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Community and Q&A

Dehumidifying a bathroom

dan7210329 | Posted in General Questions on

I have a 8′ x 8′ 2nd floor 3/4 bathroom (shower, toilet, sink, but no tub) with no window.  Paint peals from ceiling and un-tiled upper walls presumably because of moisture from the shower.  I’ve been thinking of replacing a broken ceiling fan that currently vents to the unfinished 3rd floor (not to the outside).  Venting to the outside is a poor option: it introduces an air gap in conditioned space, and a solid standing seam metal roof and soffits that are enclosed at the top of 20″ stone walls make venting a difficult choice.  Instead, I am considering a small wall/ceiling dehumidifier unit in the bathroom – one that would remove excess moisture and pump drier air back into the bathroom.  Would this work?  Would a dehumidifier rather than a vent be consistent with code requirements?  

Would a small mini-split HVAC system be a better choice than a dehumidifier or just more expensive?  The house is heated from an modern oil fired boiler connected to a radiator hot water system; 2nd floor air conditioning is a high velocity system.  

Are there passive methods for dealing with bathroom humidity, that do not entail venting?  For example, if the entire room were tiled, and wet surfaces were wiped down after each hot shower, would that protect structures from excess humidity?  You hear of indoor saunas:  how do they manage heat & humidity?

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  1. user-2310254 | | #1


    You have to control humidity--especially in a bathroom. You might need a mini-split or dehumidifier to ensure overall comfort and air quality, but that is a whole-house issue. For the bathroom (and if you cannot vent through the roof), I think the best and cheapest solution would be a high-quality, through-wall bath fan.

    1. dan7210329 | | #4

      Steve, Thanks for you response.
      A high velocity air conditioning system with the air handler unit on the 3rd floor conditions air on the 2nd & 3rd floor. Perhaps HVAC contractor would add a intake vent to the ceiling of the small bathroom, so humid bathroom air would vent to the AC unit? Should the AC be allowed to run in the winter?
      The house has 20" stone walls. A vent through exterior stone walls would compromise the historic look of the house, so I'm looking for alternatives.

      1. user-2310254 | | #6

        Hi Dan,

        You don't want to recirculate bathroom air inside your home. Venting it into the attic would be a really bad idea. On the HVAC, many thermostats have an automatic mode that switches the system between heat and cool as conditions change. But ventilation fans are needed to "vent" indoor humidity and air pollution. Can you vent through the roof? That's what most homeowners do.

        I suggest reading this article (and the other pieces in the sidebar):

  2. charlie_sullivan | | #2

    Where code requires bathroom venting, I don't think there's any provision to use dehumidification as an alternative. But I doubt anyone is going to come enforce the venting requirement, so I imagine you have leeway to do that if you want to.

    Part of the reason for exhaust fans in bathrooms is smell, as well as humidity. There's a possibility that a charcoal filter would take care of that aspect, and maybe could be run as needed independent of the dehumidifier.

    As far as reducing the source of humidity, if you make the shower enclosure more complete, for example by adding a dome on top of the enclosure (, you can keep the humidity inside the shower and then have a limited surface area to wipe down afterwards. As a bonus, you can use less hot water because the warm humid enclosure keeps the human comfortable with lower temperature water. And you stay comfortable even when the water isn't running, so you can more comfortably shut off the water flow while soaping up.

    As far as your concerns about adding air leakage to the building, some people use a heat recovery ventilator with exhaust in the bathrooms and boost the airflow rate after a shower.

  3. Expert Member
    Michael Maines | | #3

    Dan, you need to vent bathrooms to the exterior. When you have peeling paint, mold is sure to follow, if it's not already there. A dehumidifier would need to be huge in order to deal with bathroom moisture in a timely manner. If you're concerned about the energy loss you could use a balanced system, such as Lunos Ego. If the path to the exterior is long you can add a booster fan. Indoor saunas I'm familiar with are dry, but steam rooms need special detailing and a vent fan to prevent moisture problems.

    1. dan7210329 | | #5

      A veritable steam room - yup that's what I got here! Thanks for all your comments. It left me wondering if it was wrong for the previous owner to have installed this shower (1960's?) - not adjacent to an exterior wall, without proper exterior ventilation.

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