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Community and Q&A

Odor After Crawlspace Encapsulation

jweikert2626 | Posted in General Questions on

We had our basement encapsulated in the fall last year and made it through winter no issues. They used a thick mil high quality (apparently) vapor barrier and then sprayed /fogged with mold preventative. I don’t think there is any more mold – smell is not musty really and I don’t get a mold allergy reaction.

We live in a 50 year old brick ranch in Atlanta GA. Now since Spring we are smelling this bad smell. It is making me sick I believe. It smells a bit like cat urine type smell….. I have tried air cleaner, odor remover etc. with no luck. Basement is dry and at 40 percent humidity so dry. I have a dehu running as needed down there. Our furnace/AC and gas water heater are all in the crawl space as well. So we have those venting out I am hoping the smell eventually dissipates but so far it is strong and we smell it in the living space when the AC runs. A subtle yet bad smell. What to do??? I spent a lot on the encap. Didn’t anticipate this bad chemical-cat urine-odd smell.

I am worried it is toxic. I don’t know how to proceed. Recently had ducts professionally cleaned. Have had 2 HVAC guys checking duct connections and AC. Everything normal they say. Except the strong smell. I am pretty disparate for a solution. The crawlspace guy who did the encap said he never had any complaints about a smell….except early on he got one and he changed the material used afterwards.

Thanks. Jeff

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  1. Richard Beyer | | #1

    In my opinion, you may simply need to ventilate the area (install an air exchanger or place a fan in a window) for a while and see if that makes a difference now that the area is sealed tightly. Plastics in general emit some nasty by-products which need to be ventilated to the outside of the building. If this does not work you can order an air quality test to see if you have mold and also to check what level your total volatile organic compounds are at. Should the test come back as unacceptable (red zone), you may need to bring in an environmental engineer to find a solution for you. Typically ventilation will correct your problem. You may also consider testing the home for radon while your at it. Radon suction ports installed below the membrane will also eject emitting odors to the outdoors.

    Here's a simple DIY test to start with;

  2. GBA Editor
    Martin Holladay | | #2

    1. You said that you "have had 2 HVAC guys checking duct connections." But you didn't tell us whether your duct seams have been sealed with mastic. Did anyone perform a Duct Blaster test to check for duct leakage?

    2. Sealed crawl spaces can be conditioned one of two ways: either by including a register connected to your forced air system, or by installing an exhaust fan in your rim joist to depressurize the crawl space (as well as a floor grille to connect the conditioned part of your house with the crawl space air). If you choose the latter approach, you are more likely to prevent any odors from entering your house. For more information on what I'm talking about, see Building an Unvented Crawl Space.

  3. user-659915 | | #3

    Jeff, does any part of your home insulation include cellulose? It's a great material but I have heard of errant batches which have been incorrectly treated with ammonia, leading to the pervasive cat urine odor. In the case with which I'm familiar the supplier of the faulty material bore the cost of removal/replacement.

  4. user-412029 | | #4

    Jeff I have the exact same issue we used 20 Mil vapour barrier last June as you we went through winter ok but now we get that smell you described .

    We were curious how you made out with your problem

    Your help in this matter is appreciated

    Regards Gerry Dorion
    [email protected]

  5. Jay_Bryan | | #5

    We are having same issue in our crawl space. The company has replaced the dehumidifier but odor still very strong. Any resolutions found to your crawl odor? Grateful for any suggestions

  6. jweikert2626 | | #6

    It's been over 2 years and the smell is as strong as ever.....tried many things and thought of ripping up the barrier altogether. Stopped using the dehumidifier. So what I did is leave barrier intact but reopen a few vents on one side of crawl - note, these are the ones I has sealed before. And on the other side of the crawl I reopened two vents and places good fan ventilators that run all the time. This seems to have kept the odor out of our living space in the house but smell is still there in the crawl. Trade off is the humidity level is same in crawl as outside air so so much for sealing the crawl. I left barrier intact only because before all this nonsense I only had a vapor barrier and the vents were all open. So I am back to square one with the same set up except a better vapor barrier and better sealing and open vents with 2 fans. I am quite sure the smell is coming from the barrier and or sealants but decided if I can manage with the fans I will just leave it be. Hoping the smell eventually disapates but seems to be strong as ever. Very disappointed I have not found a good solution. Many others have this issue by the way. No one has cracked the code. And my crawl is clean, no leaking water, no animals nothing that would cause the smell...except the barrier and sealants and tape used. Jeff

  7. charlie_sullivan | | #7

    I would suggest finding out what material the vapor barrier is and what sealant was used. For example, vinyl sheets can be smelly as can Tremco acoustic sealant. Polyethylene as a barrier tends to be less smelly, and Contega HF is a sealant that is much less smelly to start and that stops smelling much faster (within hours).

