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Chemical/ Cat Urine Odor in Crawlspace

MSVA | Posted in General Questions on

Have a 50s built house.  HVAC company just put in a vapor barrier.  Strange smell coming from the crawlspace.  We put in two vent fans to help but it still smells and even with new sealed ductwork can smell faint plastic-y smell in house when AC comes on.
I’ve seen comments on ‘cat pee odor’ & off-gas smells after encapulation etc.  This is not encapsulated, just barrier.  I’ve read possible causes from decaying vegetation to the barrier itself.  What I’ve noticed is that most the folks with problems are on the Mid-Atlantic to Southeast US and possibly in older homes.  I do know that we have residue of chlordane (a banned pesticide) in our crawlspace (applied over 30 years ago!), which is the reason for now putting the vapor barrier and fans in…It was pretty common for termite treatment and I would bet the Mid-Atlantic to Southeast US received the most treatment for termites….

The crawlspace had an odor before but more like dirt/musty and not chemical.

Could the chlordane residue be interacting with the plastic???

I know that tapwater treated with chlorine dioxide interacts with new carpet to cause ‘cat-pee’ odors….could something similar be happening here?

Need an EXPERT please!!


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  1. Expert Member
    Zephyr7 | | #1

    The usual crawlspace liner/barrier material used is polyethylene, which shouldn't generally have much, if any, smell. Polyethylene is a very stable material.

    My guess is you have either some trapped stuff rotting, which I suppose could potentially be that pesticide residue -- I have no experience with that particular issue. The "back" of the barrier, the side against the ground/walls, is going to trap some moisture from the ground, which may make a difference with any residues. The polyethylene barrier should block anything from getting inside though. About the only normal chemical material that can get through a polyethylene vapor barrier are certain types of solvents.

    When they installed the crawlspace liner, did they cover only the floor, or did they go up the walls all the way to the rim joist too? The first thing I'd check is any areas that ARE NOT covered with the barrier material. If the crawlspace was air sealed as part of the project (and it should have been), you'll have less air leaks, so less unitentional ventilation of the space. Less air movement through the space could allow any smells to build up and become detectable by you.


  2. MSVA | | #2

    Thank you! A 'reputable' local HVAC company - they only installed it on the floor. They did not go up the walls. The plastic does smell. We asked for the brand (unfortunately after the fact - we figured they were the experts on this....) and they sent us to a website for RODEO construction sheeting, 10 mil. If this is general construction sheeting (and the website says it is used for crawlspaces) - is this the same or worse than material that's only used for vapor barrier, like Diamondback, etc.? The website says polyethylene-based plastic, but in reading through the website it looks like it could be recycled plastic (?)..... Any ideas on best type of vapor barrier? LIke I said, this is a 50s house so I'm sure there's general stuff under the vapor barrier, from residue of termite treatment, to mouse poop, etc. We live in a rural wooded area. The crawlspace dirt was relatively fine and dry. Thank you.

  3. Steve Knapp CZ 3A Georgia | | #3


    The liner needs to be completely sealed, which usually means taped or caulked wherever it meets a wall or pier. Did the HVAC company also seal off your vents and install spray foam or rigid foam on the foundation walls?

  4. MSVA | | #4

    Hi, thank you. Nope, they didn't do any of that. I guess I should have researched here online before having it done. I'm afraid for them to 're-do' it, as I don't want to kick up the crawlspace dust again by pulling it out. I suppose more could be put on top and done properly by a crawlspace company. It's so hard to find a company that just does the job right the first time. We never worried much about the crawlspace before but our youngest son died from leukemia (AML) at age 14 in 2019 - so we started searching for possible causes. I grew up in the house next door to this one, and I know that both houses were treated for termites back in 70s/80s with chlordane. I didn't imagine it would still be there - which is why it's banned. It's in nanogram amounts but I still don't like the idea of it. Not sure if it did anything or not. Our ductwork is under the house as well so with leakage, etc. - it probably was being pulled into the house too. We and other people have lived in both houses so who knows. I just want to fix this without re-fixing it several times.

  5. Expert Member
    Zephyr7 | | #5

    I like the materials from Americover, they have a lot of options for crawlspace liners.

    Recycled plastic shouldn't be any different smell-wise from new material. It will just have a little bit of "new plastic" smell, but shouldn't be too bad.

    I think an easy first step here would be to line the walls, then seal the liner everywhere -- tape the floor liner to the wall liners, caulk the top of the wall liner to the top of the wall. That should seal things off and make the "barrier" into an actual barrier. Without the sealing work, the "barrier" is more of a reducer instead.

    Mouse poo doesn't usually smell unless it gets wet. Chances are whatever is smalling now that wasn't before is either moist now and was dry before, or was getting diluted by air leaks before and is getting concentrated now. Completing and sealing the liner so that it acts as a real barrier to stuff in the ground or walls will help if there is something in the ground causing these problems. If you have a really severe case, a radon mitigation like system to depressurize the area under the barrier may help.

    Try to localize the problem first. I'd do the poly on the walls and sealing work regardless though.


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