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The Spokesman-Review Newspaper
Spokane, Washington  Est. May 19, 1883

Poorly installed bath fan vents can cause serious problems

This bathroom fan vent pipe looks like a DIY installation, but it was actually installed by a pro. Never use duct tape and flexible pipe. (Tim Carter)
Tim Carter Tribune Content Agency

DEAR TIM: I’m a do-it-yourselfer, and I’m going to remodel a bathroom. I don’t like our current exhaust bath fan and am going to install a new one. I’ve done all sorts of online research, and lots of the information is conflicting. I feel conflicted about what to do and what’s the best way to vent the fan. Can you offer advice about best practices? – Donna B., Cincinnati

DEAR DONNA: Congratulations on doing your own bathroom remodel. You’ll get all sorts of satisfaction from this experience … and possibly a dose of frustration.

You’re also to be commended for recognizing that one can get in trouble very quickly by following random advice found on the Internet. I’ve seen websites, forum discussions and videos on YouTube loaded with horrible advice that will cause you nothing but heartache and financial issues down the road.

I’m often asked about how to vet advice found on the Internet. First and foremost, you should consider just following advice offered by real professionals. A real pro is one who works for paying customers and has a successful business record doing this for no less than 15 years. Working on a house you bought to flip, or doing DIY work on weekends for your friends and neighbors, does not make you a pro. If the person can’t prove to you they have the credentials, be very careful about what that person has to offer.

Now, let’s talk about your bath fan vent. My first piece of advice is to buy the best fan you can afford. Fans that cost more money are almost always made with better parts and better motors, and they’re not noisy.

Once you get the fan, stop and read the installation instructions. Just about every great fan I’ve installed has good instructions that discuss the minimum requirements for running the exhaust piping. At the very least, heed these instructions. Failing to do so can void the fan warranty in many instances.

You live in a climate where it gets cold. Bath ventilation fans and cold weather don’t play well together. Bath fans are usually pushing lots of moist, humid air. If the vent piping is in a cold space and the pipe is not insulated, you’ll get condensation forming inside the pipe.

People often notice water dripping from the fan cover and think they have a roof leak. More often than not, the issue is water flowing back down the pipe into the fan. For this reason, you have to use the right pipe, and it needs to be installed so water never leaves the pipe if it forms on the inside pipe walls.

Some high-quality fans come with special one-piece insulated flexible piping that prevents condensation and leaks. If you decide to use it, be sure to talk to the manufacturer about the best way to connect this pipe to the fan and fittings.

To prevent condensation, it’s best to insulate the outside of the pipe. You may be able to find great foam insulation that slides over the pipe you use. If you can’t, then you can buy aerosol cans of spray foam and coat the pipe after it’s installed.

Do not count on traditional fiberglass insulation that’s secured with tape around the pipe. Duct tape, no matter how good it is, is the worst thing to use in a hidden space. The adhesive bond will eventually fail.

I’d seriously consider using foam core PVC pipe as the vent pipe for your fan. The fittings can be glued, and any water that might form in the pipe will never leak onto your ceiling or down a wall. If you do decide to use metal duct pipe that snaps together, be sure any horizontal pipe seams are at the top of the pipe so no leaks will occur. The issue with using metal pipe is that any angled fittings you need to use are all potential leak sources, especially where they connect to straight pieces of pipe.

Use an approved termination cap where the vent piping exits your home. Do not vent the bath exhaust directly into the attic hoping it will exit some other roof vent. Bath fan exhaust can and does cause massive mold blooms and wood rot in attic spaces. The fan exhaust must vent directly to the exterior of the home.

Do not put the vent termination in a roof overhang or soffit. Don’t put it on a wall near or under a roof overhang. The moist air that’s belched out by the fan can be sucked right up into the open soffit vents and be drawn into your attic.

I feel it’s best to put bath exhaust fan vents through the roof. If you live in a region that gets heavy snow, then just exhaust it through a PVC pipe that has a candy cane shape and flash this pipe as you would any other plumbing vent pipe.

Carter’s past columns are archived for free at You can also watch hundreds of videos, download Quick Start Guides and more, all for free.