to be used as per the energy code, the effect on the thermal comfort evaluation is required)
5	The thermal envelope insulation capacity directly affects the heat retained inside and the heating/cooling loads. Most energy codes only consider the insulation capacity. Detailed occupant thermal comfort modelling needs to be integrated for these to identify the effect on TC by the envelope’s insulation levels.
6	Heat leakage and the heat-retaining capacity of the thermal envelope are parameters that directly control thermal comfort. None of the reviewed North American energy codes considers this connection with airtightness/infiltration modelling for the variation in indoor TC.
7	Windows, doors, roofs, floors, and wall materials vary in terms of thermal transmission values, and the inside thermal conditions will react accordingly. The effect of the envelope materials on TC must be integrated into performance-based energy codes.
8	The window-to-wall ratio is a main factor when calculating the mean radiant temperature inside. Other than a small handful of energy codes, many still consider the dry bulb temperature in their design guidelines. The mean radiant-temperature-based operative temperature must be used if TC is to be considered.
9	Based on the HVAC air distribution methods, IAQ can be affected significantly. This aspect is considered in most of the building energy codes, unlike the TC-related factors. The detailed HVAC system designs actually consider IAQ to a very satisfactory level in most of the current North American building energy codes.
10	Heat recovery ventilation can affect IAQ by affecting humidity and heat recovery levels. This is an important finding that is not considered to a satisfactory level in many energy codes. However, the effect of this mainly affects TC, while IAQ suffers a little.
11	Infiltration and natural ventilation rates can drastically modify the air-changing cycle. However, airtight envelopes can prevent particulate pollution from entering the building. The infiltration effect of modelling on IAQ is not considered to a satisfactory level in many energy codes. This needs to be incorporated into HVAC design as an additional air pollution intake. Depending on the geographical location, this can have a significant impact on IAQ.
12	Passive cooling or natural lighting spaces can affect IAQ with undesired particulate pollution inside. These passive cooling aspects are mainly discussed in the energy efficiency certification standards rather than in the traditional energy codes. These certifications consider IAQ to a satisfactory level. However, most of the North American traditional energy codes must adopt these evaluations if they are to recommend passive cooling and natural lighting through open space.
13	Visual comfort directly depends on the lighting system and smart light control techniques. Most of the lighting guidelines consider VC. If the energy codes are to use already existing lighting guidelines, this connection is considered automatically.
14	Natural lighting is a direct consequence of the envelope elements, such as glass surfaces, which depend on the window-to-wall ratio. This is a seriously neglected factor in most traditional energy codes, where glass/transparent envelope surfaces are reduced to save energy. The envelope design must compensate for VC adequately.
15	Natural lighting can be reduced or enhanced based on the number of open-area surface elements in the building envelope. This connection is considered in the energy certification standards to a satisfactory level.
16	HVAC system duct fans, blowers, heaters, and furnaces are one of the primary sound disturbances in residential buildings. This is a mostly neglected connection in many North American building energy codes itself. However, several AC guidelines are used in some cases on top of the energy code to fix this. If integrated guidelines are to be developed, these issues will be fixed without having to adopt multiple guidelines for energy and IEQ.
17	The soundproof characteristics of the thermal envelope materials can enhance or decrease the level of indoor audio disturbances. Building-envelope-based AC modelling is not considered in commercial building energy codes itself as an integrated part. Some codes do consider this connection by adopting third-party AC guidelines. However, this is an addition to the envelope rather than being a part of the original envelope design.
18	An effective window area of a building can change the sound penetration characteristics of the building (e.g., glass is less effective than insulated walls). A similar connection to ‘17’ but related to windows and fenestration. This is a critical connection to consider, depending on the geolocation of the building.
19	Opening areas are direct sources of outdoor sound penetrations to the inside of a building’s living spaces. Only the energy certification standards consider this connection to a satisfactory level. It is a very important factor to consider in urban developments if passive designs are being pursued.

contains the detailed findings and information regarding the numbered interactions/dependabilities demonstrated in Figure 5. The interrelationships are classified using the four primary IEQ parameters identified in Section 3 and two energy efficiency enhancement approaches currently used in the industry. All the building energy codes and standards identified in Section 5 use these two basic approaches to enhance building energy efficiency regardless of whether they consider IEQ or not. The two approaches to enhance building energy efficiency are as follows:

• Approach 1: Improve the energy efficiency of equipment and appliances (e.g., HVAC, hot water systems, electrical appliances).
• Approach 2: Improve the building envelope’s thermal performance by using better insulation and reducing the air infiltration and leakage rate for space conditioning (through airtightness).

