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Community and Q&A

Controlling Stack Effect

suect | Posted in General Questions on

Last March our rim joists were sealed.  All seemed well until summer where the main level has an attic odor and the peak of the summer heat time this area seemed to be overly hot.

The plus noticed is the dehumidifier in the basement worked much less.

I have read about the Stack effect working in different directions based on outdoor temperatures.

Would there a remedy for this besides trying to undo what has been done?

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  1. Jon_R | | #1

    You can pressurize the house a little to prevent attic odors from being pulled into the house in Summer.

    Blower door testing and even more air sealing is another option.

  2. Andrew_C | | #2

    Stack effects come from pressure differentials. Typically, there is high pressure near the uppermost ceiling, low pressure in the basement, and somewhere in the middle there's a neutral pressure plane. The neutral plane can move up and down, but generally the maximum leakage comes from where the pressure differential from inside to outside is the greatest. So, most advice on air sealing, particularly in retrofits, is to focus initial air sealing effects high (at the ceiling) and low (at the rim joist).

    Having sealed the rim joists, your next step is likely to look into air sealing in your attic, PRIOR to any insulation upgrades. Blower door testing can help identify leaks and quantify improvements. Fall is a great time to do blower door testing because you can get good thermal contrast with IR camera during blower testing to help differentiate between air leaks and missing insulation.

    Good for you to get the rim joists sealed up. Air sealing is the best bang for the buck wrt energy savings, and reducing unwanted/uncontrolled air leaks should help you improve air quality as well.

  3. Expert Member
    BILL WICHERS | | #3

    Your attic may have insufficient ventilation. Stack effect draws in air in low areas like a basement, up through the living spaces, and out through the attic. This would have had the effect of helping to cool your attic before by leaking your air conditioned air into the attic. With less air coming in down low after you sealed your rim joist, the attic is likely getting hotter and that heat is radiating into the living space.

    I would check that you have proper attic ventillation at sufficinet levels. The best way to do it is usually with a ridge vent and slightly more soffit vents by unit area to maintain a slight positive pressure in the attic (which helps limit air leakage from the living spaces into the attic). You'll also want to make sure you have good insulation on the attic floor. Obviously this assumes the typical vented attic, if you have something else like a cathedral ceiling, things get more complicated.


    1. suect | | #6

      Thank you to all.

      The attic is one that I call “complex”.

      There are 2 distinct attic areas separated by 2 cathedral attics (side by side but different orientation) On either side of the 2 true attics are 2 cathedral ceiling areas (1 which is cathedral, the other is one where there is OSB against the trusses then a space before seeing the underside of the roof. The soffit vents on the last ventilate the drywall/OSB space - no exhaust to attic space.

      The home has box vents and soffit vents. Soffit is 2/1 to box vents.

      I have been told algae is seen on a nonshaded Roof located adjacent to a 5 acre pond.

      Looking to hire a professional to do blower testing and advise.

      1. Expert Member
        BILL WICHERS | | #7

        Algae on the top of the roof, which might be moss, is usually due to shading of that area by trees and not by any issue within the attic itself. If you're seeing green stuff growing on a non-shaded area, then my best guess is you have some standing water there or possibly some EXTREME moisture issues. Normally the sun bakes the roof enough to keep things from growing on it.

        If you have soffit vents only on one of those attics, you need to add some exhaust venting up near the high point of that particular attic. If you only have soffit vents you likely have stagnant air in there which could potentially be a problem with moisture accumulation.


        1. suect | | #12

          There are soffit and box vents. Are there too many soffit to box vents? Possible air flow low but not Much up high to pull it out? The other thought, would too many soffit vents increase the stack effect into the attic as there are more penetrations there? Would this positive pressure push its way to the main level if most of the rim joists are sealed?

          Thanks in advance.

    2. suect | | #17

      Thank you Bill.
      I was encouraged to have my soffit vents increased 3 years ago. Since then there has been increasing amounts of ghosting on my ceiling and walls.

