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Community and Q&A

Achieving Negative Pressure

Lindaloowho | Posted in General Questions on

I noticed a few articles on GBA regarding setting up negative pressure environments for the removal of mouldy building materials. What do I need to know about air supply/air exchanges with a longer term negative pressure set up and what are your suggestions for the set up itself?

Thanks so much!

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  1. Expert Member


    Can you flesh 0ut your plan a bit? Why do you want negative pressure, which will draw outside air thr0ugh your building envelope into the house?

    1. Deleted | | #3


    2. Lindaloowho | | #5

      I need to remove mould from the washroom. It’s a complete gut and may take a few days to complete. We may need to sleep on site during this time.

  2. Expert Member
    BILL WICHERS | | #2

    Negative pressure has to be measured as a comparison with something else. Usually on GBA, negative pressure is used to either depressurize a place with a smelly spray foam install (like an attic), or in reverse as positive pressure to help with something like a neighbor's smelly pot smoking. Negative pressures keep stuff from leaking OUT of a space. Positive pressures keep stuff from leaking IN to a space. Where things are leaking to or from can be an adjacent living space or the outdoors depending on what you're trying to accomplish.

    Let us know what it is you're trying to do here. If, for example, you have mouldy stuff in a wall, you might actually want POSITIVE pressure inside your home so that any mold smell gets exhausted OUT of your home. If you're worried about an attic, mainting the attic at a slight NEGATIVE pressure would help keep the mold from getting into your living spaces. In both cases in my examples, the positive and negative are referenced to the OUTDOOR air pressure. You can usually set up the pressure differentials with fans since it doesn't take much of a difference in pressure to affect this stuff.


  3. 1869farmhouse | | #4

    I have a small amount of exhaust only ventilation in my otherwise very well sealed crawlspace because I want that space to have have negative pressure in relation to the living space. Generally it’s a portion of the home, not the entire home.

    1. Lindaloowho | | #6

      Hi Austin,
      How do you create an exhaust only system? Mechanically? No air will come in from outside at all?

      1. Expert Member
        BILL WICHERS | | #8

        All you need is a fan blowing out. If you use a bathroom as an example, turn the fan on with the door closed. The fan pulls a slight negative pressure inside the bathroom, and air comes in through EVERY leak -- under the door, through the vent, around the window, through electrical outlets, to make up the air that the fan blows out. Since air is coming IN through EVERY leak, there is no possibility for anything to leak OUT anywhere. That's how it works.

        You can do the same thing in any room you want, you just need a fan to blow out. If your concern is mold in your washroom, run the fan (if there is one), or put a fan in the window (if there is one), with the fan blowing OUTDOORS. You don't need a hugely powerful fan, either, even a small one is usually enough.

        If you don't have any way to vent a small fan, you can rent fans with flexible ducting that you can run to wherever you can find a way out. These fans are common in the utility industry for work in manholes, but they do have other uses too. Most rental places should have them available.


      2. 1869farmhouse | | #11

        Exactly, I’m only using 70 cfm, but the crawlspace is very well sealed. I have an exterior door inside that leads down to the cellar, and it’s always under negative pressure when I open it.

  4. the74impala | | #7

    I removed mold from a small section of my basement one time. I just used a box fan and a roll of 6 mil poly. I taped the poly around the edges of the box fan and taped the seam going out to the basement window into the yard. It made a nice tube away from the house. If you are using this a couple days, it would be a big suck on your HVAC efficiency unless you have 2 windows and a door to close. One intake, one exhaust and close the door and the HVAC vent if there is one is one in the room. I guess if only one window, the air will come from elsewhere in the house, but it will make a difference if it is snowing where you are or at 70° for an overnight low.

  5. Jon_R | | #9

    If you have mold in a room that you don't want to get to the rest of the house, you can put a small fan (blowing outward) in a window in the room.

  6. Lindaloowho | | #10

    Thanks everyone! Much appreciated!

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