A total of 19 connections and dependabilities were identified between the IEQ parameters and energy efficiency enhancement methods. Mostly, many of the building energy codes and IEQ guidelines consider one or two such connections, as detailed in Table 4 above and in File S1, Table S1. This creates a massive gap between IEQ and the energy efficiency performance of a building in real life, causing all the practical problems discussed in the evaluated studies in Section 4 and Section 5. A comprehensive building construction guide should properly explore all these connections and consider the adverse effects on each other caused by changing these parameters. The development of IEQ guidelines and building energy codes must be conducted on the basis of these connections as a whole package, unlike the codes and guidelines investigated in Section 4 and Section 5. Table 5 explains to what extent these connections are used in the reviewed guidelines and highlights the importance of the findings from this review.

Among the various relationships observed in the literature, the role of HVAC systems stands out as one of the most critical. Changes in HVAC systems directly influence air change rates and the mode of indoor air circulation. The impact directly affects IAQ in various ways, such as increases in particulate pollutant levels due to fewer air changes, the indoor air velocity, and clean air distribution and circulation. HVAC systems may further affect indoor air temperature values based on the energy systems’ cooling and heating set points. Indoor conditions can be too warm or cold, negatively influencing thermal comfort. Thermal envelope designs directly affect indoor thermal conditions. Most modern envelopes are designed to retain heat inside with minimal losses. With sealed-envelope designs and fewer infiltrations, indoor environments can become too warm and highly humid, resulting in a lack of thermal comfort conditions for occupants. The thermal envelope also affects the visual comfort of occupants when the envelope is designed to save and retain indoor heat. A lack of glass surfaces (windows, door glasses, and glass walls) may result in poor daylighting conditions and thereby reduce visual comfort.

Table 5 above presents some of the major findings from this review. All these 19 connections were identified during the in-depth investigations in Section 4 and Section 5. It is critical that leading energy codes and standards attempt to integrate at least several of these connections into their design. If not, the energy code implementation process can influence all four IEQ parameters in various pathways, creating unacceptable IEQ for occupants.

6.2. Implications and Future Directions

As the above investigation proves, the lack of a connection between energy efficiency codes and existing IEQ guidelines is critical. Future-proofing existing energy codes and IEQ is essential considering emerging technologies and materials and the changing climate. There is a need for an integrated approach with all four IEQ parameters, energy aspects, and GHG emissions. In addition, the approach must be developed considering ‘occupant-centric’ concepts rather than the financial-goals-centric perspective. In the literature, two such models, ‘TAIL’ and ‘IEQ compass’, developed in the European context, are available, which combine some of the above aspects. ‘TAIL’ was developed as a part of the European energy certification method under the ALDREN project [110]. The TAIL framework uses a rating system to evaluate all the IEQ parameters simultaneously. However, TAIL does not actively consider building energy efficiency or energy performance.

The second combined IEQ assessment method is ‘IEQ compass’ [111]. IEQ compass is a tool developed to evaluate potential IAQ, thermal comfort, and visual and acoustic comfort levels [111]. The tool generates results without considering user influences, unlike other survey-based IEQ evaluations. IEQ compass can deliver an overall IEQ parameter/label for the considered building after assigning a weight for each IEQ parameter. The IEQ labelling process uses a letter-based ranking system and a color-coded score for the building design. The results were generated in this model by using a case study building through building information modelling [111]. This IEQ compass tool has paved a significant pathway towards future-proofing IEQ guidelines and energy efficiency codes. However, the IEQ compass tool has some considerable limitations in terms of its overall IEQ evaluation. The main drawback of this model is that it does not use user inputs and influences. Such an approach could mislead the final ranking results compared to the actual IEQ conditions of the physical building. Particularly for residential buildings, user influences and inputs are very important. Moreover, this tool gives the four primary IEQ parameters equal importance while calculating the overall IEQ rank. However, using parameters with equal importance without user influence can be further misleading in terms of the final performance of the buildings. With the availability of the data and other relevant information, ‘IEQ compass’ attempted an interesting approach to define an overall IEQ parameter of buildings. It provides solid ground for further improved versions to evaluate overall IEQ and energy efficiency. Figure 6 shows the IEQ compass label ratings with the performance score of the building.

In addition, the European Union’s Energy Performance of Buildings Directive (EPBD) actively pursues the importance of combined energy efficiency and IEQ as a whole evaluation metric for buildings [112]. However, in North America, similar pathways are lacking. In particular, the harsh cold climates in North America are ideal research grounds for investigating such a combined platform with energy consumption and IEQ of future building stock. On the other hand, the harsh cold climate conditions are the leading reason for this lack of guidelines in North America. The conservation of energy and mitigation of GHG emissions have been the primary target for many decades. Hence, the consideration of IEQ has been under the shadow of energy efficiency. As a result, such integrated tools from a North American perspective are not readily available, unlike in Europe. Moreover, the European regions have started pushing the requirements of IEQ as mandates and law, unlike in many North American regions. Therefore, the lack of a combined implementation for energy efficiency and IEQ is further lagging behind in North America. However, global warming effects and rapidly changing climate situations call for such initiatives now.

