What’s the best way to reduce moisture in bathrooms (from showering), and stop mold/mildew from forming?

Having the exhaust fan on the same switch as the light, seems good, but after a long hot shower, as soon as the tenant leaves the bath, and shuts off the light, the fan would go off too. To fix that, I’m thinking of going with either a timer in the light switch, to keep the light and the fan on after someone leaves the bathroom or replace the fan with a moisture sensing one.

Any thought on reducing bathroom moisture from showering? Any ideas on fans, moisture sensors, switches, timers, etc. to reduce moisture?

BTW, I rent to groups of students, so it’s not reasonable to expect them to set a timer when their done taking a shower, for example. And the bathrooms I’m working on now, are 40 square feet (8 foot ceilings), and I’m thinking of replacing the 50 CFM fans with 80 or 100 CFM fans. Thank you!!

We hard wire them to the light switch. Will they turn it off? Yes, but they can also fail to use timers or turn them off.

I hate the sounds of bath fans. I never run the bath fans. My parents never ran their bath fans. I lived for 30 years in Seattle (high humidity). I have never had mold or mildew in my bathrooms, even in windowless bathrooms.

I do like fresh air and will open windows on a sunny day.

Thanks S i d, that’s a great point: no one will be happy, in the winter, to dump 20 minutes of heated air outside, after brushing their teeth. Makes me think a settable timer, for both the light and the fan (they’re on same switch) would be the way to go. I’d put a placard by the switch saying: “please set timer for 20 min after showering. Thanks.” I’d guess 80% of my student tenants would do it. I’m thinking the “Lutron maestro 600-Watt Light/3 Amp Fan Timer” might do the trick.

RathdrumGal, I agree there’s options other than fans to reduce moisture, but my student tenants, who are wonderful, on average, have other things on their minds, and I need a solution that doesn’t require much effort on their part.

Roy, I’ve got baseboard heat in the bathroom, but one of the problems is, on this floor, I have one shower servicing 5 bedrooms, that could have up to 9 students living there.

JB, what you described, a built-in timer-switch, is pretty much what I’m thinking. I’m thinking though, a little “instruction placard” next to the switch would be nice.

Plenty and gevans, either solution sounds good, but one problem is, the room fan and light are on one switch. I looked at the Q & A of the "Decora In-Wall Humidity Sensor & Fan Control", and it does not recommend controlling a fan and a light with this humidity sensing switch. As for the, humidity sensing fan, I'm thinking I'd also run into the same problem, with the fan and light on a single switch. At this point, it seems like the timer switch, controlling the room light and fan, is the way to go.

If you like the humidity sensing fan, why not separate them into two separate switches?

Another partial solution is to trim the door on the bottom a bit more. That lets a bit more air circulate.

I'm sure this will cause more trouble than it's worth regarding noise and smell, but what about putting vent holes in the top and bottom of the bathroom door covered by vents so that when the bathroom fan is on, it sucks in air from the rest of the home into that room and would create greater flow. Also, when the main house AC is running, it would pull air from the bathroom if the door was shut. You could install a larger AC register and duct in the bathroom to up the pressure in that room forcing the humid air out into the house. Maybe install a return outside the bathroom door to help the flow. That way, both bathroom fan and HVAC system would help dissipate the humidity.

This is a theory, but I welcome experts to comment on the validity of my theory.

I recently installed 2 vents in a water heater room that was getting moisture on the walls and mold. They were 4 inch circular and I put one toward the bottom and one at top -- made of pvc. Seems to have help stop the mold.

Well, there was no hound at the house :-), so I used a tape measure, and found the space between the door and the floor is 1/4 to 3/4" so I'll shorten the door to give a 1" gap. For the 30" door, I figure that will add 15 sq*in of ventilation. For comparison, two 4" vents gives, 25 sq*in

As for the toilet paper test, the fan did hold a piece against itself, but I guess I didn't learn much more than the fan is spinning the right direction.

I have a similar situation. No fan in bathroom. Had someone come there and he said an exhaust could not be installed in that bathroom.

The only mold I've gotten so far has been in the shower. So after the first tenant messed it up I had it regrouted and now I'm using a grout sealer at each turn over. And charging them if they leave it a mess.

Moisture activated fan does not require a switch!

Panasonic Wisper fit 100 makes a low scone (quiet) 100+ CFM fan with built in timer to keep fan running for set time after . Make sure to get the one with the light

the timer setting is actually done after a whole house energy audit is done to determine the necessary 3.14 air changes an hour

with that many kids you definitely NEED it

Federal Weatherization standards REQUIRE it.

I believe in NJ you can get your local electric company to do the whole house energy audit and get a massive subsidy -50%- to pay for it all

This morning my electrician installed a 70 CFM fan, replacing a 50 CFM one. My handyman will shorten the door for 1” from door to threshold. This was spurred by Busy and myob’s suggesting to increase the area for venting.

Yesterday, I planned to install timers for the fans, but last night I realized, because my fan and light are on a single switch, a timer could make the room dark, if someone accidently set the timer to 10 minutes, before they took their 15-minute shower.

I think connecting the fan and lights to a single switch, as I have, is a good, buy not quite a guaranteed way to make sure the fan is on when someone is taking a shower. I didn’t see any easy way to add moisture sensing to that arrangement.

Well, thank you all for taking the time to share your thoughts. I think I may be implementing some of the more sophisticated moisture removal techniques suggested, in my largest house, which will be rented to 15 students (legal limit and my personal record :-) this year. Thanks again!!

I am using the DewStop condensation bathroom fan switch. You can set the moisture level and the time for the timer portion. Works very well. Here is the link to the switch at Home Depot.

Hoosier, I think because my houses are not in the "party central" area, I don't draw that type of crowd. However, If there is a giant party, and the police are called, and I'm woken at 3am, I'll let you know :-)