I’ve heard stack effect can cause problems with indoor air quality. How is this possible?
Stack effect (also called chimney effect) describes the way air flows through a building based on indoor and outdoor temperatures. It starts with the fact that warm air rises and cold air sinks. In the winter, your house acts much like a bubble of warm, buoyant air sitting on the bottom of a sea of cold, dense air. This creates a pressure difference, one of the key factors needed to create air flow. This can affect a building in different ways, but generally the pressure is positive toward the top floors and ceiling (meaning air wants to escape outside) and negative toward the bottom floor (meaning air wants to come in). To complicate matters, a taller structure with more stories will contain a taller column of air that will produce greater pressure differences.
The other key factor driving stack effect is a pathway for air to move between the regions of differing pressure–in other words, leaks in your building envelope. Even the tightest home has some amount of air leakage. As warm indoor air leaks through the walls or roof, it cools and deposits moisture along the way. The problems don’t necessarily stop there. New air to replace the exiting air must come from somewhere, and tends to take the path of least resistance. Typically air is drawn in through the lowest regions of the house (the negative pressure zone), which is why problems with soils gases like radon tend to increase in winter. Replacement air isn’t always just drawn in through the lower part of the structure; it can also infiltrate through poorly sealed or malfunctioning combustion appliances such as wood stoves and boilers, or plumbing traps that have dried out and are therefore no longer able to seal off the septic system.
The key to managing stack effect is good air sealing around penetrations in the building. If you are considering sealing air leaks in your house, it’s critical to start at the top. If you start at the bottom, you might be increasing the chances that your home will pull air from other sources such as combustion appliances. Some common air leakage points in the positive pressure zone of the house include can lights, chimneys, plumbing vents, wiring penetrations, bath fans, and range vents. Make sure you have a functioning carbon monoxide detector in your home and that your boiler and wood stove have a dedicated source of combustion air.
While you can never eliminate airflow and stack effect in your home, there are steps you can take to protect your indoor air quality.