I have a walk-in shower, roughly 2-1/2 x 5 ft, 7ft high, with a shower-curtained opening. The mildew problem on the grout is pretty bad. I've seen some good articles here on controlling mildew, but I also want to install a fan. It'd be nice to not have to mess with the tile at all, and fortunately there is a hole in the ceiling for a 6" recessed-light can. So I'm thinking this fan would fit there nicely:


It is rated for wet locations, as long as protected with GFCI. I don't think the current recessed light is GFCI protected, but I can add that easily myself. Accessing the area above the shower is going to be somewhat of a pain, but doable, though requiring some drywall repair.

My big question is about the fan exhaust: I don't want to run it to the outside, as it's going to chill the shower significantly, I fear. So I'm thinking about running it into the neighboring bathroom area instead (an area with a dual-basin sink vanity, which is open to the MBR) or maybe just into the bedroom itself. Is this a horrible idea ? I figure most of that moisture ends up in the vanity area and bedroom anyhow, so maybe not so bad. Though the fan exhaust is necessarily going to be just below the ceiling (since the shower ceiling is only 1ft lower than the other 8ft ceilings), unless I go to some heroic measures to have the vent come out lower somewhere. Though I'm in the humid south, the house interior tends to be pretty dry, with the AC running a lot in the summer and wood heating in the winter.

Or do I just need to bite the bullet and figure out how to vent the fan to the outside without chilling the shower too badly ?

  • Do you have HVAC ducting to distribute a/c and heat? Our 2-bath tract house in Dallas TX is 48 years old and the bathroom vent fans (original) exhaust into the well ventilated attic. We have a central HVAC duct into each bathroom. I don't see any mold or deterioration in the attic. Where in the south are you? Nov 6, 2018 at 11:17

3 Answers 3


You should vent to the outside. Bathroom fans are exhaust fans, why would venting to outside chill the shower? Presumably the fan unit has a flapper to prevent cold air coming in when the fan is not on. When the fan is on it will be pulling air from under the bathroom door and through the HVAC vent if there is one in the bathroom.

I think normally the bathroom exhaust fan is in the ceiling of the shower outside the shower. If it is on a timer, running the fan for 15 or 30 minutes would dry the shower enough to prevent excessive mold.

If the grout has long established mold, you might have to use bleach on the grout and then seal it.

  • Naturally when the fan is on, cooling won't be a problem, as you suggest. And I'm sure the fan has a flapper, as will likely the venting hood I'd install where the duct goes through the outside wall. But neither of these are very tight. Nov 6, 2018 at 0:19
  • 1
    Do you already have a fan elsewhere in the bathroom? If so, I would definitely not install an exhaust fan in the shower enclosure itself. If you pull the shower curtain back to one end after showering, that should allow the walls to dry fast enough to prevent the growth of mold. Of course, then mold will grow on the shower curtain or its plastic liner necessitating frequent cleaning of the liner. We have an 8 ft ceiling in the shower so there is 18 in between the shower rod and the ceiling. I pull the curtain closed after showering. Consider a wall mounted circulating fan blowing into shower. Nov 6, 2018 at 11:00
  • No other fan in the bathroom. And it's not really a bath "room" per se. There is an area open to the MBR with a wide vanity and his/her sinks. To one side is a toilet stall with a door. To the other side is the opening into the walk-in shower. Yes, I wonder if this will help enough with the mildew to be worth the trouble. Nov 6, 2018 at 20:45
  • Does the opening to the walk-in shower have a door on it? Nov 6, 2018 at 20:59
  • Opening to walk-in shower has a shower curtain. Nov 6, 2018 at 22:42

Venting the fan inside the structure would normally be a code violation. Pushing the moist air outside vs. To the next bathroom would bring in the same tempature air so venting inside don't be good for the rest of the home and make no change in the temp in the bathroom. Bite the bullet and vent the air outside, if you find that this cools the bathroom two much possibly a small heater could be added, I don't think I have seen them listed for wet locations but have seen over sink lighting with small 300w? Heaters , I thought these were a good idea compared to the ceiling fan + light + heater. Those never made sense to me. I use hydrogen peroxide at 3% it is a great mold killer and might brighten the grout with the smell of other cleaners. Since well and let dry prior to sealing.


Venting to the outside is the only option that won't result in more/different problems down the road. Bathroom vent fans have a flapper that should almost completely prevent cold air from the outside getting into the bathroom. Not only that, but the exhaust vent on the outside of the house that usually has a flapper too. Perhaps on extremely windy days, you might get a slight cooling, but the benefits really outweigh that minor drawback.

For our house, we went with what our HVAC guy recommended and upgraded our builder-grade (read: cheap and crappy) bath vent fans with some heavier duty ones. The key is a LOT of CFM to take that moisture outside. Ours are Panasonic Whisperfit that push 110 CFM. This CFM will make it so you can have the fan in a convenient location (ie, not IN the shower enclosure, which would be bad for what should be obvious reasons) and still get the humidity out of the room. Small fans (30CFM builder grade, for example) just swirl the air around without really moving it.

A fan does no good if it isn't on, so we also replaced the switch for the fan with a countdown timer. You push it on when you get in the shower, and it counts down for an hour, after which it shuts off. This is so it will continue to remove humidity even after you leave the room, because everything is still wet in there anyway. You can get a nice one from any home improvement store for about $30. We went with the Lutron Maestro 5-Amp Digital Residential Hardwired Countdown Lighting Timer from lowes. Another option is adding a switch that detects humidity so it will automatically kick on when it gets too high. I don't have any experience with these though, and the timer works well enough for us.

Yes, venting to the outside sucks, but if you want to avoid mold and moisture in your attic, which will cause health problems, rot your wood, and shorten the life of your roof (as the sheeting gets moisture), it's really the only option.

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