I have 2 existing bathroom exhaust fans for which I need to replace the ducting. The two merge at a Y and then go straight up through the roof. I bought 4" insulated flex ducting, but know the flow will be restricted by all the convolutions of that stuff, plus I do deal with rodents in the attic and want protection there if possible.
What I would really like to do is slide 4" rigid pipe inside the insulated flex, thus getting the best of both worlds. I have replaced the beginning of one vent with 4" SCH40 PVC elbows, but I hate to think of the overkill of buying SCH40 PVC for the rest of the vent run. My total distance is 16' for one vent and 8' for the other. Both are in an accessible attic area.