Ventilation guidance

Ventilation guidance

Understanding good ventilation

Good ventilation removes stale air and replaces it with fresh, clean air while maintaining a comfortable temperature and humidity levels for the people in the space.  This helps to improve indoor air quality and to reduce the transmission of airborne illnesses including COVID-19.

Our guidance has been developed to provide advice to schools on how they can improve and maintain good ventilation year-round.

Indicators that a space may not be well-ventilated include a feeling of stuffiness, lingering smells and elevated CO2 levels.

Sustained and elevated levels of CO2 can impact health and learning outcomes, can cause drowsiness and concentration issues for those in the space, and indicate an increased risk of airborne transmission.  

Action to improve ventilation should be taken if CO2 levels stay above 800ppm for an hour or more, or are very frequently peaking in the higher CO2 ranges. Actions should progressively increase if CO2 levels are sustained or continue to rise, noting short peaks of higher CO2 readings are normal and should not be a cause for concern.

Guidance for naturally ventilated spaces

General guidance

Most New Zealand schools and classrooms are designed with good natural ventilation using windows that can be opened. Schools can make the most of this by:

  • Opening windows and doors as much as practical. If it is impractical to open windows or doors fully due to outdoor conditions, occupants can consider partially opening windows (e.g., as a rule of thumb, by 5cm).  
  • Opening windows and doors on one side is known as single-sided ventilation 
  • Opening windows or doors on either side of a room will create double-sided, or crossflow ventilation. This enables an entry and exit point for airflow to help flush out contaminants and reduce CO2 concentrations more effectively than single-sided ventilation. 
  • For spaces that only have external windows and doors on one side of the building, consider also opening doors that connect the space to internal corridors to assist with airflow. If doing this, the adjoining space should be well-ventilated and have its external windows open. 
  • Ensuring teachers and staff know how to (and are able to) open windows in all spaces throughout the school, and regularly checking to ensure windows are not being temporarily blocked by classroom furniture, student artwork or teaching materials. 
  • Adjusting the room layout so that there are not any obstructions in front of windows and doors to affect airflow, and that occupants are further away from cold draughts.   
  • Pre-heating classrooms before the start of the day and continue heating throughout the day while windows are at least partially open (even when it is colder outside) to maintain a comfortable indoor temperature. 

Refresh breaks (A “reboot” of the room)

Taking refresh breaks to flush a space with fresh air by fully opening windows and doors for a short time – for example, 5-10 minutes. If this does not resolve the stuffiness and quickly reduce the CO2 levels, the windows and doors may need to be open for a longer period of time. The air in the room will also be refreshed more quickly if the space is unoccupied during the refresh break. 

Schools can check whether a refresh break was effective by using a CO2 monitor. 

Maintenance and property improvements

Schools should regularly check for any property issues that may need to be resolved, such as:

  • Ensuring any window or door that was originally designed to open, can still open easily without undue effort, and without compromising safety. 
  • Unsticking windows which may have been fixed or painted shut. 
  • Replacing or repairing missing or broken window winders, hinges, catches, handles, security stays and closers. 
  • Ensuring windows are able to be used as originally intended and are not blocked, covered over, disabled, made inaccessible or otherwise changed so that they cannot be opened. 
  • Removing artwork or decoration which may be obstructing windows that are designed to open, or the presence of which may denote that that the window should not be opened. 
  • When a room’s use has been changed or altered, ensuring it still meets minimum ventilation requirements in the current NZBC G4 Ventilation and DQLS, and that it has appropriate ventilation for its purpose and layout. 

Download the Window Replacement Requirements in Existing Buildings guidance on

Guidance for colder weather and seasons

During colder weather and especially during the winter season, it can be less practical to fully open windows and doors.  But on cold days good ventilation can still be achieved by partially opening the windows. 

This is because air flow behaves differently at different temperatures. The bigger the temperature difference between the outside and inside, the more efficiently fresh outside air is drawn in through open windows, even if the actual opening size is quite small.  This provides a flow of fresh air for the space while only resulting in minimal temperature loss.

