Summary: If exhaust fans are not properly installed in your bathroom, the cold air could be moved into your rooms. Cold air can enter a number of places including room, and the fan exhaust pipe of the bathroom. Another problem could be the inferior quality of the fans.

Question: New exhaust fans were installed a year ago in the kitchen, bathrooms and basement. Cold air fell out from the fan covers last winter while the fan was not turned on. The coldest room is now the bathroom in our home. Did we do something wrong? What is the reason for the cold air in our house? How do we prevent or reduce the infiltration of cold air into our house? With increasing heating costs, we need to do something about this.

Answer: I don’t think you did anything wrong. I think that in all probability you did not do things that could have made all the difference. These are extra things that you could have done. These extra things are also unfortunately not mentioned in the instruction manuals that come along with the exhaust fans. I have any way not seen them in any instruction manuals. One becomes aware of these things when one gains experience. I have learnt these from my experience.

We need to understand the working of the exhaust fans. Presumably you must have used the smooth-metal piping as suggested to hook up the fan with the outdoors of your home. I suppose that it is an attic space that is also cold where you have located the fan exhaust pipe. Naturally when the fan is no turned on, the pipes become quite cold. This results in the air inside the cold pipe going cold from warm. You know that cold air is heavier and denser than warm air. The cold air will go down and not up which is why that the cold air falls down out from the exhaust fan cover.

If the joints in the metal piping are not sealed, the cold air can enter it from the attic. The real duct tape is best suited to seal ducts. The heating and cooling contractors use this kind of tape in their work. This type of tape is sold where ductwork, air conditioners and furnaces are sold. This is not the same common duct tapes that are found in hardware stores and home centers.

Every fan is to include an internal flapper to block the cold air. Often these flapper dampers are not fit properly inside the fan housing making way for the air to seep through. Whenever I install high-quality exhaust fans, I make sure that a roof cap is also installed. The roof caps are self-flashing exit caps with a flapper damper having a felt seal which closes tight whenever the fan is not used. As long as there is no buildup of debris and dirt, my exhaust pipes does not allow air to work its way through. The flapper should be kept free of debris and dirt with an annual checking.

Insulating the metal pipe when it passes any unconditioned space is a sensible idea. A metal pipe that is well insulated does not become cold when it passes a cold attic. The exhaust pipe would also have some warm air from the house that would have drifted through the fan which additionally will keep cold away from the pipe.
Foam insulation is a good thing to be sprayed on the metal exhaust pipe. Aerosol cans with this insulation are available and are easy to handle. Before spraying the metal pipe, make sure that you clean the exterior of the pipe with soap- water to remove any fine oil film from the manufacturing mill itself that may have coated on to it. When using a spray foam it is better to wear clothes which are very old as the urethane component in many foams cannot be removed from clothes once it dries.

Insulation of the exhaust pipe for its full length is needed, right from one end from the fan to its exit at the underneath the roof. Condensation is either prevented or minimized inside the exhaust pipe by this insulation. Quite often people can mistake the leak during winter to be a roof leak when the exhaust fans are turned on. Invariably the leak is from the water that simply condenses against the exhaust pipes on its cold sidewalls and then runs down all the way back to the fan.

It is not as though all exhaust fans have the same quality. Some of them have low-powered fan which can scarcely push the first flapper damper open what to think of opening the second flapper that is on the roof. Just as the case in other consumer products, the more reliable and better ones are costlier. But then you will realize when you actually go shopping for it that it costs only a little more to get a high-quality fan.

It is important that you pay due attention to install a makeup air intake port. A large volume of air is consumed when you operate water heaters and fireplaces, fuel-burning furnaces, central vacuum cleaners, clothes dryers and exhaust fans. Back drafting is possibly with air being sucked in through the exhaust pipe of the bathroom fan which is a point of entry of air for use by another appliance in the house that sucks air.

It is easy to make makeup air intake ports, almost like the clothes dryer exhaust cap with holes drilled into the flapper. Sometimes the flapper is taken out from the small hoods and instead inserted with small galvanized hardware cloth. The setting of these makeup air intake points should be in the rooms where water heaters and furnaces are located.

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