It might be.

It depends on what is recommended on the installation instructions of the fan.

See fan below recommends that flexible duct for this fan.

If the run isn't in a straight line, I think they use the flexible ducting.

We have a basement bathroom that leaks condensation from the bath fan. We recently learned that the reason you should use the flexible kind is because it allows that small amount of condensation to collect and evaporate instead of running back down and dripping into the bathroom. I only wish the previous owner would have known that. :) oh- and ours has to take a turn to get outside.

The reason you have condensation is because the duct is traveling through an unconditioned area. The warm air humid air of the bathroom condenses and drips from the exhaust fan. Condensation will occur if the duct is rigid or flexible. The best way to solve this is to install a flexible insulated duct.

Tell the contractor to replace the dryer duct with an insulated type. It is not much more money and will save you a lot of problems in the future.

Can you get this type of flexible insulated duct at any Home Depot/Lowes or which one should I purchase?

Gotcha thanks, it doesn't look like a long run so 25 ft might be too much. Do they sell 8 - 10 ft lengths, if not I suppose I would have to cut it or would that compromise the duct?

I know there were shorter runs in the past that I used when I renovated my bathroom. If not then just cut it to the length you need. It is meant to be cut and will not compromise the duct.

Ok and to avoid breaking out the brick from the exterior. I was thinking if I run the insulated duct from the bathroom fan to the filmsy dryer duct they installed in the attic, I can cut the dryer duct as close as possible to the exterior wall and connect the two with PVC or is that not a viable solution?

Now that you brought up "flimsy dryer duct," I think we should discuss a bigger issue. To me, that expression suggests that flexible, light-weight stuff that looks like a Slinky toy covered in plastic (like the link in your first post), or flexible aluminum that is corrugated. Dryer ducting that is not smooth will inevitably collect lint over time and possibly pose a fire risk in the worst case. In addition, since it sounds like this is easy to cut into, I'm betting it's not insulated, which can cause a condensation problem. It would be good to see pictures of what you have in the attic.

If you expect the fan to work correctly the duct should be as straight and direct as possible. I would not connect it to a dryer vent.

Do you really want to direct bathroom odors to your dryer?

Kudzu, here is what they currently installed and have in the attic.

Mike_home, so the insulated flex would be the way to go over

https://www.homedepot.com/p/4-in-x-5-ft-Round-Metal-Duct-Pipe-CP4X60/100196725?MERCH=REC-

?

Thanks. If it was my choice, I'd got with the rigid, straight ducting you showed in the link.

Jim,

For a bathroom exhaust fan an insulated flexible duct is fine. I installed Panasonic fans in my bathrooms which have 6 inch duct connections. After 10 years I have had no issues. In both bathrooms I installed the fan in the ceiling above the shower stall. The fan is connect to a timer switch. I turn on the fan at the start of the shower and run it for 20 minutes.

The rigid works also but then you have to insulated and add elbows for turns. It would be more difficult to install in your application. I don't see the advantage of using the rigid duct.