How do you stop condensation from coming down vent pipe into bathroom?

Now that the weather has gotten nice and cold - we have a slight problem with water vapor collecting on the inside of our vent pipe leading to a fan leading to one of our top floor bathrooms. I know the vent pipe travels vertically for about 7 feet through the attic and out the roof. However, with hot showers being taken a little more regularly we have noticed water dripping from the light in our fan. It’s clearly condensing in the pipe when the outside air is freezing and inside is a veritable sauna. But this is the first time we’ve seen it dripping with such ferocity…So anyone know how what might be going on and how to fix it? This has never happened before in that bathroom.

Is it possible that the vent pipe is plugged or covered in some way? Have you been up on the roof to look at it?

Initially I would think that either the vent pipe is not fully clear, or the fan is not working as well as it used to.

Hmmm, probably a bit of both I’d better get up there and look in the morning…A blockage would cause that much condensation every day huh?

I am not a GC or Vent Expert :slight_smile: , but I have had similar issues with water in vent pipes.

Cause 1: Cap of vent pipe was dislodged, allowing rainwater to enter the house.

Cause 2: Vent pipe was stuffed with a bird’s nest and otehr debris, preventing smooth air flow and causing condensation (though not dripping).

Best of luck!

If you can get at the fan, try shutting the power off to it and then opening up the housing, removing the fan blade assembly and giving it a cleaning. I did this when I was up in the attic over my bathroom and there was a decade’s worth of dust stuck to the blades (aided no doubt by humidity). Wipe it off with a paper towel, clean all the crud out, if it doesn’t spin easily you might need to lubricate the bearing, then reassemble and try it out.

It’s not like my towels all fly out the ceiling now but it’s definitely quieter and happier.

Depending on how straight the vent pipe is to your roof, when you’ve got the fan out that may be a good time to shine a flashlight down the pipe and check for obstructions (a la Stoli’s recommendation). You could tape a plastic bag over the bottom of the pipe and use one of those dryer lint cleaning brushes to clear the crud out from the top down.

Insulate the vent or get insulated ducting.

This is the answer.
I saw this same problem on “Ask This Old House” ands the fix was to insulate the duct from the fan to the roof. Cold air in the attic was causing the moist air within the duct to condense and drip down.

Seconded. Actually Thirded.

I’m an HVAC person. The warm/humid air traveling through your exhaust duct is condensing in the cold attic. Insulate the duct with armaflex or pipe insulation.

An element I had not considered (never had an attic).

Man it would be great to have an attic.


You might have to go to a plumbing and heating wholesaler to find it, but I’ve used Reflectix pipe wrap for that task in the past with good results.

As a followup:
Once the condition is rectified you should examine the fan housing for any sign of mould and take appropriate measure.

I’m considering replacing the fan, I’ll have the housing off this afternoo to check it out. So I’ll have a good look then.