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Old 04-19-2011, 01:14 PM   #1
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bath fan exhaust pipe


what is a good vent pipe to use to run from the bath fan through the attic out of the roof? i want to keep the condensation from happening. its a short run, 10 feet or less.
ive been told pvc, or use metal but to insulate it with foam, like the great stuff in the can? seems like alot of work but id do it if thats the best way.

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Old 04-19-2011, 02:36 PM   #2
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They are referring to rigid foam or insulation batts. Keeps the moisture in the exhausted air from condensing on the colder metal and dripping back down into the fan.

Although, I am just about to start a thread regarding the theory behind that.

So what do you guys think? Is there any real benefit to insulating the exhaust run through the attic space? Seems to me, that without any active conditioning of the inside of the exhaust flue, it would become just as cold as the attic surrounding it.

Now if you ran heat through it all the time and then insulated it, I could see the benefit.

Opinions?

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Old 04-19-2011, 02:46 PM   #3
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In colder weather there is the possibility of condensation forming with the pipe warmed from the bathroom air and the colder air in the basement. Years ago they didnt do anything with them, in fact they used to run them out into the soffit. that wasnt a good idea with all that humid air from showers. Use metal pipe and just wrap a batt of insulation around it.
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Old 04-19-2011, 02:46 PM   #4
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bath fan exhaust pipe


The purpose of insulating the exhaust pipe is to try to keep as much of the water in the air being exhausted in the form of a vapor, thereby allowing it to be moved to the outside of the structure.

If the exhaust pipe is not insulated, especially in unconditioned spaces, it will become cool enough to condense the water vapor out of the saturated air and cause moisture problems and probably damage.

Normally, 4" diameter galvanized or aluminum smooth wall pipe is used, but some people use PVC now. I personally have never used PVC, but I like the idea of having leak free glued joints and PVC conducts heat less efficiently than metal...it's a win-win!
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Old 04-20-2011, 06:53 AM   #5
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I just wonder if the insulation has any effect when the air inside the vent has so little conditioning. It seems to me the walls of the vent would become as cold or nearly as cold, as the surrounding unconditioned space unless they were actively conditioned.

Seems the best thing to do would be to turn the fan on 10 minutes BEFORE you start showering, and allow it to run for 10 or 15 minutes after the shower. This would allow the vent run to heat up to the temperature of the conditioned space, before adding moisture to it.

Just me two cents
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Old 04-20-2011, 05:05 PM   #6
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The dampers are not air tight so there's always warm air rising into the pipe. For a short run like that insulated flex works good as anything.
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Old 04-21-2011, 10:26 AM   #7
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i thought of the insulated flex to, i saw richard trethewey use it on ask this old house for a similiar short run. but i figured it might start to sag or something if it got to damp. i have to do some drain work so maybe ill get a little extra pvc and insulate that as well with the batt insulation, thats a good point to that its leak free glue joints. hmmmm, desicions desicions.
thanks guys for the many answers and suggestions.
p.s. im installing a timer for the fan anyway so i would probably turn it on as i was entering the shower and let it run well after, so maybe thats good to.
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Old 05-23-2011, 04:10 PM   #8
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i have a similar problem. I would say the run is about 7-8ft. Can I just use elbow and flexible duct to connect to the fan, and then run it through the outside wall? (I do not have actual attic space)
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Old 05-23-2011, 04:20 PM   #9
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something like this
http://www.homedepot.com/h_d1/N-5yc1...atalogId=10053

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