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European citizens spend 90% of their time indoors. Meanwhile, buildings account for over one-third of emissions and 40% of greenhouse gas emissions. Clearly, the answer to this question must be ‘yes’ if we are to achieve a truly sustainable society.
Finding ways to combine health, comfort, and sustainability is of increasing interest in both research and policy. The latest revision of the Energy Performance of Buildings Directive incorporates the concept of indoor environmental quality, marking a significant step forward.
(Note: Opinions in the articles are of the authors only and do not necessarily reflect the opinion of the EU)
Between schools, workplaces, homes, and public spaces, people in Europe spend 90% of their time indoor, according to the World Health Organization (WHO) data.
It is therefore essential that these indoor environments are healthy and support the physical and mental well-being of those who occupy them. According to the WHO, the past few decades have seen a significant improvement in the indoor environmental quality (IEQ) of buildings. However, the WHO also warns that these improvements are unevenly distributed, as they are closely linked to the quality of buildings, which disproportionately affects vulnerable socio-economic groups.
Poor IEQ can lead to serious health problems, impact psychological well-being and cognitive functions, and contribute to discomfort, stress, and decreased productivity.
The IEQ, as defined by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), is ‘a perceived indoor experience of the building indoor environment that includes aspects of design, analysis, and operation of energy-efficient, healthy, and comfortable buildings. Fields of specialisation include architecture, HVAC design, thermal comfort, indoor air quality (IAQ), lighting, acoustics, and control systems.’
There is general agreement in the literature that four domains should be considered to define IEQ:
Indoor air quality (IAQ)
Acceptable IAQ is the ‘air in which there are no known contaminants at harmful concentrations as determined by cognisant authorities and with which a substantial majority (80 % or more) of the people exposed do not express dissatisfaction’ (ASHRAE 62.1, 2020).
The sources of indoor pollutants can be varied - building materials, cleaning products, the occupants themselves, kitchen fumes, outdoor air, dust etc... Some of these pollutants are CO2, Volatile Organic Compounds (VOCs), PM2.5 (particulate matter less than 2.5 micrometers), PM10, Ozone (O3), Nitrogen Dioxide (NO2), Formaldehyde, Radon and Carbon Monoxide (CO).
Thermal comfort
‘The condition of mind that expresses satisfaction with the thermal environment and is assessed by subjective evaluation’ (ASHRAE 55, 2022). It is influenced by various environmental factors such as humidity, air speed, and temperature.
Visual comfort
Primarily influenced by light intensity, reflections, glare and the balance between natural and artificial light. Visual comfort may also be affected by the availability of an outside view.
Acoustic comfort
Influenced by external noise, sounds from devices and systems operating indoors, and ambient chatter, such as conversations among colleagues in a workplace setting.
The parameters that define appropriate IEQ may vary from country to country, partly due to differences in environmental, climatic, and social conditions. The intended use of a building - for example, whether it serves as an office, a school, or a home - can also influence these parameters as well as the time of the day and other variables.
The International Society of Indoor Air Quality and Climate (ISIAQ) provides a database that contains threshold values and optimal levels for various aspects of IEQ. The aim is to ensure that people's mental and physical well-being is not adversely affected. These values are derived from existing regulations in different countries as well as from guidelines set by national and supranational organisations, such as the WHO, ASHRAE, the EU and certification systems like WELL.
By exploring the database, users can access optimal values for IAQ, including permissible concentrations of pollutants. The database also provides parameters for an optimal indoor thermal environment, such as temperature and humidity, as well as recommended noise levels and appropriate lighting standards and other aspects of the various domains of IEQ.
With the increasingly pressing need for society to move towards a more sustainable lifestyle for the planet, with building efficiency playing a central role in achieving this goal, two questions may arise:
How can the different IEQ parameters be maintained at acceptable levels without them conflicting with each other? For example, can both good IAQ and acoustic comfort be achieved at the same time?
What is the relationship between an acceptable IEQ and the building's energy expenditure? In other words: can energy performance and IEQ go hand in hand?