On colder days, take these additional steps to help ventilate your space while balancing comfortable indoor temperatures: 

  • Heat: Heat the room before the start of the school day, which will allow people to open windows earlier in the day. Keep heating on throughout the day to stay warm, with windows partially opened whenever possible. 
  • Open: Open windows by at least a crack or as much as you can while staying warm. Opening lots of windows a little can be more effective in colder weather. If the weather is bad outside, close what you need to stay comfortable. 
  • Reboot: Take refresh breaks to clear the air at different times during the day by fully opening all the windows and doors, preferably while having everyone exit the room. Aim to do this at least four times each day. 

Fine-tune your approach through the day as the weather changes. Fully opening windows will achieve the best ventilation, so increase your window openings as it warms up outside later in the day or whenever this can be done while maintaining a comfortable indoor temperature.  You can use CO2 monitors to check that these strategies are working and identify whether adjustments need to be made. 

If these tips do not provide good ventilation, schools can also try these more specific tactics: 

  • Open high-level windows first and wider than low level windows, to reduce the risk of cold draughts in the room.
  • Close the doors before you begin closing windows and reduce or close any windows directly facing the worst weather conditions while leaving others partially or fully open.
  • On a wet day, try to keep wet clothes out of the classroom as bringing them in will make the classroom more difficult to heat.
  • Adjust the classroom layout to move students away from open windows, and other areas that may have cooler air or draughts.
  • Relax uniform rules/dress codes and allowing warmer clothes to be worn on the coldest days. 

Guidance for warmer weather and seasons

When it is warmer, schools should fully open windows and doors whenever possible to make the most of natural ventilation while still maintaining comfortable indoor temperatures. 

When hot weather outside is making it too warm inside, schools should use their cooling systems (for example, heat pumps) to reduce the indoor temperature. This may require windows and doors to be open less at the warmest times during the school day. Consider resetting the temperature of the room to a comfortable level by having all occupants leave the room, open all the windows and doors fully for a short period to air the space, and then close them and run the cooling system on its highest setting before re-occupying the room and re-opening windows.

Air cleaners can also be used in spaces that are challenging to ventilate.  During warmer months air cleaners will also help remove dust, pollen and other outdoor allergens from the indoor air. 

Portable and ceiling fans can provide additional comfort to occupants in the warmer months by moving air and creating a breeze within the room, noting their use is not a replacement for opening windows.  Portable fans should only be positioned where they do not present a health and safety risk.

Guidance for spaces with ducted mechanical ventilation systems

Some schools are fitted with ducted mechanical ventilation systems that automatically source fresh air from the outside while also managing the temperature of the room. These are often referred to as HVAC or air conditioning systems. This doesn’t include heat pumps because they don’t supply fresh air from outside.

One way to identify if your space has a ducted mechanical ventilation system is to look for vents in the ceilings or walls that bring in fresh air or extract stale air.

Where ducted mechanical ventilation systems are fitted, the advice above for naturally ventilated spaces doesn’t apply unless the system has specifically been designed to work in conjunction with windows and doors being open.  If not, windows and doors should remain closed to allow the system to work as designed. 

Mechanical ventilation is designed to meet a specific occupancy rate. If there are too many people in a space, the system may not perform as intended and will not provide enough fresh air for the occupants. 

Many systems will automatically adjust to warmer or colder weather, but it may be possible for the HVAC technician to change the amount of fresh air brought in as the seasons change. The performance of a system can be best maintained by ensuring that the systems are routinely serviced, and air filters regularly cleaned. Strategies for maximising mechanical ventilation include: 

  • Ensuring the system is checked, cleaned and maintained by an appropriately skilled HVAC technician in accordance with the system’s warranty and maintenance specifications and meeting any associated Building Warrant of Fitness (BWOF) requirements for specified systems. 
  • If possible, consider upgrading the filters to a higher grade. 
  • If possible, being configured to achieve an air change rate of at 6.0 ACH or more. 
  • Configuring the system to turn on at least two hours before and stay on for at least two hours after the school day, to refresh the air before students arrive and once they have left for the day. 
  • Reduce or eliminate the recirculation of air (i.e. setting the system to “once through” or “full fresh air” mode) including running the full fresh air setting at its maximum rate, for as long as is practicable, where this can be done while maintaining comfortable outdoor temperatures. 
  • Disable any Demand-Controlled Ventilation (DCV) controls that reduce air supply based on occupancy or temperature. If the DCV controls are driven by CO2 monitoring, ensure they are set to try and achieve sustained CO2 levels under 800ppm. 
  • Closing windows and doors when mechanical ventilation is on, as most systems are designed to work in a closed room. 

