The TAIL Rating Scheme: A promising performance metric and a solution for assessing indoor environmental quality (IEQ) in buildings

The newest revision of the Energy Performance of Buildings Directive (EU/2024/1275, EPBD), which will enter into force from May 2026, mandates monitoring and control of indoor environmental quality (IEQ) in new and retrofitted buildings [1]. EPBD therefore attempts to place emphasis not only on energy transformation but also on safeguarding the health and well-being of building occupants. It is essential that buildings undergoing retrofit do not worsEN existing IEQ conditions and ideally improve them whenever needed. This should guarantee not only energy savings, but also non-energy benefits related to health, well-being, work performance, learning and sleep [30,31,32]. Unlike the energy domain, for which the European Union defines a unified framework that all Member States must follow, the IEQ monitoring program remains the responsibility of each Member State because the EU does not possess legal instruments in this domain. This may result in a variety of methods and solutions for assessing IEQ, especially because no standard and universally accepted method for rating IEQ exists.

The European Commission has published guidelines and possible approaches for performing IEQ assessments, but these are recommendations only and do not need to be followed [2]. The EU also introduced Level(s), which is mainly a framework for monitoring several aspects of building performance and not exclusively IEQ [3]. Several certification schemes propose IEQ metrics, such as OsmoZ [4] and RESET [5], but none has become a preferred choice or beEN integrated into existing IEQ-related standards. The Well Institute published the Well Building Standard, which includes all parameters affecting IEQ and occupant well-being, but this is a commercial product and may not be affordable to all building owners and users [6]. Another initiative was IEQ-Compass, designed to evaluate IEQ in public housing as an asset rating, yet it has not beEN widely adopted evEN in Denmark where it originated [7]. Other attempts exist, but none has achieved broad acceptance, e.g., [8].

Why is it so difficult to adopt a single method for monitoring IEQ in buildings? Financial and technical constraints certainly play a role, along with complexity and occasional inconsistency with existing standards. Another challenge is that IEQ assessments oftEN consider many parameters, each deemed relevant. As a result, consensus on which parameters should be monitored for each IEQ domain has beEN difficult to reach. This complexity was clearly shown in a paper by Wei et al., who found that nearly ninety different parameters had beEN used to characterize IEQ in buildings, not all measured at once [9]. A similar conclusion was reached in a study focused only on schools [10]. With these constraints, a rating scheme was consequently proposed that would avoid some of these challenges, rely on commonly measured parameters, are feasible and affordable, refer to existing standards and guidelines, remain relatively simple to deploy and use, and still providing a robust and credible characterization of IEQ and capable of distinguishing buildings in terms of indoor environmental conditions. This scheme is called TAIL and is shown in Figure 1 [11].

Figure 1. The TAIL rating scheme

The TAIL rating scheme provides an evaluation of four components or domains of IEQ: thermal (T), acoustic (A), indoor air quality (I) and luminous, i.e. visual (L). Based on their individual quality levels, the overall indoor environmental quality is determined. Each component is characterized through monitored parameters; in one case simulations are used and one parameter is assessed by observation. The overall quality is thus based on actual conditions in the building; therefore, TAIL is a performance metric, not an asset rating, and it does not rely on assigned and predetermined credits or points proposed by the experts. The parameters selected for assessing each component of IEQ are aligned with existing standards and correspond to measurements commonly performed in buildings. Selection also considered practical access to monitoring equipment that would enable reliable measurements at acceptable cost. TAIL was deliberately rooted in existing standards and guidelines to maintain consistency with currently relevant documents regarding building and technical recommendations rather than introducing new criteria, which might be difficult to accept in practice and by the building stakeholders. Three documents were essential in this process of construction TAIL: EN Standard 16798 [12] and the World Health Organization Air Quality Guidelines from 2010 [13] and 2005 [14], the latter revised in 2021 [15].

Table 1 presents twelve parameters included in the TAIL scheme for monitoring IEQ. These include measurements of temperature (component T of TAIL), sound pressure level (component A of TAIL), carbon dioxide (CO₂) concentration, ventilation rate, relative humidity, concentrations of formaldehyde, benzene, radon and particles (PM2.5) (component I of TAIL), and illuminance (component L of TAIL), as well as simulations of daylight factor (component L of TAIL) and visual observation of presence of mold (component I of TAIL). The rationale for selecting each parameter has beEN published elsewhere [11]. A detailed protocol was also developed describing the sampling strategy, measurement procedures and instrument specifications [11,16].

