Natural Ventilation

Natural ventilation is the use of wind and thermal buoyancy to create air movement in and out of your home without the use of mechanical systems, with the goal of bringing fresh air into your home. During nighttime in the summer months natural ventilation can provide free cooling and reduce your home energy use. Natural ventilation design is typically considered during the home design phase.

The following video demonstrates how natural ventilation works. Then we discuss the factors that affect natural ventilation, and the types of natural ventilation techniques typically included in home design. Lastly, further design considerations are introduced.

Video by: Hologram Digital 

The effectiveness of natural ventilation varies based on:  

  • Dominant wind speed and direction

  • Surrounding environment

  • Building footprint and orientation

  • Outdoor temperature and humidity

  • Window sizing, location, and operable

There are two commonly used natural ventilation techniques: wind driven ventilation and stack ventilation. 

wind driven ventilation

as wind blows across a building, it comes into contact with the windward wall, which develops a positive pressure. Simultaneously the opposite wall, also called the leeward wall, develops a negative pressure. If there are any openings on the windward and leeward walls of a home, fresh air will enter through the openings on windward wall and exit through the leeward openings. With stronger wind and larger openings, more air can pass through the building.                                                                                                            Source: gbtech.emsd.gov.hk

stack ventilation

also known as buoyancy or thermal ventilation, is primarily induced by temperature differences within a home. As air in a home heats up it becomes less dense, which causes the air to rise. This warm air will leave your home through a window or opening located higher in the home, which results in cool fresh air entering through lower openings. Because stack ventilation does not rely on the wind, it can take place with relatively stable air flow on hot summer days with no wind.                                                                           Source: gbtech.emsd.gov.hk

If you're considering natural ventilation in your new home you should consider the following main design techniques:        

Building orientation and location

homes should be oriented so that the windward wall is perpendicular to the summer wind. Check out the wind rose map to determine your local prevailing wind direction. 

Building form and dimensions

naturally ventilated buildings should not be too deep because it will be difficult for wind driven air to pass through the home.

Window typologies and controls

different types of windows will result in different ventilation rates.

External elements

such as trees, adjacent buildings, or other structures may obstruct the wind.

Air inlets and outlets

air inlets should be low in the room and air outlets should be higher and across the room from the lower inlet.There should be a notable vertical distance between the inlet and exhaust openings to take advantage of the stack effect.

Ridge vents

 can enhance the air movement.

Be careful not to compromise fire exits

of enclosed staircases in your building design.

 

 

   References

1 "Natural Ventilation" by HK Green Building Technology Net 

In depth considerations for natural ventilation design in new builds

Wind Rose Diagram

Discover where the prevailing summer winds will hit your home. This is an important design consideration in natural ventilation. 

"Natural Ventilation" by the Department of Energy (DOE) 

Short introduction of natural ventilation.

"Ventilation" by the Departmnet of Energy (DOE) 

Brief overview of all ventilation strategies, including natural ventilation.