Looking for some advice from any ventilation experts that saunter around these forums.  We have a new ensuite going on in the middle of the house so wanting to figure out the best solution to ensure we have good ventilation in the bathroom to remove moisture.  I'd love to just throw in a Showerdome but wife thinks they're ugly so that's that decision made

We're also about to get the house re-roofed, which leads me to ask, any tips on roof cowl fan installation? I've got an inline fan sitting directly beneath a roof cowl in our main bathroom, total duct length of less than 1500mm, and it does condensate over winter. I'm presuming this is due to the temperature difference in the roof space and the air coming up through the fan.  I could insulate the duct work (but not the in-line fan), but wondered if there were any other tips before having to install a condensate trap? I had somebody tell me that there must be a horizontal drop in the duct work, and can't run straight up and down, but I can't see how that would help. I'm planning on doing the same for the ensuite so would be good to get it right from the first time. The soffits are at least 5m away at best, and have very narrow (-100mm) gaps to get ducting through, so not really an option.

Secondly, a question on return air ventilation.  From what I have read, most of the off-the-shelf axial fans move enough air to adequately ventilate a small bathroom, but the issue can often be that there isn't enough area for air to be pulled into the room from. I've also seen mentioned that we should be undercutting the door into the ensuite by ~25mm to ensure air can move through, but that would considerably effect noise transfer between the ensuite and the bedroom. I wonder if there were any smart ideas around how to tackle this? One of the walls of the ensuite is shared by the hallway, could we potentially put in an insulated floor vent between these two rooms? Or is that a bad idea?

Thanks Geeks!