Classroom overheating in Barcelona: HVAC simulation study
EnergyPlus simulation study on classroom overheating in Barcelona, thermal comfort, solar orientation and HVAC strategies for schools.
Classroom overheating in Barcelona: HVAC simulation study
Introduction: why classroom overheating matters
As climate change intensifies summer heatwaves, preventing classroom overheating has become a critical design challenge for educational buildings in Mediterranean cities such as Barcelona. Schools must provide healthy, comfortable indoor conditions while also reducing energy consumption and operational complexity.
This article presents a thermodynamic building simulation study assessing overheating risk in two primary-school classrooms in Barcelona. The analysis uses DesignBuilder with the EnergyPlus calculation engine to evaluate whether a centralised 100% outdoor air HVAC system can maintain thermal comfort during school hours from May to September.
The study is relevant for architects, engineers, school operators and public authorities seeking evidence-based strategies for thermal comfort in schools, especially where cooling is delivered through a central air-handling unit rather than room-by-room terminal units.
* Schools shut in Spain from the end of June until September. However, for the simulations, July and August were included in the simulations, to have results under more challenging climate conditions. This was also done as the climate file is based on historical data. Currently, weather conditions that typically occurred in July now often occur in May.
Study objectives
The simulation was designed to answer three practical questions about classroom overheating, supply-air temperature and the impact of solar orientation.
- How do east- and west-facing classrooms behave differently due to solar exposure, and what does this mean for HVAC control?
- How many teaching hours exceed an operative temperature of 27°C despite cooled air being supplied at 14°C?
- What relationship between outdoor temperature and supply-air temperature helps avoid both overheating and overcooling?
Simulation tools and building model
The dynamic simulation was carried out in DesignBuilder using the EnergyPlus engine. The model represents two identical classrooms located on the third floor of a primary school in Barcelona: one facing east and one facing west, with a north–south corridor between them.
The Barcelona–Airport IWEC II weather file developed by ASHRAE was used for the climate assumptions. July and August were included in the simulation period even though schools in Spain typically close during summer holidays, because these months provide a more demanding stress test for the system and help evaluate future climate resilience.
Internal floors and partitions were modelled as adiabatic. The envelope parameters comply with the minimum requirements of Spain’s CTE HE1 code for Climate Zone C. Windows are double glazed with Ug = 1.80 W/m²K, 79% visible transmittance and a solar factor of 59%. The external wall has U = 0.49 W/m²K, the roof has U = 0.40 W/m²K and air permeability is n50 = 3 ach.
Each classroom includes fixed external shading devices with a 50% reduction factor, representing expanded metal mesh shading. This shading reduces solar gains but does not eliminate the impact of orientation.
Internal loads and HVAC configuration
Each classroom has a floor area of 60 m² and is occupied by 31 pupils and one adult teacher during school hours. Lighting and equipment loads operate during teaching hours, Monday to Friday from 08:00 to 18:00, although lighting is assumed to be off during summer.
Ventilation is provided by a central air-handling unit supplying 100% outdoor air at 45 m³/h per person, equivalent to 1,395 m³/h per classroom. The system includes heat recovery with 79% sensible efficiency and 62% latent efficiency.
Cooling is provided by an air-to-water heat pump producing chilled water at 7°C for the AHU cooling coil. During summer school hours, the supply air temperature is assumed to be a constant 14°C at classroom supply level. In practice, duct heat gains and pressure losses may mean that the delivered supply air temperature and airflow differ from the simulation assumption. For May and September, a dual-setpoint control strategy is used to reduce overcooling: when outdoor temperature is below 16°C, supply air is delivered at 20°C; when outdoor temperature rises above 17°C, supply air is delivered at 14°C.
Thermal comfort criteria
Thermal comfort was assessed using operative temperature thresholds during school hours only. The comfort band was defined as 22°C to 27°C, with indoor relative humidity targets between 30% and 60% at 26°C. Conditions below 22°C were classified as too cold, while conditions above 27°C were classified as too hot.
Using operative temperature rather than air temperature alone provides a more representative assessment of how occupants experience the room, because it accounts for both air temperature and radiant temperature from surrounding surfaces.
Key results: overheating and overcooling risk
External Climate Overview (08:00–18:00, Mon–Fri)
The analysis shows that the central HVAC system generally maintains classroom comfort for most teaching hours. However, the results also demonstrate that orientation, solar gains and supply-air control are decisive factors in determining real comfort performance.
