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I have a room mate who loves to just sit in the shower tub with the shower raining over him. He sits in the shower for 2 hours or longer easily (not over exaggerated), just watching youtube videos. He's also taking steam showers. He showers very very hot, producing a lot of steam and causing water to literally drip down the drywall.

The fan in the bathroom simply cant work that hard and suck up all the moisture fast enough. I have told him many many times to take a bath instead because we're guaranteed getting mold development, but he gives me an excuse that he prefers the rain showers.

If the bathroom fan is on AND the bathroom door is open, keeping the temperature in the bathroom cooler, does that resolve the moisture/mold problem? Or are you guaranteed getting mold no matter what if you steam shower and shower that long?

I am also not sure if leaving the bathroom door open would be a good idea, potentially having the moist air going outside of the bathroom as well.

We constantly argue over this and we're both adults so I can't really do much to him in terms of treating him like a child and shutting off the main water supply or something like that so I'm really looking for a second opinion that I can show him.

A compromise would also work, such as a fan on AND bathroom door open. If that's the golden solution to prevent mold with long steam showers then that's acceptable for me as well. He is paying for the extra heat and water charges anyway. I just don't want mold in this still brand new house.

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    There is zero chance someone who showers like that is willing to have the door open. And they can't anyway if they want steam to build up. If they're going to anyway, might as well confine it to the bathroom, and paint it with Zinsser's or Kills' special mold bathroom paint when necessary. Or jump the gun and paint it now and forget about it.
    – Mazura
    Dec 20, 2020 at 2:17

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A you hear all of the time mold needs moisture to get moving.

But, it also needs time, and a food source.

The Drywall and most surfaces give it that food source, but the time and moisture you can control.

If the house is new, the fan should be fit to size. I say this, but sadly it's often not true. Also, if the roommate likes steamy shower rooms, a good fan and an open door will likely remove the "steam room like" environment.

So, my opinion-ated answer based on these thoughts & some known facts:

Fan & venting:

  1. Ensure the fan is the right size for the room. Please see what it is rated at, increase it to a higher cfm fan if need be.
  2. Check the vent tubing, it should be rigid, not ribbed flexible like most contractors and builders, and homeowners use.
  3. The shorter and straighter the run for the vent tubing, the better. Use 6" vs. 4" even. Also, make sure it is properly supported as moisture has to get out asap! this is the key really. It might be full of water, make sure it is not obstructed

The walls, cracks and crevices are where mold will grow, so a circulation fan in conjunction with the vent fan is a good idea. The home will need a source of fresh air or circulating it in the living space works, but is worse. Your best bet is to get it out of the house. A fresh source, (even opening a window a crack will suffice).

After the shower, wipe the walls down, keep the vent fan running for a while. you can install a timer too to control it and not have it run 24/7 (although that might be what you will need).

The sooner you keep the surface dry, the mold wont grow.

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  • Thank you for your answer. This bathroom has no window unfortunately. It is in between 2 bedrooms. I noticed that when the door is left open (also with the fan on) the mirror doesn't fog up at all. So I was hopeful of that solution, but do you think that will be enough? My homebuilder told me to do the "2 sheets of toilet paper" test with the fan. If its strong enough to hold 2 sheets of toilet paper its fit for the size of the bathroom?
    – Martin
    Dec 20, 2020 at 1:47
  • I would look at the rating of the fan cfm, and the tubing as I mentioned. "can it suck" is not the same as "cant it remove moisture effectively and properly"
    – noybman
    Dec 20, 2020 at 1:57
  • Another point is understanding attic air temps inducing condensation in the exhaust tube. Ideally, the attic air temp should be the same as the outside air temp. This means cold winters can induce the moist air in the exhaust tube to condense before evacuating the house and possibly flowing back toward the exhaust fan. Be sure to effectively insulate the exhaust tube to minimize the potential for condensate to create mold growth in the tube/fan assembly. And as others have said before, CFM will help too. Get it out fast. Closing the bathroom door chokes air flow so the fan can’t pull air. Bad!
    – riffin-rich
    Dec 20, 2020 at 3:04

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