Night Cooling Ventilation

A passive cooling strategy that removes stale air at night to cool down the building’s thermal mass, reducing the demand on energy-intensive mechanical cooling systems.

Contact a Specialist

What is night cooling or passive cooling?

Also called night purge ventilation or night flushing, it uses the building’s thermal mass as a sink during the day as it absorbs heat gains from occupants, equipment, and the sun. Then, when evening temperatures drop, the building envelope is opened, allowing for the cool air to enter the building and dissipate the stored heat via convection.

Because the building’s thermal mass is cooled down at night, the building can take advantage of radiant cooling during the day while the building is occupied. The result is a reduced reliance on mechanical cooling systems during the daytime which are peak hours in terms of energy costs.

Why do buildings need night purge ventilation?

New buildings are becoming more and more insulated due to regulatory requirements, lowered U-values, and the continued focus on building heating usage. In a typical new office building with normal heat loads, the need for cooling can be required even at outdoor temperatures below 0 degrees. This is primarily due to high heat loads generated from people, lighting, and equipment. Often this leads to buildings which have a decreased need for heating while increasing the need for cooling.

Rocky Mountain Institute 03 Web

How does night flushing or night cooling work?

Night cooling can be used in a natural, mechanical, or mixed-mode ventilation approach.

Passive cooling with natural ventilation

If your building employs natural ventilation, then night flushing is achieved by opening the windows at night and letting wind-driven or thermal buoyancy-driven airflow cool the space. To ensure that this is handled properly, we recommend a natural ventilation solution that can offer feedback about window opening position and/or that provides window status directly to the building management system. This will help maintain the security and integrity of the building without compromising on the benefits of night cooling.

Night cooling with mechanical ventilation

If your building uses purely mechanical ventilation, then nighttime flushing will occur by forcing the air out of the building via ventilation ducts. Air is then usually supplied to the building throughout the day as required by regulations or demand. This technique will not offer the same energy saving benefits as a natural approach.

Passive cooling with mixed-mode ventilation

In the case of a building making use of mixed-mode ventilation, night flushing is achieved through a combination of natural ventilation strategies and mechanical ventilation. This is often very effective when a building has spaces that are especially deep or that can’t make use of cross ventilation. 

How to enhance the effects of night purge ventilation

To enhance the effects of night cooling for a new project, you can consider building materials with high thermal mass. What determines how much heat and cool the mass can store are its heat capacity and density. Exposing the thermal mass will also help enhance the thermal effect as covering it will hinder its ability to store heat or cool. Another way to enhance night cooling is by using solar shades in the design. Solar shades prevent the building from gaining too much heat from the sun, increasing the effectiveness of night flushing as well as providing several other benefits to the building.

Learn more about passive ventilation
Clayton Heights Community Centre Rendering Interior Web Credited

Key benefits of night purge ventilation

  • Icon Num1

    Reduces daytime use of mechanical cooling systems

  • Icon Num2

    Lowers energy bill

  • Icon Num3

    Enhances performance of the building envelope

  • Icon Num4

    Improves occupant health and comfort

  • Icon Num5

    Low carbon method for building cooling

In which climates is night ventilation most effective?

The efficiency of night cooling depends on the thermal properties of the building and on the local climate conditions, i.e. nighttime wind speed and the temperature swing of the ambient air. It is particularly effective in climates that have a marked swing between the day- and nighttime outside temperatures. This is not to say that night purging cannot be effective in warmer climates. Even when internal and external temperatures are very similar, night flushing can still provide a means for airborne pollutants to be exhausted and allow fresh air to enter.

Rocky Mountain Institute 01 Web

Common concerns with night purge ventilation

Security is a common concern when night purge ventilation is considered. This concern is alleviated by selecting high-level openings which minimize many risks such as break-ins. Insurance companies often prefer small openings at higher levels when night ventilation is present.

The risks from entrapment can be lowered by using intelligent actuators which are programmed with a pressure safety function. Furthermore, by using an intelligent actuator, in combination with an intelligent control panel, the building management system can receive feedback about the status and degree of the openings.

Normally, with a night purge ventilation strategy the windows do not have to open fully to achieve effective cooling. Therefore, the system will help the building cool while maintaining the security of the building. The intelligent control system should also include wind and rain sensors which are able to detect when rain and wind speed limits are exceeded in order to close the windows and protect the building.


Get in touch