# Preferred way to vent a bath fan



## brokenhammer (Jan 11, 2009)

I'm doing a bath remodel and am taking out the window in it. I know I need to add a bath fan and vent it out. What's the best way to run the vent out of the house in a 1 story home. Is it out the roof of can it usually be easily run out the side of the home? 
I'd rather do it that way and have one less hole in the roof.. if it's not a ton of extra work.

thanks for your insights.


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## Just Bill (Dec 21, 2008)

Long duct runs can reduce the efficiency of the fan, but for short runs, a side wall is preferrable to the roof.


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## ultimatetouch (Oct 8, 2010)

Most bathrooms we remodel vent out of the soffit or the attic and through the roof. Flexible insulated duct is what we use. Usually the heating contractor will take care of it if were doing a large renovation or new house.


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## concretemasonry (Oct 10, 2006)

Don't be cheap with the fan and buy an undersized unit.

Since it probably the only vent for the bath, get enough capacity to remove the moisture from washing and showering. Odors can be handled easily and with a lower speed setting.

It gets used many times a day, so do not skimp since undersizing or an improper installation is costly and long term evacuation is embarrassing.

Dick


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## Gary in WA (Mar 11, 2009)

Try to use smooth wall straight pipe for better efficiency. The bathroom air is loaded with moisture during a bath or shower, and for some time afterward, depending on the size of exhaust fan and replacement air. Code does not allow exhausting into an attic as the moisture could condense there. The same reasoning not to exhaust out a soffit near the supply vents, it would be drawn right into the attic because of the pressure differences and heat build-up there. Flex duct has more surface area than straight ducting, especially the foil flex. Flex also creates air turbulence inside the ducting effectively forcing the moisture to land on the duct’s sides, keeping them wet long after the fan stops. If you must use flex, the fan has more resistance to overcome and it moves less air, so keep the run under 6’. When using rigid pipe and elbows, be sure to tape all connections and joints over the required 3 screws. Insulate and vapor barrier wrap when in the attic or concealed space to prevent condensation. Use a hood with a back-draft damper. Your local Building Department may have other added restrictions.
http://www.pse.com/SiteCollectionDocuments/residential/1210_RES_Moisture.pdf

Gary


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## concretemasonry (Oct 10, 2006)

Keep in mind that the code is the minimum you have to do and still be legal, but may not be the best way to do it. You also may choose to decrease the fan manufacturers capacities, since they are for advertising and are done in lab tests to prove them under ideal conditions.

Dick


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## ultimatetouch (Oct 8, 2010)

GBR makes some sense on not running it over six feet and taping seems. But there really should be no seems if your making a short run. As far as code goes hes off. You can run duct work through an attic but it must be sealed and insulated. Many times in our remodels its the only option to vent through the soffit or roof. There is nothing wrong with it if you use the proper duct work and make sure there is no kinks in it. Most full bathrooms are on 2nd floors over here and in many parts of the country.

_________________________________________________________________
Bathroom Remodeling Contractor, 
Bathroom Remodeling Contractor


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## Gary in WA (Mar 11, 2009)

Read it again, slowly this time: 

"Code does not allow *exhausting* into an attic" 

"Insulate and vapor barrier wrap *when in the attic or concealed space*" 

"When using *rigid pipe and elbows*, be sure to* tape* all connections"

I spaced it out and put some in bold so as not to confuse you again......

Gary


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## Michael Thomas (Jan 27, 2008)

Real world, based on my experience at home and moisture intrusion inspections here in Chicago, the problems with soffit vents as compared with roof vents usually involve installation issues, including:

1) They often involve longer runs that through-roof vents.

2) By their nature, they involve horizontal runs.

3) They are more often run in flexible ducts.

4) These three conditions mean that they are usually _much_ more subject to low spots and other causes of condensation problems:










5) Because they are on the attic floor, and usually in flexible duct, they are considerably more likely to experience mechanical damage from foot traffic and stored household goods:











6) Especially at roofs with a slope lower than 4/12, it is often somewhere between difficult and impossible to correctly install the duct and terminations, during my inspections this is a _very_ common location at which to find disconnected or damaged ducting: 











The bottom line is that assuming a properly pitched vent is run in properly insulated hard pipe and correctly and permanently connected to a properly installed termination, and as long as the attic is not negatively pressurized (for example by a powered attic ventilation fan) and exhaust air is not drawn back in through the soffit vents, then a soffit vent is fine - but at least in my area (Chicago) the chances of finding such an installation are pretty slim.


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## brosea (Oct 7, 2010)

I am in total agreement with Gary, always use ridgid whenever possible.
If you must use flex make sure that it is fully stretched out. It is better to use a little extra and do a large radius turn than to kink it in the corners. And dont forget to add a timer!!!


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## federer (Aug 20, 2010)

the joys of homeownership...........sigh


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