Certain measures and technologies clearly make the answer to both questions ‘yes’: green roofs improve air quality and passively cool the indoor environment, affecting positively thermal comfort and energy efficiency. Another example, electrochromic glass, a special glass that becomes opaque when current flows through it, positively influences lighting, thermal comfort and energy savings on heating and cooling. Furthermore, there are thermal insulating materials, such as expanded polyurethane foam, expanded polystyrene, cotton, cork, sheep's wool, wood fibre, cellulose, rock wool, and glass wool, that are also used in structural applications (e.g., roofs, walls, floors) and can provide a good degree of acoustic insulation; in some cases, however, insulation materials can release pollutants into the air, therefore, it is essential to ensure a proper ventilation system. Several studies investigate sustainable methods for producing effective thermal-acoustic insulators.
In general, though, the problem is more complex, and much research is being conducted on this topic.
For example, an 2023 study investigating the first question examines how classroom ventilation - both natural and mechanical - in schools and universities affects the acoustic perception, or the environment as perceived and understood by a student in a context. The study finds that fan noise, for instance, negatively impacts learning abilities compared to an absence of noise. External noises from natural ventilation, however, have a negligible effect compared to anthropogenic sounds (e.g., traffic, voices, construction sites) and may even have a positive effect when natural in origin.
Another recent study addresses both questions - can we achieve adequate levels of the various IEQ parameters simultaneously, and can we also maintain good energy performance? - by examining the interplay between IAQ, thermal comfort, and energy efficiency through a review of several papers on the subject. The findings suggest that a holistic approach to the problem is needed:
Expanding the research, which currently focuses largely on residential buildings, to include non-residential ones.
Considering more the variables related to occupancy (number of occupants, behaviour, activities, position) and to the building (orientation, envelopes, windows, materials and insulation).
Future research in this area should incorporate more indoor environmental factors (acoustic, lighting).
The paper highlights the potential of using evolutionary algorithms, artificial intelligence, and machine learning techniques as promising avenues for further exploration.
Considering just these three variables, however, presents a complex optimisation challenge. Nonetheless, the paper offers an optimistic outlook on addressing such challenges in the future, incorporating additional parameters that contribute to defining IEQ.
Solutions to these complex optimisation problems appear to emerge from the fields of building automation, the Internet of Things (IOT), and Artificial Intelligence (AI) and Machine Learning (ML).
It is particularly interesting what could be achieved when these fields intersect: building automation systems, which centrally control various building systems (HVAC, shading, lighting, electricity, etc.), can leverage sensors capable of communicating and exchanging data (IoT). Based on this data, these systems can predict the need for and consumption of various systems and optimise the regulation of parameters (AI) such as temperature, humidity, lighting, or ventilation to achieve optimal internal conditions in terms of comfort and energy consumption.
An example of a technology that partially integrates these principles is EQ-OX, developed by researchers from various institutions led by Eurac Research. It is a device that simultaneously measures up to 15 IEQ parameters. Although it does not specifically use ML algorithms - instead, it relies on lightweight algorithms that enhance its versatility at the software level - EQ-OX presents itself as a promising model to produce low-cost monitoring devices that, due to their affordability, are suitable for large-scale use.
Initiatives such as the Horizon 2020 CULTURAL-E Project promote the development of Plus Energy Buildings (PEBs) - buildings that are not only highly efficient but also produce more energy than they consume - by developing technologies that not only enable normal Plus Energy Building operation but at the same time ensure improved levels of IEQ.
The effort is both scientific and political. Not only must steps be taken to ensure that these technologies are applied safely and consciously, but it must also be ensured that they are adopted inclusively, without widening the gap between those who enjoy good IEQ and those who do not. With 97% of Europe’s buildings in need of renovation, (BPIE, 2017), this challenge is also an opportunity - a chance not only to enhance energy performance but to transform these buildings into spaces that support both body and mind, ensuring that everyone, regardless of background, benefits from a healthier, more inclusive indoor environment.
The recast Energy Performance of Building Directive (EU) 2024/1275 (EPBD) - the main EU-wide legislative tool for promoting and improving energy efficiency in the building stock - incorporates for the first time a definition of IEQ: ‘Indoor environmental quality’ means the result of an assessment of the conditions inside a building that affect the health and well-being of its occupants, based on parameters such as those relating to the temperature, humidity, ventilation rate and the presence of contaminants.’
Article 3, which governs building renovation plans, specifies that each Member State must include in its national plan an evidence-based estimate not only of the anticipated energy savings but also of other factors, including those related to IEQ.
When setting minimum energy performance standards, Article 5 reads: ‘‘Those requirements shall take account of optimal indoor environmental quality, in order to avoid possible negative effects such as inadequate ventilation, as well as local conditions and the designated function and the age of the building’.’
