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I have a mystery on my hands, I believe I have finally come up with a theory that explains it. I am hoping that someone with drywall expertise can verify it.

I moved back into my place (after renting it) several months ago. Last weekend, I started smelling an unpleasant pungent scent in one of the corners.

I was not sure if it was mold or pet odor (the previous tenant had a cat).

I finally got a person in, they established that the moisture in that corner is much higher than anywhere else. They opened up the drywall and from photos, we see evidence supporting pet-damage.

I kept asking them how can moisture be higher in that corner (I don't have a pet). They kept giving me the same answer, the moisture must have seeped into underfloor remained there and then gets pulled into the drywall.

I respect their professional opinion and that is definitely one of the theories.

They asked me to increase air circulation in that area to dry out the drywall. Which is what I am doing (hoping to exhaust the source of moisture)

But I am wondering...if what is really happening is as follows:

  • The drywall is damaged
  • The damage makes it porous and susceptible to moisture in the air
  • I had my windows open during a rainy week
  • The damaged drywall sucks up the moisture activating the pet-odor in the drywall

But if my theory is right then as soon as air moisture goes up the drywall in that area will suck up the moisture and the issue will repeat.

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  • So there's a source of moisture, unlike your title question states.
    – Ecnerwal
    Oct 29, 2023 at 15:44

1 Answer 1

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I think your theory is all wet. (Sorry, I couldn’t resist.)

Cat urine and all urine contains various substances, including urea and the chlorides, sulphates and phosphates of sodium, potassium and ammonium. All of these substances are hygroscopic (absorb moisture from the air) to a greater or lesser degree. Both urea and calcium chloride are actually deliquescent when exposed to high relative humidity, meaning that they will absorb enough humidity from the air to form a liquid puddle.

If a cat has peed in a particular location repeatedly and the urine was allowed to dry out on its own, then you’ll find a high concentration of the above substances in that location, either in the drywall, the wood or both. That can easily explain why that corner attracts moisture. It’s not due to the porosity of the wall or floor, it’s due to the contamination with substances found in urine.

There’s no reasonable way to get those substances out of drywall or wood. You’ll have to replace any building materials that were contaminated by urine.

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