Ventilation for school buildings

All New Zealand schools are designed to be well ventilated, either naturally or mechanically. We encourage schools to review our guidance and adjust ventilation year round to cater for seasonal and outdoor conditions.

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  • Boards
  • Proprietors
  • Principals and tumuaki
  • Consultants

Overview of good ventilation

  • All New Zealand schools are designed to be well ventilated, either naturally or mechanically. When airborne illnesses are prevalent, maintaining good indoor air quality reduces the risk of airborne transmission by regularly refreshing the air in a space.
  • The best way to achieve good ventilation is to open windows and doors, either fully or partially, whenever you can.
  • Spaces that are fitted with ducted air conditioning systems are an exception because they do not rely on opening windows to bring in fresh air.
  • Heat pumps are not ducted air conditioning systems as they do not bring in fresh outdoor air. They only cool/heat and recycle the existing indoor air.
  • Our guidance is provided to help schools understand how ventilation works and to assess and mitigate the risks caused by poor ventilation.
  • We encourage schools to review our guidance and fine tune ventilation strategies year round to cater for the seasonal and outdoor conditions.

Why you should maintain ventilation

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
  • elevated CO2 levels.

Sustained and elevated levels of CO2 can impact health and learning outcomes, can cause drowsiness and concentration issues for those in 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

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, consider partially opening windows (for example, by 5cm as a rule of thumb)
  • opening windows or doors on either side of a room to create double-sided or crossflow ventilation (opening windows and doors on one side is known as single-sided ventilation). Crossflow ventilation enables an entry and exit point for airflow to help flush out contaminants and reduce CO2 concentrations more effectively than single-sided ventilation
  • considering also opening doors that connect the space to internal corridors to assist with airflow for spaces that only have external windows and doors on one side of the building. If doing this, the adjoining space should be well-ventilated and have its external windows open
  • making sure 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.

Take refresh breaks ('reboot' the room)

Take refresh breaks to flush a space with fresh air by fully opening windows and doors for a short time – for example, 5 to 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. You 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.

  • Make sure any window or door that was originally designed to open, can still open easily without undue effort, and without compromising safety.
  • Unstick windows which may have been fixed or painted shut.
  • Replace or repair missing or broken window winders, hinges, catches, handles, security stays and closers.
  • Make sure 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.
  • Remove artwork or decoration which may be obstructing windows that are designed to open or whose presence may denote that that the window should not be opened.
  • When a room’s use has been changed or altered, make sure it still meets minimum ventilation requirements in the current New Zealand Building Code 'G4 ventilation' and Designing Quality Learning Spaces. You will also need to make sure that the room has appropriate ventilation for its purpose and layout.

Colder weather and seasons

During colder weather and 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. Aim to do this at least 4 times each day.

  • Heat the room before the start of the school day to 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 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 outside is bad, close what you need to stay comfortable.
  • Take refresh breaks to clear the air at different times during the day by fully opening all the windows and doors, preferably while the room is empty.

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 allow warmer clothes to be worn on the coldest days.

Warmer weather and seasons

When it is warmer, 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, use your 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, opening all the windows and doors fully for a short period to air the space, and then closing them and running the cooling system on its highest setting before reoccupying the room and reopening 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. They don’t include heat pumps because heat pumps do not 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

  • Make sure 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.
  • Consider upgrading the filters to a higher grade, if possible.
  • Configure the system to achieve an air change rate of 0 ACH or more if possible. Configure the system to turn on at least 2 hours before the school days and stay on for at least 2 hours after to refresh the air before students arrive and once they have left.
  • Reduce or eliminate the recirculation of air (that is, set the system to 'once through' or 'full fresh air' mode). This includes 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, if the DCV controls are driven by CO2 monitoring, make sure they are set to try and achieve sustained CO2 levels under 800ppm.
  • Close windows and doors when mechanical ventilation is on. Most systems are designed to work in a closed room.

Use CO2 to assess effectiveness

Systems should preferably be activated by CO2 thresholds. This enables them to identify when they need to supply more fresh air 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, try a refresh breaks. This is where you 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.

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 it 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 reoccupying the room and reopening windows.

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

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 (for example, 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 over 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 CO2 monitor and air cleaner device support

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.

Assessing ventilation

There are several ways to quickly assess whether a space is well ventilated.

Your senses can give a good immediate indication of whether a space has good airflow. For example, a room may not be well ventilated if it feels stuffy or has lingering smells. You will be able to verify this using a CO2 monitor.

Carbon dioxide (CO2) monitoring

Monitoring CO2 levels is one way to quickly assess whether a space is well ventilated when it is occupied. Elevated CO2 levels for a sustained period of time (for example, over an hour or more) indicate that fresh air isn’t flowing into a space quickly enough to meet the needs of the space’s occupants.

