What Is Cross Ventilation?
Cross ventilation refers to one form of naturally occurring ventilation in a building. The basic requirements for this type of ventilation are that an entrance and an exit for air has to be present, and the pressure of the air entering the space must be different to the pressure of the air leaving. At its simplest, this pressure differential occurs where two windows in one room are open, and they face different directions.
Air inside a building needs to be replenished regularly. Stale air can hold moisture and cause mold. It can also smell bad, and make a building uncomfortable to be in for an occupant. Without a supply of new air, sources of irritation and allergy in the air, such as dust or hair, can also adversely affect health.
A typical house contains several features that allow fresh air in and stale air out. Windows and wall vents are the most common options. These entrances and exits for air are passive features of a building. Even though vents and windows are traditional methods, engineers and architects can still design them for optimum efficiency to suit a particular building.
The basis of cross ventilation involves specific differences in air pressure between incoming and outgoing air. As building interiors tend to be warmer than the outside environment in cool climates, the interior air is at a low pressure, as it expands with heat. The cooler air outside has a high pressure as it is denser with the cold. Another source of pressure differential, which is important in both cool and hot climates, is the wind.
Wind provides outside air with more force than the air inside the building. Both of these sources of pressure tend to drive the air into buildings. The air comes in through the entrance, which is a window or vent on the side of the building that faces the wind. Where cross ventilation comes into play is areas where the incoming air can displace the interior air and push it out of the building.
To do this the outgoing air needs to have an escape route. Generally, in cross ventilation, these escape routes are located in the opposite direction to the air entrance or to the left or right of the entrance. Architects with a knowledge of ventilation mechanics can plan appropriate air exits relative to the air entrances for maximum efficiency.
Although cross ventilation does apply to single rooms, it can also play a role in buildings with many rooms. One problem with cross ventilation in a structure with a complex interior is that people who work or live inside the buildings can block ventilation through closed doors or windows. In order to get around this problem, construction companies can put duct systems in place that channel the fresh air around the building and act as an exit for stale air.
One benefit of engineering in cross ventilation into a building is that the process occurs without the need for any extra energy use, as in air conditioning. On the other hand, cross ventilation typically affects the temperature of the interior of a building. This can necessitate the use of central heating, which requires energy, if the building is located in a cold part of the world.
@pleonasm - I guess it's also the explanation for why opening the wrong combination of windows in a car results in all the papers in the area heading for the window. I didn't realize it was to do with air pressure, I always thought it was something to do with the wind blowing through.
But I guess the wind is created by a difference in air pressure from warm air and cool air moving around each other, so it's basically the same thing.
@browncoat - It really depends on what the weather is like. Sometimes even cross ventilation isn't enough to get the air moving. And sometimes the air is moving a little bit too much outside and it's better to just open one window.
This is also the reason why you can't open a window when you've got air conditioning on, because the difference in air pressure means that the cold air is all just going to flow straight outside.
This always makes a huge difference in my house, but I didn't know the term for it until now. If I just open one window it doesn't seem to cool down at all, but if I make sure there is another one open somewhere else (not in the same wall) then it cools down quickly. It's just one of those little common sense things that you don't really notice until someone points it out I guess.
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