I don't care about volume. Hear me roar! Or not.

I have a bathroom that gets really humid really fast with exhaust fan running. My problem is that the humidity sits behind the mirrors & then mold grows behind them. Will a larger CFM fan fix this issue?

Some Cabanas have a wet towel hanging area Outside the wet area that helps a bit

Pelonis portable floorBox Fan 20 Inch @ 2295CFM .. $50

Panasonic FV-30VQ3 Ceiling Exhaust @ 290CFM .. $235 + new vent hose OUT the roof/soffit
on a timer or humidity switch

Extreme remodeling problems have considered a lighted kichen hood over lavatories @ 900CFM+ .. $300+

Switching to faster drying tile surfaces may help .. $$$$$$$$$$$

Rare cases of louvered (passive venting) door to hallway have helped

will.mcg said:

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I don't care about volume. Hear me roar! Or not.

I have a bathroom that gets really humid really fast with exhaust fan running. My problem is that the humidity sits behind the mirrors & then mold grows behind them. Will a larger CFM fan fix this issue?

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Could you perhaps provide some clarity as to the source of the humidity? Is it when the shower is running, or like on any ordinary day you just turn it on and BOOM! the humidity goes up from some unknown source? Knowing what is producing the humidity might help get you more effective solutions.

Ah, yes.......my original post makes it sound like the humidity exponentially increases when the exhaust fan is turned on. That is not the case..

Humidity generated by the shower running. Existing exhaust fan doesn't touch it during the shower.....apparently doesn't get it all sucked out before it has time to cling to the walls.

A few thoughts:

As someone has mentioned they make switches that detect the sudden rise in humidity and turn on. This should sense the shower running and kick on.

You can also swap to a timer switch. It's common to have 4 times on one switch (15, 30, 45, and 60 min).

There is a formula you can google for the recommended minimum number of air exchanges to keep humidity down. You enter the volume of the bathroom and it spits out the minimum CFM your fan should be. You can always go a little bigger.

Double check your fan exhaust path is not restricted and also not vented straight into the attic.

tgivaughn said:

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Panasonic FV-30VQ3 Ceiling Exhaust @ 290CFM .. $235

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Once you get past the $60 Broan fart fans the outlet goes up to a 6". Most houses only have a 4" flex duct/hose/connection. You end up fighting the static pressure.

Do you have HVAC supply air in the bathroom? I added a small one to our old hall bath. Eliminated musky and other odor issues.

We have added a register to this part of the bathroom & it does help.

Thanks everyone In going to measure & figure out if my fan is undersized or not.

will.mcg said:

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I don't care about volume. Hear me roar! Or not.

I have a bathroom that gets really humid really fast with exhaust fan running. My problem is that the humidity sits behind the mirrors & then mold grows behind them. Will a larger CFM fan fix this issue?

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How many square feet is the bathroom and what height ceilings ?

Take the grill off the fan now and look up the model number and see how many cfm the existing fan is rated for

Have you followed where the fan exhausts to? Maybe there's an issue in the attic preventing proper operation…

Turn it on a couple mins before the water so it starts moving air

Koko Chingo said:

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A few thoughts:

As someone has mentioned they make switches that detect the sudden rise in humidity and turn on. This should sense the shower running and kick on.

You can also swap to a timer switch. It's common to have 4 times on one switch (15, 30, 45, and 60 min).

There is a formula you can google for the recommended minimum number of air exchanges to keep humidity down. You enter the volume of the bathroom and it spits out the minimum CFM your fan should be. You can always go a little bigger.

Double check your fan exhaust path is not restricted and also not vented straight into the attic.

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Ours runs 18-20 hours a day. Gonna have to disconnect it this weekend.

I have heard horror stories of the humidity sensing switches, especially in the swamp know as Houston.

I like timer switches, hit 45 minutes when you turn the water on and don't worry about it.

I like the timers as well.

Oh man. I can see that here. I put a timer in the kids bathroom here in College Station. The house I had while stationed in Tucson had the senor and it worked great.

Brings up memories.

The master bedroom in that house had carpet. Especially in the winter, when you walked across the carpet, you made sure not to touch a screw when you turned turned on the light. If you did you would see the arc, hear the snap, and get that good ol Tuscon static shock. You also had to touch and hold metal as you got out of the car to avoid getting zapped. You could feel the static build as you pushed a shopping cart in the store. Yea low humidity.

Can't find a model number but it looks just like a Broan 70 CFM fan. In the attic the fan box has a 4" diameter plenum going into a 6" diameter duct hose that goes up to a vent in the roof. No obstructions. I think the fan is just undersized for the combined 90 square foot room. We used to keep the door shut in the bathtub/shower toilet room but the ceiling molded so then started keeping that door open & when possible the door to the hallway open. I'm just going to try a higher CFM fan & see if that yields noticeable results.

I would try a 110 cfm version


https://www.amazon.com/Broan-A110-Invent-Single-Speed-Ventilation/dp/B016UQ2YME/ref=mp_s_a_1_2_sspa?dib=eyJ2IjoiMSJ9.ROGgkyqjYfQlCVE2Anm-zqjcQau6qT2TljfR2roLWI7uAvVuxBprKaEmGbt5gB4sgtp0hNSs0O4rbvZ0Ga-Bie8ivEK7PxAB1gBzsfyas9jvmyN5KKdSUSgTr-lR-rq3-S7FQSB43vFNTDDsYTrF1lZXzgT5pNPFHhv9PYlxSZNaz2b_DzEmWTGpRp9YG3bfeVqs9Rk-jl_vD0Y5pOLarA.haRTzWT2KnwDwq7U4HzqYwr3TGU3gzNSSIFTGiZGzXY&dib_tag=se&keywords=broan+bath+fans&qid=1756748581&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9waG9uZV9zZWFyY2hfYXRm&psc=1


A general guide is 1 cfm per sq ft for bath venting

Thanks. Yes I read you should add 50 CFM for a shower/bath that's in its own part of the bathroom.

Just upgraded bathroom fan 3/3 with a timer this weekend, the housing for this one feels a bit cheap but works great:

https://www.homedepot.com/p/Leviton-Decora-15A-60-Minute-Indoor-In-Wall-Countdown-Timer-Switch-1-2-HP-9-8A-Fan-Motor-No-Neutral-Required-Single-Pole-White-R02-DT160-0LW/325797188

It can also supposedly operate without a neutral wire present, though all of our boxes had one available so this capability was not needed in our case.

I've concluded the existing fan is a 50cfm. The box is too small to accommodate an upgrade kit. It does suck some TP to it but have to be within 2". I'm going to put a 110cfm fan in there & see what happens. Thanks for all the input.

Yes 50 cfm is definitely undersize for 90 square feet

THEN in that case, replace with 6" so things will work besides, all moot concerns
" Can't find a model number but it looks just like a Broan 70 CFM fan. In the attic the fan box has a 4" diameter plenum going into a 6" diameter duct hose that goes up to a vent in the roof. " posted later

Actually I think the original was 3" into 4" duct & the replacement is 4".