Lesson -1: Principles of Eco-Design(Part Three)

3.Stack effect:

-when air warms it expands, becomes less dense than the surrounding air, and rises,this process is called convection and is the main process by which heat moves around a room and the house, when rooms are sealed, convection is a sealed circuit of hot air rising over radiators and then sinking as it cools to be heated again.

-in reality, though, we don’t want rooms or houses to be fully sealed, ventilation with fresh air is vital in a healthy house, and convection plays a leading role in natural ventilation, hot air rises and escapes through small gaps in the building fabric at the top of the house, as it does so it draws in new cold air through similar gaps at the bottom of the house,the powerful suction created by escaping warm air is called the stack effect, or sometimes the chimney effect because it is the same process that draws smoke up a chimney or smokestack.
-stack effect is the movement of air into and out of buildings, chimneys, flue gas stacks, or other containers, and is driven by buoyancy,wich occurs due to a difference in indoor-to-outdoor air density resulting from temperature and moisture differences, the result beeing either a positive or negative buoyancy force,as  the greater the thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect.

-like many other environmental principles, the stack effect can either be a problem or an opportunity, it is the main motor generating draughts and the loss of hot air; and often the largest single cause of heat loss in a home,it requires particular attention when draughtproofing a house, however, when carefully controlled it can produce a low and effective level of natural ventilation and if respected and built into the house design, the stack effect is by far the most effective way of keeping a house ventilated in summer.

- over the past ten years environmental building has paid increasing attention to generating a stack effect to create natural ventilation, especially in large buildings, in a typical design tall chimneys at the top of the building create a powerful draw and fresh air is pulled into the building through specially placed controllable vents around the outside wall. 

-stack effect ventilation is an especially effective strategy in winter, when indoor/outdoor temperature difference is at a maximum, a chimney heated by solar energy can be used to drive the stack effect without increasing room temperature.

-in a modern high-rise building with a well-sealed envelope, the stack effect can create significant pressure differences that must be given design consideration and may need to be addressed with mechanical ventilation, also stairwells, shafts, elevators, and the like, tend to contribute to the stack effect, whereas interior partitions, floors, and fire separations can mitigate it, especially in case of fire, the stack effect needs to be controlled to prevent the spread of smoke.