There is a smell in an interior bathroom with no windows. The tub is rarely used. When I noticed the smell, I got on my hands and knees to sniff everywhere, stuck my head in the toilet tank, shoved my nose in the tub drain, put my head in the vanity, and so on. Nothing smells up close, so I threw bleach down the sink, toilet and tub. Since the tub hasn't been run since I last cleaned it about 3-4 months ago, I figured that was the culprit. I ran hot water in the tub, cleaned it again for good measure, and thought the smell went away. That was when I noticed that the smell goes away when the AC is running. If there is no air being pumped in, the smell is there. That allowed me to rule out a dead critter in the duct work. Any ideas???? Thank you!!!

  • Well, any room w/o ventilation will start to smell bad sooner or later. Sweat, damp towels, or whatever (uck) could stink up the room. If you own this property, consider putting in an exhaust fan in the bathroom ceiling (will require ducting to outside). BTW, in many locales, either a window or an exhaust fan in a bathroom is required by code. Commented Jun 23, 2016 at 15:15
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    Sounds like something in the room is slowly emitting odor, and the longer you let the air sit the denser the odor becomes. Commented Jun 23, 2016 at 15:19
  • Carl, there is an exhaust fan. I am trying to figure out what could be emitting the odor. I can not pinpoint it with my nose, and I am trying to avoid a large bill for a plumber to try and locate it. Thanks.
    – acgirly
    Commented Jun 23, 2016 at 17:02
  • Try checking the fan itself. Sometimes mold can begin to grow in the exhaust vents.
    – paas_shill
    Commented Jun 23, 2016 at 17:23
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    Does the stench remain if you keep all your traps full of water for a few days? When water evaporates from a trap, sewer gases can flow into your room. Even if the drain/trap doesn't stink at the time you check it, an open trap could have previously conveyed a smelly cloud of gases into the bathroom. Commented Jun 23, 2016 at 18:33

3 Answers 3


To me it sounds like the vent from the bathroom to the outside must have moisture in it and growing something. I say this because many HVAC systems have a small percentage of fresh air added when they are running and this keeps the air moving out of the bathroom. When the System is off the smell wafts back into the room. You may be able to use a spray disinfectant, Remove the cover, turn the fan on and direct the spray into the fan (not the motor) and spray for a few seconds,,, or minutes. put the cover back on and see if the smell is still there in a day or so. I am guessing you have a fan since there are no windows and this has been code for decades.

  • Thanks Ed, I will try this and let you know how it works out!
    – acgirly
    Commented Jun 23, 2016 at 20:05
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    Well, my boyfriend went in the crawlspace and the dryer vent hose was three quarters of the way blown off the aluminum part of the vent. There is about an 8-10 foot diameter of lint strewn about the floor of the crawlspace. He duct taped the hose back on to the aluminum part. The smell dissipated a lot, but it is still slightly there. I figure that must have been it. Vacuuming the dirt of the crawlspace is out, so I am looking for something to alleviate the musty smell now.
    – acgirly
    Commented Jun 25, 2016 at 4:51

Just a thought, I worked in an office with an internal bathroom, i.e. no exterior walls and we noticed a off smell there as well. The toilets and sinks worked fine, it wasn't until we realized the floor drain was the source. There was a P trap on the floor drain and the water had evaporated in the trap, allowing air to escape into the room.

You might want to make sure there is enough water flowing in the tub drain to seal.

The other possibility is there is a negative air pressure and air is being pulled (bubbling up) past the gooseneck or P trap into the bathroom. that might be why the smell goes away when air is being pushed into the room by the AC.

  • I think you're on to something here. My guess is that the reason it goes away when the AC is on is that the AC essentially "pressurizes" the room and forces the air out the vent, drawing the stink with it. Also if it is sewer gas coming through the p trap it might be inconsistent, so it might not smell all the time. So, keep your p traps full.
    – Tim Nevins
    Commented Jul 5, 2018 at 18:57

Looking under the vanity, does your sink drain have a simple p-trap that goes straight into the wall? Or is there something more in there, something sticking straight up with a bunch of vent slots/holes in it? You could have a failed air admittance valve, allowing sewer gasses to leak out.

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