Authors

Francesco Babich, Senior Researcher at Institute for Renewable Energy - Eurac Research
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Giulia Torriani, Junior Researcher at Institute for Renewable Energy - Eurac Research
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This article presents the reason why a POE is necessary to verify comfort and energy efficiency design targets and the challenges of applying this evaluation to educational buildings.
Post-occupancy evaluation can be defined as the examination of the effectiveness for human users of occupied, designed environments [1].  In other words, POE is a method that is used to evaluate an occupied building during its typical use, with the aim of better understanding the disparities between predicted and observed energy and comfort performances. There are multiple reasons for conducting a POE. The most common reasons are to fine-tune a new building for maximising occupants’ wellbeing and productivity, to inform designers and contractors about the actual energy performances of their ‘product’, to establish benchmark data, to improve certification schemes, and to support the investment decision process. 

Why is a post-occupancy evaluation necessary to verify comfort and energy efficiency design targets?

POE comprises objective (e.g., measurements) and subjective (e.g., surveys) techniques to assess the users’ comfort and satisfaction, the energy efficiency and energy consumption, and the environmental performance and sustainability in an occupied building. The results of this evaluation provide valuable insights for making enhancements, adjustments, and optimisations to the building to ensure it meets the initial comfort and energy efficiency design targets.
User behaviours, ranging from occupancy patterns to usage of energy-intensive appliances, wield a significant impact on the energy profile of a building. These behavioural dynamics, when harmonised with the building's operational strategies, can either amplify or mitigate disparities between predicted and observed energy performance. By scrutinising these facets within the context of a POE, stakeholders can gain a comprehensive understanding of the factors influencing energy performance.

To ensure that a building meets occupants’ needs, long-term and detailed spot measurements of thermal, visual, acoustic, and indoor air quality (IAQ) comfort parameters are conducted. Long-term measurements are important to monitor the building for long periods (e.g. months, or even years) in order to identify trends and possible issues, and to measure several parameters simultaneously to spot correlations. Knowing trends and correlations among different parameters is relevant also to identify typical periods (e.g. a week) and exceptions. On the other hand, detailed spot measurements are typically used to obtain more detailed measurements of only a few parameters in order to verify the magnitude of likely issues. Hence, while long-term measurements are used to identify possible problems, more accuracy is needed to fully understand the issue. Spot measurements are performed with higher-quality devices, but for shorter periods, as there is often the need for the presence of experienced personnel. POE surveys are adopted in combination with measurements as a method to capture the subjective perception experienced by a building’s occupants. Starting from a typical common structure, these surveys are adapted to each specific building, and are repeatable over time to better evaluate the indoor experience.

Regarding energy performance, the building's energy consumption is monitored to assess its alignment with design forecasts and expectations. It is verified whether energy systems, such as insulation, lighting, and HVAC (Heating, Ventilation, Air Conditioning), are operating as intended and contributing to the anticipated energy savings. Furthermore, the extent to which the building is achieving the established environmental sustainability objectives, such as greenhouse gas emissions reduction or renewable energy utilisation, is evaluated. Environmental indicators like the building's carbon footprint are also monitored.

General challenges of post-occupancy evaluation applied to educational buildings

Educational buildings comprise schools and universities. Considering school buildings, different European countries have adopted different school systems in terms of educational stages (grouping according to pupils’ age) and methods (type of activities and consequent usage of the indoor spaces). 

This means that a first challenge that almost all international research projects have to address is to define a grouping (by age) strategy that enables a country-to-country comparison while capturing the specificity of each national system. This is essential because different ages require different methods for posing questions, or even completely different approaches for evaluation of the perceived environmental quality (if possible at all, for pre-school children) and for collecting long-term measurements. While older pupils can be instructed about the need of not altering the measurement equipment, with younger children their safety is the primary issue, and hence instruments must be placed in a meaningful but out-of-reach location. Moreover, the consent procedures are usually more complex when underage people are involved as participants. 

Another key challenge refers to the fact that educational buildings often have complex HVAC systems, as well as lighting systems tailored to different educational activities. Assessing and optimising the energy performance of these systems while accommodating varying occupancy and activities can be intricate. Furthermore, educational buildings experience varying occupancy and energy use patterns across different seasons, which can complicate the assessment of energy efficiency and comfort.

Lastly, standards and guidelines for assessing indoor conditions vary over time and among similar countries despite targeting similar occupants, evaluating indoor air quality (IAQ) and thermal comfort independently, and they do not include any specific adaptations to children [2].

Interpretation of long-term measurements of indoor air quality and thermal comfort in school buildings

The European standard EN 16798-1:2019 [3] recommendation for schools ‘category 1’ is the highest quality category whose thresholds are CO2 equal or lower than 950ppm (assuming 400ppm outdoor concentration), and indoor operative temperature equal or higher than 21°C in winter and equal or lower than 25.5°C in summer (for buildings with mechanical cooling systems). Different thresholds might be set at the national level.

Data collected from school buildings suggests that there is considerable variation even within the same region, and possibly within the same building. While in many classrooms the values were below or slightly above the recommended CO2 values for most of the time [2], in other cases thresholds were considerably exceeded with sporadic peaks above 5000ppm [4]. Although the EN 16798-1:2019 states that a lower level (i.e., category higher than the first) does not provide any health risk but may decrease comfort, evidence suggests that reduced classroom air quality will cause a reduction in cognitive performance of pupils [5]. 

