WHAT IS 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 warmer 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.
Understanding the Stack Effect Stack Effect:
High & low leakage. Warm air tends to leak out near the top of the house, causing outside air to leak into the house lower down. The same convection force that makes a hot-air balloon rise can cause a house to have major comfort and energy problems.
Everyone knows that hot air rises and that cool air sinks. Convection: It’s a basic law of thermodynamics aptly demonstrated by a hot air balloon that ascends as it’s filled with heated air. When the air inside the balloon cools down and becomes denser, the balloon begins its journey back to earth.
Exfiltration + Infiltration = the Stack Effect
The connection between convection and home energy performance is known as the Stack Effect. In a house, this air movement phenomenon is most noticeable during cold winter months.
Here’s how it works: Indoor air that’s been warmed by the heating system rises by convection to the upper areas of the house. Unfortunately, this warm air doesn’t stop moving when it gets to top floor’s ceiling. In a typical house, hundreds of gaps and cracks provide passageways for warm air to escape into the attic.
Holes for recessed ceiling lights, gaps around a drop-down attic stairway and wall cavities used as electrical or plumbing chases are common leak locations. Interior air that leaks out of the house is called “exfiltration”.
Losing the air you’ve paid to heat is bad enough, but this escaping air creates negative pressure inside the house –a vacuum that causes air infiltration. Cold outside air is sucked into your living space through other cracks, gaps and openings. In the winter, cold air infiltration usually occurs in lower parts of the house –like the basement and/or crawl space. But air infiltration can also occur around window and door framing and around electrical outlets located in exterior walls.
Is the stack effect at work in your house? Check for these signs.
Symptoms of the Stack Effect at work in your house include cold drafts you can feel near exterior walls (especially around electrical outlets and window trim) and cold drafts in the basement. You may also notice that the floor surface directly above a basement or crawl space is uncomfortably cold during winter months.
Up in the attic, evidence of air leakage and Stack Effect energy loss can often be found on the fiberglass insulation installed over the attic floor. Insulation installed near an air leak will often discolor from the dust deposited on the fine fibers by rising air.
The comfort problems caused by the Stack Effect also cause energy problems. With heated air leaving the house and cold air entering, your heating system must work longer and harder to maintain comfortable interior temperatures.
During cold weather when the Stack Effect is strongest, heated air is constantly escaping, and it’s being replaced by an equal volume of cold outside air. Because the cycle continues 24/7, you’re going to be spending a lot of money to keep warm.
Sealing air leaks short-circuits the Stack Effect.
The Stack Effect has three “drivers” that determine its magnitude: the overall height of your total living space (multi-story buildings tend to have stronger convection), the interior-exterior temperature difference (the larger the difference, the stronger the effect) and the amount of air leakage in the house.
Since you can’t do anything about the first two drivers, it’s necessary to focus on the third: air leaks. Having your home professionally air sealed will effectively short-circuit the Stack Effect by dramatically reducing air exfiltration and infiltration. It’s especially important to seal attic air leaks since this is where most warm air escapes in the winter.