Is it normal and/or code compliant to have a bathroom exhaust fan venting to the attic?

I've seen this a couple times now, and the first time I saw this I thought it was very odd.

But I mentioned this to a couple people around here and they thought it was 'normal' for this area (I live in a part of southern California where it is rarely below freezing).

  • 7
    It's common because, sadly, a lot of contractors are lazy and cut corners whenever they can.
    – DA01
    Apr 9, 2012 at 13:19
  • 3
    I've seen bathrooms and dryers vented into attics. It leads to wood rot and moisture saturated insulation. Sounds like a Contractor Shortchange technique. Oct 3, 2012 at 1:19
  • In my last house the lav fan vented across the hall and down the basement stairs inside the ceiling. Since there was not a shower or tub in there I never bothered to change it. The upstairs bathrooms were vented through vinyl ducting to the soffit vents. My current house has all metal ducting in all toilets to the outside.
    – sborsher
    Apr 30, 2014 at 20:18
  • Mine vents right onto my patio :( May 6, 2017 at 0:21

4 Answers 4


Bathroom vent fans must be vented to the out of doors. Venting this fan into the attic is simply asking for problems. The excessive moisture will cause condensation on the roof members, insulation and eventually cause mold. It is never OK to vent directly into an attic even if the attic itself is vented.

So, the simple correct answer is NO. Your friends are misinformed.

  • What if it's vented to the attic where it connects to the main house vent, is that typical or desired? This way there is only one hole in the roof where we could have a leak in.
    – Scott
    Apr 9, 2012 at 13:30
  • 6
    A bathroom vent or appliance vent cannot be put into a common vent line. When more than one source of air/moisture is connected to a vent line, there is situation where the moist air from one item fills the entire vent plenum and can back feed at the other appliance or condense in it's plenum. I have actually seen a range hood drip water that was connected to a bathroom fan vent line in the ceiling. Apr 9, 2012 at 16:02
  • 3
    @kellenjb; Wrong is wrong and is never looked at as "normal" just because it is a common mistake. KeithS has given a very good and accurate description of what happens in these cases. Apr 9, 2012 at 16:17
  • 6
    It is true that 'normal' and 'to code' can often be exact opposites. ;)
    – DA01
    Apr 10, 2012 at 0:06
  • 1
    'Normal' is what causes code to be written because accumulated evidence that doing it that way causes a high percentage of fail. Jul 16, 2014 at 0:38

See this question. The long and short of it is that a vent fan, whether for a bathroom, a range hood or a clothes dryer, is normally removing very humid air from the room. If that air is not exhausted beyond the waterproof "skin" of your house (up through the roof beyond the shingles, or out the side of the house beyond the siding or brick), then you are not removing the moisture, you're just hiding it.

On top of that, your attic is a "cold zone"; it is not insulated or climate controlled and is generally the same temperature as the outside. So, in colder weather, the moisture in the air will precipitate out. It doesn't have to be anywhere near freezing for this to happen; just cool enough that the air being vented, when cooled to outdoor temperatures, falls below its "dewpoint" and can't hold that amount of moisture.

The end result is that it will literally "rain" inside your attic; condensation will form on the wood rafters and sheathing, and that's an open door to mold (especially considering your bathroom's already a breeding ground for all sorts of nasties that can hitch a ride out the vent fan). In just a couple of years, the mould could make your house unlivable, and your rafters will be rotting until the next severe weather could collapse the entire roof.

So, it is required by code in virtually all jurisdictions that ventilation fans terminate in a vent cap on the outside of the building. There should be no leaks in the vent line that could allow the humid air to escape into an interior space. Bringing the vent to an existing attic vent isn't enough, because the air can still mix before it leaves the space and form condensation. You must either seal the vent line to that roof vent (which would also require you to have enough roof vents that the attic can still "breathe" with the loss of the one you sealed up), or cut a new hole through the roof for a new vent cap.


DO NOT VENT TO THE ATTIC. I speak from experience. While trying to sell my (town) house I discovered a mold problem in the attic. We had a bathroom venting to a duct that was just stapled to a roof joist. I knew this was there and monitored it over the course of the 5 years I occupied it. Of course, it was never a problem until we went to sell it, then suddenly mold appeared. Fortunately it had hardly progressed enough to be a rot concern but it was a major sales problem (it cost me a private sale where I could have avoided paying a sales commission, ouch). I think the cause of the sudden change was a particularly snowy and cold winter that just encouraged condensation buildup in the attic space. Also, the environmental company inspector noted that the roof work that was done not too long before we bought the place had left excessive paper and shingles hanging over the ridge vent. This greatly restricted airflow which would otherwise likely have prevented the mold problem. I cleared the vent but had to spend about $1,200 for the mold neutralization and prevention treatment. The company also extended the bathroom vent down and out a soffit.

Take the time and vent properly but also evaluate the roof's ventilation. Even if you have moist air entering the attic, if the roof is venting properly there's a far lesser risk of mold which prefers stagnant air. The mold guy told me that as long as there's sufficient regular airflow, mold won't grow.


Listen to these kind people, DO NOT VENT TO THE ATTIC! This could cause problems with mold. Pumping all that humidity from the bathroom into the attic can lead to more serious problems. I've heard cases where the rafters needed to be repaired within 6 months of using this method. READ THIS!!


It's a shortcut that can be avoided by routing the venting system 6 feet higher. Pop your head in your attic and make sure that you're contractor doesn't do this. Not sure if it's against code but it should be!

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