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Community and Q&A

Battling Second-Hand Smoke With Fan and Positive Pressure

Garsagil1 | Posted in General Questions on

Hi everyone!

Have been battling with second hand smoke after I bought a condo. It is a nightmare. The two neighbors above spend the whole day home smoking cigarettes’ and weed, and now that we hit summer the negative pressure pulls the air into my house.

I am considering selling the house for a loss, but I wanted to try a few things before.

Could you recommend a window fan with a carbon filter that would push inside so much air that will keep the house in positive pressure?

Just in case it exists… and as a separate question, an AC unit that generates positive pressure?

I have two AC window units in my condo.

If the unit can also dehumidify, great, if not I can buy an extra dehumidifier.

Thanks so much guys, really appreciate it!

PS: I am considering breaking all the ceiling to spray foam everything myself, but still think that will not work 100% and positive pressure could be an easier and better solution

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  1. Jon R | | #1

    Positive pressure is an effective way to stop infiltration. But how much airflow is needed varies with conditions (like temp & wind) and it does have energy consequences. So ideally you would find a way to measure the pressure differential and use this to control fan speed. But I don't know of an off-the-shelf system do to this. Also note that air sealing reduces the amount of airflow needed for any given pressure.

  2. Expert Member
    Zephyr7 | | #2

    I would check if you have any obvious leaky spots that could air seal first. Classic spots are electrical boxes and fixtures. Seal any leaks that you find, which will minimize how much smoke gets in to begin, making the problem easier to deal with.

    Check if you have an existing fresh air intake somewhere. If you do, that's where you want your activated carbon filter to go -- you don't want your HVAC system sucking in smoke.

    After you've done those things, it won't take much positive pressure to deal with whatever is left. You don't even need an entire window fan if your home is otherwise pretty well sealed -- even a small muffin fan or blower will do. You can use some of the activated carbon air filter inserts that Honeywell makes to rig up your own fan with a small box.

    Honeywell makes a lot of small and mid size filters for use indoors that are standalone units. I've had good luck with these to help with allergens in the bedrooms of people who are very sensitive, and I use one in the bathroom where my cat's litter box is to keep dust down (that one runs 24x7 all year long).


  3. Expert Member
    DCContrarian | | #3

    I was talking to my local AeroBarrier rep recently. He said that since DC legalized cannabis his biggest source of business is air sealing multi-family dwellings against second hand smoke. He claims they can get a very tight seal and solve the problem. See:

    It is expensive though, probably around $5k for finished space. Most of the expense in finished space is the labor for masking to protect the finishes and possessions.

    1. Andrew C | | #5

      @ DCContrarian - air sealing is a big deal. In retrofits, there are usually some big holes, but also a lot of small holes and seams to seal that can be hard to get at in a finished house. Aerobarrier seems like an attractive option for retrofits. That observation about the need for air sealing after legalizing weed makes me want to look up what it would take to become an Aerobarrier installer.

  4. Garsagil1 | | #4

    Thanks everyone.

    DC, I spoke with aerobarrier but they need to access every unit that connects with me to put air purifiers there... and that is impossible.

    So I am out of luck there.

    My plan is to remove the ceiling drywall. Then get flex and canned foam all over it, then repair drywall. Hopefully that covers all holes that leak air.

    On top of that, get a window fan... although I was hoping maybe there was some type of window fan that was powerful that could save me having to remove drywall and make the house airtight. It is going to be crazy to remove all the ceiling and seal it myself.

    Thanks again!

    1. Expert Member
      Malcolm Taylor | | #6


      Don't forget your ceiling is also a rated fire separation. Any work done to that needs the approval of your condo board.

    2. Jon R | | #7

      Just about any fan will work. It's cheap to try.

      1. Garsagil1 | | #9

        This is what I needed to hear before going crazy with other stuff.

        Will get one today, and run a test.

      2. Garsagil1 | | #10

        This is what I am thinking as a solution to try.

        For the vertical window I have in the kitchen:

        Add a fan with a carbon filter:

        What do you think?

        1. Expert Member
          DCContrarian | | #12

          Why a carbon filter?

          1. Charlie Sullivan | | #21

            If the intake is near windows that the neighbor opens while smoking, it could be pulling smoke in.

        2. Jon R | | #13

          > VIVOSUN Inline Duct Fan

          Should be plenty of CFM and the variable speed allows setting as low as possible (for energy savings and less humidity impact). Let us know how it works out.

    3. Expert Member
      Zephyr7 | | #8

      I don't see any point in removing the ceiling drywall to air seal behind it -- the ceiling drywall already IS an air barrier. All you need to do is seal the perimeter, which is probably already tied into the drywall on the walls (another air barrier), and any penetrations like electrical boxes for light fixtures. Air sealing the middle of the floor above, then replacing the drywall, doesn't really accomplish anything.

      What MIGHT help, if the smell is migrating through the drywall somehow, is to apply a coat of primer to the drywall -- something like BIN, that is often used to seal smells from things like smoke.


      1. Expert Member
        DCContrarian | | #11

        Good point. And fighting fire with fire, as it were, an effective way of finding air leaks is to pressurize the space with a fan and then use an incense stick or other source of smoke along seams to detect air movement.

        1. Expert Member
          Zephyr7 | | #14


          Using a smoke stick and a fan here is an EXCELLENT idea.


          1. Expert Member
            Malcolm Taylor | | #16


            Or a cigarette ;)

        2. Jon R | | #15

          My understanding is that it's better to use small amounts of smoke with negative building pressure. True??

