Correct diameter for bathroom fan outlet duct & vent itself?

[AcidWater]

Where can I find good practices or code for the size of bathroom vent piping & outlet? My hall bath vented into the attic. A few years ago I replaced the fan, and it was extremely quiet and kept the mirror from fogging up. I just paid a plumbing contractor $$$ to vent it outside, and he said it would be best on the roof not under the eave. But now the fan is noisy & the mirror fogs up. I don't think I oversized it (much) for the room (about 6x8). IIRC 110 ft^/min. I have to dig up the specs for the fan... But the vent on the roof is pretty small; they used decent sized flex duct until the vent itself.

 

[ImOld]

But now the fan is noisy & the mirror fogs up

Sounds like there is some air restriction. Is there a flap on the air vent?

 

[Sponsor]

 

[Jadnashua]

8-10 exchanges per hour. Some fans can accept multiple duct sizes...larger will tend to be quieter and have higher flow rates; it depends some on how long the run is.