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Classroom heat is usually filed under summer, which means it is usually filed as somebody else's problem. The European Environment Agency's assessment of school heat exposure says otherwise. Between 70% and 90% of the days above 30 °C during the European school year fall before the summer holidays. Around 16,000 schools, about 5% of the European estate, already see those days with classes in session, and that number is expected to reach roughly 31,500 by 2050.
That changes the question for anyone running a school building. Ventilation in classrooms has been managed for years around one measurement, CO₂, and one device, the window. From March onwards that approach starts working against itself. Opening the window lowers CO₂ and raises the temperature. Closing it protects comfort and lets CO₂ build up. You cannot manage that trade-off with a single number, and you certainly cannot manage it from the corridor. The wider case for continuous IAQ monitoring in schools and universities is covered separately. This article is about the specific problem of spring heat, and what measured data says about it.
The climate signal is recent and easy to check. Copernicus recorded European spring 2026 at 1.13 °C above the 1991 to 2020 average, the third warmest European spring on record, and described the heatwave that arrived in western Europe around 20 May as one of the most intense ever observed that early in the year. May temperature records fell in France, the United Kingdom, Ireland and Portugal. April 2026 was Spain's warmest April since 1962.
The buildings absorbing that heat were not designed for it. In Italy, of just over 40,000 public school buildings, close to 80% were built before 1990, and the large majority still rely on open windows as their only means of ventilation. Location makes it worse: the EEA finds that 43% of European schools sit in areas at least 2 °C warmer than their surrounding region because of the urban heat island effect, most of them in Italy, Spain, Greece and France.
The result is that a classroom in May behaves nothing like the same classroom in January, while the ventilation habits stay the same.
This is one of the better documented findings in indoor environment research, and it gives us a number to work with.
A 2025 systematic review in the peer-reviewed journal PLOS Climate pulled together studies covering around 14.5 million students in 61 countries and found the same threshold appearing again and again: 26.7 °C. Above it, cognitive performance starts to fall away faster. The review also found that mathematics suffers more than reading, which matters because the harder subjects tend to be timetabled in the morning and the heat peaks in the afternoon.
The experimental work points the same way. A meta-analysis of 18 studies, led by Pawel Wargocki of the Technical University of Denmark and published in 2019 in the peer-reviewed journal Building and Environment, concluded that performance on school tasks improves by around 20% when classroom temperature drops from 30 °C to 20 °C, and that the best results appear below 22 °C.
The useful part is where 26.7 °C sits. It is not an extreme value. It is the sort of afternoon temperature a naturally ventilated classroom in southern or central Europe reaches routinely in May and June, without anybody describing it as a heatwave. A room at 26 °C looks unremarkable on a walk-through. It is already at the edge of a measurable penalty, every afternoon, for the last two months of the school year.
Here is where the two parameters collide, and the research is more specific than general intuition suggests.
A 2025 study in the journal Energy Efficiency modelled a school in Lecce, southern Italy, testing ventilation rates from 1 to 10 air changes per hour. In winter it works: low rates hold classrooms comfortably around 21 to 22 °C. In May and June, no rate tested keeps the rooms within the comfort range, because the outside air is no longer cold enough to do the cooling. Ventilation stops being a temperature tool and becomes only an air quality tool.
The opposite behaviour fails on both counts. A study of a classroom in Gliwice, Poland, published in the journal Energies, took as its baseline the common habit of opening windows only during breaks. CO₂ peaked at 2,900 ppm, and the room spent 44% of class time outside the comfort range and about 30% of the time above 30 °C. Closed windows do not protect comfort. They lose both.
The same study shows where the answer is. When window opening was modulated according to measured conditions instead of the timetable, CO₂ stayed between 400 and 950 ppm for 80% of class hours and time outside the comfort range fell from 44% to 18%. Nothing was installed. The difference was knowing, in that room, at that moment, which of the two problems was the binding one.
There is a second reason this is becoming urgent, and it comes directly from the European renovation agenda.
A 2025 study of four Madrid school buildings, published in the journal Applied Sciences, modelled a standard envelope upgrade with external insulation and better windows. Heating demand fell by between 31% and 77%. Cooling demand rose by 42% to 49% in three of the four buildings, and the number of hours outside the summer setpoint rose by between 42% and 64%. Adding solar shading recovered part of the loss, but not all of it.
For anyone specifying or signing off a school renovation, that is the risk in one sentence: the works can improve the energy certificate while making the classroom harder to teach in. Measuring temperature before and after the intervention is what turns that from an assumption into evidence.
The recast EPBD, Directive (EU) 2024/1275, is what turns this into an obligation. Until now classroom temperature was a comfort question, settled by judgement and rarely recorded. The directive places it inside indoor environmental quality, which it defines as covering temperature, humidity, ventilation rate and pollutants.
Two consequences follow directly. Member States have to set indoor environmental quality requirements for buildings. And building automation and control systems have to be capable of monitoring indoor environmental quality, a requirement in force since 29 May 2026.
The directive's recitals add guidance that speaks to the problem in this article: they give priority to avoiding overheating through shading and thermal mass, and to developing passive cooling, ahead of adding cooling capacity. Worth being precise about this, because it is a common misattribution: it sits in the recitals, not in the operative articles.
The practical consequence for a school is that temperature moves next to CO₂ as something the building has to be able to demonstrate, continuously, with a record that stands up to review. That is exactly what a one-off measurement does not produce.
