In Green Buildings emphasis is given to utilize renewable energy sources and natural ventilation rather than mechanical ventilation. I came across a plan a few days back which had windows and doors but didn’t follow the principal of cross ventilation. There are instances where making minimum of 2 openings on adjacent/opposite walls which are open to the outdoor environment is not possible, but if the builder is planning to make the building Green and get it certified by any of the authorities like IGBC, LEED, GRIHA etc., special attention should be given by the team to provide openings to facilitate cross ventilation in the design stage itself.
In one of the rating systems Cross Ventilation and Natural Ventilation can get you a 2-4 points.Cross Ventilation also leads to decrease in use of Air Conditioners thus reducing electricity consumption and ultimately reducing Green House Gases emissions.
What is cross ventilation?
This form of ventilation drives air from openings at one side of a building through to the other. It’s success depends on the tightness of the building envelope – the physical separator between the interior and the exterior environments of a structure.
The direction and amount of wind available, the potential passage of air through vents, chimneys and other openers and how well wind is able to travel through a building also affects ventilation.
When constructing a building, it’s important to know which way the wind is likely to blow if you want to make the most of cross ventilation. The position and size of vents, windows and doors also has a bearing on the effectiveness of this form of ventilation.
Why use it?
This natural form of ventilation is effectively free and will ensure a building is properly cooled. There are no mechanics or electronics to be maintained and it is possible to prevent excess moisture and heat building up in ceiling cavities.
Wind ventilation also delivers on all three drivers of ventilation: maintaining minimum air quality, removing heat and other pollutants and facilitating air movement to enhance thermal comfort.
Wind ventilation is the easiest, most common, and often least expensive form of passive cooling and ventilation. Successful wind 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.
Using the wind for passive cooling and fresh air
Strategies for wind ventilation include operable windows, ventilation louvers, and rooftop vents, as well as structures to aim or funnel breezes. Windows are the most common tool. Advanced systems can have automated windows or louvers actuated by thermostats.
If air moves through openings that are intentional as a result of wind ventilation, then the building has natural ventilation.
When placing ventilation openings, you are placing inlets and outlets to optimize the path air follows through the building. Windows or vents placed on opposite sides of the building give natural breezes a pathway through the structure. This is called cross-ventilation. Cross-ventilation is generally the most effective form of wind ventilation.
It is generally best not to place openings exactly across from each other in a space. While this does give effective ventilation, it can cause some parts of the room to be well-cooled and ventilated while other parts are not. Placing openings across from, but not directly opposite, each other causes the room’s air to mix, better distributing the cooling and fresh air. Also, you can increase cross ventilation by having larger openings on the leeward faces of the building that the windward faces and placing inlets at higher pressure zones and outlets at lower pressure zones.
Different amounts of ventilation and air mixing with different windows open
Placing inlets low in the room and outlets high in the room can cool spaces more effectively, because they leverage the natural convection of air. Cooler air sinks lower, while hot air rises; therefore, locating the opening down low helps push cooler air through the space, while locating the exhaust up high helps pull warmer air out of the space.
Cross Ventilation is one of the most important aspects to improve the Indoor Environmental Quality and should be kept into mind while designing the plans .The criteria states
Minimum 50% of the regularly occupied spaces(by area) in each dwelling unit shall have an opening (doors/ventilators/windows) to the outdoor environment in at least two of the orientations.Regularly occupied spaces include living rooms,bed rooms,dining rooms,study rooms,kitchen etc.