I agree for the most part with the above post. However, I think the article on the provided link goes a bit overboard. It is definitely a bad idea to vent a bathroom fan into the attic. However, flexible ducting is commonly used for bathroom fans (not for dryers which should have smooth metal duct). It is also fairly common to terminate the duct by butting it up to a vent opening in the wall or roof. I have seen many installations like this and have never seen them cause mold or wood rot (the amount of humid air that would get back into the attic is minute). While it probably wouldn't hurt to insulate the duct, I've rarely seen that done, with no adverse effects.

I'm curious only.. where do you live? I was suprised to hear you have mold in the attic from the amount of moisture from a bathroom. If you're in Houston or other humid area I can understand the moisture never leaving. I would ask about the 'as is' sale of your house. that is, if it was not 'as is' then you have recourse especially if the mold is due to the duct not being connected. Where do you live? I say I'm surpised to hear of the dramatic effect of the fan from a bathroom into the attic I say this as in my experience it's usually little moisture from a bathroom fan and the attic being warmer or vented can get rid of the moisture pretty fast. Good luck. I'd figure your home purchase is going to be backed up via the real estate agent who sold the property. Anyone know where he goes first to complain? I'll have a look back in a week or two. Andre

Some of my info will repeat what has already been written. In the first photo: It looks like the duct is looped around to go behind the fan and then it goes up to the roof to exhaust. I would have rotated the installation of the fan housing itself to give a shorter path to the exhaust boot. 2) Rigid duct is better than flex duct. The wire coil in the flex offers more resistance to air movement through the duct resulting in your fan moving less air. 3) Insulate the duct for less chance of moisture vapor in the duct condensing and pooling in the duct, or running back into the fan housing. 4) It looks like light can be seen coming from the bathroom below. If so, the fan housing installation is poorly sealed to the bathroom below. Seal those gaps/holes, either with canned foam or aluminum tape, etc. Free air movement between conditioned living space and unconditioned space is bad for myriad reasons. See the dark/discolored/dirty fiberglass insulation? That discoloration is usually a result of the fiberglass insulation acting as an air filter. The discoloration is from the fiberglass filtering pollutants put of the air as air flows through the fiberglass. The second fan: Comments remain the same, with one addition. Since the duct is longer, when you replace it with rigid duct, have the duct leave the fan and run it vertical to a height a couple of inches above the height of the gable wall's exhaust boot. Then as it runs horizontal to the exhaust boot, pitch the duct downward a bit. That way if there is any condensation in the duct, it'll run out versus pooling in the duct.

The main reason is laziness and cost. What they should have done is installed an insulated 4 inch flexible duct to a capped roof vent. It needs to be insulated so the warm air does not condense in the winter and drip into the fan. Your old fan probably got rusty because there was no roof cap and the duct was not insulated.