DEAR TIM: I'm remodeling a second floor master bathroom and will be installing a new ceiling-mounted ventilation fan. The fan itself will only be three feet from an exterior wall so I could easily extend an exhaust pipe through the attic and have it terminate at the soffit overhang. Do you see any problems with this method? I can extend the exhaust pipe towards a roof ventilation hole but I am worried that water might condense and run backwards towards the fan. What would you do? Larry L., Bloomfield Hills, MI

DEAR LARRY: I can only think of one other method that might cause more damage and destruction than the two methods you have proposed. Some builders, subcontractors and unknowing do-it-yourselfers simply let bathroom exhaust fans blow air directly into the attic space. Talk with any seasoned certified home inspector and she / he may tell you tales of horrible attic mold and wood rot in the roof framing and roof sheathing.

You are definitely thinking about doing the right thing by getting the exhaust air to the exterior, but dumping it under the soffit might create massive problems for you. The moist humid air that is exhausted when someone is showering will create a vapor plume at the exhaust fan vent. Some of this cloud will waft over the soffit and dissipate into the outdoor air, but some of the sinister water vapor will sneak its way into the attic space through soffit ventilation intake vents, cracks and gaps in the actual soffit materials and seams between the gutter board and soffit.

When this moist air gets into the attic, it will readily condense into liquid water on the cool or cold attic framing surfaces. This liquid water is the needed spark to ignite active mold growth and fungi growth that causes wood rot. The actual buildup of water on these surfaces can turn to a thick layer of frost in very cold weather. I have seen it and it is a very eery feeling.

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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.

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This same problem can happen if you extend a bathroom exhaust fan and terminate it at or near a rooftop ventilation hole. Some of the air may make it outdoors, but some will undoubtedly find its way to other places in the attic. The best way is to simply extend the pipe through the roof and end it with a special bathroom exhaust fan termination cap that includes a damper.

I prefer to use smooth galvanized steel pipe to duct the air from the fan to the outdoors. Your concern about condensation forming inside the pipe is valid. It can and does happen to thousands of people because the installer failed to insulate the pipe. For this task, I prefer to use spray foam that comes in an aerosol can. Clean the exterior of the pipe with a rag soaked in mineral spirits to remove the residual oil film from the manufacturing process. The pipe should be covered with at least 1.5 inches of dried cured foam, and you must cover all of the exposed metal pipe from the fan all the way to the exhaust termination cap at the underside of the roof.

Be sure the foam insulation is approved by your local building and fire inspector for use in your attic space. Some local codes do not allow flammable insulations in accessible locations.

Don't try to wrap the pipe with standard fiberglass insulation attached with duct tape. High attic temperatures can cause many traditional duct tapes to deteriorate in attics. If you want to wrap the pipe, use special duct insulation and tape that can be purchased at heating and cooling supply businesses.

If you do not feel confident installing the exhaust fan termination cap in the roof, hire a roofer to do this. It is really not that hard, but it is a job that must be done correctly to ensure you have no leaks from rain or snowmelt.

Never underestimate the amount of damage simple water vapor can do. This danger is real for all homeowners, even those who live in very dry climates. Those who live in warm humid climates are at grave risk for rapid mold growth and wood rot. Don't think for a moment that you are immune from damage because of where you live.

It is also very important to install the exhaust piping as directed by the fan manufacturer. Short runs are better and you can only insert in the line a given amount of bends in the piping. The pipe and bends create friction to the moving air. If you install too much pipe or too many bends, the fan motor simply will not be able to push the air to the exterior of your home.

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Comments: Welcome! I, Tim Carter, don't answer questions here. If you post a question here in the Comments Area, perhaps another visitor will help you. You need to go to the Ask Tim page if you want a question answered. Once there, look closely at how many weeks behind we are. Please be patient as you use this free service. If you have an emergency and need to talk to me, there is an option there for you. Anouck 27 Nov 2007, 07:32 I have a very noisy fan in the bathroom and wish to replace it. I bought a Panasonic fan but the diameter is 4 inch. The exhaust pipe in the ceiling is 3". Do you know of a good, not too noisy fan with a 3" exhaust? AL ATB 27 Nov 2007, 07:49 No. Why not do this the right way? Get up in that attic and install new 4-inch insulated flexible pipe. The job could be over in as little as 20 minutes. chuck 27 Nov 2007, 12:13 i just replaced my bathroom cieling exhaust fan. problem is that the old sheetmetal duct going up to the roof is 7 inches the new fan has a 4 inch exhuast. is there a problem with going from 4 inches to the 7 inch duct? thanks ATB 27 Nov 2007, 12:18 There is not a problem if you make the transition from 4 to 7 as close to the roof as possible. Geo 30 Nov 2007, 16:32 I plan to vent a future basement bath fan through the roof in my south Michigan ranch. I can route it through a section of non-load-bearing 2x6 interior wall with studs already exposed on the main floor -- it has no obstructions in either the basement or attic. I can insulate the outside of the pipes to prevent external condensation, but should I be concerned about condensation inside the pipe running back toward the fan? Would 4" PVC be a good choice for the vertical run, to help reduce condensation? ATB 30 Nov 2007, 19:12 PVC will not lower the chances of condensation. Insulate the pipe very well! Jay 01 Dec 2007, 09:43 I have three bathrooms, and in the mornings, when the outside temperature has dropped below freezing, all of the exhaust vents drip water onto the bathroom floors. Our house is six years old. I have my own theories about the cause, but would like to hear an official opinion. George 05 Dec 2007, 13:53 Here is a cautionary tale regarding condensation in a bathroom fan vent line. A previous owner vented my bathroom with flexible, un-insulated tubing, which was draped along the ceiling joists and terminated at a gable vent. The low points in the tubing collected enough condensate to completely fill up last winter. I went up to investigate, expecting a birds nest or something, and found the water. I emptied it into a bucket and poured it out. The next day I came down with what first seemed like a cold, but quickly got worse. The day after that I was in the ER, and spent 5 weeks in the ICU! Turns out I had contracted legionnaire's disease from the standing water in the vent tube. I very nearly died. Moral: Wear a mask when you deal with standing water in attics, and vent your fans properly, with insulation. micahel 09 Dec 2007, 11:05 I a installing a panasonic bath fan in my upstairs bath,I have a flat roof with fairly little attic space to work with.Their is a 4 inch exhaust duct being used for the downstairs bath fan running up to the roof near my upstairs bath. Is their a problem with tying in my new fan to this existing duct? AsktheBuilder 09 Dec 2007, 15:14 Mike, What do the instructions say to do? I feel you might exhaust air into the other bathroom.      View all comments *Name: Email: Notify me about new comments on this page Hide my email *Text: Security Image: Security Image Text:   Remember, Tim Carter doesn't answer questions here in the comments. You need to go to the Ask Tim page.