My home in the Chicago suburbs has a third level that is a mostly finished master suite with a full bathroom. As the weather warms up we notice an obvious “body odor” type smell in that room. When the AC runs you can’t smell it.

I'm worried what I'm smelling is mold. There is no obvious water staining on the ceiling. There is a small crawl space but I can’t see any signs of moisture there. Where do I start to find the source? If this Is not DIY friendly, what type of professional would I call?

  • 1
    Could the smell be from mice or something that has died in your ceiling? Is the smell more noticeable when you are up there?
    – JD74
    May 8, 2023 at 2:31
  • @JD74. Thanks for the reply. But don’t think it’s mice. Smells more like musty or BO. Also, it’s only noticeable during the summer months. If it was mice, I’d expect to smell more than just when the weather warms up
    – Jdv590
    May 8, 2023 at 2:38
  • Probably mold in the walls, there needs not to be a clear visible leak on the outside, a small continuous dripping would do it
    – Traveler
    May 8, 2023 at 3:04
  • Try closing the sink and shower tubs with rubber plugs and observe if it is from drain
    – Traveler
    May 8, 2023 at 3:06
  • 1
    What type of AC do you have and where is it located? Maybe a condensate pan is not draining and the stagnate water is making the smell. May 8, 2023 at 13:49

1 Answer 1


No one answer, but speaking as a desperate homeowners and not so much a DIYer-

I'd check the attic first; look with a flashlight and blacklight for poo and pee or any nesting in the insulation. If a rat has died in there, a rat has lived in there, and the evidence is apparent. Check the roof while you're there as well, look for any evidence of water intrusion. Also if you have attic vents, ensure they're operational.

Next, I'd check the vent fan. Make sure the ducting isn't blocked, say with a dead rat, (or even a birds nest), and that the CFMs are adequate and that the run and dimensions are OK. Ensure the fan is clean, still gets a good pull and, if you haven't, install a timer so that it can continue running 15m after every shower.

Third I'd check for any leaks in or around fixtures. Ensure the p-traps have water in them so sewer gas isn't seeping in. A "BO" smell is very non-specific, but sewer gas doesn't necessarily smell like poo. Ensure the plumbing vent is working; this may be a job for a plumber, but if you have any complaints like a slow draining toilet, or odd interplay of plumbed fixtures, the vent may be non-functioning or improperly supplied.

Fourth I'd check if the window has any issue of water intrusion. This can happen everywhere but if it happens anywhere, it's often at the window, and often because of improper flashing. Water goes behind the siding at the top of the window, and saturates the walls around the stool, sometimes with signs of rot around the corners.

If 1-4 are OK give everything a good wash with bleach, wash your towels with a splash of bleach (and dry them on high), replace your bathmat, your shower curtain, old bottles of lotions and stuff, and add a nice airfreshener. See if the smell comes back and, if so, I'd get an inspector.

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