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Flex duct bath exhaust vent fan terminating straight up through the roof surface (C) InspectApedia Bathroom Ventilation Fan Duct Routing FAQs
Q&A on routing a bath vent duct

Bath vent fan duct routing questins & answers.

This article weries describes routing bath exhaust fan duct upwards through an attic or roof space or downwards through a floor or crawl space.

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Q&A on How & Where to Route Bathroom Exhaust Vent Duct

Wet attic around bad bath vent fan (C) Daniel Friedman

These FAQs about how to install and route the ductwork for ventilation in bathrooms were posted originally

at BATHROOM VENT DUCT ROUTING - be sure you review the guidelines given there.

On 2019-02-01 by (mod) - good fan velocity can reduce moisture condensation on siding or decking

Yes, good velocity, short run, helps.

Follow-up with periodic inspection, especially for clogged jammed exit flapper door on the vent.

I look fwd to seeing a photo as installed.

On 2019-02-01 by Joel

Thanks Dan. I have all the same concerns, but can't see a better solution. It's a 300CFM fan, & the run would be max 5 feet, so I'm thinking the higher velocity will throw the exhaust far enough to allow adequate dilution. Thanks again. I love these forums

On 2019-02-01 by (mod) -

Joel,

You might see some algae or mold damage, but if it were my house and I had 4 ft or more of height to ground I'd give it a try.

You might also not like steamy exhaust coming up through the deck when you're out there but that's another gripe.

On 2019-02-01 by Joel

Thanks for the quick reply Dan. Decking is all redwood. Back of house is totally exposed to free air, with fairly consistent light breezes. Picture shows where termination needs to be. It would come right through the 2X8 ledger.

Venting a bath exahust duct through a deck ledger board (C) Inspectapedia.com Joel

On 2019-02-01 by (mod) -

It might be a problem, Joel, depending on additional variables; deck material, ventilation, air movement, etc.

My best advice is at BATHROOM VENT DUCT TERMINATION

On 2019-02-01 by Joel

Due to existing framing design, the only way I can vent the bathroom is down the wall 1 foot, then exit back wall right under an overhead patio deck.

Exit point would be 8 feet from nearest door, with no nearby windows. My concern is the warm, moist air eventually damaging the decking just above the termination point. But, would this meet code in the first place? Located in SF Bay area

On 2018-10-01 by (mod) - Does it matter how close to the edge the vent is on the roof?

That is probably ok Jason, but I'd be more-confident if I could see a sketch of your situation; try attaching a photo or sketch to a comment.

On 2018-09-30 by jason

Does it matter how close to the edge the vent is on the roof? My fan is near the exterior wall and going straight up is easiest route out, but with the soffit, it puts the vent like 2-3 feet from the edge of the roof. Any issues there?

On 2017-10-31 by gfc

Can you vent through a wall to a connected garage?

On 2017-10-23 by (mod) - do not connect a bathroom vent fan to a plumbing vent system

No, Bob, you would not connect a bathroom vent fan to a plumbing vent system. Aside from that being unsafe, creating drain failures when the fan is on, risking drawing sewer gases up through the building by losing the water seal in nearby traps, the pipe would be much much too small in diameter.

You can vent a bath exhaust fan out through a building soffit instead of through the roof, as you'll find in examples in this article series.

On 2017-10-23 by bob

Can a bath fan be vented through the existing plumbing drain vent by using a T-connector? Rather than creating a new hole in the roof or wall to vent it outside. Or, can it be vented to the soffits?

On 2017-08-21 by (mod) - any vent installation has to worry about duct routing, slope, condensation, condensation leaks

Bill,

Thank you for taking the time to write with criticism and suggestions for better content. You're quite correct that more needs to be detailed about bathroom venting.

I agree that any installation has to worry about duct routing, slope, condensation, condensation leaks. And I agree that a long vent run across an attic to a gable end may be prohibitive. If I were doing one I'd:

- use solid metal ducting

- come up high enough off of the attic floor that I can get enough slope to the exit point

- insulate the ducts and tape joints with foil tape.

Blowing air "down" isn't easy but then also we don't want water from a long duct run falling back out of the ceiling.

I'm a fan of larger capacity low-sone bath fans, which of course is more expensive.

Where we have, for example, had to vent a bath fan installed close to a steamy shower through an insulated cathedral ceiling, we had a successful install: the vent is in a sloped cathedral ceiling. Its exhaust was taped aluminum vent ducts routed between rafters, down, and out through a soffit where we installed a vent terminator that protects against backdrafts.

The vent works perfectly in freezing weather - we've published photos of clouds of moist air blowing away from the building.
Indeed the vent needed enough CFM capacity to push air down and also to overcome the resistance of the length of the duct and of two 90's.

No water and no "spigot" of condensate falls down the siding.

Air resistance wasn't a problem. But I would be very reluctant to claim that "one size fits all" - in venting installations.

I will think more about your concern and do some research. If you'd like to send photos, sketches, or other remarks to us directly our email is at the page top or bottom CONTACT link.

Thanks.

Daniel

On 2017-08-20 by Bill

Related to installations where an attic has to be used: I'm concerned by

A.) Absence of pictures showing this done the way you recommend. partials don't tell the full story.

B.) Discussion on going out the gabled end, with it slanted in that direction, so that water goes that way introduces new concerns:

1.) To get the rise/fall/slope for a long run, on the 10' -12' mentioned above, requires about 3" at the start point, so an elbow is needed, at least two are, since you need to raise it, using a 45 or 90, then rotate down, thus there will be a lump in your run at the start the water is almost certainly vapor here, but this is a slow down point in CFM.... darned if you do, darned if you do not.

2.) Hot air and water vapor, BOTH rise. so if you think you are sloping to allow water to go out the house, this is not going to occur until that water vapor has become water again, which, as the cool air passes over it at the end of the shower IT WILL cool quickly, there is still a chance in a undersized fan situation, or unequal pressure in the home, that water vapor will sit close to the fan, and can, cool there, and water vapor could remain at the fan housing area (just really unlikely), so

3.) The water vapor very likely becomes water before making it outside especially in colder climates, and with the extra air resistance, SO what does this mean AT the vent exit, near the gable?

Are you now turning on a tiny little spicket spilling water onto your siding?

Behind your siding? Down an interiror wall? Well installed probably not, but still, at best, down your siding! Then when its in a winter climate, you are creating an icicle start point, freezing the door shut?

Same concerns with a roof vent,but at least with a roof vent, both the hotter air and water vapor have a chance of getting OUT fast. Still ice possibilities on the roof

On 2016-10-15 by Sam

I live in Northern California and have a question about bathroom fan duct routing. I have a Panasonic bathroom fan. The manual for the fan recommends a two feet straight (horizontal) run before attaching any elbow connectors. The location of the roof exit to the fan makes this difficult to do.

Currently the vent ducting is set up with a right angle elbow immediately connected to the bathroom vent that then goes straight up to the roof exit.

This setting is loud, and I would like to reduce the sound. The ducting is all smooth galvanized steel.

My question is would it be okay to use three adjustable elbows after a two-foot horizontal run to create a spiral up to the bathroom vent exit? Are there any other recommendations to reduce the noise?

Would using a combination of flex duct and ridged duct be better? Thank you very much for your advice.


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