It is preferable for these systems to be activated by CO2 thresholds, enabling them to identify when they need to ensure more fresh air is supplied into the space. You can use CO2 monitors to assess whether the system is working effectively.

If CO2 levels become too high in mechanically-ventilated spaces, schools can also employ refresh breaks where they purge the room of old air by opening windows that can be opened (if there are windows in the space), or by having occupants leave the room and turning the system on full until CO2 levels drop. 

Schools that are not able to achieve acceptable CO2 levels, despite employing all strategies to improve mechanical ventilation, should work with their Ministry Property Advisor to identify other solutions. 

Configuration and maintenance of ducted ventilation systems should only be done by appropriately skilled technicians.

For more information, refer to our technical advice for ducted mechanical ventilation systems

Guidance for using heat pumps and other heating systems

You can continue to use heat pumps to heat or cool spaces, even when windows and doors are open. Using a heat pump with windows open will be less efficient and may incur some additional power costs but can help balance increasing fresh airflow with comfortable indoor temperatures. 

Heat pumps and many other heating systems only heat or cool recirculated air within the space. They do not bring in fresh air, so to achieve good ventilation they are to be used alongside a means of providing fresh air.  

When using heat pumps and other heating systems:

  • Pre-heat the space to a comfortable temperature before the school day to improve the draw of fresh air through partially opened windows.
  • Increase indoor heating or cooling during the day, if you need to, to offset the impact on temperature of having the windows open.
  • Reset the temperature of the room to a comfortable level after it has been vacated and aired out, by briefly closing all windows and doors and running the system on its highest setting before re-occupying the room and re-opening windows.

Your heat pump or heating system should be regularly checked and serviced, including cleaning its mesh dust filter, to make sure it is operating efficiently.

Other supplementary solutions

Portable air cleaners (purifiers)

Air cleaners are a supplementary solution that filter and recirculate the air within a space.  This can improve indoor air quality and reduce the airborne transmission of illnesses including COVID-19. Air cleaners do not replace good ventilation practices as they do not supply fresh air or reduce CO2 levels.

Air cleaners can offer a modest improvement to air quality in spaces that are challenging to ventilate (e.g. well where air flow is low, the air is stale, CO2 levels remain elevated for one hour or more) and when it is impractical to sufficiently open windows to flush the space with fresh air. Their effectiveness is dependent on the air cleaner being correctly sized for the room, running on a high fan speed, and having a quality HEPA filter. 

In 2022 we distributed more than 13,000 air cleaners to state and state-integrated schools to use at their discretion in spaces that are challenging to ventilate well or may have a higher risk of COVID-19 airborne transmission. This may include staff rooms, music rooms, high-use meeting and break-out rooms. They can also be used in classrooms and other spaces when adverse weather conditions make it less practical to open the windows. 

Schools, early childhood services and other education providers can also purchase air cleaners at a discounted price, directly from the Ministry’s two suppliers: Samsung NZ and Rentokil. 

Portable air cleaners
Request additional portable air cleaners for your school
Request additional portable air cleaners for your school

Ceiling fans

Fixed ceiling fans help to circulate warm or cool air around the room. They can provide a small improvement to ventilation when windows are fully open, however are unlikely to result in an improvement when windows are only partially opened.

Fixed extract and supply fans

Fitting well-designed and positioned extract and supply fans that bring in fresh air or push out the stale air can boost natural ventilation in conjunction with, or as an alternative to fully opening all windows and doors.  If you are considering fitting extract or supply fans, please discuss with your Ministry property advisor first to ensure that they will successfully supplement the existing natural ventilation.

Portable fans

Most non-industrial portable fans do not produce sufficient air movement to offer a notable improvement to ventilation, but they can provide additional comfort to occupants in the warmer months by moving air and creating a ‘breeze’ within the room.

We recommend limiting the use of portable fans as it can be difficult to determine whether they are assisting or interfering with air flow. Portable fans are also noisy and can be a safety hazard depending on how they are positioned in the room.