Natural Ventilation

Natural ventilation, also called passive ventilation, uses natural outside air movement and pressure differences to both passively cool and ventilate a building.

Natural ventilation is important because it can provide and move fresh air without fans. For warm and hot climates, it can help meet a building's cooling loads without using mechanical air conditioning systems. This can be a large fraction of a building's total energy use.

Successful natural ventilation is determined by having high thermal comfort and adequate fresh air for the ventilated spaces, while having little or no energy use for active HVAC cooling and ventilation. 

You can choose the right strategy based on the temperature and humidity of your site. The following chart shows how much these different strategies can extend the comfortable climate range for people. 

Different passive cooling strategies can keep people comfortable at different ranges of outside temperature and humidity.

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When not to use natural ventilation

Sites with high levels of acoustic noise, such as near heavy traffic zones, may be less suitable for natural ventilation because large openings in the building envelope can make it difficult to block outside noise. This can sometimes be solved by using acoustical ventilation louvers.

Also, sites with poor air quality, such as adjacent to busy freeways, may also be less desirable for natural ventilation. Such sites may overcome poor outdoor air quality with filters and ducting, though this usually requires some mechanical fan systems. 


Quantifying Ventilation Effectiveness

To measure the effectiveness of your ventilation strategies, you can measure both the volume and speed of the airflow. 

The volume of the airflow is important because it dictates the rate at which stale air can be replaced by fresh air, and determines how much heat the space gains or losses as a result. The volume of airflow due to wind is:

Q_wind = K • A • V

Q_wind = airflow volumetric rate (m³/h)
K = coefficient of effectiveness (unitless, see below)
A = opening area, of smaller opening (m²)
V = outdoor uninterrupted wind speed (m/h)

The coefficient of effectiveness is a number from 0 to 1, adjusting for the angle of the wind and other fluid dynamics factors, such as the relative size of inlet and outlet openings. Wind hitting an open window at a 45° angle of incidence would have a coefficient of effectiveness of roughly 0.4, while wind hitting an open window directly at a 90° angle would have a coefficient of roughly 0.8. 

When placing ventilation openings, you need to place both air inlets and air outlets; often they do not have the same area. The opening area used in this equation is the smaller of the two. 

Air Speed and Temperature in Buildings

In addition to volume, you should design for the wind speed inside your building. Wind speed is a component of human comfort, and the speed you want depends on the climate. 

Higher velocity air causes more effective cooling, because it pulls heated air away faster, and because it helps sweating be more effective by evaporating it faster. Even a moderate wind speed can cool perceived temperatures 5°C (9°F) compared to still air. This is how fans make people feel cooler even though they do not change the temperature of the air. 

However, the ability of air movement to cool people depends on whether it is the air itself that is hot, or if the radiant temperatures of the room’s surfaces are hot. The hotter the air itself is, the less it helps to move it. If people are primarily hot from surrounding radiant temperatures, however, moving air helps more. The ASHRAE 55 standard provides guidelines for how much cooling is possible with air movement at different speeds, for different mean radiant temperatures. A 3°C temperature rise can be nullified by a 0.8 m/s increase in air speed when air temperatures are 5°C below radiant temperatures, but if air temperatures are 5°C warmer than radiant temperature, it would require a 1.6 m/s increase in air speed. This is far above what is acceptable wind conditions for light office work.

Comfortable air temperature vs. wind speed depends on mean radiant temperature


You’ll need to make sure that wind speeds inside the building aren’t so high that they disturb the occupants. Fast winds can blow papers around on desks, blow people's hair around, etc (refer back to the Beaufort Wind Scale). Referring to ASHRAE 55 for thermal comfort guidelines regarding air speeds for interior spaces, the standard suggests that air speeds appropriate for indoor environments do not exceed 0.2 m/s or 0.447 mph. ASHRAE also accounts for elevated air speeds that will increase the acceptable temperature. The maximum allowable elevated airspeed is 1.5 m/s or 3.579 mph.

It is also important to consider how often the air in a room is replaced, as an important feature of natural ventilation is that it supplies occupants with fresh air. The number of times the air in a room is replaced is known as air changes per hour, ACH, or the air change rate. It is determined by both the size of the room and the volumetric flowrate of air (Q). Q_wind, referenced above, is a component of this overall flow rate.

There are standards and recommendations for how much fresh outside air should be delivered to different building spaces, and to people within the building. For example, ASHRAE 62.2001 specifies 0.35 air changes per hour for residential living areas, but also specifies a minimum volumetric flowrate of 15 ft3/min (CFM) per person.

The equation is:

ACH = (Q / V) * (conversion factor)

Q = volumetric flow rate of fresh air

V = Volume of room or space

Conversion Factor = If the volumetric flow rate, time scales, and volumes are incongruous units. For example, if Q is in cubic feet per minute (CFM) and volume is in ft2, you’d need to multiply by 60 to get it in terms of hours. If Q is in cubic meters per second, or Liters per second, the conversion factor would be different.


Thermal Mass

Thermal mass can also have an impact on natural ventilation. Sometimes a space can get too hot for natural ventilation to have an impact on thermal comfort. However, you can use thermal mass to help maintain a consistent temperature and avoid big jumps. By stabilizing the temperature swings, you have a better chance of using natural ventilation effectively. Best practice design strategies for enhancing natural ventilation with thermal mass is explained further through night purging.