First, I'd redirect the bathroom vent out of the home. Best place is on the gable side of the house, but straight out the roof would also work. Depending on how much condensation you're seeing, and where it travels after condensing, this could be the cause.
Then I'd do some detective work. Start removing the insulation on the other side of the mold. Look for signs of water damage and follow them back to their source. There could be lots of causes, including a leak in the roof, plumbing problem, a clogged hvac drain line, or condensation on uninsulated hvac ducts. Water can travel a long distance before it finally settles on a low spot, so search far and wide.
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.
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.
Best Answer
First thing you want to do is verify that the fan is working. If there is an attic, you need to get up there and check the venting to the outside. If no attic, then remove the fan so you can stick your head up there and see what's going on with the vents. Poor ventilation is a major cause of mold and mildew. All my bathroom vents have 60 minute timer switches.
Next, mix 1 part bleach and two parts water in a spray bottle and spray down the whole bathroom. I don't usually use bleach, I use vinegar because it soaks into porous materials and bleach doesn't. In this case I'd use the bleach: spray, scrub, wipe down, spray, scrub, wipe down. Once everything is clean, wipe the floor down with vinegar. It will soak into the grout and prevent the return of mold. Keep in mind that the walls are probably not made of that plastic. The plastic is a wall covering and there could be mold behind it. If any of the plastic is loose, pull it out and check for mold behind it.
Last but not least, think about replacing all the caulk in the bathroom. Good luck.