The east-facing classroom experiences the highest overheating risk, particularly in July and September, because morning solar gains coincide with early occupancy and rising outdoor temperatures. In July, the east classroom exceeds 27°C during approximately 7% of teaching hours despite the supply air being delivered at 14°C.
The west-facing classroom performs slightly better in terms of overheating, with only a small proportion of hours above 27°C in September. However, it is more exposed to overcooling in the morning during shoulder months, because delayed afternoon solar exposure does not compensate for early cool supply air.
This finding illustrates a common HVAC design challenge in schools: a single central control strategy can perform adequately on average while still creating different comfort outcomes across zones with different orientations.
Why orientation matters in classroom comfort
Solar orientation plays a significant role in overheating risk. East-facing classrooms receive direct solar radiation in the morning, when pupils arrive and classrooms are already occupied. This can quickly raise operative temperatures before the cooling system has fully compensated for the solar load.
West-facing classrooms receive more delayed solar radiation in the afternoon. This can reduce early-morning overheating risk but may create a different comfort profile later in the day. In the simulation, the west classroom was also more prone to mild overcooling during May and September.
For school buildings in Barcelona and other Mediterranean climates, façade orientation should therefore be considered alongside HVAC design. Identical classrooms can require different control strategies even when they have the same area, occupancy and envelope specification.
Recommended strategies to prevent classroom overheating
The simulation confirms that HVAC control is important, but it should be combined with passive design measures to create resilient school buildings. The most effective design approach is to reduce heat gains first, then use efficient mechanical systems to manage the remaining cooling load.
- Improve external shading, especially on east and west façades where low-angle solar radiation is difficult to control.
- Optimise glazing specifications, particularly solar factor, visible transmittance and frame performance.
- Improve airtightness and insulation to reduce unwanted heat transfer and improve system predictability.
- Reduce internal heat gains from lighting, equipment and operational schedules.
- Consider night ventilation or free-cooling strategies where climate, acoustics and security conditions allow.
- Commission airflow rates and supply-air temperatures carefully at classroom level rather than only at the AHU.
HVAC design lessons for schools
For 100% outdoor air systems serving multiple classrooms, ductwork design and commissioning are critical. Long duct runs can introduce pressure losses and heat gains, reducing airflow and increasing supply-air temperature at the classroom grilles. These effects can be especially important when the design relies on cool supply air to manage sensible loads.
The study also highlights the value of simple but intelligent control logic. A dual-setpoint strategy based on outdoor temperature can reduce overcooling in shoulder months without adding excessive complexity. However, zone-level monitoring or balancing may still be needed when classrooms have very different orientation or solar exposure.
Energy simulation with tools such as EnergyPlus and DesignBuilder allows design teams to test these strategies before construction or refurbishment. This supports better decisions, reduces performance gaps and helps align thermal comfort with energy-efficiency objectives.
Conclusions
The results show that a centralised 100% outdoor air HVAC system can generally maintain acceptable comfort in classrooms in Barcelona, but overheating risk is not eliminated. The east-facing classroom remains more vulnerable to overheating, especially in July, while the west-facing classroom shows a greater tendency toward mild overcooling during shoulder-season mornings.
Preventing classroom overheating requires a combined strategy: accurate energy simulation, façade-specific passive design, robust HVAC controls and careful commissioning. For Mediterranean school buildings facing warmer and more variable climate conditions, this integrated approach is essential for health, comfort, educational performance and energy efficiency.
FAQ: classroom overheating and thermal comfort
What causes classroom overheating?
Classroom overheating is caused by a combination of high outdoor temperatures, solar gains through windows, internal heat gains from pupils and equipment, insufficient shading and poorly matched HVAC control.
How can overheating in schools be prevented?
The most effective approach combines passive measures such as external shading, better glazing and reduced internal gains with HVAC strategies such as adaptive supply-air temperature control and careful commissioning.
What is a comfortable classroom temperature?
In this study, the comfort range was defined as an operative temperature between 22°C and 27°C during occupied school hours.
Why use EnergyPlus for school overheating studies?
EnergyPlus enables hourly dynamic simulation of weather, occupancy, solar gains, envelope performance and HVAC operation, making it suitable for evaluating overheating risk before decisions are implemented on site.