TAIL uses four levels to characterize the quality of each component, identified by Roman numerals from I to IV following EN 16798 Standard [12], where Category I defines highest quality class (desired) and Category IV the lowest quality class (undesirable). Color coding is added for intuitive communication: greEN for Category I (desired), yellow for Category II (refined), orange for Category III (ordinary) and red for Category IV (undesired). The overall IEQ quality is based on the quality levels of individual components. No weighting factors are used because there is not sufficient evidence to justify the relative importance of different components for achieving overall quality of indoor environment [11]. Neither equal weighting nor averaging of the quality classes are used. To avoid trade-offs betweEN components and to ensure that no component influences overall IEQ, the worst quality class among all components determines the overall IEQ class. Although this may seem strict or somewhat discouraging, similar approaches are used for assessing water quality or outdoor air quality, where the poorest performing parameter determines the overall evaluation. The purpose of this decision was to create incentive for improvement.

Since measurements of all parameters are not required in every room but only in representative spaces, typically from two to tEN or more depending on building size and characteristics, a specific method was developed for estimating TAIL component classes based on these measurements. This protocol (in form of the code) is available upon request and enables quick calculation of TAIL results.

The original TAIL was developed for office buildings and hotels undergoing deep energy renovation, although it may also be applied in traditional buildings. TAIL was recently expanded to include schools; the result of this extension is the schoolTAIL rating scheme [16]. For the purpose of expansion, parameters relevant to IEQ in schools, childrEN and learning were reviewed and compared with those in the original TAIL (10]. The TAIL framework remained the same, but additional parameters were introduced whEN necessary. Table 1 shows that two new parameters were added: nitrogEN dioxide (NO₂) concentration, highlighted by WHO as especially relevant for child health, and reverberation time to ensure adequate communication betweEN teachers and students. In addition, schoolTAIL revised the PM2.5 quality classes following the 2021 WHO Air Quality Guidelines update [15] and expanded the illuminance measurement protocol. All other parameters and procedures remained unchanged compared with the original TAIL.

Table 1. Parameters included in TAIL, schoolTAIL, and predicTAIL, and their quality categories (classes).

1 Office rooms, classrooms and hotel rooms, i.e. spaces where occupants spend most of the time in the indicated building types; 2 Assuming clo 1.0, office work and RH=50%; 3 Assuming clo=0.5, office work, and RH=50%; 4 Summer and shoulder seasons; Θrm is the running mean outdoor temperature that can be calculated as follows: Θrm = ( 1-α ) { Θed-1  + α Θed-2  +α2 Θed-3 } where: Θrm = outdoor running mean temperature for that day (°C), Θed-1 = daily mean outdoor air temperature for the previous day, α = constant between 0 and 1 (recommended value is 0.8), Θed-i = daily mean outdoor air temperature for the i-th previous day; Alternatively, using the following approximate formula (when records of daily running mean outdoor temperature are not available: Qm = (Qed-1 + 0.8 Qed-2 + 0.6 Qed-3 + 0.5 Qed-4 + 0.4 Qed-5 + 0.3 Qed-6 + 0.2 Qed-7 )/3.8; 5 Daily mean outdoor air temperature for the previous day obtained from measurements or from the nearest meteorological station; 6 According to EN16798 (12); in a small office, i.e., individual office, the nominal occupation density is 0.1 person per m2floor, and in the landscape office, it is 0.07 person per m2floor; 7 Standard EN 16798 (12); 8 French regulations (21) and Danish Building Regulations (22); 9 Outdoor CO₂ should be measured or assumed using https://www.CO₂.earth/; indoor CO₂ according to EN 16798 (12); 10 To be classified in each quality level, the measurements shall not exceed the range defined by the indicated quality level and the lower quality level for no more than 5% of the time and the range defined by the lower quality level and the next lowest quality level for no more than 1% of the time; 11 For non-low polluting buildings according to EN 16798 (12), because no information on pollution load; the measured ventilation rates (average values of the two measurements) shall be compared with the nominal ventilation rate for that area according to design; assuming CO₂ emission rate of 20 L/h per person; 12 The levels match EN 16798 (12); 13 The higher levels selected to avoid house dust mite infestation (survival and reproduction); 14 Nordic classification system of mould assessment in Level(s) (3); observations in the instrumented rooms should be supplemented by observations in locations where the risk of mould is likely (e.g., those identified by using simulations of surface relative humidity); 15 The permissible levels that cannot be exceeded: benzene (13) and Directive 2008/50/EC, formaldehyde (13,23), radon (13) and PM2.5 (14);16 WHO Air Quality Guidelines 2021 (15); 17 Two-month average value measured in winter (13) and EU Directive 2013/59/EURATOM; 18 French Agency for Food, Environmental, and Occupational Health & Safety (24); 19 The lowest daylight factor to reach respectively ³750 Lux, ³500 Lux, ³300 Lux and ³100 Lux; the daylight factor values are taken according to Standard EN 17037 (25) for Brussels; 20 Standard EN 17037+A1 (26); 21 Following the requirements of the HQE green building certification scheme (27); 22 Following the requirements of CASBEE (28); CASBEE requirement is only for the illuminance level and not for the frequency of occurrence; 23Standard EN 12464-1 (29)