      I decreased the size of most. Some reversal seen. Thank you!!

  4. Jon_R | | #4

    Summer (with AC) stack effect creates a low pressure at the uppermost ceiling and draws in air from high areas (like an attic).

    1. Expert Member
      BILL WICHERS | | #5

      True, stack effect goes with convection. I'm stuck in heating-mode thinking with the cold weather :-)

      Same basic principle though. Probably needs more air sealing and quite possibly some attic ventilation work too.


  5. GBA Editor
    Kiley Jacques | | #8

    Hi Sue,

    Martin Holladay provides a detailed description of how stack effect works in this article, An Introduction to Pressure Diagnostics.

    1. suect | | #14

      Thank you Kiley.

  6. user-1140531 | | #9

    I don’t understand how stack effect could produce the symptoms described in the original post.

    Stack effect happens when interior air is heated and becomes less dense than outside air which is colder. When this happens, the cold outside air forces its way into the house through any opening it can find. Once inside, the cold air sinks downward to displace the warmer interior air. The force comes from the weight/density of the heavier cold air, and not from an upward pull of the heated interior air.

    If there is an elevated outlet for the warmer air, the incoming cold air flows in and displaces/lifts the warm air and will force it out of the house though the elevated outlet.

    So this tendency for cold air to displace warm air is a pressurizing event in the upper regions of the house caused by the incoming force of the cold air which is heavier than the interior warm air. So the cold air pressurizes the house and forces out the warm air. There is no interior suction created in this scenario.

    Without the incoming push of the cold air, the warm air has no tendency to rise, and thus cannot exert a pull or a suction effect that can draw air into the house through air leaks. The air leaks let the cold air force its way into the house, and this raises pressure in the house.

    It is just like a hole in a boat letting water flow in as it naturally will. If the boat has a sealed deck, the incoming water will pressurize air as the water flows and fills in under the air pocket, and presses upward against the air pocket. Then if you open a valve in the deck, the trapped air will be forced out by the pressure created by the incoming water rising in the boat hull and raising the pressure of the trapped air pocket.

    1. Jon_R | | #10

      You can review below, paying special attention to "Imagine reversed arrows for summer (cooling season)."

    2. Expert Member
      BILL WICHERS | | #11

      Imagine a vessel of water with a hole in the bottom and a hole in the top. As the heavier water flows out the hole in the bottom, air comes in through the hole in the top to fill the void. If you plug that hole, you’ll feel some suction. The same basic principle applies to air with the stack effect: heavier air will flow out the bottom while drawing in new air from the top to replace it.


  7. user-1140531 | | #13


    Yes, that makes sense when considering the summer effects that Sue CT was referring to. With hot summer air existing outside and interior cold air-conditioned air falling out of the house, that would pull the hot air into the house with an actual suction force.

    I guess I am always thinking in terms of stack effect in winter, and my comment above applies only to that situation when I say there is no interior vacuum or suction created by stack effect.

    Where this really matters is with the theory that in winter, heated air is rising and pulling a vacuum inside the house attic insulation space because the rising warm air has an upward force of its own. And this belief leads to the conclusion that soffit vents should have a larger area than the ridge vent. The reason given is that the if you have a larger ridge vent, air flow will be restricted at the intake of the soffit vents, and this will cause a vacuum inside the attic that will pull conditioned air from the living space into the attic through air leaks.

    1. Expert Member
      BILL WICHERS | | #15

      I think most of us who are concerned with energy efficiency tend to be from colder, heating dominated climates so we tend to think about heating concerns first. I made the same mistake myself earlier that Jon pointed out.


      1. suect | | #18

        This home is in the colder portion of the country.

        Thank you all!!

    2. Jon_R | | #16

      > cause a vacuum inside the attic that will pull conditioned air from the living space into the attic

      Or maybe in the Summer, such low pressure in the attic will exactly balance the low pressure below the ceiling, resulting in the ideal: no induced air movement. But there seems to be an almost complete lack of data on measured attic vs interior pressures.

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