  • A space under 800ppm is considered well ventilated.
  • When levels start to climb above this level, associated risk of airborne transmission of illnesses also begins to rise.
  • We advise schools to take action to improve ventilation if CO2 levels stay above 800ppm for an hour or more.

Actions to consider when there are sustained CO2 levels

Green: Under 800ppm

Your space is well ventilated for its current number of occupants and their level of activity, continue with the current approach.

Green: 800 to 1,250ppm

Open windows more if this can be done while maintaining comfortable indoor temperatures.

Consider lowering the level of activity in the room, briefly vacating the room and/or purging and refreshing the air in the space (a reboot).

Amber: 1,251-2,000ppm

Take action to introduce more fresh air. You might:

  • open all windows and doors as much as possible, and whenever it is practical, do so while maintaining comfortable indoor temperatures
  • reboot the room by fully opening all windows and doors for a short time (5 to 10 minutes), preferably while vacating the room, to purge and refresh the air in the space
  • reduce the level of vigorous activity performed in the room or lower the total occupancy
  • consider using other protective measures such as face coverings and increased physical distancing.

Red: Over 2,000ppm

If you have followed the above advice and still have persistent CO2 levels over 2,000ppm for a sustained period of time (for example, over the course of an hour), or have very frequent CO2 peak levels exceeding 2,000ppm, contact your Ministry property advisor.

In addition, instigate at least 4 refresh breaks through the school day where you reboot the room with all windows and doors open for a few minutes, and preferably with the room unoccupied.

Use your portable CO2 monitor to perform spot checks

Spot checks provide an immediate indication of current CO2 levels. If the levels are high, follow our guidance to try to lower them and consider if you should also monitor the space’s CO2 levels over a longer duration as explained below.

Spot checks only provide a snapshot of the current CO2 level and may not represent peak or sustained levels. Short, intermittent peaks in CO2 levels are common. Very frequent peaks or persistently elevated CO2 levels for a sustained period of time (for example, over an hour or more) indicate that fresh air isn’t flowing into a space quickly enough to meet the needs of the room’s occupants.

Spot checks are best carried out toward the end of a teaching period while the space is fully occupied and in use. This will provide the most representative reading. For a more accurate understanding of ventilation, carry out a full day reading as per below.

  • Take the CO2 monitor to each space and place it somewhere around student head height, away from doors and windows, out of direct sunlight, and at least 1 metre away from the closest Note breathing directly into or over the device will cause it to report high CO2 levels.
  • Leave the device in the room for at least 5 minutes before checking the CO2 levels reported on its If temperature readings are also required, extend this to 30 minutes to allow the device to report this accurately.
  • Repeat this process in a selection of spaces, or all spaces on a regular basis (for example, fortnightly). Look for patterns and relationships between CO2 levels, who is in the room, doing what, and with windows and doors open or closed.

Use your portable CO2 monitor to gather a full day’s readings

If you have a concern with how the space’s ventilation is changing through the day, you can leave the CO2 monitor in the room for a longer period for it to automatically collect its readings. This will provide a more representative picture of ventilation for that space compared to performing spot checks. The devices will typically store 3-7 days data that can then be downloaded.

  • Take the CO2 monitor into the space and place it somewhere around student head height, away from doors and windows, out of direct sunlight, at least 1m away from the closest people and in a place where it will not be disturbed or
  • At the end of the day, use the smartphone app to view and download the CO2 When downloading and assessing the data, ensure you only review the data linked to that space on that day. Take note of how CO2 levels change based on who is in the room, doing what, with windows and doors open or closed at various times throughout the day.
  • When discussing any concerns with your Ministry property advisor, provide a copy of the downloaded

Device support

In 2022, we distributed more than 12,500 portable CO2 monitors and 13,000 portable air cleaners to state and state-integrated schools to support them with good ventilation, and as part of our response to COVID-19.

These devices were provided at no cost to the schools.

All schools that have received devices are to record them on their fixed asset register. Schools are responsible for any ongoing maintenance, consumables or replacement devices.

See more information about these devices, including warranty and purchasing details:

Portable CO2 monitor and air cleaner device support

Schools, early childhood services and other education providers can still purchase CO2 monitors at a discounted price directly from the Ministry's supplier, Butler Techsense Ltd, via the web site below. Use the voucher code ‘3DUCATION’ at checkout to access the discount.

The Temperature Shop(external link)

Ventilation research and studies

Our ventilation guidance has been informed by international and local studies and research to investigate and confirm the appropriate evidence-based strategy for the context of New Zealand classrooms.

Ventilation research and studies

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