natural and mechanical ventilation in cooling strategies

Natural ventilation can be a key element of cooling strategies in many climates. Mechanical ventilation can also be part of cooling strategies (that's the case of the ceiling fans), but it involves also other goals: remove stale and polluted air, distribute fresh air, etc.

More than just opening windows

In its simplest version natural ventilation is opening windows and doors, to benefit from breezes. But natural ventilation - cross and stack ventilation - can be part of a more elaborate approach for cooling our homes, involving features like home design, landscape, placement and size of openings, etc. Air movement and types of ventilation
Air movement is the key element of passive cooling, either in the form of 1) cross ventilation, 2) convective ventilation or 3) mechanical ventilation.
Cross ventilation is very effective for building cooling and is based on fresh air entering in one side of the house, pushing warmer inside air out of the building through openings in the opposite side. Convective ventilation involves fresh air entering the home by lower openings, while warmer air rises and goes out through upper openings (often in the second floor of the house.
Mechanical ventilation (fan-assisted) can be especially suitable for people cooling or for stale air exhausting.

Cross ventilation

When people open windows in opposite sides of the house to cool the indoor temperatures, they are using cross ventilation.

The image below (from the Division of Architecture of California Government) illustrates the process.

Natural Cross Ventilation

See, for more details: Ventilation and Home Design

Stack (Convective) Ventilation: the «Chimney Effect

Natural stack ventilation (also called convective ventilation) uses a physics natural law: the chimney effect or air buoyancy...

In natural stack ventilation, the warmer indoor air rises up from lower living areas and escapes through the upper openings of the building, causing cold air infiltration through windows or other lower openings…

The image below, from the Division of Architecture of California Government, illustrates it.

Natural Stack Ventilation

See, for details: Convective Ventilation and Two-Story Homes

climates and natural ventilation

Natural ventilation loses much of its power in cold climates or even in hot-humid climates. Unlike mechanical air-conditioning, natural ventilation doesn’t reduce the humidity of incoming air, making the cooling process rather ineffective during periods of high humidity (in hot-humid conditions).

Higher humidity conditions requires greater air-speeds and greater ventilation to provide comfort. And the breezes and shaded spots associated with natural ventilation may not provide those pre-conditions.

If natural ventilation can’t provide significant comfort, and you live in hot climates, you may try a closed building-air-conditioning approach for some parts of the house

Deflecting the wind: Wing Walls and casement windows

Wing walls are vertical panels placed next to windows to intercept and direct the colder breezes into the house, via open windows or other openings. They are specially useful to deflect breezes, whenever they don't blow in the more convenient pattern (see image). Wing Walls and Casement Windows: deflecting the breezes

Casement windows can work as wing walls. Just have into account the way the casement windows open relatively to the dominant breezes, in order to direct the wind into the home. Casement hinges can be located on the left-hand side or on the right-hand side, taking into account the dominant direction of the breezes...

Mechanical ventilation

Mechanical ventilation involves 1) ceiling fans and other assisted-fan ventilation like attic and crawlspace ventilation, but also 2) simple and intermitent exhaust fan ventilation (kitchen exhaust fans and bathroom exhaust fans) and 3) the more sophisticated HRV and ERV ventilation: continuous whole house ventilation to remove polluted air and stale air, and to supply fresh air throughout the building.

While natural ventilation or ceiling fans are instruments of cooling strategies, the exhaust ventilation systems main goal is to exhaust stale air and/or to supply fresh air (a particularly important feature in cold and temperate climates, in tightly insulated homes).

See:
Home Cooling in Hot Climates
Natural Ventilation in cold and temperate climates

Natural ventilation, cracks and air leakage

Air leaks and gaps were traditional ways of supplying fresh air to homes, in cold climates. They acted like vent openings, contributing to the ventilation needs.

But since these gaps and sources of air leaks are also a source of heating and cooling loss, and many modern well insulated and airtight houses do not provide that source of outside air, mechanical ventilation becomes crucial for reasons other than cooling goals.

Ventilation & Health

In cold and mild climates, gaps and leaks in the walls or next to windows and doors may be a source of contaminants such as formaldehyde or radon, which are a cause of health problems; besides, these same gaps can also be a source of moisture and lead to mold growth and cause damages…

That's why ventilation - namely mechanical ventilation, in cold or even in temperate climates - is a key element to solve these problems.

See:
Natural Ventilation vs. Mechanical Ventilation
Summer cooling with natural ventilation in temperate and cold climates