If the exhaust fan was not turned on until after the shower is complete (say, 30 min. after the water is first turned on), is that enough to cause mold/mildew/paint issues? The mirror is foggy during these 30 minutes.

Or is 30 minutes of humidity per day a short enough period to avoid any negative effects?

  • 2
    Depends on the specific conditions. Certainly doing it once shouldn't cause a problem, but doing it daily for years could result in several problems.
    – Hot Licks
    Mar 29, 2020 at 14:06
  • One problem I've run into is fungus growing on the ceiling paint. Had to repaint with an anti-fungal additive in the paint.
    – Hot Licks
    Mar 29, 2020 at 19:40
  • It probably depends on the construction as much as conditions, to be honest. I've been living in brick/concrete buildings for decades, and none of those had any fan in the shower/bathroom at all. Yet it didn't cause any significant problems.
    – Alice
    Mar 30, 2020 at 11:56
  • 1
    There is no complete answer to this question. It depends entirely on your climate, the time of year, and the construction details of your specific house and bathroom as well as your own personal habits living there.
    – J...
    Mar 30, 2020 at 15:29
  • You're asking a question about what will happen if you take a particular action. The best way to find out is to try it and see what happens. There's no substitute for experimentation! Mar 30, 2020 at 17:35

5 Answers 5


Condensing water is never good. The vapor will creep in places where it may not quickly evaporate once it has condensed. It will condense easily but dry slowly in cold places and badly ventilated corners (like around the window) and on a cold floor. That means that these places stay moist for longer than one would think. Take a "finger test" in such places an hour after you have finished.

If water vapor condenses between layers like under wallpaper, window paint, between plywood layers or similar, it will start to do progressive damage there, even if it's only present for an hour each day.

Getting the vapor out immediately is much better. It will prevent some of the condensation from happening in the first place, which is much preferable.

  • Especially in the interior of exterior walls, when its cold outside. This can rot out the sheathing of the house + mold etc. Mar 30, 2020 at 17:10

Not a big problem but moisture is almost never a good thing in a house unless you live in a very dry climate. Why not turn the fan on first thing? In addition to mildew, moisture isn't good for medications, for example. A number of rx medications will state not to store them in your bathroom for that reason.
Always better to keep the air moisture low.

  • As to why not turn the fan on first thing, it gets a bit cold as the exhaust fan pulls the heat out.
    – kostmo
    Apr 15, 2020 at 0:47

30 minutes is not an issue as long as it drys out in between. I live in the Pacific Northwest and know such things... You'll be fine running the fan at the end

  • 4
    In case anyone was wondering, PNW = Pacific Northwest (Oregon, Washington [the state, not the city], British Columbia, northern California, Alaska panhandle).
    – Vikki
    Mar 29, 2020 at 15:32
  • 2
    @Sean, and specifically the parts west of the Cascade Mountains. The east side of those states is a fair bit dryer, and excess humidity is generally not a problem.
    – Mark
    Mar 29, 2020 at 22:33

I grew up in a wood house with a wooden bathroom that was well-used each morning. We saw first-hand the effects of rotting wood from moisture. I also rented from landlords that didn't take care of their property, causing black mold to form from excess moisture.

The primary concerns are the following:

  • Mold
  • Mildew
  • Rot
  • Hydration of permeable substances
  • Warping of wood (structure, wooden decor or wooden instruments)
  • Rust

These effects depend on the bathroom construction, the ambient relative humidity, ambient temperature and the humidity generated by the shower.

- - - - - - - - - - - - - - - - - Problems - - - - - - - - - - - - - - - - -

Mildew and Mold

Mildew and mold can each cause a host of respiratory/health issues. They are not something to mess around with. They thrive on moisture and darkness, and if you don't ventilate your bathroom, you may be inviting mildew and mold.


Moisture can seep behind wallpaper and thin or cracked paint to rot the wood behind the surface. Wooden floors can begin to slowly rot. If moisture in the air is insufficient to stay around for long, condensation on some surfaces can trickle down and pool in cracks and crevices to cause more damage. You can generally expect the highest risk of rot to be where the most moisture is -- in the bathroom -- but it could also be in an adjacent room or one connected by an air vent. If the wood in your house is rotting, it can lead to expensive repairs, can attract termites, and can cause structural damage if unchecked for too long.


Wood can warp when exposed to moisture and temperature gradients (more info). This generally won't be a problem for structural wood, but it could be an issue for wooden instruments or other wooden decor -- not only in your bathroom but also elsewhere in your home.


Most people don't have many things in their home near a bathroom that would be subject to rust, but I wanted to include this to be more comprehensive. Things that might rust include knives, cast iron cookware, art & decor, and more. Usually this won't be a problem unless the moisture levels are too high.

- - - - - - - - - - - - - - - - - Causes - - - - - - - - - - - - - - - - -

All of the above problems are caused by excessive moisture in one way or the other. Here are some of the factors that can affect those problems.

Construction Materials

If you have lots of wood in your bathroom, rot may be a concern. Moisture can seep behind wallpaper and begin to very slowly rot and/or warp the wood behind it. If there are surfaces where water can condense and pool, these are especially areas of concern. Other materials may also be vulnerable to rot, mildew and mold.

Ambient Temperature

The cooler your house is compared to your shower, the lower the dew point and the more condensation there will likely be on the surfaces of your bathroom and house. The more moisture condensates in the bathroom, the less there will escape into the rest of your home in the short term, but you should expect it to distribute.

Relative Ambient Humidity

If you life in a humid place, you might need to worry about mold/mildew forming from the moisture, or rot developing. If you live in a very dry place, you might be able to use the shower as a humidifier and not even use an exhaust fan. Mold/mildew/rot will not form as quickly in dry environments. Many houses in humid climates will have bathrooms that are better designed to withstand moisture, but don't count on it, especially for older homes.


None if you open your window for few minutes after you shower. In winter also keeping the door open is a good choice because heated air inside home is usually dry enough to remove the moisture.

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