Wet attic around bad bath vent fan (C) Daniel Friedman Bathroom Ventilation Fan Installation FAQs
Questions & answers on bathroom exhaust vent codes, specifications, advice

  • BATHROOM VENTILATION FAQs - CONTENTS: How to install, specify or improve bathroom venting, reduce indoor condensation, avoid bathroom mold. Bathroom vent fans, required bath vent fan capacity, fan noise and sones. Bathroom vent fan ducts, where to route vent air, duct condensation, ceiling leaks; Photographs of bad or ineffective bath fan installations
  • POST a QUESTION or READ FAQs about bathroom vent fan and fan ducting installation procedures, codes, standards
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Bath vent fan installation, troubleshooting, repair FAQs:

Questions & Answers on how to design, size, install, wire, or fix bathroom vent fans, ducts, switches & controls.

This article series describes how to install bathroom ventilation systems, fans, ducts, terminations. We include bathroom venting code citations and the text also explains why bathroom vent fans are needed and describes good bath vent fan choices, necessary fan capacity, and good bath vent fan and vent-duct installation details.

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>Bathroom Ventilation Code, Fans & Ducting Questions & Answers

Bath vent spills into attic (C) D Friedman


(Sept 26, 2014) James said:
I have come across your site 3 times, as I continue to get our recently purchased 1950's home up to snuff. Excellent resource, great photos, and very detailed explanations. Just wanted to say thanks for your effort!


(May 16, 2014) Kevin said:
Are there any restrictions on how close an exhausting bath vent may be placed to an exhausting vent for a gas water heater? Both exhaust vents will terminate outside through a side wall. Thanks.



Clearances for gas fired appliance sidewall vent terminations are given in detail at - side wall & direct vent clearances

You will see that the required distances range from 1 foot to 7 feet depending on what's being cleared-from.

If the bath vent respects the gas fired exhaust (chimney / flue) vent clearances you should be ok.


(May 21, 2014) Mary. Question said:
My builder has installed a small 4" extractor fan in a newly created utility room with a door to outside, but no windows. The fan has been installed on the outside wall which is 30 cms thick. It will blow out through the old metal ventilation grille. My worry is that he has not lined the 'duct' with anything other than plaster. I cannot see any plastic or metal material lining the hole. Is this acceptable? Won't I be getting damp air in my brickwork?


Mary I'm not sure I've got the whole picture, but I'd agree that a metal or plastic duct liner would have made cleaning easier and would have reduced the chances of damage should moisture accumulate in or condense on the sides of the air path. If it's easy to paint and thus seal that surface that'd help. Sounds a bit odd.


(Aug 20, 2014) rob said:
we had rough in electrical inspection and it passed and went back and seen our electrician vented ours to the attic and there it lay i live in pa how did that pass? other than their all close friends with each other?



about the mistake of incomplete routing of a bath vent fan I can't *know* but I can offer speculation based on experience:

1. The electrician is not someone who is going to want to cut a hole in a roof, gable end nor soffit - nor might s/he know how to properly flash and seal that opening against leaks.

2. The electrician wants to finish the job quickly and get paid and go home for a beer and to watch the World Cup on TV.

3. The building inspector doesn't want to crawl into a hot attic, and furthermore, cannot possibly inspect every detail of every job - so over time the inspectors tend to get to know individual contractors and to trust (or not trust) their work, making just spot checks on it. It's not a matter of being close friends or of bribes.

You need to vent the fan properly to avoid a moisture problem down the road.

Question: Can I vent a bathroom into the attic space that has soffit vents and a ridge vent?

(Sept 7, 2014) Anonymous said:
Can I vent a bathroom into the attic space that has soffit vents and a ridge vent?



Physically you "can" but it's a bad idea, sending moisture into a building cavity rather than outdoors invites mold, insect damage, rot.

Vent bath exhaust to the outdoors not into an attic.

(Sept 15, 2014) Anonymous said:
Thanks, will look to vent to the outside but was hoping that the air flow from the soffit to the ridge vent would carry out the moisture



Well air flow may carry moisture but enroute, flowing across attic surfaces it will also deposit it on cooler surfaces - leading to mold-sorrows later.

Question: how can I keep the bath fan from sucking the heat out of my building?

(Sept 17, 2014) Redpoint said:

I want to install a four port remote bathroom fan in my house. I have a conditioned attic where the 4" thin walled PVC pipes will run. I have two concerns; first when I have this running for one bathroom i don't want it sucking all the heat out of my house (in winter) from the other 3 bathrooms. How would I prevent this from happening? Second I have some long runs in my conditioned attic (60-70ft) is this too long and should I pitch the pipes or worry about condensation ? Thanks


You might want to use an air to air heat exchange ventilation system instead.

Question: Can you vent a bathroom fan into the chimney?

(Nov 5, 2014) Peter said:
Can you vent a bathroom fan into the chimney?


No. Don't do that.


Nov 13, 2014) Gene said:
We get a sewer smell in two of our four bathrooms in the fall when it turns cold and also in the spring. It's usually short lived but is lasting longer this time. The same smell is also in a few spots outside our home. Toilets are not loose and bathrooms are well vented. Any suggestions ? Any help would be appreciated. We live on an acreage but have city septic.



Even if a toilet is not loose it could have a failed wax ring seal.

You might also be facing odors exacerbated by partly clogged drains or defective vent piping.

Question: what's the best way to venta guest bath with a room above it

(Dec 8, 2014) Anonymous said:
What's the best way to vent a 1st floor guest bath that has a half story above it? Should I just go straight to the outside wall of the bathroom, through the brick? Or should I try to go around the upstairs rooms to get to the gable attic, and to a vent there? The second just seems like a long distance, but I can't find much info about venting out the side of the house.


Anon the best answer is site specific - depending on framing and construction details such as which way ceiling joists run I might go into the ceiling and then out through the wall, or I might place a vent right into the exterior wall.
Shorter distances are of course better.

Question: stop blown insulation from entering the bathroom through the ceiling vent

(Dec 24, 2014) joyce said:
how might I stop the blown in attic insulation from entering the bathroom thru the ceiling vent when the wind blows thru the attic window vent?



Your installers needed to adequately seal or baffle around ceiling vents, ducts, HVAC air intakes, etc. to keep insulation in its place.

There is a more serious worry here if your bath vent is for a ceiling exhaust fan: loose fill insulation that enters a power-operated bath vent can clog it leading to overheating and a fire.

(Dec 24, 2014) Anonymous said:
Dan, thanks so much for this reply. Yes, the bath fan is for exhaust of moisture during shower, etc. I live in a home well above a lake, when the wind blows hard (often) I then note fine particles of insulation on the bathroom floor and other surfaces....I have an attic vent on a gable style roof line and hear the wind blowing this time, I have in place plastic over the vent until I can figure out what to do. Not using the fan. Yes! conerned re: fire..thanks so much.



take a look at our notes about wind-washing - blowing attic insulation from air entering at soffits


(Dec 29, 2014) Katrine said:
We moved into a house built in 1999 in north Florida and learned that the bath exhaust fans are connected to the HVAC ductwork and not to the outside. Is this acceptable? We have had problems with high indoor humidity even when the air conditioning is running.



I can't be as smart as an on-site expert who will see important details we can't, but what you describe sounds wierd to me. Exhaust fans should exhaust to outdoors, not blow moisture or odors into the HVAC system. It sounds to me as if someone took an improper shortcut.

(Dec 30, 2014) Katrine said:
Thanks for your reply. I am following up on this with the buildings inspection office and getting everything rerouted properly.


(Apr 23, 2015) Jim said:
Why is the air from my bathroom exhaust fan blowing down into the bathroom instead of blowing up and out?



Some bathroom fixtures combine a light, exhaust fan and heater. If your ceiling fixture includes a heater it may be on and blowing air down.

If your fan is ONLY an exhaust fan then it's running backwards OR there is no exhaust vent.

Question: insulation leak at bath ceiling vent?

(Aug 3, 2015) Carole said:
I have a combination light/exhaust/heater in bathroom ceiling. It looks like blown insulation in the vent blades. Hate to think I am breathing this. What do I do?


As the bath is normally going to blow air out rather than into the bathroom, it ought not be blowing anything onto you. However if the fan is not properly-installed or if its ducting is damaged, attic insulation could be falling into the room below.

Start by identifying the suspect material; most-often you'll see gray dust accumulating on the intake louvers of a vent fan as house dust, principally fabric fibers and skin cvells, is deposited to the surface of the fan while air passes into it.

Question: birds moved into bath vent ducts

(June 26, 2016) Rgrigsby said:

Question, several years ago we had 2 bathrooms redone! The venting was redone at the time. The vent material was light weight foil. Run through the ceiling and the second floor.

Bird move's in to the cheesy duct and in the process of the bird evacuation the line is damaged and is not accessible without removing the drywall in the ceilings. What are the odds of those lines meeting code.


I can't guess at the code compliance issues, though you could ask your local code inspector for an opinion; the installation is largely hidden by reality (the ceiling) and remoteness (e-text). However there should have been an appropriate vent guard on the exterior wall = that usually is what keeps the birds out.

Since you've got who-knows-what bird pathogens in the ceiling it might make sense to do enough demolition to remove the nasty old birded-duct and install new material.

See BATHROOM VENT CODES for details about the allowable screen size on bath exhaust vent terminations.

Question: wind tunnel and flapping noise at bath exhaust vent

(Aug 18, 2016) Toni said:

I recently had a contractor come in and replace a 20 yr old bath exhaust fan in my condo. I did not watch the process, however due to the "accessibility" of the existing ducting, he was forced to use flexible ducting to connect the new fan. I intentionally requested a 1 sone fan for obvious reasons, however the fan noise level is inaudible due to the now very low "wind tunnel" noise.

The contractor advised that because he was forced to install flexible ducting, this is what I was hearing. In order to repair with rigid ducting, he would have to open the ceiling up further. Is it likely that the flexible ducting is what is causing this flapping wind tunnel noise? Thanks

sorry my last comment should have stated " very LOUD "wind tunnel" noise...not LOW.

Reply: sources of extra or abnormal noise in a bathroom vent fan

Tony I can't know quite what's going on from an inspection by e-text, but a key word in your note is "flapping" - something is loose. I'd want to inspect and find what that is before spending a lot on a new approach to the exhaust duct.

Flex duct, properly installed in a bath fan, would be stretched out with no extra material and ought not be flopping nor flapping around.

Also check for a loose flapper door on the vent duct termination outdoors.

Noisy bath vent fans can also be traced to use of a reduced exhaust vent duct size. The duct size must be at least the same as the diameter of the fan's discharge port. Here is a typical manufacture's warning:

Duct work size must be a minimum of the discharge and should not be reduced. Reducing the duct size can increase fan noise.

To avoid motor bearing damage and noisy and/or unbalanced impellers, keep drywall spray, construction dust, etc., off power unit. - Ventilation Fan with light and Heater, Model RAD80L, Delta Breez Ventilation System, [PDF]

Question: can bath vent fans share ductwork? How does that affect noise?

2016/09/12 Jane said:
Hi Guys

Can someone tell me whether you can run 2 fans (in the same bathroom) through one lot of ducting? One appears to be quieter and less powerful than the other.

Many thanks for any advice offered.



You can vent two fans through a common duct provided the duct size, length, number of bends meets the manufacturer's specs. Watch out for back-drafting out through the fan opening that is not running when the other one is. If that problem occurs your installer will need to either vent the fans separately or add a one-way backdraft preventer in the ductwork .

Fans vary in noise - measured in sones. A problem is that noisy fans tend to be left OFF so a bath may not be vented when it should be.


Also see AIR LEAK NOISES in BUILDINGS as duct leaks can also be noisy.

Question: OK to vent a bath exhaust fan down and out under an addition?

2016/09/12 Todd said:

We have built an addition to our house. The bathroom fan vent was run to the outside of the house but was removed as the addition surrounded vent. The contract nicely wrapped the exhaust hose around the fan and installed the roofing with out re plumbing it. All the floor joists have some kind of wiring or vents running through them.

Would it be alright to exhaust the bathroom fan air under the addition as there is to vents on either side of the addition. I would have to cut a hole through the wall to get to the next room to install the vent outside. This is a down stairs bath.


In general, you can try venting a bath vent fan "down" but the combination of down-direction, elbows, and total run length may mean that it's ineffective. Take a look at the installation manual for your fan brand and model. The manufacturer will give guidance on the total duct length permitted. Adding elbows and bends increases the "effective" duct length (shortening what's allowed) as would blowing "down".

Please see BATHROOM VENT DOWN THROUGH FLOOR / CRAWL AREA for a detailed reply to your question.


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