How do we pinpoint the source of an odd smell in our finished basement?

We first noticed a stench a few months ago (thought it might be a dead mouse), and then the smell went away. Now, there's another stench, noticeable as we descend the stairs. It smells like dirty cat litter (we don't have a cat). Could there be something in the ducts or walls? Any ideas...?
  10 answers
  • KMS Woodworks KMS Woodworks on Mar 22, 2012
    In my experience a dead mouse tends to turn into a mummy and not really throw that much smell into a space. I would lean more toward a possible sewer gas issue. I have know of some infrequently used showers in guest baths etc. where the water in the P trap may evaporate to a point where the seal is lost. Is there carpet down there? old is the home / space.

  • Jessica T Jessica T on Mar 22, 2012
    The house is ~30 years old. The basement is fully carpeted except for the bathroom, part of the laundry room, and there are only carpet remnants in the utility room. There IS a shower that is seldom used...definitely something to look into. Thank you!

  • KMS Woodworks KMS Woodworks on Mar 22, 2012
    Most folks will use sinks in these seldom used spaces way more frequently than a tub or the traps in the sinks see use to keep them full. I'm not a huge "carpet" fan and prefer hardwoods and tile. we cover these with areas rugs which can be pulled and cleaned easily. In my home only two rooms have carpet, we try to have these cleaned every few years to keep them "fresh".

  • Jessica T Jessica T on Apr 20, 2012
    It's definitely not the bathroom. The odor still comes and goes. The utility room is next on the suspect list. I prefer hardwood floors over carpet as well. But the carpet was new when we bought the house three years ago, and projects take money.

  • KMS Woodworks KMS Woodworks on Apr 21, 2012
    Does this utility room have sinks and drains etc?

  • Jessica T Jessica T on Apr 26, 2012
    Yes, but the smell isn't from that room. It seems to lurk in the air.

  • KMS Woodworks KMS Woodworks on Apr 27, 2012
    I have seen some P-traps dry out enough to allow sewer gas to enter the room. This often happens in place were they are not used regularly. On a slightly windy day the downdraft in the vent stack can send smells into the room. If these have not been used in a long time run a gallon or so of water through them to fill the traps.

  • Tee B Tee B on Apr 27, 2012
    ...with a litte clorox to kill any bacteria adding to the odors.

  • Linda Linda on May 14, 2012
    If it smells like a dead animal, it could very well be a dead mouse. They can get trapped in your walls and die and then stink for a week or two until they dry up. It's not nice, but it will pass.

  • Marilyn Lefler Marilyn Lefler on Oct 30, 2015
    If it's coming from the P-trap, pour some cooking oil in it. It traps the odors and keeps them from getting out into the room.