Wally’s Workshop

Question: I want to install a new ventilation fan that is ceiling-mounted in the master bathroom in my second floor. If I locate the fan three feet away from the exterior wall, the exhaust pipe can be easily extended through the attic and end it at the soffit overhang. Is there something wrong with this idea? Would there be problems? Of course, an alternative is to take the exhaust pipe to the roof ventilation hole. The water then could condense and this may run down back towards the fan which is a cause of worry. How would you address this?

Answer: There is only one other way that I can think of that could be damaging more causing more destruction that what you have proposed. And that is blowing air with the bathroom exhaust fans straight on into the attic space. This is often done by the naïve do-it-yourselfers, and some subcontractors and builders.  A certified home inspector who has seen many homes will tell you how bad this has been with wood rot and attic mold in the roof sheathing and roof framing.

No doubt you are right in wanting to move the exhaust air outdoors. But you will face terrible problems if you plan to dump it underneath the soffit. A vapor plume that gets created from the moist and humid air when someone takes a shower in the bathroom gets exhausted at the exhaust fan vent. A part of this cloud will drift over to the soffit and disperse into the air outdoors. A part of the cloud could steal into the attic space through the gaps and cracks in the soffit materials, the soffit ventilation intake vents and the seams in-between the soffit and gutter board.

The cold or cool attic framing surfaces cools this moist air, condensing it into liquid water as it gets into the attic. Fungi growth and mold growth is then triggered off by this liquid water to cause wood rot. The increase of water on a cold frosty day could transform itself into frost laid out in a thick layer. It gives you a creepy feeling when you see it.

If the bathroom exhaust fan is extended and terminated near or at a rooftop ventilation hole, the same kind of trouble can occur. A part of the air finds its way outdoors, but another part could make its way without doubt to the attic. To avoid this problem, what can be done is to extend the pipe further through the roof and terminate it with a termination cap of a special bathroom exhaust which incorporates also a damper.

A galvanized steel pipe that is smooth is preferred to channel the air to the outdoors from the fan. Your worry about condensation within the pipe is compelling. The failure of installers to insulate or properly insulate the pipe has created similar problems to thousands. Spray foam available in aerosol can is good for the task. Residual oil film could be deposited on the exterior of the pipe from the manufacturing procedure. This is first to be cleaned and removed for which you can use a mineral spirits soaked rag can be used. Dried cured foam of no less than 1.5 inches should cover the pipe. This foam coat should cover all the exposed parts of the pipe to the point of termination underneath the roof all the way from the fan.

Select the foam insulation that you want to utilize in your attic should be permitted for use by the local building and fire inspector.  Flammable insulation are not permitted to be used in locations that are accessible under local codes in some areas.

The pipe should not be wrapped with standard fiberglass insulation fixed to duct tape. A number of traditional duct tapes are found to weaken in the attics due to its high temperatures. Special tape and duct insulation could be bought from the heating and cooling supply stores to cover the pipe if you want to.
The exhaust fan termination cap can be installed in the roof by the roofer in case you are not sure of doing this. Though this job is not so difficult, yet this needs to be carried out properly so that there are no leaks from snowmelt or rain.

Water vapor can lead to severe damage. You may not believe how nasty this could get. It is not as though that this threat is real for some homeowners in wet or humid climate. This could occur even in areas with exceedingly dry climate. It is true that the risk of wood rot and mold growth is more for those who reside in climates that are warm and humid. Don’t be under the false impression that you could be free from this damage because of the place where you live.

The exhaust piping should be installed as per the direction provided by the manufacturer. Short length will have only a limited number of bends which means a better flow of moving air with lesser friction. If the pipe is too long or if the bends are too many, it may not be possible for the air to be pushed by the fan motor outdoors

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