Understanding Our Cross-ventilation

Cross-ventilation is a form of passive ventilation that uses wind to bring air to the building.

It cools our homes without air conditioning, requires no energy and makes our interior ‘green’. It allows cool air to enter our homes through one opening and swiftly extracts warm air through another.

While this seems simple, there are various factors in designing a home that affect a natural, wind-driven ventilation. I took my house as the prototype in explaining the effects of cross-ventilation.

Orienting a building correctly determines the inlet for cold wind.

Meteorologically, I assessed the wind rose for my town to determine the wind pattern (Meteoblue). A wind rose provides pictorially the direction from and the speed at which the wind blows in a city all year round.

Narrow buildings have better cross-ventilation.

To have maximum effect, the short axis of the building should be parallel to the windward direction. However, this can vary depending on side obstructions like trees and adjacent buildings.

At least two outlets must be present on the adjacent or opposite side of the inlet. Same-side openings provide limited flow.

Total area of operable openings must be at least 15% of the floor area.

Wind rose of my town based on the data for past 30 years

Natural ventilation tends to be most effective on cool, windy days for narrow buildings. If the building is wide and exposed to direct sunlight, and the wind speed in vicinity is low, buoyancy-driven stack ventilation becomes necessary.

Stack effect states that air gets warmer and lighter near the ground and is replaced by cooler air. The denser cool air enters the building from lower levels, heats up through various heat sources and vents out at the highest level.

It is essential to identify the stack ventilation, especially in high-rise buildings.

It is also a good idea to put ventilating fans in the vents close to the stack ventilation. The figure below indicates a narrow-width flat with the provision for stack ventilation.

Stack ventilation provided in my apartment

Winds in my town flow from south-west directions. There appears no opening in those sides around the apartment. A window in the living room (1) is in the interior of the apartment building, which is obstructed by adjacent constructs.

For natural ventilation to have its effect, the axis width must be five times to the floor-to-ceiling height if windows are on both sides, and 2.5 times if windows are on one side.

Identifying appliances that emit heat is important.

The more the heat generated in the room, the more will be the requirement to vent it out. Hot air reduces the atmospheric pressure of the room and can be easily replaced by the cool outside air if properly channeled.

Therefore, a room with greater heat sources (like TV, washing machine, sunlight, conventional bulbs, gas-driven stove, laptops, cell phones etc.) require an efficient air-circulation irrespective of the season.

It is equally important to identify devices that aid in venting-off hot air.

This includes air-conditioning indoor units, ventilator fans, and ceiling fans. They aid in mechanical ventilation and choosing the appropriate size and placement affects cross-ventilation.

Heat sources and sinks in an apartment

As a rule of thumb, sunlight should approach at least 50% of the floor area for effective cross-ventilation. But sunlight generates heat. As such, exposure to sunlight and natural air flow is a trade-off.

A way to direct this trade-off for our comfort is to have a gallery with plantations that cool the air around. However, it should be spacious for a swift wind flow to reduce heat. One can shade this area by having an insulation wall (paint or plaster) or by adding awnings or pale curtains.

Modeling an Apartment for Cross-ventilation

My home is subject to direct sunlight with a slow, hot wind during summers, and a swift, cold wind during winters.

Default ventilation of the apartment.

Having a narrow geometry does not help in cross-ventilation. Therefore, mechanical ventilation with air conditioning is the need of the hour. Stack ventilation is limited and there is a high heat build-up in the kitchen due to one-sided outlets and large heat sources. It has small heat sinks that do not compensate for the heat generated.

A few modifications that have proven useful are

• installing ventilator fans near gallery and stack ventilation

• keeping opened bathroom and laundry doors

• venting heat from the kitchen and preventing its diffusion in other rooms

• shading and planting in the gallery

Better choices before buying an apartment can be made, like

• choosing an apartment that follows the wind rose of the city

• laying out the map for heat sources and sinks

• double-sided vents for cross-flow

• identifying natural cold spots in the house

Cross-ventilation map of the apartment after identifying heat sources, heat sinks and mechanical ventilation