Two deployments illustrate different halves of the problem.
The Navarra regional education authority installed 700 MICA Mini devices across its public school network, with visual indicators in the classroom and a central dashboard for the authority. The point was not that teachers did not know how to open a window. It was that an authority managing thousands of rooms cannot rely on individual judgement in each one. With data, it could set a protocol and check whether it was being followed, so ventilation happened when it was needed rather than continuously, which limits both the CO₂ peaks and the temperature penalty of over-ventilating.
In Southampton City Council, a Defra-funded project put eight MICA WELL devices into classrooms, dining rooms, gyms and corridors, measuring NO₂, ozone and carbon monoxide alongside the usual parameters. It found a clear link between outdoor activity and indoor particle spikes. That is the input most overheating discussions are missing: whether opening a window at 3 pm improves the room or imports traffic pollution depends on what is outside at 3 pm. Indoor CO₂ alone cannot answer that.
The measures that work are neither expensive nor complicated. What they need is data, because each of them can be applied wrongly.
Ventilate on a schedule, not in reaction. CO₂ build-up is predictable, so a short window-opening period at the start of each lesson prevents most of it, and in warm weather it moves air renewal into the coolest part of the day.
Use night ventilation, carefully. It is the most effective passive cooling measure available to a naturally ventilated school, and also the easiest to overdo: without measurement it can turn an afternoon overheating problem into a morning heating problem.
Prioritise shading over ventilation rate in the warmest rooms. Orientation decides which rooms those are, and the difference between a south-facing and a north-facing classroom is large enough that treating them the same is a wasted intervention.
Where mechanical systems exist, drive them from measured conditions. This is where BMS integration matters: with CO₂, temperature and humidity available over Modbus, BACnet, MQTT or API, ventilation and cooling respond to the room rather than to the clock, which is the demand-controlled approach the EPBD anticipates.
For any of this to be verifiable rather than assumed, four things need logging continuously, per room rather than per building:
Classroom overheating is not new, but it is newly measurable and newly regulated. The evidence puts a threshold around 26.7 °C, European classrooms in the existing stock pass it regularly in April, May and June, and the two obvious responses, ventilating more and ventilating less, each fail on one of the two things that matter. The only way out is to stop managing one variable at a time.
inBiot builds continuous monitoring around that requirement: up to twelve air quality and comfort parameters per device, including temperature, humidity, CO₂, particulate matter, TVOC, formaldehyde, NO₂, ozone and carbon monoxide, with a real-time indicator the room can see and native integration into building management systems. The My inBiot platform holds the record, reports by occupancy period, and exports it in the formats an audit or a renovation sign-off asks for.
For a school, a local authority or a consultant checking a renovation, that answers a question a one-off measurement never can: not whether a classroom was fine on the day somebody tested it, but whether it stays within limits across the months when both temperature and air quality are hardest to hold.
If your next school project needs air quality and temperature data that holds up across a full academic year, start here.
European Environment Agency, European Climate and Health Observatory, Heat exposure of schools, March 2026. https://climate-adapt.eea.europa.eu/en/observatory/publications-data/analysis-data/calendar-of-heatwaves
European Environment Agency, Exposure of social infrastructure to flooding and urban heat island, March 2026. https://climate-adapt.eea.europa.eu/en/observatory/publications-data/analysis-data/exposure-of-social-infrastructure
Copernicus Climate Change Service, Surface air temperature for May 2026 and for April 2026. https://climate.copernicus.eu/surface-air-temperature-may-2026
Vasilakopoulou, K. & Santamouris, M. (2025). Cumulative exposure to urban heat can affect the learning capacity of students. PLOS Climate, 4(7), e0000618. https://journals.plos.org/climate/article?id=10.1371/journal.pclm.0000618
Wargocki, P. (Technical University of Denmark), Porras-Salazar, J.A. (University of Costa Rica) & Contreras-Espinoza, S. (Universidad del Bío-Bío) (2019). The relationship between classroom temperature and children's performance in school. Building and Environment, 157, 197–204. https://doi.org/10.1016/j.buildenv.2019.04.046
Congedo, P.M., Palmieri, S. & Baglivo, C. (2025). Climate resilience strategies for schools in Mediterranean areas. Energy Efficiency, 18, 24. https://link.springer.com/article/10.1007/s12053-025-10311-7
Grygierek, K., Nateghi, S., Ferdyn-Grygierek, J. & Kaczmarczyk, J. (2023). Controlling and limiting infection risk, thermal discomfort, and low indoor air quality in a classroom through natural ventilation controlled by smart windows. Energies, 16(2), 592. https://www.mdpi.com/1996-1073/16/2/592
Rodríguez-González, A. & Barbero-Barrera, M.M. (2025). Energy performance and thermal comfort in Madrid school buildings under climate change scenarios. Applied Sciences, 15(18), 9980. https://www.mdpi.com/2076-3417/15/18/9980
Ferrari, S., Puglisi, G. & Cardelli, R. (2025). Heat recovery ventilation in school classrooms within Mediterranean Europe. Energies, 18(19), 5069. https://www.mdpi.com/1996-1073/18/19/5069
Directive (EU) 2024/1275 on the energy performance of buildings (recast). https://eur-lex.europa.eu/eli/dir/2024/1275/oj/eng
