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Building Science

Understanding the Stack Effect

Hot air rises, and cold air sinks, but stack effect ventilation reverses in summer

Hot air is less dense than cool air, so it rises. During the winter, the warm air near the top of a house is pressurized with respect to the outdoors, while the air near the bottom of the house is depressurized. The air at high pressure escapes though cracks, drawing outdoor air into the house through cracks near the bottom of the house. This phenomenon is called the stack effect. Image courtesy of Fine Homebuilding.

Like wind, the stack effect can move large volumes of air through a building envelope. In the winter, the warm air in a heated building is lighter (less dense) than the cold air outside the building; that warm bubble of air wants to rise up and out. The flow of air leaving the top of the building draws cold air into cracks at the bottom.

The reverse happens in summer when hot air outside of an air-conditioned house can push cooler indoor air down from the ceiling and out of cracks in the basement (see drawing). At least in theory, this can lead to moisture problems on the top floor.

But the differences in temperature and pressure aren’t as great during the summer as they are during the winter. When it’s cold outside, the pressure created by the stack effect is 4 pascals per story of height; when it’s hot, about 1.5 pascals per story of height.

Leaky buildings consume tremendous amounts of energy. Air leaks can contribute to condensation, compromising the quality of the indoor air. For high-rise residential buildings in cold weather, that isn’t the worst-case scenario — it’s the normal scenario (see “Why revolving doors were invented,” below).

If the stack effect is a big deal in two-story houses, imagine what kind of pressure it causes in high-rise buildings. This pressure is so significant in fact that “when skyscrapers were first developed at the turn of the century, people also had to invent revolving doors because you couldn’t open the front door due to the stack effect pressure,” says Straube. “The cold air was rushing in with so much pressure that it was difficult to push the exit doors open.”

Stack effect feeds on itself

“Air entering the building make the downstairs people cold, so they turn up the thermostat. When the people upstairs get all that heated air, they open the windows to cool off. This increases the flow of air leaving the building, which increases the flow of air coming up from the bottom floors — so the people downstairs plug in space heaters.

“You wind up with this merry-go-round — sucking air up the bottom, heating it up, and blowing it out the top,” says Straube. “There’s so much airflow in the elevator shafts that you can float! You can just put out your arms and you’ll float in the middle of winter in many of these buildings.”


Related Articles

Questions and Answers About Air Barriers

Airtight Wall and Roof Sheathing

One Air Barrier or Two?

Blower Door Basics

Pinpointing Leaks With a Fog Machine


  1. Hal Dolin | | #1

    Stack Effect
    Mr Morrison: I work for a company that offers a fresh air system that encapsulates the crawlspace and pulls air out of the crawlspace and subsequently out of the house. I read your article on 'The stack Effect: When Buildings act like Chimneys' and it is changing my thinking on how this all works and it does seem quite logical. However, there is a question I would very much like to hear your comments on please.
    When we cut an opening in the main level floor for a vent, in the summer time, with the A/C unit cycling, the air rushes up thru the newly cut opening from the crawlspace. After reading your article, I would expect it to go the other way, to sink. Looking forward to your comments, thanks

  2. GBA Editor
    Martin Holladay | | #2

    Response to Hal Dolin
    Of course I'm also eager to hear Dan Morrison's response. In the meantime, here's mine: If you cut a hole in the floor of an air-conditioned building and feel air rushing up at you from the crawl space, that means that the home is depressurized.

    The two main causes of depressurization are the operation of exhaust appliances (for example, range hood fans, bathroom fans, whole-house exhaust ventilation systems, clothes dryers, power-vented water heaters, or central vacuum cleaner systems) or an unbalanced forced air system (for example, a forced-air system with leaks in supply ductwork located in the attic).

  3. user-1055261 | | #3

    Stack Effect and the Atmosphere
    Just joined your site even though I've come across it before during internet searches. Lots of great stuff here.
    I'm a guy that likes to find out how and why things work. While your article above explains how stack effect occurs within a building or house, what I've been mulling over lately is what might be the "prime motivator" behind stack effect and summer "reverse" stack effect.

    How I'm looking at it is atmospheric pressure being the primary driver, following the dictum that greater pressures always move toward lesser pressures. In our wonderful internet age we can pull up weather trend data for almost anywhere in the good ol' USA. Which I did so for my own, specifically barometric pressure trended over the course of a year. And what I saw is that during winter, barometric pressure trends higher than during summer. This would be consistent, in my mind, with a sinking air mass in winter due to the air being cold, and a rising air mass in summer due to the air being warm.

    How this ties into stack effect is that we purposely counteract the seasonal outdoor conditions for our comfort, which results in a pressure dynamic in opposition to the atmosphere. In winter we heat the dwelling, causing the interior air mass to expand and become less dense, hence rising toward the top of the structure. The outdoor air mass, being colder and heavier, pushes into the house to make up the difference in pressure created by the warm air rising and escaping at the upper regions of the structure.

    In summer this is reversed since the air mass outdoors is rising, and air conditioning the indoors makes the indoor air mass fall. Unlike winter stack, where the pressure delta is greatest at the highest ceiling plane (adjoining a ventilated attic in a house), the greatest pressure delta is perhaps at the lowest floor or a walkout basement (where I live we have mostly slab on grade construction), although concurrent to the article above the Pascal reading per story is less than in winter.

    I offer this not to counteract anything written in the article above. It's just my tendency to find the "ultimate prime mover" in these scenarios, which I think helps put all of it into better perspective. At least for me. :) I'd like your thoughts.

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