Since ancient times, humans have used the prevailing wind to cool their homes and provide fresh air. Egyptians took things a little further and added screens of wetted reeds to their window openings, creating the forerunner of modern air conditioning. Sometimes called wind effect ventilation, cross ventilation is an ideal way to cool down the home and provide fresh air without using electricity.
Reasons to Cross Ventilate
The air inside buildings needs to be replaced regularly to prevent it from going stale and posing health risks to people working or living inside them. According to the EPA, “indoor levels of pollutants may be two to five times –
and occasionally more than 100 times – higher than outdoor levels.” That’s especially concerning given that the average Canadian spends 90 percent of their time indoors. And for people with health conditions such as asthma and allergies or for people who need to use CPAP machines or home hyperbaric chambers for increased air it becomes even more critical to ventilate the home to keep oxygen levels up and carbon dioxide levels down.
The cooling effect is only one of the many benefits of cross ventilation. Other advantages include:
- Fresh air is introduced into the room, increasing oxygen and decreasing CO2 levels
- Saving energy costs from not using air conditioning
- Reduced maintenance costs – HVAC units are working less, less buildup of dust on appliances, less moisture related issues
- Improved air quality – particularly important for people with health conditions such as asthma and allergies or for people who need to use CPAP machines or home hyperbaric chambers
- Cooler temperatures during the hot summer months
- Volume of cooling air is easily controlled by the size of the openings
- Reduces humidity and moisture in rooms and prevents mould and mildew growth
Cross Ventilation Strategies for Houses
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Under normal conditions, introducing external air using cross ventilation is the most efficient way of ensuring a continuous supply of fresh air into the building. A building creates a barrier to the prevailing wind, and having an opening on the side of the building facing the wind and another opening on another side of the building will cause the wind to blow through the room, causing a cooling effect and changing the air in the home.
Cross ventilation works best when openings are on opposite sides of the room, with one opening on the side facing the prevailing wind. The size of the opening and obstructions such as large furniture impede the amount of airflow. The effectiveness of cross ventilation is also influenced by the distance the air travels from the inlet source to the exhaust opening. The farther the distance, the less effective the cross ventilation will be.
To enhance the cooling effect of cross ventilation in the home, careful attention needs to be given to the design of the windows. Single-hung windows, for example, only allow the wind through fifty percent of the opening, while casement windows allow almost one hundred percent of the opening to the wind. In addition, casement windows are also able to redirect the wind when the wall is not perpendicular to the wind direction.
One problem with windows that open facing the direction of the wind during rain showers is that the opening allows rain to enter the home. This problem can be overcome by having wide overhangs on the windward side of the house. Windows that are hinged on the upper side as opposed to a vertical side have an advantage here in that they can remain open during rainfall without water getting into the house. Louvre windows can also be safely left partially open during rainfall.
Cross Ventilation with Fans
While the days of having loyal servants gently waving fans to keep the aristocracy cool have passed, cross ventilating with fans is still a great way to keep the home cool. Fans are fairly energy-efficient, and placing them in strategic positions can have a marked effect on comfort levels in the home.
While ceiling fans gently move the air around a room, you can position fans strategically to cross ventilate a house and cool it down. Place a fan facing away from the open window on the side of the house that gets the most wind (windward or upwind side) and place another fan on the opposite side of the house (leeward or downwind side) facing out the window to exhaust the air. Doing this will have a pronounced benefit in drawing air into the house through the windward opening and moving it throughout the house.
Attic Cross Ventilation
Cross ventilating attics is important to prevent mildew and mould from forming. Moisture-laden hot air rising into the attic needs to be exhausted before it causes damage, and the most efficient way to do this is to fit ridge venting.
Soffit or gable vents are efficient at allowing air into the attic area, and together with exhaust vents located higher up towards the pitch, they create good airflow.
Some exhaust vents use solar or wind power to effectively remove air from the attic area. Complying with building regulations regarding the size of vents required in attics is important, and as a rough guide, for every 150 square feet of attic space, one square foot of venting is required, according to the 2018 International Residential Code (IRC).
Cross Ventilation and Stack Ventilation
Stack ventilation combines the positive effects of cross ventilation with the natural tendency of warm air to rise. Cool air enters the building and exhausts through venting situated higher up in the building. The cooler air entering through the lower openings forces the warmer air out of the building.
Wind always blows from high to low-pressure areas, and as air higher up is less obstructed and moves faster, it has a lower pressure that effectively sucks air from lower areas. This creates an air movement from lower to higher areas, an updraft which forces warm air out of the home and introduces cooler air at lower levels.
In practice you can achieve this cooling effect by opening windows of different heights. Open a lower window on the windward side and a higher window on the leeward side. In multi-storey homes, opening windows on the windward side on the lower level and higher windows on the leeward side on the upper level will efficiently exhaust warm air from the home.
Using methods that have been refined since ancient times to keep your home cooler in summer and providing fresh air has the dual advantage of saving you money and positively influencing the environment. In areas where the ambient air is fresh and clean, it seems a pity to waste it on energy-hungry air conditioning.
To learn more about this topic, read our Green Building Guide to Indoor Environmental Quality
