Stack Effect

What is Stack Effect
by Erik North on October 11, 2011

stack-effect

If you’ve ever seen an eagle or other large bird gliding above a canyon’s warm air flows, then you can understand the stack effect. The eagle is taking advantage of the warm air that flows up between the canyon walls and buoys the bird aloft.
A similar effect happens within your home when warm air flows up through the building enclosure. The stack effect is based on buoyancy and encompasses the air either moving into or out of your home.
Stack effect is the natural buoyancy of warm air, made stronger by larger temperature differences and taller buildings. In temperate climates, stack effect is a lesser issue. In colder climates, like Michigan the stack effect becomes more pronounced. Along with wind pressures, stack effect is one of the main drivers of air movement within a building.
Stack Effect & Air Pressure
The stack effect is caused by a difference in temperature between the interior and exterior spaces of your home, thus creating air pressure. The air pressure acts similarly to an earthquake force behind a tidal wave, in that the pressure pushes air around your home. Most of the time, this pressure and the air pushed doesn’t do any damage on its own. But there are some cases where this can cause serious issues. The air itself can be a strong force and it can carry particles with it, like water vapor, mold spores and chemicals.
Why Is Stack Effect Important?: The Financial Wind Tunnel
Having the stack effect occur in your house is like having a wind tunnel where your hard-earned dollars fly out of the chimney! The air is buoyed up and out, and you are currently paying to heat and cool that air. However, it is impossible to prevent some flow of air into and out of your home. But you can reduce the negative aspects of the stack effect and minimize the air leakage flowing out of your home.
Is There a Draft in Here?
Drafts are one of the most telltale signs of the stack effect in place. Air is being pushed around your home and when it connects with outside air, the temperature drops and you feel the force of the air. It’s most common around windows and doors. Unfortunately, many older homes are very drafty because they’re not well air sealed and the stack effect causes cold air to enter the house through many different entry points. One customer of mine had a large central chimney with an unsealed chimney chase. The chase ran from her basement to the attic. It was sucking the warm air from her home and whooshing it up and out. She was heating the air outside her house and never got to enjoy the warmth!
What To Do?
A temporary plug to a draft does not solve the underlying problems of air leakage caused and exacerbated by the stack effect. Capping the top of a house is a little like holding your finger on one end of a straw. Naturally warm buoyant air causes the stack effect. A thorough air sealing of the attic space can short circuit this conveyor belt of warm air. Sealing light fixtures, plumbing chases, top plates of all walls, chimneys, skylights, and other major attic leaks can improve your comfort and save money. Gable walls at vaulted ceilings are another major loss of air due to being improperly air sealed and insulated.