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Odor from Sealed Crawlspace

bobhoperises | Posted in Pretty Good House on

I’ll try to be concise.  I’d hoped not to have to ask for help but I’m out of ideas.

The house is 75 years old, pier and beam, climate zone 2. The crawl space has been encapsulated in the past couple years. Mold treatment was applied, but I don’t know what kind. Probably the cheapest kind.

The smell is strong, consistent, and rapidly causes throat irritation in the same way sawdust does. Wearing only sediment filters (the pink ones) did not feel like it was completely effective.

The humidity is controlled via a whole house dehu in the crawl space. I’ve not run a dedicated return nor a dedicated supply vent, nor have I installed a jumper. That said, much of the original flooring is still in place, so there’s leakage.

It’s not a chemical smell. It’s not a “cat pee” smell. It’s not from animal leavings nor from a dead animal. It’s not any of the smells I could find as “common odors after crawl space encapsulation.”

My guess from reading the resources on this site, as well as energy vanguard, and hell even yahoo answers, is that it’s from 75 years of mold that is now dead from dehydration getting in the air, but if that were the case I’d expect the sediment filter to be closer to 100% effective.

At this point, it’s more important to me just to figure out exactly what the problem is. Then it’s just a matter of solving it.

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  1. user-2310254 | | #1

    Old crawl spaces often benefit from a bit of ventilation. You might want to install a small exhaust fan to address the odor issue. Allison Bailes has some specific guidance here:

    1. bobhoperises | | #7

      Thanks for the reply. I have read this article backwards and forwards. I have applied Allison's fourth solution:

      "Conclusion: Although more expensive, installing a dehumidifier is the best method for treating the air in an encapsulated crawlspace."

      I can see how a bit of exhaust might still benefit the space, and have considered it, but it would be a ton of work to install so I'm trying to be damn sure before I do.

  2. Expert Member
    PETER Engle | | #2

    Sometimes it is very difficult to figure out exactly what it is, but if you can make it go away it might not matter. Depressurizing the crawl with a small exhaust fan can sometimes work at a relatively low overall energy cost. It is often also pretty cheap to do a test by rigging up a fan in one of the old crawl vents and testing it out. If it works, you can do a more rugged and permanent installation.

  3. walta100 | | #3

    When I see the word “encapsulated” I think someone is looking for a free lunch. If you can convince yourself to call it a “conditioned crawlspace”. I suspect only then will you be willing to spend the money needed to keep the air in your crawlspace more or less the same temp and humidity as the rest of your home.

    If your question said what the humidity level is in the crawlspace, I miss it. My wild guess is that it gets over 60% sometimes and mold had started growing and that is what you are smelling but that is purely a guess.


    1. Deleted | | #5


  4. user-2310254 | | #4

    If humidity is too high in your sealed crawlspace, it may be too high in the rest of your home. I'd follow Walta's suggestion and put in a couple of inexpensive humistats to track indoor conditions. It may turn out that you need mechanical dehumidification (and ventilation) to get rid of the smell.

    Something like this:

  5. bobhoperises | | #6

    There is a dedicated dehumidifier in there. The crawl space holds around 40%. The living area of the house is not so well controlled, but I do have central air.

  6. nynick | | #8

    My first inclination is that the smell is from the unidentified mold treatment chemical off gassing.
    You should start by seeing if you can mechanically eliminate the smell temporarily with fans and the like. If so, does it return afterwards? If it does, I'd repeat the process, but for an extended period of time. If it still returns, you'll either need to treat the source or provide continuous ventilation.

  7. bobhoperises | | #9

    Update: I installed a 40cfm exhaust fan and it's been running continuously for a few weeks now, but I'm really not seeing much of a difference. Perhaps there's too much moisture underneath the vinyl barrier?

  8. user-2310254 | | #10

    Maybe take a smoke pencil to the crawlspace and see if air is getting past the liner.

    1. bobhoperises | | #11

      Will that really work without a blower door to create a pressure gradient?

      1. Expert Member
        BILL WICHERS | | #12

        It takes suprisingly little air movement to make the smoke move if you're careful. Your existing vent fan may well be enough, especially if most of the air leakage is coming from only one or two places. It's worth trying before you do anything fancier.


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