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Community and Q&A

bathroom vent sucking in attic air

reggie99 | Posted in General Questions on


I moved into a house a few years ago and installed a bathroom vent. It previously functioned with just a window, but it would create a hurricane with the shower curtain when left open while showering so I decided to try upgrading to a venting system.

Venting works fine but every time I turn on the vent the upstairs bedrooms get filled with attic dust, has an odd smell and makes my lungs and eyes super irritated. If I crack open any window in the house and run the shower vent I have no issues. I also found out that running a window fan in the basement will suck attic dust into the upstair bedrooms if I dont crack a window open.

Is there a way to control where the bathroom fan sucks in air? At the moment Ive just been cracking open the bathroom window when the vent is running and Ive had zero issues. However 2 times this year I accidentally ran the fan without cracking open a window and had to vacuum/air purifier the bedrooms.

I live in a 2 story cape cod with kneewalls. I ran a window fan in the bedrooms with the door closed and the air/dust seems to be sucking in from where the wall meets the floor. The bedroom and kneewall attic share the same floor boards, its just separated by a wall. The walls are batt insulation while the floor is some type of cotton ball like blown fiberglass insulation.

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  1. brad_rh | | #1

    What 'venting system' & bathroom fan do you have? It sounds like a lot of CFM if it's sucking in that much dust. You might need to look at air sealing inside your home.

    1. nynick | | #2

      I was thinking the opposite. Maybe he needs a fresh air supply.

    2. questiondude99 | | #3

      Sorry didn't realize someone replied. I believe its revent 50 cfm.

      Only times I have issue with the dust being pulled i to the bedroom is if I run a window fan upstairs or run the bathroom fan without opening a window. I did manage to flood the house with dust when I ran a large lasko box fan in a basement window. I closed the upstairs windows and forgot it was running so it sucked in air on MAX for 12 hours. Took about a week to fully remove all the dust.

      I've run a window fan in the bedroom and taped the door closed to see where the air is coming from and its being sucked in from where the wall meets the floor. After a few hours there was a decent amount of dust piling up by the edges of the baseboard all around the room.

      1. Expert Member
        Michael Maines | | #4

        It sounds like you need to do some air-sealing between the attic and living space.

        1. questiondude99 | | #5

          I'm honestly thinking I might just need to replace the batt fiberglass/blown fiberlgass in the kneewall with spray foam but thats going to be expensive. I already went into the attic a few times with great foam cans but it still seems to be creeping in from somewhere. I think sealing the subfloor to the baseboards probably would help the issue but that would require removing all the carpet yet again.

          I'll continue to leave the bathroom window cracked during operation and just save up money. The bathroom fan is still nice because it used to just be a window and it caused mini hurricanes in the shower when I left the window open for ventilation.

          1. Expert Member
            Michael Maines | | #6

            Kneewalls shouldn't be insulated anyway, the roof slope should be. More info here:

            To seal the joint where the kneewall air is getting into your living space, you could remove the baseboards, caulk the gaps and reinstall the baseboards.

            Canned foam doesn't do a very good job of air-sealing in many cases. It's good for contained spaces, such as around windows and in holes, but it's not meant for narrow linear gaps. Caulking does a much better job.

  2. questiondude99 | | #7

    One last question. This dust issue became a problem after I hired an insulation company to block off the floors of the kneewall from the bedroom and add blown fiberglass to the floors and ceiling. I'm told by mutiple people that blown fiberglass isn't dusty but when I roll the blown stuff that was added between my fingers, it evaporates into pixie dust. The wad of fiberglass will dissapear and my fingers will be sparkly. Is that normal?

    I have batt insulation in my basement joists and the only problem I have downstairs is pollen when I run the dryer for extended periods of time during the spring/summer. I can constantly feel cold air leaking into the basement but it hasn't causes any problems like the upstairs has.

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