According to the Directive, by 2028, all new public buildings must be zero-emission, and from 2030, this requirement will extend to all new buildings without exception. Article 7, however, goes beyond energy-related requirements, mandating that Member States take measures to ensure optimal indoor environmental quality (IEQ). This objective is reiterated in the following article, which specifically addresses the renovation of existing buildings.
In Article 13 (§4, §5, and §10), Member States are required to establish standards for adequate IEQ and to equip zero-emission or deeply renovated non-residential buildings with air quality monitoring devices.
Starting in May 2026, control systems must be capable of monitoring IEQ.
In accordance with the definition of IEQ in article 2, the minimum parameters that must be monitored to ensure optimal IEQ are: temperature, humidity, ventilation rate and presence of contaminants, and lighting.
A comprehensive document, signed by authoritative organisations within the IEQ sector, explores practical approaches to achieving these objectives in alignment with the provisions of the EPBD. We summarise below some useful information from the text:
Facility management plays a pivotal role in ensuring adequate IEQ, as it oversees critical systems such as HVAC and lighting. Given this centrality, it is essential that Member States:
Recognise facility management in their transposition of the EPBD into law, highlighting its role as a catalyst for improved IEQ.
Implement training programmes for facility managers, focusing on the new EPBD provisions, and particularly the application of Smart Readiness Indicators (SRIs).
Provide ongoing funding for facility management training to ensure continued professional development.
Article 13 §3 of the EPBD mandates that Member States ensure, where feasible, that new buildings are equipped with self-regulating temperature controls for each room or zone, along with hydronic balancing where appropriate. This requirement also applies to existing buildings when heating or cooling systems are replaced, provided it is economically viable.
Common room temperature control devices include thermostatic radiator valves, room thermostats for surface heating, fan coil unit regulators, and standalone heater controls.
In addition, it is necessary to maintain the relative humidity at adequate levels using humidifiers and dehumidifiers, when required, for the health, comfort, and structural integrity of the building.
The EPBD (Article 13 §5) requires IAQ monitoring devices for non-residential zero-emission buildings only, and when technically and economically feasible.
Good indoor air quality requires source control, filtration of external pollutants, and effective ventilation. Monitoring all pollutants in residential buildings is expensive and complex, but CO2 and PM2.5 are two key indicators of air exchange and pollution control respectively, CO2 in particular is used as a proxy for ventilation. Proper IAQ management involves the use of low-polluting materials, appropriate ventilation rates, and air filtration.
Article 13 §12 affirms that Member States must require non-residential buildings with heating, air-conditioning, or combined systems over 290 kW to have automatic lighting controls by 31 December 2027, and those over 70 kW by 31 December 2029, if technically and economically feasible.
In general, the use of LED lights and lighting control systems boosts energy efficiency and supports visual well-being. Integrated Building Automation and Control Systems (BACS) further enhance efficiency and indoor environmental quality, not only for lighting but also as part of a holistic approach to improve IEQ in its entirety. Finally, Article 15 indicates that the smart readiness of a building can be measured by its capacity to optimise operations in relation to factors including IEQ and, finally, Article 17 specifies that IEQ must be included among the parameters on Energy Performance Certificates
LIFE VISIONS logo
LIFE VISIONS, Start Date: September 2020 - End Date: February 2024
The VISIONS project aims to improve indoor air quality and energy efficiency by developing photocatalytic paints incorporating TCM-1, targeting pollutant reduction and thermal comfort. It includes DSS development, real-scale applications, EU compliance, and market penetration efforts.
TwinAIR logo
TwinAIR, Start date: September 2022 - End date: September 2026
TwinAIR improves urban life by addressing IAQ through innovative tools for tracking pollutants, assessing health impacts, and enhancing building management. It empowers citizens and policymakers to make healthier choices, with applications across diverse European pilot sites.
FaceINQ, Start date: January 2023 - End date: March 2024
FaceINQ is a project aimed at designing naturally ventilated buildings with complex glass facades. It uses advanced models, validated by occupant feedback, to balance energy efficiency and indoor air quality, enhancing health, comfort, and productivity through interdisciplinary research and industry-academia collaboration.
ZERAF logo
ZERAF, Start date: July 2022 - End date: June 2026
The ZERAF project aims to enhance building facades using smart materials to improve energy efficiency, heat transfer control, and occupant wellbeing. It integrates advanced technologies to optimise thermal comfort, natural light, and ventilation, while reducing environmental impact and improving IEQ.
