What size of insulated flex hose do I need to install for bathroom exhaust. Fan is 110cm. Length of hose in attic is approx 15-20 ft, going just about straight to the exit site. Bathroom size is 100 sq ft. There is a 5" one there now and we are replacing it. Both fan entrance and fan exit to outside is 4". We do not know why the builders did it this way. House is 5 years old.
110 cm fan requires 5" by code and manufacturer's standards. I think you can possibly go up to 4" at 100 cm if you use rigid duct and it is a relatively short distance - probably 4-8' at the most.
With flex duct you cannot for sure go out at 4" more than a few feet and also 4" flex I think maxes out at around 80 cm.
Honestly I would not worry about the 4" if you can go rigid. However if you can't that is a super long distance for flex and expect the duct to fail or pool water and possibly put strain on fan motor. (This is not end of the world stuff you giving you my opinion) If being inspected 4" flex will not pass unless your inspector just doesn't care about that at all - some just care that you vent outside, some want it done exactly right.
IRC Chart is below.
Now would I install a 4" flex duct in my own home with a 100 cfm fan. Yes if there were not a lot of bends and it was generally straight. It also depends on how often the bathroom will be used and the bathroom size. If it is a heavy use bathroom and smaller in size - well wow a bend in a flex duct and you have a small pond after a week and mold and whatever kind of issues once things start breaking down.