Air Movement and the Thermal Gradient

Stack effect is due in part to thermal differences between outside and inside air. Cold air is denser than warm air, so does this mean that cold air is higher pressure than warm air and, given equal moisture content, air will move from a heated area toward a cold area? Building Science Corp. says *moisture *will move from a heated area toward a cold area. Is the way in which air moves relative to thermal differences determined by the amount of moisture vapor it carries (humidity level)?

I don’t know if any of these might help you Kenton;

Stack effect in buildings

Since buildings are not totally sealed (at the very minimum, there is always a ground level entrance), the stack effect will cause air infiltration. During the heating season, the warmer indoor air rises up through the building and escapes at the top either through open windows, ventilation openings, or leakage. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through either open doors, windows, or other openings and leakage. During the cooling season, the stack effect is reversed, but is typically weaker due to lower temperature differences.

Atria involve the following two natural phenomena: 1) Greenhouse effect: Short wave radiation from the sun passes through glazing to warm interior surfaces. The re-radiated heat in the form of long wave radiation is not able to pass back through the glass. This effect is of benefit in the winter and a liability in the summer. 2) Stack effect: The stack effect is the result of convection within an open space. The warmer, less dense, more buoyant air rises to the top of the space, and tries to exit. This results in a positive pressure at the top of space and a negative pressure at the bottom. If the air cannot exit, then this results in the stratification of air by temperature especially apparent in a tall closed volume.

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.

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. 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.

Just a few pieces of information for you Kenton. If it does not help you a little, they call it recycle bin. ha. ha.

Marcel :slight_smile: