Q&A / 

Bathroom Exhaust Fan Leaks Cold Air

DEAR TIM: My husband and I purchased a small ranch home built in the 1950's. A year ago we installed new exhaust fans in the bathrooms, kitchen and basement. Last winter I noticed cold air would fall out of the fan covers when the fan was not operating. The small bathroom is now the coldest room in the house. Did we make a mistake? Why is cold air coming into the house? Because heating costs are rising, we must stop or minimize the cold air infiltration. Jill L-H., Maumee, OH

DEAR JILL: My gut reaction is that you did nothing wrong. My experience tells me that you probably didn't do some extra things that would have made a big difference. Unfortunately these extra added touches don't come as part of the written instructions with many exhaust fans. I have never seen these tips on a sheet of paper. When you install as many fans as I have, you start to learn what works best.

Let's briefly discuss the dynamics of what is happening with the exhaust fans. I assume you used the suggested smooth-metal piping to connect the fan to the exterior of your home. My guess is the bathroom fan exhaust pipe is located in a cold attic space. If so, the pipe gets very cold when the fan is not in use. This cold pipe can cause the air inside the pipe to go from warm to cold. Cold air is denser and heavier than warm air and wants to go down, not up. This is why you feel it fall down out of the exhaust fan cover.

This exhaust fan has a built-in damper, but most fit poorly allowing cold air to drift back into a home. PHOTO CREDIT: Tim Carter

Cold air from the attic can also be entering the joints in the metal piping if they are unsealed. It is an excellent idea to use real duct tape that heating and cooling contractors use to seal ducts. This is a special tape you often can only find where furnaces, air conditioners and ductwork are sold. Do not confuse this professional tape with the common duct tape sold at home centers and hardware stores.

The fan itself should have an internal flapper damper that is supposed to block cold air, but these dampers usually fit poorly inside the fan housing and air can seep by them. I prefer to install a roof cap that is made to compliment the high-quality exhaust fans I install. These self-flashing exit caps have a great flapper damper with a felt seal that really closes tightly when the fan is not in use. Virtually no air works its way back down my exhaust pipes so long as the flapper is checked annually for dirt and debris buildup.

I have found that it is often best to vent fans and dryers through the roof. I urge you to watch this video of mine to see how easy it is to install the correct vent-cap flashing on a roof. Have no fear - if done right you will have no leaks.

It is also a great idea to insulate the metal pipe in any unconditioned space where it passes. If the metal pipe is in a cold attic space but well insulated, the pipe should not get cold as some of the warm air from your house will drift up through the fan and into the exhaust pipe.

I prefer to spray the metal exhaust pipes with foam insulation. This insulation is available in aerosol cans and is quite easy to work with. Be sure to clean the exterior of the metal pipe with soap and water as the metal pipe often is coated with a fine film of oil from the manufacturing mill. Wear very old clothes when working with the spray foam. Many foams have a urethane component and if the urethane foam gets on the clothes and dries, you can't get it off.

Be sure the entire length of the exhaust pipe is insulated all the way from the fan to the underside of the roof where it exits the house. This insulation will provide a secondary benefit by preventing or minimizing condensation inside the exhaust pipe. Many people think they have a roof leak during winter months as the exhaust fans operate. In most cases the water is simply condensation that forms against the cold sidewalls of the exhaust pipe and then runs downhill back to the fan.

Not all exhaust fans are created equal. Some have low-powered fans that can barely push open the first flapper damper much less the second flapper at the roof. As with many consumer products, the things that work better and are more reliable often cost more. When you shop for fans you will be shocked to discover it only costs a little more money to get a very high-quality fan.

Keep in mind that your house should also be equipped with a makeup air intake port. When exhaust fans, clothes dryers, central vacuum cleaners, fuel-burning furnaces and water heaters and fireplaces operate, they consume vast amounts of air. It is entirely possible that back drafting is happening in the bathroom fan exhaust pipe as it becomes the point of entry for air needed by another appliance that is sucking air from the house.

Makeup air intake ports can be as simple as a clothes dryer exhaust cap that has holes drilled in the flapper. I have also seen small hoods where the flapper is removed and small galvanized hardware cloth is inserted in its place. These makeup air intake points should be located in the utility room where furnaces and water heaters are located.

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6 Responses to Bathroom Exhaust Fan Leaks Cold Air

  1. Hi, We have water leaking through our bathroom vent only when it is cold outside. We do not take showers in this bathroom. The voting pipe goes straight up from the bathroom then approx. 16ft throughout the attic to the roof. We have completely insulated the pipe. It is still leaking like crazy through into the bathroom and now there is water leaking into the kitchen but only when it is cold outside. Our kitchen ceiling is now covered with wet splotches. What do we do now??

    • The water is almost certainly condensation - this means that warmer, more humid air (i.e. the air inside your house) is escaping through this exhaust vent.

      Hot air rises, so your home's warmest, most humid air is always up at ceiling level, seeking a way to get even higher. It'll use any crack it can find... and apparently your exhaust vent pipe is a nice open highway for air that wants to escape.

      As it goes, up, it cools and the water condenses on the inner sides of the pipe and makes its way back down.

      Basically, you've re-invented a distillation still, only instead of making moonshine you're making ceillng water 😉

      Your base problem is that air can escape the house via that vent too easily. It sounds like the pipe has nothing to act as a damper/flapper. That's why these dampers exist; to make sure the pipe is "closed when not in use".

      Fix the air escape by installing either an internal or rooftop damper.

  2. Tim. We just bought this home and have a gas fireplace . When it is windy out the vent has something outside that bangs. Hope you can help.. Jean

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