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lola123go 05-07-2010 11:38 AM

Bathroom exhaust fan. Is it ok to vent it through attic & up to ridge vent?
I'm not sure what to many opinions!

Do you think I will have problems with condensation dripping down through the bathroom exhaust fan or moisture in the attic?

If I ran a duct from the bathroom ceiling over to the roofs ridge vent, it would travel approx 12 feet. Is this too far?

Thanks! :)

(I live in Baltimore MD)

Nestor_Kelebay 05-07-2010 04:07 PM


No, that's not too far. The bathroom ceiling fans in my basement suites have 20 feet of vertical duct to go through before coming out the vent caps on my flat roof. But, if I were you, I would go the shortest straightest route to get out your roof.

The bottom line is that the shorter, straighter and smoother your bathroom fan vent ducting is, the easier air will flow through the duct and so the higher the volume flow rate through the duct, and the better the fan will work. The following web site shows the "length equivalents" for common fittings for water flow through various sizes of water pipe.

You can see that any perturbation on the ID of the pipe (such as would be caused by a valve or a tee) or change in direction of the flow (as would be caused by a 45 or 90 degree elbow) offers resistance to flow which is expressed as an equivalent length of pipe. There are similar charts for air flow through various size ducts. The length of your duct and any bends in it, and even the roof vent on the end of the duct are all going to increase the resistance to air flow, and reduce the air flow rate out of the bathroom when the fan is on.

Since many of the most common problems associated with bathrooms arise from poor or non-existant ventilation, ensuring you have good ventilation in a bathroom is a wise investment, and you don't want to cut corners here.

So, if it were me, I would use an insulated duct (or insulated it yourself) and have your bathroom fan duct go straight up from the fan through your roof. Any shingle roofing company would be able to install a fan vent on your roof, and this is the kind of thing that you typically only do once in a blue moon, or whenever you add a bathroom to your house, so it's not like it represents a major expense when you consider it's something that's done seldom and greatly affects the comfort of your house. (In some houses even here in Winnipeg, there are windows instead of ceiling fans (the building code here required one or the other at one time), and the condensation problems some people have on those windows in winter is Gawd-awful. A good bathroom ceiling fan that works well will make your house more comfortable to live in by reducing condensation and mildew problems and eliminating excess humidity from your environment.)

I have also heard of people running the fan duct out a soffit along the side of their house. The problem that may occur there, however, is that an condensation that does form inside the duct will leak out at the duct joints and possibly cause water damage to the ceiling in places inside your house.

oldognewtrick 05-07-2010 06:24 PM

You are better off running the shortest route to a dedicated roof exhaust vent. They make vents that have flaps that close when not in use and prevent back drafts into the vent pipe. I believe that venting to a ridge or to a sofit is against most local building codes.

abrowning 05-10-2010 09:38 AM

I echo the previous answers of taking the shortest route to a dedicated roof vent.

And I'm curious about how you would connect to the ridge vent. Is there a special fitting for that? If not, you would be asking for a mold problem. My neighbor let his bathroom exhaust fan duct in to the attic for just a few weeks (not sure why) thinking that the open attic windows would allow the moisture to escape. He ended up with a BAD mold problem up there.

oldognewtrick 05-11-2010 06:33 AM

The problem is the ability of the fan to push heavy moist air in a verticle flow. I know of no attachments for ducting to a ridge vent, doesn't mean there isn't one made tho.

handyguys 05-11-2010 03:03 PM

code in my area allows the bath vent to be directed into a soffit vent. That's the easiest and cheapest thing to do for the builder and how mine was done. I intend to do a proper vent out the gable end someday.

oldognewtrick 05-11-2010 03:11 PM


Originally Posted by handyguys (Post 44739)
code in my area allows the bath vent to be directed into a soffit vent. That's the easiest and cheapest thing to do for the builder and how mine was done. I intend to do a proper vent out the gable end someday.

Handyguy, if you get one of the roof vents with a flap, keep the run short, it will work better than running to the gable vent. The vent with the internal flap helps reduce back drafting of cold air. They are fairly cheap ($20 or so) and pretty easy to install.

handyguys 05-11-2010 03:49 PM

hey, thanks for the advice oldog/newtrick. I should have been more precise. I want to put an exhaust vent into the gable end wall which is near the bathroom. I prefer to go that route, versus having a roof penetration.

Wuzzat? 05-11-2010 04:12 PM


Originally Posted by Nestor_Kelebay (Post 44628)

I think this link is for water flow, not air flow.

kingz1032 05-13-2010 03:58 PM

bathroom fan problem
This is where I got my bathroom fan. It works really well in terms of exhaust, and they help you set up and answer any questions. It also looked very nice in my bathroom. :)

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