We just moved into a rental home. It turned out after living there for a few weeks that we discovered the vents were innudated with mold. They were so bad the landlord had to replace all the vents in the home both upstairs and downstairs.

Toward the end of the job, the guy in charge of the job told me that a major cause of the mold was likely a bathroom floor vent close to a shower. Water dripped slowly over time into the vent and pooled in the vents causing mold to flourish. Now there's a couple exacerbating factors. The AC only runs in the summer. So a few drips during the summer were not a big deal. They would dry up as the AC ran. In the winter, though, the home doesn't use the AC vents. Instead radiator heating is used. So water pools from the vent, but no AC comes on to dry up the water.

Additionally, the bathroom has no exhaust fan. So with the door shut, the room gets rather steamy. I imagine that in the winter, the hot steam can easily travel down the vent and condense underneath the home. This might be a bigger cause.

What can we do to deal with this situation? How can we keep our vents dry and still take showers in this bathroom? Keep in mind we're renters, so we have to ask for everything. It seems likely that the mold problem will repeat unless we can find a way to deal with this problem.

2 Answers 2


Cover the vent

They sell magnetic vent covers at nearly all hardware stores. Buy one, and cover the vent when it's not being used. This will prevent moist air from entering the vent, though it may not stop water if it pools around the vent.

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Install an exhaust fan

Install a properly sized exhaust fan in the bathroom. Let it run during showers, and for about 20-30 minutes after showers.

WARNING: Make sure you vent the exhaust properly.

Sizing an exhaust fan

Exhaust fans are sized using Cubic Feet Per Minute (CFM), so you'll have to first figure out how many cubic feet your bathroom is. To do this, measure the length, width, and height of the room.

  • Length = 10 ft.
  • Width = 8 ft.
  • Height = 8 ft.

To find the cubic feet, simply multiply these numbers together.

10 ft. * 8 ft. * 8 ft. = 640 ft.3

Next we'll divide the cubic feet by 60, to figure out how many CFM is required to exchange the air one time in an hour.

640 ft.3 / 60 Minutes = 10.66 ft.3/Minute.

It's recommended that the air in a bathroom be exchanged 8 times per hour, so you'll have to multiply our CFM value by 8.

10.66 ft.3/Minute * 8 = 85.33 ft.3/Minute

So in this example, we'll need an exhaust fan with at least an 86 CFM rating.

Quick formula: (Length * Width * Height) / 7.5 = Required CFM rating.

Making sure the fan runs long enough

When installing an exhaust fan in a bathroom, it's a good idea to put the fan on a timer. This allows you to run the fan for the recommended 20-30 minutes after a shower, without having to worry about remembering to turn it off.

Warning: Make sure you get a timer that is rated for motor loads.


Ultimately, your issue is caused by not having an exhaust fan. Ask your landlord whether he could have someone come out and install one for you? You can push this as being a health concern to you and your family (which it is). Also, it's an investment and/or protection for his property. I'm pretty sure having someone come out to clean the vents on a regular basis can be very pricey.

Being this is a rental you can't do much yourself without the approval of your landlord (such as installing an exhaust fan yourself). If he is not willing to install an exhaust fan, try a mini dehumidifier. There are some out there, like these, that will possibly help your situation out.

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