Assuming you don’t have a family of cats under there, the problem is probably natural soil gas. It is not the fault of your crawlspace membrane. A soil vent fan system will remedy it. Read my whole story below for details, hopefully this helps someone out!

The problem is that my crawlspace smells like cat pee. After a couple weeks of being sealed up it makes the first floor in my house smell like cat pee. We noticed this was exceptionally worse after we have been gone for a while, or after the air conditioner had not run for a few days. This might happen in the spring and fall when the weather is good. It also seems to happen more a couple days after it rains. This was all very strange.

The smell all started about 10 months after we “encapsulated” the crawlspace to help with humid conditions under the house. My contractor for this pretty well sucked and was called out 4 times to patch up holes and gaps they left behind but finally we had a pretty good dry space down there.

There is a confusing matter of some area rugs in our house inherited from prior owners that do actually smell like dog pee. They had been cleaned but if the room stays closed without air on for a while then you can smell that too. That’s mostly in the bedrooms though upstairs. The first floor is where the more pungent cat pee smell is. Also confusing things we got a new puppy during this time who took some heat for making the smell undeservedly a while.

I went down into the crawlspace to check some things and noticed the smell was extremely pungent down there. I don’t have such a great sniffer myself, so I wasn’t really sure what I was smelling. I had my wife confirm that it was indeed the same type of smell but about 10 times the strength.

I actually became worried that it might have been a gas leak of some kind. I had to Amazon overnight me a cheap gas wand, about $30. To my alarm the next day when it came it actually would go off when I was in the crawlspace. See video of me using the wand to detect the smell:

I called the gas company who sent out a tech with an expensive wand and could not detect any “explosive natural gas”. He said he could smell the odor but that it wasn’t a “gas leak”. Looking around with the gas guy it became clear there was some rodent activity. I called a rodent company and paid $3000 to seal and guarantee my whole home from rodents. We trapped a total of one medium mouse.

I read and read and read online. It seems this cat pee smell is stronger in the southeastern states. There is a school of thought that blames the smell on the encapsulating membrane with polyester fibers Dura Skrim2 12WB which is popular in this industry and there happens to be a manufacturing plant right here in Atlanta. I checked the MSDS on this product and it is described as totally inert except when on fire. There are crawlspace encapsulating contractors who swear this product creates the stench about 9 months after being installed - exactly what happened to us!

Another school of thought puts then blame on the soil. Most soil (such as damp red clay) harbors various natural soil bacteria that work in the natural nitrogen cycle and produce soil ammonia gas. Ammonia gas rises, though when humid can stick to water vapor that sinks. Maybe something about sealing up my crawlspace allows this soil ammonia IN but it cannot ventilate OUT as easily except through the house?!

Cheap gas detecting wands also detect ammonia NH3 gas by accident because their sensors are imperfect! So my cheap natural gas wand was actually (maybe) a soil ammonia gas detector! Indeed if I was careful I can go in the crawlspace and find that the levels are higher up at the “ceiling” floor joists and lowest down on top of the vapor barrier. Of interest I could not get the wand to detect higher levels under the vapor barrier (I thought it should, but maybe its just that leaky?)

So I decided to install a quick DIY soil gas exhaust system. I bought a 200cfm fan, some flexible ductwork, and 100 feet of perforated landscape drain. I ran this system UNDER the vapor barrier and out an old side vent. No doubt this is leaky so when I run it about 2 days the entire air smells fresher in the crawlspace - there are plenty of leaks still in my vapor barrier so the exhaust system does pull air backwards from the airspace back under the barrier then out the side vent.

I also diverted a small AC duct to keep some conditioned air down there and try to make a net positive pressure from house to crawlspace to try to keep smell “pushed out”. Also I have a gas furnace and gas water heater down there that need to draw some air to be safe. Those heaters do have an unsealed flue that lets air escape up the chimney.

I ran my fan system a week straight. No more smell, then put it on a timer to run 4 hours a day. Works great, THEN I revisited this issue during some renovations- the fan and AC were shut off for a few weeks. The smell built up again. HVAC guys offering to sell me all sorts of air cleaners (none will work to remove ammonia gas molecules of course). I started some experiments:

Sample 1: I removed a large piece of the vapor barrier and sealed it in plastic bag.

Sample 2: I put some crawlspace soil in bag

Sample 3: I put a bucket AND a cardboard box upside down on top of the crawlspace soil in the crawlspace.

5 days later:

Sample 1 vapor barrier smells like pee!!! I then let it air out a few days. Gas wand goes off inside the plastic bag.

Sample 2 Soil in bag did not smell but the gas wand went off a little.

Sample 3 Gas wand goes off like crazy under the bucket and under the box, 10x higher relative to the alarming in the crawlspace air.

Next after a few days of vapor barrier airing out, I did this:

Sample 1 aired-out vapor barrier was rolled up and put in plastic bag again with a little water and soil again.

Sample 4 (new) Shredded some of the sample 1 barrier in a paper shredder, putting in plastic bag and with moist soil. Trying to see if something about the vapor barrier components would react and stink.

Results after 5 days:

Sample 1: The vapor barrier no longer smelled like pee, no smell at all.

Sample 4: There was no noticeable pee smell from the shredded vapor barrier and soil.

CONCLUSIONS FROM EXPERIMENTS:

1. Soil gas, likely ammonia, accounts for the smell.
2. Vapor barrier will absorb this smell and off gas it a few days afterward but is not guilty of producing the smell.
3. Sealing the crawlspace is not good enough to keep the smell out but is good enough to trap it until it clears out via the house.
4. A soil gas vent system below the vapor barrier, even good homemade, is the best bet to evacuate the smell before it reaches the crawlspace interior. A radon system does the same thing.
5. If you have gas appliances in the crawlspace you will need some kind of fresh air down there for combustion and for allowing flue to draw. A small AC vent is good for this.
6. Even with a great vapor barrier it is worthwhile to install at least a simple dehumidifier to keep humidity low in your crawlspace, especially if you have a gas appliance down there (gas produces CO2 and water vapor when it burns).

I did also buy a $200 radon monitoring sensor that connects wirelessly to smartphone. I use this in the crawlspace to monitor effectiveness of my soil vent system and timer settings. Not perfect but it helps!

A note on “wet” crawlspace: We live atop a small hill but there is a little runoff from rear of house that keeps that area of crawlspace soil damp. We have a perimeter drain and a sump pump. There has been sign of standing water in a couple areas after massive flooding - like a 5 year flood. But even the highest points of the crawlspace still have “damp” red clay. I think there is no such thing as dry red clay :) if it were practical to totally desiccate several feet of soil beneath the home then that might zap the smell too, but I think that is impossible.

I hope all this helps someone!