The Stack Effect and Mold

The stack effect is a cyclical flow of air. It is driven by differences between indoor and outdoor air densities and temperatures. There are three forces that move air through a house: HVAC equipment, wind, and the stack effect. Of these, the stack effect is the least understood. At times it is the most powerful. By understanding this effect, you can increase the comfort, energy efficiency, and healthfulness of your home.

The air pressure within a house decreases with height. The air pressure on the ground floor is higher than the air pressure on the top floor. Air always flows from areas of high pressure to areas of low pressure. When air warms it expands, becomes less dense than the surrounding air, and rises. The stack effect is a naturally occurring condition in buildings, where colder outside air enters the house in a low spot, warms and exits at the top. This process is called convection and is the main process by which heat moves around a room and the house. As heat escapes a roof, cold air is sucked in through the basement and first floor windows.

The stack effect shown in winter and summer

The stack effect. In winter, a house is like a bubble of warm air in an ocean of cold air. As warm air rises and escapes through cracks, cold replacement air is drawn in at the bottom. In the summer, the reverse happens: Cool air sinks to the bottom of the house, drawing in hot replacement air at the top.

Like wind, the stack effect can move large volumes of air through a building envelope. During the winter, the difference in air pressure between the ground level and the top floor of a three-story house will be a lot more severe outside than inside, where the air is warmer and less dense — the warm air in a heated building is lighter (less dense) than the cold air outside the building; that warm bubble of air wants to rise up and out. The flow of air leaving the top of the building draws cold air into cracks at the bottom. The greater the difference between the indoor and outdoor temperatures, the more significant this airflow becomes.

The reverse happens in summer when hot air outside of an air-conditioned house can push cooler indoor air down from the ceiling and out of cracks in the basement. This can lead to moisture problems on the top floor that can lead to mold growth.

But the differences in temperature and pressure aren’t as great during the summer as they are during the winter. When it’s cold outside, the pressure created by the stack effect is 4 pascals per story of height; when it’s hot, about 1.5 pascals per story of height.

Air is lifted over heating ducts then dropping as it cools, rising again as it is heated. This is convection in action in a sealed house. Fresh air is important for our health so we add the element of ventilation. Warm air rises and escapes where it can from the roof. In doing so it draws in cooler air from lower portions of the building. The warm air leaving from the top is actually sucking air in from the bottom. This is the stack effect in action.

The stack effect in building design

When the stack effect is part of the building design it is a very effective method of natural ventilation. When it is not controlled by design the stack effect can be the biggest cause of heat loss and create cold draughts throughout a house.

In recent years creating natural ventilation with the stack effect has become an important part of the design process. Environmental designers are especially conscious of it for large building projects. We are seeing more designs that feature high “chimneys” at the top of the structure. These make a strong upward draw of fresh air through the building.

Tight houses are better than leaky houses — with a caveat: tight houses without a ventilation system are just as bad as leaky houses with no ventilation system; maybe worse. Energy efficiency requires a tight shell; good indoor air quality requires fresh outdoor air. Ideally, the fresh air should come not from random leaks but from a known source; for this to happen, the house needs an adequate air barrier and a controlled ventilation path (see Mold Problems with R-2000 homes).

In leaky homes, large volumes of air — driven by exhaust fans, the furnace fan, the stack effect, and wind — can blow through the home’s floor, walls, and ceiling. Because air contains moisture as water vapor, these uncontrolled air leaks can cause condensation and mold -  especially basement mold, crawlspace mold and attic mold. (also refer to Causes of Mold in Homes)