  5. C L | | #17

    I had this same issue in a condo I own. I was amazed at how much it helped to seal. About $30 of acoustic sealant and a few cans of fire barrier spray foam, and putty pads made an immense difference.
    1. Remove all electrical outlet and switch covers. Spray foam big gaps, use acoustical sealant at smaller gaps. Gaps between boxes and gyp board caulked/sealed. Holes at back of boxes sealed with pieces of putty pads. DO NOT put any spray foam inside the box.
    2. Check all plumbing. Seal between plumbing and walls using foam.
    3. Check bath exhaust fan and kitchen exhaust. In my case they were tied together and if they turned on their kitchen exhaust it came right into my unit. I installed a damper after my exhaust fan and a charcoal filter. These two actions helped a great deal.
    4. Check all closets, hvac rooms, etc for holes in walls and floors. My HVAC closet had holes from old condensate lines, etc. Seal all holes.
    5. Check all access panels. Had an access panel on the other side of the wall from the shower, and opened it up to find it was open to the units above and below, which is probably a fire code violation. Put in a two horizontal seals in the stud bay with fire rated spray foam.

    1. Expert Member
      DCContrarian | | #18

      I imagine this would help with soundproofing too.

    2. GBA Editor
      Kiley Jacques | | #26

      This is excellent information, CL. I'm glad you shared it here. This is such a bummer of a problem and you have offered an affordable fix.

  6. Walter Ahlgrim | | #19

    Think of the condo as a balloon it will be easier to fill it up if you patch as many leak as possible before you start blowing.
    I would start with a window fan blowing out and find as many leaks as possible even if the leaks do not lead to the other unit every leak you seal will make pressurizing your unit more effective and lower the energy penalty you will be paying.

    A fan like this should move enough air to help you find the leaks with a bright LED flashlight and incense sticks.

    This thread is worth reading I would caulk every gap in the ceiling electrical boxes while remembering not to reduce the air space inside the boxes as it is carefully calculated to the number of wires to provide the necessary cooling.

    You could cover the existing ceiling and walls with 2 layers of plastic sheeting caulking and staggering the seams and adding another layer of drywall over everything.

    Some locations require multi unit building to have ventilation systems that have no occupant controls.

    Most condo boards are unlikely to approve of anything visible from the outside so the window plate kit you linked to could be a problem but maybe not if window ACs are allowed.

    Given the current real estate market if you can stay more than a few months it seem likely to sell at little or no loss.


  7. Garsagil1 | | #20

    Thanks everyone!

    CL and Walter, totally agree.

    I bought that fan you pointed in the link yesterday and have it running.

    I think that is worth a shot.

    The problem right now is the humidity. Introducing hot humid air Vs cold air inside the house, I have right now a battle. AC units reducing the temperature and need extra dehumidifiers to keep the humidity in my house below 50%.

    I have "window AC units" that are in the wall (not the window).

    My question now is: if I substitute one window/wall AC unit for a mini-duct AC unit, wouldn't that work as a fan and generate positive pressure?

    If the machine to generate the cold air is outside the house, the AC unit would not generate negative pressure, but positive by only injecting cold air inside the house. So I would have the window fan and the cold air, without humidity issues.

    Can you correct me if I am wrong?

    1. Expert Member
      DCContrarian | | #25

      I would only pursue positive pressure as a solution if you have done air sealing to the absolute limit of your ability and the problem still remains. As you are finding, positive pressure introduces a raft of other issues.

      Even if you end up needing positive pressure, the more tightly sealed you are the better. The tighter you are the less air flow you'll need to maintain pressure, and less air means fewer problems.

  8. Garsagil1 | | #22

    What about a machine like this with the hose connected to my house? Positive pressure?

    1. C L | | #23

      You are overthinking this.
      1. Seal all penetrations. To add to my list above, remove HVAC supply and returns and seal between the gyp and the metal duct. Work really hard on sealing. Get a good smoke pen ( - full disclosure I have not used this product) to help locate ALL air infiltration. Any infiltration not on an outside wall can be assumed to be coming from the other unit and even infiltration from outside may be carrying smoke from their unit. The work of finding all the infiltration is time consuming but does not take alot of skill - it is easy. You care the most about this, so you will do the best job as opposed to hiring this out.
      2. Use regular HVAC system to create positive pressure
      3. Only resort to removing ceiling drywall and other significant measures if the above don't work. But the above should work.

      1. What year was the condo built, and is it a high rise or a stick/wood frame structure, and which part of the country is it in?
      2. Are your HVAC systems completely separated? Does that include heat/cool, bath venting, kitchen exhaust and dryer venting? Many condos and apartments have shared bath, kitchen and/or dryer venting.

      1. Garsagil1 | | #24

        Thanks buddy.

        1. The building was built in 1990. It has no insulation between drywall and the wood above, which is the floor of the second unit. This is a first unit, concrete under me. Illinois. Wood frame inside, outside is brick. Only two floors, first and second.

        2. I don't have HVAC, only 2 wall mounted AC units (like window AC units). The heat is through electric radiator.

        Thanks again!

        *I will work on sealing well everything. I do have a smoke pen already. Thanks!

  9. Andy S | | #27

    Another thing to consider is using an air purifier. This would help knock down any lingering smoke, or smoke that comes through when you open the door or window.
    New York Times' Wirecutter did a review on them:
    I'd grab one before mid summer as that's when the fires start in the West and then good luck getting anything!

  10. Walter Ahlgrim | | #28

    I say forget about the weather turn on the fan and fix the leaks. Yes when you are done the AC will run for a few hours. I say it is money well spent and it will be a drop in a bucket compared with the operating costs of keeping your unit pressurized My guess is you will need more AC and heater capacity to condition all the air you will be brining in. More or less it is like having an open window 24/7/365 it will not be cheap.


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