all. I live in an old Edwardian home where the attic above is a finished loft space so I can't run any ducts upward. I have a second floor bathroom below that needs to be vented correctly. Right now it's only venting directly between the joists. What's really unfortunate is that the current exhaust in the ceiling is literally less than two feet away from an outside wall. But the joists run the wrong way. I don't know a lot but I know not to cut a four inch hole into a joist.

Since the joists run the wrong way there is a bedroom next to the bathroom. I think it'll be around thirteen feet, including the bathroom area to run a duct from that point above the ceiling along the adjacent bedroom to get to an outside wall. My question is, will this be too long to run a duct or will I encounter problems of some sort?

Here is the model that I'd like to get. It has good reviews and appears to be the quietest:


Unless anyone has any ideas for a wall mounted exhaust that is of similar quality and as quiet I feel like doing a ceiling mounted exhaust is my best option.

Hope to receive some suggestions.


  • 2
    When I have to run long on vents I won't use the flexpipe but smooth walled. 13' is a long run but if only 1 or 2 90's it will be much better than venting inside.
    – Ed Beal
    Commented Jan 16, 2018 at 9:14
  • Hi, Ed. Yes, I did read that the smooth-walled vent tubing was the way to go when working with long lengths. Other than the 90 angle of a vent directly from the exhaust itself it will run a straight line until it reaches the outdoor vent.
    – Adrien
    Commented Jan 16, 2018 at 10:05
  • This sounds like a good plan 1, 90 smoothe wall pipe I don't foresee any problems with this , good luck.
    – Ed Beal
    Commented Jan 16, 2018 at 14:02

2 Answers 2


The Panasonic product you listed supports 4" or 6" round duct, according to the installation manual. 13 feet should not be too far at all, especially if it is a relatively straight run. And to be extra safe, use 6" round duct. The rigid kind, not the cheap flex stuff! The increased size will allow less restricted airflow.

Also, try to avoid using any 90 degree turns. The model you referenced has a side port, which means if you install it between joists you should be able to take the duct straight out of the side of the fan and straight through the exterior wall to the vent cap.

In reality, you should be fine with a straight run of 4" rigid duct. But 6" would give you some peace of mind.


Typically in all bath fans I seen go in, a 30' run was allowed IF there was no elbows used. If elbows are used, which will be pretty much the case in any install, you need to deduct 5' for each elbow added. For example, if a given run needs 3 elbows, your total length of run cannot exceed 15' and still have an effective draw. The static pressure will be too great and the rated CFM of the fan will not be realized.

  • Good news! I took out the old exhaust and learned that area above the ceiling is part of a soffit. I obviously had no concept of what things looked like from the outside. This isn't my house. It's my partner's home that I am trying to improve. Looks like I'll be able to run a flexible exhaust directly straight up. And it's only about 20" or so from the ceiling level. Thanks, everyone, for your help!
    – Adrien
    Commented Jan 17, 2018 at 23:05
  • Please at least use semi rigid, and not the plastic stuff with the coil wire in it
    – Jack
    Commented Jan 17, 2018 at 23:28
  • Are you suggesting I use the semi rigid for a duct that is going to go upward to vent out through the roof of a soffit?
    – Adrien
    Commented Jan 19, 2018 at 2:55
  • Yes, the coiled plastic type that is the real cheap stuff available any hardware place degrades over time and it might just shred apart under its own weight after a period of time passes by
    – Jack
    Commented Jan 19, 2018 at 19:49

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