Unidentified Smell - any ideas?

1

Client has a home built in 2019 - Crawlspace - on a gravity fed septic.

Client advised that when it rains heavily, he notices bad, musty smells. He swears that is isn’t sewer gas, but I cannot confirm. The smell is present ONLY when it rains hard.

He was concerned with mold - we did a look around for any moisture intrusion; everything looks good. An indoor air quality test was completed; everything looks good.

Has anyone experienced this before or have any ideas?

2

Based on AI ChatGPT…otherwise I no clue… :thinking:

A bad smell in your house after a hard rain is usually caused by one or more of the following:

1. Sewer Gas Backup

  • Cause: Heavy rain can overwhelm the sewer system, causing sewer gases to back up through drains, especially in basements.
  • Smell: Rotten eggs or sulfur-like odor.
  • Check: Floor drains, toilets, sinks (especially unused ones), and sump pumps.

2. Mold and Mildew

  • Cause: Excess moisture from rain can seep into walls, floors, or the foundation, creating a damp environment perfect for mold growth.
  • Smell: Musty or earthy odor.
  • Check: Basements, crawl spaces, behind walls, under carpets, or around windows.

3. Wet Building Materials

  • Cause: Rainwater infiltrates the home through leaks in the roof, windows, or siding, soaking wood or drywall.
  • Smell: Damp, musty smell—sometimes similar to wet cardboard.
  • Check: Attic, roof, around windows, or any recently patched areas.

4. Clogged or Poorly Ventilated Drains

  • Cause: Rain can push debris into drains, causing blockages or slow drainage, which can lead to stagnant water and odors.
  • Smell: Sour or decaying odor.
  • Check: Kitchen sink, bathroom drains, laundry area.

5. Foundation Cracks or Drainage Issues

  • Cause: Water pooling around the foundation may leak inside, especially if the grading or gutters are poor.
  • Smell: Musty or muddy, sometimes like stagnant water.
  • Check: Basement corners, sump pump area, crawl space.

Would you like help narrowing it down based on where the smell is strongest or what it smells like?

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Musty smell and sewer smell are very distinct smells. So the range of possibilities is somewhere between mold and sewage.

Where does the client smell it, in the bathroom? Outside? In the crawlspace? The garage? Are the windows open or closed? What did you smell? Did you inspect it when it was raining? Why not?* Is the crawlspace ventilated? Is there a vapor barrier? Is it sealed? You “looked around”. Where did you look? The interior? The attic? The crawlspace? The exterior? Where are the plumbing vents? The gas regulator vent? Are they backdrafting into the house? Where is the inspection port, the D-box, and the perimeter drain discharge in relation to the house? How close is the neighbors house? Does your client live near any industries? What direction are the prevailing winds? You performed an indoor air quality test. Where? In the interior of the house? In the crawlspace? In the attic? Did you do an outdoor test for comparison?

Valid questions when our only clue is, “My client smells something when it rains but he doesn’t know what it is”. Humid air can enhance smells. Sometimes recreating a condition is the only way to diagnose a problem. Let us know what you find.

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Very helpful! Thank you for those clues!

For some additional context; we only did an IAQ test. During that test we did a short walkthrough to look for anything obvious. I am going to personally go back to that house and dig further and I’ll keep this thread updated with the findings. Rain is coming Friday and I want to experience this first hand.

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Did you enter the crawl?