Natural ventilation

Natural ventilation is the process of naturally delivering and removing air from an interior space (room or building) without the mechanical assistance — only by means of natural forces, typically by wind speed or differences in pressure internally and externally.

To ways to ventilate or cool buildings - actively and passively

  • Active ventilation/cooling - with active system or mechanical components, requires energy
  • Passive ventilation/cooling - is a technology and/or design feature of a building, typically does not require energy

Ventilation consists of three main parts:

  • Ventilation rate – the amount and quality of the outdoor air,
  • Airflow direction – the flow of air within a building, from 'clean' to 'dirty' areas, and
  • Air distribution – the delivery of outside air and removal of pollutants.

When designing a ventilation system, factors to consider include but are not limited to:

  • Type of building/structure,
  • Location of building and outdoor climate
  • Usage of the space,
  • Volume of the area,
  • Number of airflow changes required per 24 hours,
  • How exhaust air will be vented,
  • Structural requirements and availabilities, Electrical requirements and availabilities, and,
  • Budget available.

Passive cooling is a measure that uses no energy to cool buildings. It involves at least three concepts:

  • Solar shading
  • Thermal mass
  • Ventilative cooling

One of the best known passive cooling technieques is providing natural ventilation by openable windows.

Natural ventilation

Natural ventilation uses natural forces to exchange the air in a building. The driving forces are typically wind and temperature differences. However it is important to ensure an efficient airflow through the building.

Natural ventilation and riving forces

Air moves through an opening (i.e. window) when there is a pressure difference across the opening:

  • greater pressure difference = higher airflow
  • larger opening area = higher airflow

Natural ventilation uses also pressure difference 

  • difference between densities of interior and exterior air given by temperature
  • difference wind velocity providing on windward façade positive pressure and on leeward negative pressure

Did you know?

Natural ventilation is also influenced by wind turbulance and flow pattern around a building (affected by neighbouring buildings, building exposure and building orientation).

Building shape also influences the natural ventilation, i.e. flow direction throughout the building and ventilation intesity.

Other points to know about natural ventilation

Controls for natural ventilation can be based on sensors for indoor air quality, meteorological weather data and smart control/management systems.

Mechanims for natural ventilation

Natural ventilation pressure differences driven by two mechanisms - stack and wind, or combination.

Stack effect (air density difference)

  • A combination of inlets via typically controlled window openings and shafts as outlets, creating a stack effect and using pressure difference in different heights
  • warm air is less dense than cool air (more buoyant)
  • works when indoor air is warmer than outdoor air
  • but it is harder to achieve stack aiflow in summer
  • typically used for multi-floor apartment buildings

Wind (aeration)

  • creates varying surface pressures (positive and negative) around the building, i.e. inlet at the ground via openings and outlet via skylights in roofs
  • wind velocity is typically lower near the ground and increases with height above ground
  • on the windward (upwind) side of a building, air velocity slows and pressure rises
  • on the roof and sides of the roof, air velocity accelerates and pressure drops
  • on the leeward (downwind) side of a building, air flow separates from the roof and sides creating a low pressure recirculation zone
  • typically used for industrial halls or stables
  • the ventilation is regulated by inlets and outlets openings.

Pros and cons for types of natural ventilation

Advantages of natural ventilation

  • Suitable for many types of buildings located in mild or moderate climates.
  • The 'open window' environment associated with natural ventilation is often popular, especially in pleasant locations and mild climates.
  • Natural ventilation is usually inexpensive when compared to the capital, operational and maintenance costs of mechanical systems.
  • High air flow rates for cooling and purging are possible if there are plenty of openings.
  • Short periods of discomfort during periods of warm weather can usually be tolerated by building´s occupants.
  • No room space for ventilation equipment is needed.
  • Minimum maintenance.
  • Can be less expensive to install and operate than HVAC but this need not always be true.
  • No fan or system noise.

Disadvantages of natural ventilation

  • Inadequate control over ventilation rate could lead to indoor air quality problems and excessive heat loss.
  • Air flow rates and the pattern of air flow are not constant.
  • Fresh air delivery and air distribution in large, deep plan and multi-roomed buildings may not be possible.
  • High heat gains may mean that the need for mechanical cooling and air handling will prevent the use of natural ventilation.
  • Natural ventilation is unsuited to noisy and polluted locations.
  • Some designs may present a security risk.
  • Heat recovery from exhaust air is technically feasible but not generally practicable.
  • Natural ventilation may not be suitable in severe climatic regions.
  • Occupants must normally adjust openings to suit prevailing demand.
  • Filtration or cleaning of incoming air is not usually practicable.
  • Ducted systems require large diameter ducts and restrictions on routing.
Opening windows is more than just ventilation – opening a window creates the link to the outside and is a symbol of affection for your family.
Velux