The stack effect, also known as the chimney effect, describes the air movement in and out of buildings due to principles of physics. In the winter, warm air rises and escapes out of air leaks near the top while cold air is drawn in from the base… just like a chimney.
During winter (heating season), outside air is drawn in through basement air leaks while heated furnace air rises through the house and out of the home through leaks in the attic and roof. When hot air escapes out of air leaks, outside air is pulled in to replace the internal pressure.
During summer (cooling season), the stack effect is reversed, but is typically weaker due to lower temperature differences.
The more air leaks you have, especially in the top and bottom of the structure, the worse the stack effect is. This contributes to high energy bills and a drafty, uncomfortable home. Also, with outdoor air infiltration, you get the accompanying moisture and outdoor pollutants.
What Causes the Stack Effect?
The main thing that drives air movement is pressure. Air pressure depends on the density of the air, which is dictated by its temperature. The hotter the air, the farther the molecules are from each other (less dense), creating less air pressure.
Since air moves from areas of high pressure to areas of low pressure, warm air (less air pressure) will rise and cold air (greater air pressure) will sink. It also why wind moves from areas of cold (high pressure) to warmer (low pressure) areas.
As with the stack effect, the greater the pressure difference, the more powerful the air movement. Meteorologists use a barometer to measure air pressure when forecasting wind and speed directions.
There are three main things that affect the stack effect:
- Height of the building
- Temperature/pressure difference between the indoors and outdoors
The pressure difference (ΔP) is the main factor for determining the stack effect. Keep in mind that pressure difference can be affected by wind, exhaust fans, ventilation fans, chimneys and flues.
Along with differences in air pressure, air leaks are the greatest contributing factor to the stack effect.
Why You Should Invest in Basement and Attic Air Sealing and Insulation
Source: Energy Star
The image above shows the stack effect in motion, warm air rising out of your house and cold air being pulled in through the bottom. Imagine reversed arrows for summer (cooling season).
As discussed, air leaks and pressure differences exacerbate the stack effect. And in most homes, air leaks are greatest in the attic and basement. These air leaks cause your expensive conditioned air to escape and unwanted outdoor air to enter.
In the heating season, warm air rises up and escapes out of windows, ventilation openings, and unwanted gaps and cracks.
The air that escapes out of the top of the building will reduce the pressure at the bottom of the home, drawing in old air through doors, windows, cracks, and other openings.
The air movement is reversed during the cooling season but is usually less pronounced since the temperature difference tends to be lower.
Insulation must work in conjunction with a well-sealed building envelope. Sealing air leaks prevents the stack effect and air movement between the indoors and outdoors. Insulation slows conductive and convective heat flow. For added attic insulation, consider adding radiant barriers to reduce radiant heat gain.
Common Attic Air Leaks
Any openings in the building envelope will result in air leakage and contribute to the stack effect. In addition to replacing weatherstripping around doors and windows, you’ll also want to check around plumbing pipes, wiring, light fixtures, kneewalls, and more.
Here are some of the most common air leaks found in and around attics:
- Attic hatch/door
- Behind kneewalls
- Recessed lights
- Open soffit (box that hides recessed light)
- Furnace flue
- Plumbing vent
- Electrical Wiring
- Exhaust vents
In the attic, look for any holes in the ceiling (areas where light is coming up from below), dark spots in the insulation (indicates air movement), and areas where pipes, wires, and ventilation equipment enters the attic.
Common Basement Air Leaks
The area beneath the first floor is similar to the attic. Look for any areas where light penetrates through gaps in the floor. One common air leak in the basement occurs near the top of the basement wall where it comes into contact with the wood frame.
The top of the basement wall is above ground so pay special attention to cracks and gaps along where the house frame sits atop the foundation.
Here are some of the most common air leaks found in and around basements:
- Gap between sill plate and foundation
- Rim joists (where foundation meets wood framing)
- Anywhere where electrical, water, gas lines, or venting ducts enter
- Windows and doors
- Gaps and cracks in the floor
The stack effect is all about air being pulled and pushed in and out of your home. With summer fast approaching, take the time to seal air leaks and improve insulation before the stack effect kicks in. Don’t make your HVAC system work harder than it needs to.
Speak with the professionals at Service Champions to learn more about energy efficiency, air sealing, insulation, indoor air quality, and more.
Service Champions offers ComfortCloud™, our signature attic insulation system to properly contain warm and cold air.
- Learn more about attic air sealing projects.
- Learn what to know before sealing attic air leaks and improving insulation.
- Don’t forget about sealing and insulating your air ducts.
Extend the life of your HVAC equipment, protect your possessions, and decrease your energy costs with Attic Insulation by Service Champions.