    To figure out where the smell is coming from, take a sample of each proposed or existing material, and put it in a glass jar with a tight lid for a few days. Then open the jar and smell it.

    Polyethylene is the most common vapor barrier material, and I have yet to hear of anyone finding it smelly. Perhaps a bad batch? You can buy a small package of poly sheeting at a hardware or home store for under $5 and test it to see if it has the same problem for you. If you did find that to be a problem, another inexpensive plastic sheet material that is low smell/taste/toxicity/outgassing is polypropylene. It's harder to find in big rolls of the kind of thickness you'd want, but you could probably find something you could use. Another option might be radiant barrier foil, which is Al foil laminated to a reinforcing scrim. Foil is highly impermeable, so would block any smell from underneath it, and although I don't know what scrim material and adhesive are used, it would probably seal in any smell that might result from that. It would be more expensive than polypropylene, but might be easier to find.

  8. user-2310254 | | #8


    Did the company install open-cell foam as part of the process? Improperly installed foam can give off a very bad odor.

  9. Scott64 | | #9

    Hey Guys,

    Hey Jeff, I'm in the Atlanta area and install a number of crawl space encapsulations. This is not a solicitation for business but to offer let you know what we have done to help resolve the odor. Over the past 5 years we have had this come up only 2 times for our installs but we worked with some other companies to help. It's a strange problem which appears to be specific to that particular homes soil. We have done adjacent houses in the same neighborhood and have had no issues with the adjoining neighbor. We also use the products from probably the largest manufacturer of polyethylene material. Since we also install Radon Mitiagtion systems, we installed a vapor intrusion system (fancy term for a radon system) in these homes and problem/odor is gone or at least unnoticeable. We're basically ventilating the crawl space but from between the ground and poly. The benefit with this method is we're not pulling (as much) conditioned space out of the crawl/house.

  10. L3Boats | | #10

    Anyone find a fix for this issue?

    I am experiencing the same issue of a cat urine smell in my encapsulated crawl space in the Atlanta area. I started by contacting the installer, which contacted the manufacturer of the polyscrim material used as the vapour barrier. No luck there... The manufacturer said they didn't have issues and said its more than likely off gassing of the soil.

    As a temporary solution I opened up 2 of my crawl space vents tand instaledl vent fans to get some air movement. It helped a little, but still not happy with it. Especially since I opened the envelope to my encapsulation.

    I also have a SanteFe Max Dry Dual XT dehumidifier running all of the time.

    So the next step I guess is the radon system under the vapor barrier. Anyone install the radon system and successfully remove the "cat urine" smell??

    Interested in a cost effective solution if anyone can help.

  11. user-6640994 | | #11

    We are having the same problem in Washington DC. We had to dig out a dumpster worth of soil in our crawl space in order to run pipes. We then encapsulated with 12 mil Dura Skrim on the bottom and spray foam on all four walls. This was 6 months ago and when we moved in mid-October the smell was permeating the 1st floor. It is a cat urine smell. The crawl space is very dry, 50% humidity, and no rodents. I have had four different crawl space companies and the company that encapsulated come to inspect the smell and no one knows what it is. They suggested running a dehumidifier which we have done for two months now with no improvement. Finally I called 2 companies that sell the DuraSkrim product and I received two different responses but have at least narrowed our problem down to the soil. The first told me this is a soil gas problem and that before the plastic was installed we need to install a gas mitigation system with a fan to carry the gases out. He said now that the plastic is down and we have limited access, we need to open up the vent to the outside which was closed upon encapsulation and install a fan to blow the gases out. Not ideal and defeats the purpose of the encapsulation, but he said we have no other choice. The second company told me it is a problem with the microbes in the soil. That when we installed the plastic barrier we killed the aerobic microbes and they are now decaying and the anaerobic microbes are flourishing. He said we need to find a company that performs subsoil fogging and that we will need to do this procedure every three years. He said he has heard of this problem in 1 out of 1000 houses. Any thoughts on these two opinions? And does anyone know of a company in the DC area that knows about these crawl space encapsulation problems and can help us out? Thanks!

  12. GBA Editor
    Martin Holladay | | #12

    Sorry to hear about your problem. As GBA readers contribute to this thread, I'll learning, too.

