Bathroom Exhaust Fan Leaks Cold Air
Bathroom Exhaust Fan Cold Air Leak TIPS
- Fan dampers meant to stop mice, not air
- NEVER install bath exhaust in an attic or under a roof overhang
- WATCH my Two VIDEOS below about bath exhaust fans
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DEAR TIM: My husband and I purchased a small ranch home built in the 1950s. 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? What's the best way to exhaust the air from the bathrooms? 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.
Cold Air Falls Down
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.
Seal Exhaust Pipe Joints
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 aluminum duct tape that heating and cooling contractors use to seal ducts.
This is an inexpensive 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.
Venting a Fan to the Outdoors
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. If you want to stop cold air, you need to have a great damper, or double damper at the exhaust hood where the air leaves your home.
Some vent hoods made by the fan manufacturers have swinging doors that have a felt seal. These are not bad. I've had decent success with these.
But the best vent hood I've ever used to stop cold air is one that exits the side wall of your home and has a double damper. These are ingenious and inexpensive bathroom exhaust vent hoods that are very easy to install. Here's the one I have at my own home:
You have two choices when exhausting bathroom air. You can go out the roof or you can go out a side wall of your home.
Exhausting bath exhaust out a roof is the safest and best method. This ensures all humid air from showers and baths will never rot any wood. If you exhaust bath air under, or near, a roof overhang, the humid air can get sucked into the attic through soffit vents. This moisture can condense on the trusses, roof rafters and roof sheathing and cause wood rot and mold growth.
I prefer to install a roof cap that is made to compliment the high-quality exhaust fans I install. Most high-quality exhaust fans, and I've installed Fantech in my own home with great results, come with a special hood that's made to be used with shingles, shakes or metal roofs. The Fantech exhaust fans are very quiet because you install the motor up in an attic or at some other location away from the bathroom. These fans vacuum the air out of the bathroom and you simply don't hear much noise at all.
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.
Insulating Metal Pipe
It is also a great idea to insulate the metal pipe in any unconditioned space where it passes. If the metal pipe is not insulated, the moist humid air in the pipe will condense on the side walls. Often this water finds its way back to the fan and drips into the bathroom. Many a homeowner thinks they have a roof leak, when in fact the issue is condensation inside the 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.
WATCH these Bath Fan VIDEOS:
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.
Remote Fan in Attic!
If you want the best bathroom exhaust fan, I recommend you get the Fantech fan that I had in the last home I built. I loved them so much, I had two of them, one for the master bathroom and another one exhausted the air from two back-to-back bathrooms in my home.
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.