4" diameter since that's what it takes. Rigid or semi-rigid (plastic flexible hose is a fire hazard, at least when used with dryers). Don't think you need to insulate it (except for soundproofing where it may rattle). Just make sure there are as few angles/turns as possible, and that any are large radius.

I have 2 Panasonic fans. The secret to any fan noise is proper duct work. This means as long a run as possible directly off the fan without changing direction or reducing size. Avoiding 90 degree bends - multiple 45's are better. Avoid flexible duct work like the plague. It's horrible stuff. Not only will it collect dust and moisture, the turbulence of the air caused by all those ridges contributes a lot to the noise level. Both my fans are exhausted to the outside with rigid duct work properly installed, taped (not duct tape - silver foil tape) and of course screwed together. They're very very quiet. Any fan is going to be much quieter with no ductwork attached, which is how they're demonstrated in big box stores which also are massive, tons of ambient noise etc. all making the fans seem quieter. Also don't oversize the fan for the cubic feet of the room. Very important, as is ensuring there is enough supply coming in under the closed door etc. to feed the fan. As much air as it pushes out of the room has to be replaced with new air coming in under that small crack under the door. If it's not getting enough replacement air, it will be noisier.

I have one. It's not installed yet. It IS big :o I was going to send mine back and get a smaller cfm since I don't have a shower but this is the only exhaust/heater they make so I'm keeping it. As for the heat/fan situation, I've wondered the same thing. The discussion here is that you don't use both at the same time. Monica

Auzzy, I haven't been happy with the color of the CFL's I've tried so far, but something still sounds fishy to me that you're finding it too dark. A 32 watt CFL should be brighter than a 150 watt incandescent bulb -- which should be very bright in your shower. So I'm still wondering if something could be wired or configured wrong such that you're not getting the full power of the 32-watt CFL. Or could the bulb which came with the fan accidentally be a lower wattage one? Or could there be a problem with the transformer in the fan unit. Unless you have a super high ceiling, or a huge room, I don't think you should be perceiving the light as too dim. You might want to call Panasonic tech support to see if they can tell you anything that might be causing the light not to reach full power. As Brickton says, changing to a warm color temperature CFL bulb -- more yellow light instead of blueish-wite light -- could improve the overall feel. -- Eric