Is water in a bathroom vent pipe avoidable?

Question

My bathroom's fan vents through the ceiling, with a unit like this one:

https://www.homedepot.com/p/Broan-NuTone-3-in-to-4-in-Roof-Vent-Kit-for-Round-Duct-Steel-in-Black-RVK1A/100344509

We had some water dripping into the ceiling. I found all the shingles and roof looked good, but the box had a lot of leaf litter in it. No telling if it was birds or just accumulated 15 years of sitting there, but I cleaned it all up, verified that the flap was in good working order and was clean. I cleaned and replaced the screen that should have kept that stuff out.

Then I went into the attic and verified that the outside surfaces of the roof vent was dry and it was only the inside that was wet. Again, I cleaned it all out - lots of evidence that leaf litter had found its way into the housing and down into the vent pipe. All the building materials looked first rate, no faults or corrosion, no reason to complain about the installation.

I put it all back together again. After some reading, I found that I should add insulation to the pipe, but I don't want to do that until I'm sure I won't be taking it apart again. So I waited for a rainy day when nobody took a shower and returned to the scene.

But alas, the inside of the pipes were damp, with a couple of stray drops of water. Again, no moisture outside of the vent, except for a drop hanging from the bottom that snuck out a joint. The insulation underneath it didn't look damp. Yeah, it was wet, but not like before I cleaned it up.

I then dried it thoroughly with a paper towel, waited a couple hours, and went back up there. Sure enough, moist again. All the way around top of the vent, not focused in one area. Again, the cap operated freely, no problem with the mechanics.

Note that I live in the Pacific Northwest. Wind isn't a thing here, but rain sure is.

At this point I want to believe it's some kind of condensation - house air making its way up to the cold metal. Sure, I could insulate the pipe, but it's metal, and it's directly connected to the vent cap, which is on the roof. That metal is going to get cold, insulation or no. If it's a case of warm house air making its way up there and condensing, well, I don't know what can be done about that.

I suppose I could go ahead and wrap some insulation around the pipes, but I don't want to make a maintenance hassle for myself if I'm going to be repeating the ritual.

Answer 1

Insulating the pipe reduces the scale of the problem, since the pipe is not as cold, overall, even if the very end of it is just as cold. So, you are aggravating the scale of the problem by not doing that.

Solving the problem "for good" tends to require venting out a sidewall with slope downward away from the fan, towards the exit, rather than taunting physics by venting moist air straight up and pretending that condensate won't happen and flow straight down.

I suppose you might be able to get creative and vent to a Tee that has a drain in the bottom so the "straight down" route leads to the drain, and the fan is off the side of the Tee, heavily insulated. I know venting out a sidewall properly sloped meets code, not sure if code is happy with "creative drain Tees" as an alternative.

Answer 2

I have a similar problem with 2 exhaust fans in my home that are not used regularly. Re-vented fans coinciding with a roof replacement in 2018, water drain-back was not an obvious problem until this year. The cause appears related to high-humidity air in the house, which comes from our Pacific Northwest high relative humidity (RH) outside air. Even at 26 degF outdoor temp today, the outdoor relative humidity is 87% (inside I have 63 degF and 70% RH). The warm, and moist, indoor air rises through the fan vents where the moisture condenses on the cold vent tubing (mine are not insulated). The condensate flows to the low point, which in my case happens to be at the fan box. After enough condensate accumulates, the water presentes itself on the floor. Thankfully thus far, there has been no water dripping on the gypsum sheet-rock ceiling.

Insulating the vent pipe should help, but for the best chance to be effective, there must also be a thermal break between the metal vent pipe and metal roof cap. A thermal break should be created with a low thermal conductivity material such as rubber, plastic, foam, or foam tape, yet the seal must be maintained. Running the fan regularly may reduce moisture content in the vent pipe, but at the same time it is exhausting the heated indoor air, in turn increasing heating requirements to keep your home at the desired temp. Some heating systems may have a feature to reduce indoor humidity beyond what happens by simply heating the air.

Unfortunately, some condensate accumulation is likely unavoidable due to our climate.