When analysing IAQ data collected from buildings in operation, it is important to not only look at the mean values and the overall distribution (e.g., by using boxplots), but also to evaluate the trends to see when the peaks occur and the duration of the period in which a given threshold was exceeded. An example of guidelines that support this type of analysis is the UK Building Bulletin 101 (BB101) [6]. For teaching and learning spaces, the BB101 states that the maximum concentration CO2 should also not exceed 1500 ppm for more than 20 consecutive minutes each day during the occupied period when mechanical ventilation systems are used (2000ppm for naturally ventilated buildings). Lastly, although no standard indicates to jointly analyse CO2 and thermal comfort, this is highly recommended for balancing the need for fresh air with the need of not getting too cold or hot (depending on the seasons), but also for understanding whether a higher ventilation rate would imply higher energy consumption or not (e.g., if in winter time the CO2 is too high but the temperature is considerably above minimum threshold, then a higher ventilation rate would not imply more energy consumption as a lower temperature would still be acceptable).

The impact of perceived control on thermal comfort, indoor air quality perception and energy efficiency in schools

While measurements provide an objective overview of the indoor environmental conditions, questionnaires enable the capture of how people perceive a given environment. The subjective perception is usually affected by a combination of factors, such as the physical quantities (temperature, concentrations of non-odourless contaminants, etc.,) and psychological aspects, such as the view through a window or the perceived control over the indoor environment (i.e., the level of control that a person perceives to have, for instance by setting a thermostat or opening a window). 

A recent three-month field study conducted during the heating season and involving 26 school classrooms showed that the perceived control allows a better judgment regarding the IAQ and can also improve students' thermal comfort and reduce energy consumption [7]. By jointly analysing 859 questionnaires collected at every educational stage, the measured values taken in the classrooms, and the actual energy consumptions, this study demonstrated that the thermal neutral temperatures of the occupants (i.e., the temperature perceived as most comfortable) with and without perceived control are 21.7 °C and 22.2 °C, respectively. Thus, overall evidence recommends enabling perceived control in educational buildings. The challenge in its practical implementation is that classrooms usually have, in the best cases, some level of temperature and ventilation control at room level, but almost never at personal level (by means of the so-called personal environmental control systems – PECS [8]).

Beyond thermal comfort and indoor air quality: indoor soundscape, speech perception, and cognition in classrooms

The link between IAQ and thermal comfort is usually rather clear since in most cases the same systems (either natural ventilation or mechanical systems) are used to control both. However, in educational buildings, the indoor acoustic environment plays an essential role, and it is therefore important to investigate how different types of ventilation (operated according to IAQ and thermal comfort needs) affect the acoustic aspects.

A recent review of the published evidence investigated the interaction between acoustics and ventilation modality and especially the effects of sound stimuli related to ventilation on students’ speech perception, cognition, and acoustic comfort [9]. Nearly 40 studies ranging from primary school to university were included in this review.  The typical sound related with the mechanical ventilation is the noise of the fan that moves the air, while the sounds connected with natural ventilation are those entering a room when windows are open (i.e., traffic noise, aircraft noise, railway noise, human noise, sirens and construction noise, and natural sounds).

Fan noise was perceived as negative. Anthropogenic sounds entering the classroom in natural ventilation conditions were perceived as negative or without effects (i.e., they had no relevant impact in comparison with a quiet baseline condition) depending on the specific task and noise characteristics. Natural sounds from open windows were instead found to consistently generate a positive effect on students’ learning and comfort. Thus, overall, ventilation may have different effects on the acoustic environment and related aspects depending on the type of noise/sound and the activity performed, and thus attention should be paid also to this while analysing IAQ and thermal comfort data both to better understand them (perhaps a peak of CO2 is due to the intentional choice of reducing ventilation for noise issues) and provide effective recommendation to better operate the building.

Recent and on-going EU-funded project on IEQ in schools

• QAES project was an Interreg Italy-Switzerland whose aim was to develop technological solutions, with low architectural impact, to manage IAQ inside school buildings. The main objective was to act on the air quality inside school buildings so that pupils´ indoor comfort and learning environment would improve.
• LEARN project aims at learning more about IAQ in school buildings, while also taking into consideration the cognition of children. One of the main objectives is to develop innovative sensors to detect the pollutants of indoor air.
• InChildHealth project will conduct an environmental epidemiological study and controlled interventions in schools based in three European cities and will assess the health effects of multi-pollutant airborne exposure.
• SynAir-G project  will develop and employ novel sensors for chemical and biological pollutants which will be tested in schools of five countries around Europe, and eventually combined in a multi-sensing platform. Eco-friendly air purifying will also be assessed.

Conclusion

Overall, ensuring healthy and comfortable indoor environments in schools is extremely important as children and young people represent the future of our society. Post-occupancy is a fundamental piece of the puzzle and it enables to understand the current situation in schools and universities, but also to provide evidence when possible enhancements are implemented (for instance, means to improve the perceived control). Furthermore, POE is fundamental to assess the energy use and energy efficiency once the building is occupied and to compare them to the initial design targets. When conducting a POE, both measurements and people’ perception must be considered, and especially the latter deserve specific adaptation according to the pupils’ age and conditions. In this framework, multi-domain research is fundamental to improve the conditions inside educational buildings without leaving any key aspects behind, the methods used to the design them, and also the capability of evaluating them in-use (i.e., the POE).