Indoor air quality · Classrooms · Natural ventilation

CO₂ ventilation analysis for schools and classrooms

TopicTopic icon for classroom CO₂ ventilation with occupants, an open window and outgoing room air
Make healthy indoor air predictable

Does window ventilation really work at full occupancy?

CO₂ concentration can rise quickly in densely occupied classrooms. A transient airflow simulation combines occupancy, window geometry, opening position and weather. It reveals how long a ventilation strategy remains effective and whether comfort, temperature and energy use are compatible.

CO₂ development over time

When is each CO₂ concentration reached?

Occupancy and CO₂ generation by pupils are used to calculate the concentration throughout the lesson. The assessment identifies when defined CO₂ values are reached and how tilted windows or purge ventilation change the curve.

  • Increase from the start of occupancy
  • Times at which 1000, 1500 and 2000 ppm are reached
  • Reduction and recovery time after ventilation

The times shown are illustrative. Reliable values follow from the room volume, pupil count, activity, window position and weather of the specific project.

Chart showing the rise of CO₂ over time in an occupied classroom with marked concentration values
Time chart of CO₂ concentration and room air temperature during winter purge ventilation
Winter purge ventilation

Clean air, but rapid cooling

Fully opening the windows replaces stale room air very quickly. At the same time, the indoor air temperature responds to the cold outdoor air and can fall noticeably within a few minutes.

  • Rapid reduction in CO₂ concentration
  • Simultaneous decrease in room air temperature
  • Optimised opening duration and ventilation interval

Indoor air quality, thermal comfort and heating energy are therefore assessed together. The goal is short effective ventilation without excessive cooling.

Classroom CFD result

Where does CO₂ accumulate in the room?

A room average does not describe indoor air quality completely. The simulation reveals the spatial distribution in the occupied zone, local concentration peaks above pupils and the removal over time after the windows are opened.

Spatial CO₂ distribution

CFD result showing the spatial CO₂ distribution in an occupied classroom

Development during occupancy and ventilation

Three operating states

Closed, tilted and fully open windows compared directly

Every option uses the same room geometry, occupancy and outdoor conditions. This makes the real effect of each window position directly comparable.

Fully occupied classroom with closed windows shown from inside and outside
01

Windows closed

The reference case shows CO₂ accumulation from occupants. Outdoor air exchange is limited to joints and other leakage paths.

Fully occupied classroom with tilted windows shown from inside and outside
02

Windows tilted

Tilted windows provide continuous but weather dependent air exchange. CO₂ removal, winter draught and heat loss are assessed.

Fully occupied classroom with fully open windows for purge ventilation
03

Purge ventilation

Fully open windows create a high short term air exchange. The reduction in CO₂, required opening time and thermal comfort are evaluated.

Method

From occupancy and weather to a transient ventilation assessment

The room, windows and opening angles are modelled according to the design. CO₂ generation by occupants, lesson schedules, outdoor concentration, temperature and wind form the boundary conditions. Each window position is then simulated to determine airflow, air change, CO₂ concentration and room temperature over time.

  1. 01

    Transfer room geometry, window areas and opening positions from the design

  2. 02

    Define occupancy, schedules and CO₂ generation by people

  3. 03

    Set weather, outdoor concentration and thermal boundary conditions

  4. 04

    Simulate closed windows, tilted windows and purge ventilation over time

  5. 05

    Compare CO₂ development, air change, temperature, draught and heat loss

Assessment variables

Assess the effect in the room, not only the air change rate

A high theoretical flow rate is not sufficient on its own. Fresh outdoor air must reach the occupied zone and remove stale air effectively.

01

CO₂ concentration

Temporal development and spatial distribution in the occupied classroom.

02

Outdoor air exchange

Actual airflow through the windows under the relevant weather conditions.

03

Temperature and draught

Local cooling and air velocity in the occupied zone.

04

Ventilation duration

Required frequency and duration of purge ventilation during lessons.

Result for design and operation

A robust ventilation strategy instead of generic opening times

The assessment shows whether window ventilation is sufficient, which opening position is effective and when mechanical assistance is required. It provides clear guidance for design, automation and school operation.

CO₂ development during lessons and breaks Air change and airflow for each window position Recommended opening duration and ventilation intervals Guidance on draught, heat loss and possible mechanical assistance
FAQ

Frequently asked questions

Why is a simple air change calculation not always sufficient?

Outdoor air may be distributed unevenly. Airflow simulation shows which occupied areas are actually ventilated.

Can summer and winter conditions be assessed?

Yes. Temperature difference, wind and opening duration are defined for each season and assessed separately.

Can the results inform window control?

Yes. Opening angle, duration and intervals can be derived for manual or automated ventilation strategies.

Next step

Verify indoor air quality during design

We assess occupancy, window design and relevant weather cases and develop a transparent ventilation strategy.

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