Just finished my basement. Went to take a shower in my new shower for the first time, and was shocked to see the exhaust fan dripping here and there while showering. I installed everything on my own - per code. It's a 6x8 bathroom with a 110cfm fan and a straight 15' run put the wall with an insulated duct. The fan is getting good suction as it holds a piece of paper up no problem and the airflow from the outside vent is strong enough to hold up the flapper. Any ideas? Could it just be related to the first use? I'm stumped. enter image description here

  • Is the exhaust fan directly over the shower? Commented May 7, 2021 at 3:29
  • Good question. I edited my initial post to include a picture. No it's not.
    – EGrant23
    Commented May 7, 2021 at 3:33
  • 2
    If warm humid air is hitting a cooler surface, condensation will occur and you will get drips.
    – crip659
    Commented May 7, 2021 at 12:14
  • 1
    Does the duct slope up or downwards from the fan to outside?
    – crip659
    Commented May 7, 2021 at 12:55
  • Was the fan running while you were showering or only afterwards? I often got condensation build up on my bathroom mirror until I installed a humidity sensing fan that would automatically kick on during the shower.
    – FreeMan
    Commented May 7, 2021 at 14:41

2 Answers 2


In my state bathroom exhaust fans are required to stay on 15 minutes after the lights go off for this reason.

Actually the time allows the excess humidity in the bath to be removed and the dry air blowing over the fan and duct help to dry it out also.

We don’t hear about it often on this site but prior to the 15 minute requirement new homes we built did have some of the same issues.

Increasing the home ventilation or make up air and the 15 minute times really has eliminated this issue I haven’t received a call back for this in years.

Add a fan timer to your switch and it will probably eliminate the issue since it sounds like you have good air flow.

  • I turned it on prior to getting in which is why I was surprised to see it dripping while I was in there. First time it was used, I'm stumped.
    – EGrant23
    Commented May 7, 2021 at 21:41
  • Pryor to use is fine but unless it really sucks, pun intended in an hvac way, the ductwork, possibly all metal and cooler than the hot steam will condense moisture in the pipe, give it some time after the shower and the lines dry out. Turn the air flow off prior to drying out and well the water follows the law of Mr gravity and drips on you.
    – Ed Beal
    Commented May 7, 2021 at 23:22
  • I'm a little confused by this. If the fan is actively running while I'm in the shower (which it was), there shouldn't be water dripping, right?
    – EGrant23
    Commented May 7, 2021 at 23:44
  • No that humid or damp air condenses on cold parts and restrictions points in the flow drop the moisture out of the air then it will drip. By allowing the fan to run once the high humidity is gone it drys out the fan assembly and duct work. I understand not wanting to purchase a timer just leave it on so it can dry out after the shower to prove it to yourself. If it is dripping while running it is sucking a lot of steam and the weight of a water drop is more than the air flow can push.
    – Ed Beal
    Commented May 8, 2021 at 13:39

Water is going to condense in the duct. if the duct slopes towards the bathroom it will drip in the bathroom.

Increating the flow ratr in the duct can reduce the humidity of the bathroom air and thus reduce condensation in the duct.

Ensure that there is a way for dry air to enter your bathroom.

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