    This sounds like a rare but frustrating problem. If I were you, I would take Scott's advice and install a radon mitigation system under the vapor barrier on the floor. I know that solution is expensive and unsatisfying, but it's the best proposed solution I've heard.

  13. richardhale | | #13

    I had the same problem. I had a cat pee smell (ammonia like) in my crawl space. Why I chose to encapsulate my crawl space with that smell is beyond me. I was told it my vapor barrier but that wasn't the case. The cause was my soil, apparently from decomposing plants. The science is a little bit over me but this article I found describes it.

    I was able to get rid of the smell by balancing the soil. The PH range goes 0-14 with 7 being neutral. I tested the soil and was at a 11 PH. I was able to get to a 7 PH and I no longer have that cat urine smell. Unfortunately, I had to replace my vapor barriers but that was my fault. Again, if you have this smell in your crawl space, don't encapsulate it first or you'll get that smell on your vapor barriers. They don't cause the smell, but they retain it for sure. Solved my problem though. Hope it helps.

  14. gozags | | #14

    How did you neutralize the soil PH? A surface product then put the plastic back down?

  15. Expert Member
    Michael Maines | | #15

    Sean, sulphur reduces soil pH. You can get it at farm supply stores. You wouldn't think "sulphur" and "neutralize smell" as being complementary, but that's how it works. Here's one source:

    1. Geosmell | | #16

      We have the same odor problem after we encapsulated our crawl space. It comes up through the registers. So we need to remove the plastic material, spread sulpher all over the dirt and cover it back up? If so will this iliminate the problem for good or just temporarily?
      Thank you

      1. Expert Member
        Michael Maines | | #17

        Geosmell, this isn't something I have first-hand experience with, I was just sharing one way to reduce soil pH. To raise soil pH, agricultural lime--powdered limestone--is what works for farming and gardening, and should work on crawlspace soil as well.

  16. Jenna | | #18

    Hi all - I realize this is an old thread. We think soil is the cause of our house odor. Our basement is encapsulated so wondering what we should do. Do we rip up the encapsulation to fix the ph and then reencapsulate? Is that the only option? We have two crawl spaces. One of them is cemented and interestingly has no odor! I’m told it will costs about $10k to lay concrete in that crawl space but there’s no guarantee it will solve the problem. I’d have no problem forking over $10k if I knew it would be a fix. Anyone find an economical solve? My husband has been complaining about the smell of our house since we moved in last September. I did not immediately notice it, but just went on vacation and I could smell the house on all of my clothes! Also worth noting we had a mold test and jo mold and there’s not a lot of humidity and we have air purifiers all over. Help!! :)

    1. user-5946022 | | #19

      Based on the replies above:
      1. Have you tested the ph of your soil? Your local extension service should be able to do that for $15 or so.
      2. You need data before you move forward. I would also get something like the Airthings Wave Plus and start tracking radon (it does not have any odor) /voc, and other air quality etc. in the house.
      3. A radon mitigation system may pull out enough air to reduce or eliminate the odor. If your house is prepped for one the fans only cost about $150 and are easy to install.
      4. How long ago was your basement encapsulated? I noticed a very strong odor, but just on certain days, for almost 2 years after my home was spray foamed, but fortunately it was still being finished up during part of that time, and the odor is no longer as noticeable.

      1. Jenna | | #20

        Thank you for your response! We have not tested the soil yet. Was not sure if we could cut through the encapsulation to test. But I’m told we can and can just tape back up. Will do that and if high going to try the radon remediation fan to see if we can exhaust the gas to the exterior. Will also look into the air things Wave plus. Not sure when the encapsulation was done. We moved in in Sept. We noticed a slight smell to the house but wasn’t off putting at the time. Not sure if it’s getting worse or if we are just now noticing the smell.

  17. Ryan_SLC | | #21

    I know this is old, but not one person got it right.

    It's the liner. Cheaper vapor barriers will give off a cat urine smell.

    1. freyr_design | | #22

      #7 suggested that

      1. Ryan_SLC | | #23

        It's true, I hear ya. But that is the answer to this many response thread.

        I am looking at the liner specifically used and see that the company now has a page dedicated to claiming the smell is 100% caused by rotting material under the liner.

        This is at odds with other companies (Ninja, Stego) that specifically state cheaper liners do give off a cat urine smell.

        Well worth the mention now given I randomly came across this old thread using google to find a free lunch new vapor barrier liner that isn't Stego pricing. Just doesn't seem possible.

        And too, the company has dedicated page to cat urine smell is not their product...which is questionable now that we know cat urine smell is vapor barrier liner caused...That's a company creating confusion with folks like me and I just don't like that at all :)

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