Stack Effect – Defined
Air Infiltration is caused by wind, stack effect, and mechanical equipment in the home (see Figure 1 below).
The “stack effect” is when warm air moves upward in a building. This happens in summer and winter, but is most pronounced in the winter because indoor-outdoor temperature differences are the greatest. Warm air rises because it’s lighter than cold air. So when indoor air is warmer than the outdoor air, it escapes out of the upper levels of the building, through open windows, ventilation openings, or penetrations and cracks in the building envelope. The rising warm air reduces the pressure in the base of the building, forcing cold air to infiltrate through open doors, windows, or other openings. The stack effect basically causes air infiltration on the lower portion of a building and exfiltration on the upper part. Mechanical equipment such as fans and blowers causes the movement of air within buildings and through enclosures, which can generate pressure differences. If more air is exhausted from a building than is supplied, a net negative pressure is generated, which can induce unwanted airflow through the building envelope. Bathroom exhaust fans, clothes dryers, built-in vacuum cleaners, dust collection systems, and range hoods
all exhaust air from a building. This creates a negative pressure inside the building. If the enclosure is airtight or the exhaust flow rate high, large negative pressures can be generated.
Here is a great video that demonstrated the stack effect and shows why it is important to air seal a home. https://www.youtube.com/watch?v=a3rKGMIqWis&t=787
This video is also good and is great for youth. http://www.waptac.org/MediaModule/video/450/Grandmas-House.aspx