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Mistakes to avoid when installing a bathroom exhaust fan

This exhaust pipe for a bathroom fan is a ticking time bomb of mold, mildew and wood rot. (Tim Carter)
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Q: Mr. Tim, I’ve had three contractors over to my house and I’ve received three different answers about how my bathroom fan should exhaust. I’m beyond frustrated. How are we homeowners supposed to protect our largest investments, our homes, if maintenance advice is wrong? I’ve read online about serious mold and wood-rot problems caused by bath fans. I’m confident you’ll be able to give me the correct answer. I only wish you lived in my town so I might bump into you for advice at the grocery store from time to time! — Sharon P., Evanston, Ill.

A: Do you wring your hands like Sharon is undoubtedly doing each time you have to spin the wheel hoping it stops on the correct contractor to work on your home? Can you imagine signing a contract for $1,000, $5,000 or even $50,000 hoping everything is going to work out? I don’t know about you, but I only hope for things that I can’t control, like the weather, and the next time the giant fault 60 miles west of Portland under the ocean is going to unzip its entire length unleashing a mammoth tsunami. Yes, geology was my college major, but I digress.

Before I dive deep into helping Sharon, I’d like to share a sobering story that should shake you to the core. Two years ago I attended a preconstruction meeting with the builder of my daughter’s new home that was being built in cold and blustery Bar Harbor, Maine. The company that was going to prefabricate the exterior and interior walls for the house wanted to make sure everything was right before they started to bang together all the studs and oriented strand board.

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After most of the questions were answered, I asked, “Is it possible for you to cut all the interior walls where they touch the exterior walls 3/4-inch short?” The builder and the panel manufacturer looked puzzled. “Why?” They both asked at the same time. “Well, I want to run the vapor barrier on the exterior walls continuous around the entire house so there’s only one seam. This gap will also allow the drywall hanger to run full 12-foot sheets around the exterior walls.”

The builder scoffed and said, “Vapor barrier? We don’t install those here. They cause mold and rot problems in the walls.” I wasn’t about to get into a science debate with a builder who had no idea what he was talking about. In the meantime, I heard the wall-panel manufacturer rep’s gears spinning and he blurted out, “That’s a brilliant idea. I can’t believe this is the first time in my career I’ve ever heard someone suggest it. Yes, we will shorten all those walls.”

I subsequently asked the Bar Harbor building inspector about the vapor barrier requirement and she said the current code didn’t require it but that she always recommends one be installed. Are you frightened? If not, you should be.

Now think about what a bath fan really is. When you take a hot steamy shower, it’s a pump. It pumps hundreds and hundreds of cubic feet of moist air to some remote location. The last place you want this air to be is inside your attic space. The moist air will fuel the growth of mold and mildew on the roof framing and the sheathing that covers the timbers. Wood rot is an absolute certainty over time should you dump bath exhaust into an attic.

This moist air must exhaust to the exterior of your home. This is nonnegotiable.

But it can’t just exit anywhere. The best place for it to exit is through your roof. The hot moist air is dumped onto your shingles or other roof covering that’s made to get wet. However, you can’t do this if you live in a cold climate where snow can build up on a roof. Here in New Hampshire where I live, it’s entirely possible to have two feet, or more, of snow cover on a roof! Snow can cover the vent outlet and the moist air has nowhere to go.

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The next-best location for a bath fan cap is a wall that’s at least three to five feet from a roof overhang. Most roof overhangs today have continuous soffit ventilation strips. Bath exhaust is almost always warm and this air will float up and into the soffit ventilation openings. You’ve now put the water right back up into the attic without realizing it.

Just a year ago I saw the absolute best bath exhaust fan roof cap. I wish it had been available when I built houses. It’s made by Lifetime tool, the manufacturer of the best plumbing vent-pipe flashings. This cap will work well for tens of millions of homes that never see a snowflake.

The pipe that extends from the fan to the exterior of your home needs to be a solid galvanized metal pipe or an approved insulated flex duct that might be part of a kit for a remote bath fan where the fan is far away from the bathroom. If you use sections of metal pipe, be sure to tape each joint with the true metal duct tape used by HVAC contractors. Don’t use cheap fabric duct tape from a home center.

Pay very close attention to the installation instructions published by the fan manufacturer. They state how many elbows can be in the piping and how long the piping can be. Don’t exceed these limits.

I urge you to always read installation instructions before you talk to contractors. Ask pointed questions of the contractors to see if they really know how to install something. Remember, pro contractors will never be offended by your questions. Ask away!

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