InChildHealth logo
InChildHealth, Start date: September 2022 - End date: August 2026
InChildHealth will research indoor air quality (IAQ) in environments children occupy, focusing on pollutants’ impact on health. The project involves studies in Europe and Australia, developing tools for risk assessment, exposure measurement, and IAQ improvement, with citizen involvement and educational resources.
SynAir-G logo
SynAir-G, Start date: September 2022 - End date: August 2026
SynAir-G aims to study and quantify the synergistic effects of indoor air pollutants on human health, focusing on highly susceptible groups like children and those with asthma. It will develop advanced sensors, monitor air quality in schools, and provide eco-friendly solutions and personalised interventions using AI.
SUREFIT logo
Surefit, Start date: September 2020 - End date: September 2024
Surefit aims to retrofit homes with eco-friendly technologies to achieve near-zero energy consumption while enhancing IEQ. The project integrates systems like solar heat pumps and bio-aerogel panels for energy efficiency, comfort, and minimal disruption to occupants.
JUSTNature logo
JUSTNature, Start date: 1 September 2021 - End date: 28 February 2026
JUSTNature aims to activate nature-based solutions for a just transition to low-carbon cities, focusing on clean air, thermal comfort, and biodiversity. It promotes co-design and co-decision through governance, system maintenance, business models, and innovative technologies in seven European cities.
SSID, Start date: March 2018 - End date: February 2025
Eighty million EU citizens face excessive environmental noise, prompting billions in noise control spending. Traditional sound level reduction is ineffective. Soundscape studies aim to improve quality of life through interdisciplinary research, developing soundscape indices (SSID) to guide built environment design and noise management.
Cultural E+ logo
Cultural-E, Start date: October 2019 - End date: March 2025
CULTURAL-E aims to create adaptable, cost-effective solutions for Plus Energy Houses (PEHs) that account for climate, culture, and energy needs. It focuses on renewable energy, efficient design, and user-centric technology to ensure affordable, comfortable living while reducing environmental impact and promoting e-mobility.
X-tendo logo
X-tendo, Start date: September 2019 - End date: 31 August 2022
The X-tendo project advances next-generation EU energy performance certification with a toolbox of 10 innovative features, including comfort and smartness indicators, enhancing compliance, usability, and policy and market impact.
ALDREN logo
ALDREN, Start date: 1 November 2017 - End date: 30 September 2020
ALDREN aims to improve renovation rates and quality through standardised energy ratings, healthy indoor environments, financial incentives, and a building passport, driving energy savings and supporting EU policies for deep renovation.
U-CERT logo
U-CERT, Start date: September 2019 - End date: August 2022
U-CERT aims to introduce user-centred certification schemes for buildings, focusing on quality, reliability, and innovation. Its objectives include facilitating certification implementation, promoting user-centred solutions, and supporting EU-wide adoption, with attention to smart readiness and IEQ.
Health and well-being are fundamental pillars of sustainability. Therefore, it is essential to live in healthy and comfortable environments. This is why IEQ and energy efficiency must go hand in hand to achieve comprehensive sustainability in the building sector.
The urgency of building renovation plans is becoming increasingly evident. These plans must aim not only at energy efficiency but also, simultaneously, at achieving adequate IEQ.
Existing technologies and strategies enable the integration of certain IEQ aspects with strong energy performance. However, adopting a holistic approach - that is, prioritising energy savings without compromising essential IEQ parameters, while considering all the variables at play, from occupant behaviour to building characteristics - presents a complex optimisation challenge. New smart technologies offer promising solutions in this regard.
IEQ is taking on an increasingly central role within the Energy Performance of Buildings Directive (EPBD), receiving, for the first time, a formal definition within the text. The directive calls on member states to adopt various measures to preserve IEQ, with a focus on integrating control devices, where possible, for real-time parameter monitoring.
Furthermore, European projects demonstrate that renovation and construction can improve IEQ while balancing it with energy efficiency. This can be achieved through the engagement of citizens and communities, as well as by developing innovative tools and technologies, promoting nature-based solutions and introducing adequate certification schemes.
Nevertheless, achieving true sustainability requires political action to ensure these tools are accessible to everyone, allowing society to genuinely claim to be sustainable