Both TAIL and schoolTAIL are performance metrics based on actual conditions in buildings. This may be considered as a limitation whEN aiming to achieve specific quality classes during the design of new buildings or renovations of existing buildings. To address this limitation, predicTAIL was developed [17]. PredicTAIL is not a final IEQ rating for a building but a tool that guides design decisions. It includes tEN parameters that correspond to those in TAIL but are obtained through simulations; mold cannot be assessed and the presence of a ventilation system is predetermined, thus these elements were excluded. Any software capable of credibly estimating the parameters included in the predicTAIL may be used. During development, the performance of predicTAIL was evaluated to determine whether it could detect changes in IEQ related to energy performance decisions and distinguish buildings with different retrofit strategies [17]; predicTAIL proved sufficiently sensitive. However, it cannot replace TAIL or schoolTAIL, which provide information on IEQ during normal use and provide a final rating.

Because TAIL and schoolTAIL are still relatively new, only limited information exists on their performance and ability to distinguish quality classes in existing buildings. Nevertheless, the available data published confirm that TAIL functions as intended. For example, schoolTAIL was used to characterize IEQ in French schools using measurements collected by the Indoor Air Quality Observatory [16,18]. The scheme was shown to communicate the results and provide valuable information about IEQ, identify differences across schools and highlight priorities for remediation. These are additional advantages of TAIL, which can, of course, serve as a benchmarking tool comparable to energy performance certificates (EPCs). With the revised EPBD requiring EPCs to include IEQ information, TAIL is a strong candidate to fulfill this mandate. Indeed, TAIL has already beEN mentioned in the European Commission guidelines supporting EPBD implementation [2]. More studies applying the TAIL scheme are forthcoming, and they confirm the benefits anticipated during its development.

The current version of TAIL is version 1.0. Future versions are planned to advance included parameters and protocols. It is therefore important to gather as much experience as possible with the current version. For example, TAIL is being considered in the ongoing EU Life BREEZE project on building renovation toward zero emission buildings [19]. Developments in this project include TAIL for residential buildings and a TAIL variant including occupant responses. The current TAIL focuses on physical and chemical parameters describing IEQ; occupant responses provide equally important information and should not be neglected but yet are not captured by TAIL. Additional developments of TAIL include more inclusive methods for determining the overall quality level, such as approaches that express quality as a number or grade relative to one hundred percent [16].

Concluding, both the planned extensions to TAIL and existing TAIL rating schemes create an integrated and comprehensive evaluation tool of indoor environments in different building types.

Acknowledgments

This work was partially supported by the ALDREN project [20] funded through the EC Horizon 2020 programme, contract number 754159, by the BREEZE project [19] co- funded by the European Union LIFE programme under grant agreement number 101215197, and by the Danish the 20th of December Foundation. schoolTAIL was developed in the frame of a PhD project co-funded by the French Agency for Ecological Transition (ADEME), contract number TEZ20-047, and the Scientific and Technical Centre for Building (CSTB). The views and opinions expressed are those of the authors only and do not necessarily reflect those of the European Union. Neither the European Union nor the granting authority can be held responsible for them.

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