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Crawl Space Odors

Should you leave the old vapor barrier and debris behind when performing crawl space encapsulation? Can leaving old vapor barrier and debris be causing odors? We had a great Ask a Ninja from Georgia and wanted to share our thoughts:

I purchased a home in March which had mold remediation and a 20 mil Dura Skrim vapor barrier installed (ground only). A Sante Fe dehumidifier keeps humidity in the high 40's; there are 2 open vents out of 9 in a 1400 sq ft space; no active vent fans. The crawl space smells, but not like cat urine. I believe it to be soil off gassing and it gets pulled into the HVAC. Can CSN add-on to my existing (newish) 20 mil barrier to complete an encapsulation or must we start over?"

Crawl Space Odors - Cat Pee

Not every odor you find in the crawl space smells like cat urine. Cat urine odors are prevalent in many crawl spaces. Crawl space encapsulation is not a guarantee your crawl space will not smell like cat pee. We feel the biggest reason for cat urine odors in crawl spaces is cheap reinforced vapor barrier. If you recently had the crawl space encapsulated with reinforced vapor barrier consider removing and replacing the plastic.

Crawl Space Odors - Soil Gases

Crawl space odors can also come from under the plastic and the odor can be hard to identify. Many homeowners want us to leave down the old vapor barrier. This can be a problem if the old vapor barrier was ever exposed to sewage or moldy debris. Most will not take time to wipe down the plastic before installing new plastic on top. trapping odors between plastic can create a hot bed for more bacteria. More bacteria can become more odors after crawl space encapsulation.

Debris Under the Vapor Barrier Causes Odors

This is one of the biggest overlooked items when dealing with crawl space odors. The debris under the plastic vapor barrier can grow mold, wood rot fungus, bacteria and slime molds. The percolate together and off gas some powerful odors. make sure you remove as much loose organic debris as you can prior to installing vapor barrier.

Will radon Mitigation Lower Humidity


Do you need help battling an odor in your crawl space? If so, please contact Crawl Space Ninja and we will be glad to help. Also, let us know in the comments below if you'd like to suggest a future blog post.

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About Michael Church

Michael Church has been involved with indoor air quality since 2005 and feels the unhealthy crawl space is one of the major problems causing poor indoor air quality.


  1. Richard on December 23, 2022 at 10:30 pm

    Hi Michael,

    What might be the cause of our crawl space liner to be billowing? This appears to be in correspondence to a slight odour and the liner has numerous air pockets underneath, whereas the liner used to lay flat. 🤔


  2. Aaron on February 6, 2023 at 12:26 pm

    Hi Michael,

    I purchased my home in October of 2016. Had fiberglass insulation in the floor joists but with the humidity a lot was wet / sagging, just in bad shape. I removed all of the insulation and fully encapsulated the crawl space and added a dehumidifier both in 2017 / 2018. A few months ago we started noticing a smell in the HVAC, when the air handler fan is running. It’s not mold, cat pee or any of the usually suspects but it smells horrible. I had my HVAC ducts professionally sealed about 4 years ago as well. I added 4″ perforated, corrugated pipe under the vapor barrier and added a soil gas mitigation fan (outside the house) as well opened up two of the vents (opposite corners of the crawl) and installed an intake and exhaust fan. Nothing has worked, I still have the smell. I have since removed the intake fan and left the exhaust fan. I faintly smell the odor in the crawl space when I’m under there but not like when it comes through the HVAC vents. Any ideas / suggestions on how I can remedy this? It’s driving me nuts and I’m at a loss for solutions.

    • Michael Church on February 8, 2023 at 3:10 pm

      Hi Aaron, if it is primarily coming from and through HVAC system, it could be an odor from the coils of the unit. Have you seen our video on dirty sock syndrome? It may help. https://youtu.be/pJsTgVCLX6M

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