The bathroom/laundry room fan is provided to remove excessive moisture in room, for example shower/washing. The fan pushes moisture to exterior of home, they are often considered scent fans, but the purpose is moisture fans.
We recently observed bathroom exhaust fan duct hoses in attic full of water. With further review determined that the duct hoses were installed in attic “Christmas Light” style or hanging, they are installed with dips or low spots in ducts. This created moisture the opportunity to build up in ducts. Once the duct was full of water it ran back into the fan unit in bathroom ceiling it created a very bad organic growth condition (mold) throughout the bathroom ceilings and walls within days. Recommend checking these flexible ducts in attic and verify no low areas present giving moisture opportunity to build up.
Often builders will use a more rigid ducts in attic (right image below) and install to with a constant slope to termination through roof or walls with no low areas. This provides little opportunity for moisture to build up.
I have provided two different photo examples of duct materials used for bathroom fans.
