Cat Pee Smell In The Crawl Space

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So lately there has been a huge movement to confuse homeowners and professionals alike as to what actually causes this cat pee smell that about 5% of encapsulated crawl spaces get.

I don’t know why, but self identified “Experts” have blamed this cat pee smell on everything from reinforced plastic to the poor hygiene of a homeowners cat. We have seen online video’s from the “ninja” explaining how reinforced vapor barrier is “glued” together and when that glue gets wet it falls apart and then the string in the middle gets wet and smells like cat pee. Personally, I have never experienced making contact with any kind of string that smells like cat pee when it gets wet. This company’s ideas are clearly born from the purest form of ignorance, lacking the knowledge but claiming to be an expert anyway. After all they have some “ninja plastic” they bought from the home improvement store to sell you, without the reinforcement that smells like cat pee of course.

The REAL Cause

In the videos we have posted online about this problem, we have identified the odor originates from the decomposition of plant material in the ground under the home. We also identified the elevated moisture levels in the ground, post encapsulation, plays a part in the cat pee smell complaint. We have continued to collect data on the conditions that surround this cat pee smell complaint, but have not been able to draw any conclusions....until now.

I don’t want to repeat the information that we have already posted so you can find it here- Crawl Space Odor.

Cat pee smells like ammonia, mostly because cat pee often has ammonia in it. Nearly everyone can identify what a claim of “cat pee smell” would smell like and ammonia is the common denominator. Ammonia is made up of Nitrogen and Hydrogen, NH3. When a plant breaks down in the soil, the bacteria decomposes the plant back into minerals other plants can use. If you look at the life cycle of Nitrogen you will see how ammonia NH3 (ammonium NH4) plays a role. (Info graphic courtesy of

So what is the difference between ammonia and ammonium? The simple answer is pH. Higher pH levels in the soil support ammonia, cat pee smell and lower pH in the soil support a less toxic ammonium. Ammonia is one part nitrogen and 3 parts hydrogen, NH3. Ammonium is one part nitrogen and 4 parts hydrogen. Here is a great YouTube video explaining the relationship between ammonia and ammonium-

As you can see from this video, the relationship between the more toxic and cat pee smelling ammonia and the less toxic ammonium is a matter of pH balancing.

How To Prevent The Cat Pee Smell...

As a prevention to this bad cat pee smell showing up in your new encapsulated crawl space you will need to do one of two things; test and pH balance the soil as needed prior to encapsulating or install a soil gas mitigation system that will remove the ammonia smell (cat pee) from your crawl space once it arrives.

The pH scale is from 0-14 with 7 being neutral. Ammonia lives around 11 on the pH scale. The key is to get your soil pH as close to 7 as you can to prevent/eliminate this cat pee smell.

This cat pee smell does not come from polyester string, nor does it come from any of our products. The level of ammonia in the soil can permeate polyethylene and make the plastic smell as bad as the crawl space....but that is not the source