Moreover, the usefulness of such pathways for North America will help the building sector move towards smart buildings and smart cities in regions with a cold climate. More data can be monitored and accumulated through smart building instrumentations for IEQ parameters. Therefore, dynamic IEQ scales can be developed and integrated with building energy performance to deliver real-time overall building performance data. Figure 7 shows what such an integration would look like when combined performance scales are developed for energy efficiency and IEQ. Hypothetical values are shown in this figure. The proposed integration is shown for the IEQ index with the EnerGuide energy rating system for homes. EnerGuide is a commonly used energy rating scale in Canadian houses, particularly for small residential buildings [113]. The EnerGuide rating system is used here as a sample, since it is nationally recognized across all the provinces in Canada. The United States also has a similar energy rating system for homes called the HERs index [114]. The development of a combined index will require extensive data collection on local construction practices, surveys from occupants and the construction industry, the consideration of suitable weighting and aggregation methods, and testing on actual case study buildings. Performance verification based on sensor-monitored data as well as post-occupancy surveys would ensure that buildings meet the desired IEQ and energy standards. In addition, the developed performance scale should be flexible enough to consider adaptive strategies, such as increased cooling load requirements under a changing climate and the latest trends in building technologies and materials. Lastly, the importance of different occupant needs and behaviors should be a central consideration of this performance framework. Such an integrated model would massively benefit building policy, mandate, and regulation developers, government authorities, building constructors, and building designers. A balanced performance in terms of both energy efficiency and indoor environmental quality levels would improve the well-being and health of North American dwellers.

7. Conclusions

Poor IEQ can cause various health-related problems for occupants, including, but not limited to, sick building syndrome, a lack of sleep, and a reduction in working efficiency. Understanding and improving IEQ is a complex process, and the recent changes in occupant lifestyle patterns (for example, increased hours spent working from home following COVID-19), the changing climate, and an aging population have made IEQ pertinent for a sustainable future and for the health, satisfaction, and well-being of occupants. This review was conducted to identify the current status of IEQ standards/policies, evaluate the overall IEQ of buildings, examine IEQ-related data, and examine the extent to which building energy codes and green building standards account for IEQ parameters. The scope of the review was kept within the North American region when evaluating the energy codes and IEQ guidelines.

As a result, the current relationship between IEQ and building energy codes and standards were explored. This review identified 19 different interactions and dependent relationships between the primary IEQ parameters and energy efficiency enhancement methods. It was found that most of the reviewed IEQ standards and guidelines lack comprehensive methods to cover all IEQ parameters comprehensively. Designers and constructors may have to use multiple standards to achieve satisfactory levels of IEQ. Moreover, these IEQ guidelines generally exclude energy efficiency parameters. Hence, building energy codes must be used in addition to IEQ guidelines. A few energy certification standards, such as LEED, CASBEE, and the WELL Building Standards V2, discuss all four IEQ parameters in parallel with energy efficiency practices. However, their implementation requires expertise, and they are often not possible to implement by small construction companies. Moreover, these certifications are based on voluntary adaptation and are not a law or mandate. IEQ parameter values and energy efficiency codes can be in synergy or in conflict depending upon the type of codes, buildings, and users. From the perspective of building energy codes, many fail to evaluate the identified 19 connections. Several traditional building energy codes consider some connections to IEQ, but it is not feasible enough to provide a balanced IEQ for occupants. Hence, there is a need to revise the generic North American building energy codes and IEQ guidelines to consider complex interactions.

The most viable and long-term solution is to update and future-proof the currently available energy codes and IEQ guidelines through a combined platform. If an integration is achieved between IEQ guidelines and building energy codes by considering all the above-identified 19 connections, the buildings developed through the combined platform will provide a perfect balance in terms of IEQ and energy efficiency. Based on the findings of this review, it is proposed to develop a model for the North American regions in which IEQ and energy efficiency are represented on a benchmark scale. As the initial steps, research-based methods should be explored using European models, such as TAIL’ and ‘IEQ compass’, as starting points. As the proposed scale is updated and improved with data collection, surveys, expert opinions, and real-world case studies, more sustainable and healthy buildings will be possible. Likewise, future building energy codes, IEQ standards, and guidelines can be connected in a single platform to each of the ultimate sustainability goals of future building designs. An occupant-centric, human health and well-being-focused, energy-efficient, and sustainable building stock could significantly change the North American construction industry and occupant lifestyle.