Panasonic has very quiet fans. It sounds like it's not actually venting to the outside. Are you sure the ductwork hasn't become disconnected inside the attic? How long is the run to the outside? Your current fan may not be powerful enough if it's more than a few feet. It could also be clogged. Do you clean it out regularly?

The inexpensive low-end flex duct could be enemy #1. The rough inner surface can greatly reduce airflow. I usually use rigid metal duct, then insulate it. Not all flex ducts are bad. There are good quality insulated flex ducts out there that do a better job than the inexpensive flex duct commonly found at the box stores.

The length of the run, though not mentioned can be an issue as well. If there is any drooping of the duct in the horizontal run, is could be filled with condensate, that can partially or even fully block the duct.

I encourage you to look at a Fantech remote fan. The fans are strong, but the remote mounting will minimize or even eliminate fan noise heard in the bathroom.

If you are used to $60 fans from the box center, the $150-$200 price tag on a good Fantech or good Panasonic fan might scare you off. But trust me, the extra money is worth it for true ventilation performance and a better bathroom environment.

When you size the fan, you don't just size it to the bathroom. You also take into account the type of duct, the length of duct, and the number of turns in the duct. More turns and more length reduce air flow, it's referred to as "duct loss". So you'll need a higher CFM fan to get the desired CFM movement at the fan grill in the bathroom.

Thank you for the response. I am fine with a $200 fan if it will solve the problems. The attic run that is horizontial is a good distance..probably 15 feet or so. It is laying on top of the insulation so there probably are dips and such.

Is it ok to go horizontal if we change it to solid duct work? Or is it better to vent through the roof? I would still need it to go some distance because the bathroom is on the frontD of the house and I would prefer the vent pipe to not stick up on the front of the roof.

Remote fan sounds great. Is there a site to calculate what size fan is needed based on all the factors?

here's the Fantech website - click on the bath ventilation solutions brochure. They have info about what size you need along with pics and cutaways that show the installation. I love my Fantech. It is almost perfectly silent!! And it would suck the dust off the floor, it's so powerful.

Here is a link that might be useful: Fantech info

"We run the fan for a long time maybe 30 minutes after each shower."

How about running the fan till the bathroom is dry?

You might want to have xomeone come in and check your wall voids. If mold is appearing this easily, it could very easily be migrating from inside the walls, and then you could be talking 10's of thousands of dollars, as well as an extreme health risk. That's the bad news. The good news is that most, iof not all homeowners policies will cover the remediation of severe mold growth, no matter what it costs.

We just went through this! Our master bath had a real problem with mold. Our fan didn't really do much. We'd run it every time we showered and the walls would be wet, mirrors fogged up and the fan didn't seem to be doing any good. We have a very large master bath and the fan was 80 CFM. Our contractor recommended a NuTech fan and he calculated that we needed 420 CFM! We killed off all the mold, sealed it, used a high quality kitchen and bath paint and installed a 370 CFM NuTech fan over the shower. We also have a high tray ceiling in the bathroom so we installed a beautiful decorative 80 CFM fan light in the high point in the ceiling to catch anything that got by the fan over the shower.

What a massive difference!!!! The new fans are very quiet. The NuTech fan motor is up in the attic closer to the roof where the exhaust is mounted and it is so quiet! Now I can take a long, scalding hot shower and when I get out there isn't any fog or dampness in the bathroom. The mirrors are not fogged up AT ALL. Amazing difference. We have the fans running on a timer that we turn on when we use the shower. Has been installed since October and we couldn't be happier. Best of all, no sign of mold at all!

Got both my fans on Amazon for a great price.

Good luck!

Our master bathroom had a terrible mold problem as well. Moisture would condense on the ceiling and drip down, and then mold grew on the ceiling and the upper edges of the walls.

You will certainly need to ensure that your ductwork vents properly to the OUTSIDE, first of all, as other posters have indicated. Also as others have indicated, the length, duct size, and especially turns will have an impact on how strong the fan will need to be.

When we remodeled the bathroom I wanted to take no chances with mold regrowth. Since it was growing on the drywall I had it all ripped out. Since we were going with a large number of recessed fixtures anyway, it made sense.

Instead of ordinary drywall we installed greenboard, which is better for damp environments like bathrooms.

Then I sized the fan properly. We went with Panasonic Whisper Fans in each of our bathrooms. Since our bathrooms are small sized, and the duct runs were short, it turned out that the smallest fans were sufficiently powerful. They are also very quiet.

Finally, I had installed a timer switch in the bathroom. The switch has three positions. On, off, and timed.

On turns on the lights and the fan. So any time the lights are on and someone is in the bathroom, the quiet little fan is running.

Off is just off.

The third position is set to a 30 minute timer on the fan.

Simply running the fan while you shower is insufficient to properly ventilate the moisture in the bathroom. To ventilate the moisture, the fan needs to run for about an additional 20 minutes AFTER the shower is over.

So we take a shower or bath, and when we walk out we flip the switch all the way down. The timer turns the fan on (no light) for 30 minutes and exhausts the moisture.

Voila! No more mold problems (and it has been seven years now).

I think we are going to try a fantech or other type of remote fan. I also think we need to change the type of ducting used since all we have is the flexible stuff with no insulation. Love the timer idea. No matter how long I run my current fan, the bathroom is never dry. Even if I shower with the bathroom door open, the room remains very damp.

So, next question, can I run the new duct pipe to my gable vent or does it need to go throug the roof?

In winter climates (snow in the roof) I prefer to run it out of a gable wall. I'll insulate the duct and pitch any horizontal run for condensate.

Really good information already posted. Just wanted to add two thoughts.

In our master bath, about 10 x 11 ft. we have two fans. One goes on when there is any movement in the room, its near the toilet. The other is over the shower/bath area and has an on/off switch (a timer would be good too). So when someone is taking a shower both fans are running. If someone forgets to turn on the shower fan, there is still a fan running.

So while you might not need two fans, consider the placement of the one you have.

The other thought... in our house everyone agrees that if the bathroom feels cool they turn the water temp up higher than normal. This creates more steam. To correct this we have installed underfloor heating in one bath already and in the process with the other bathroom. We make sure that the room temp is comfortable and the hot water use really drops.

So make sure you are not making more steam just because the bathroom temp is too cool.