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Community and Q&A

What’s That Smell?! (Basement Edition)

RH68 | Posted in General Questions on

Our partially submerged basement has developed an unusual smell in the past few weeks, and I cannot figure out what it is.  It’s making me crazy to not have resolution.  Below is the best I can describe it.  Any suggestions are welcome!

Location:  Chicago, IL  (Zone 5)

When it started:  A few weeks ago.  Peak of winter.  Coincidentally or not, around the same time as severely frigid weather here.  In our 4 years of being in this house, this smell has never occurred previously.

Basement description:  Unfinished; solid concrete walls; concrete mud slab floor with lots of cracks; drain tile and sump at perimeter; about 3′-6″ below grade, 3′-6″ above grade; glass block windows above grade; boiler in basement, hydronic baseboard heating system throughout house; basement temperature stays around 60-ish to 70-ish, unless it’s super cold outside, in which case we’ve seen 50-ish or below down there.

What it smells like:  To me, it honestly smells like melted butter for movie popcorn, but without the popcorn part.  The alternate, less pleasant analogy has been sweaty socks.  It’s prevalent and not a great smell, but I wouldn’t say it’s completely revolting either (not like sewage would be).  To me, it DOESN’T smell like mold, mildew, dead animal, sewage, sulfur, natural gas, chemicals, spoiled food, or general basement.  I can’t even trace a source, although I got the impression that it might be coming from a foundation crack under the front porch.  The underside of the porch is solidly filled in between the walls and under the concrete porch slab.  I don’t see any way for an animal or other organism to even get into that area behind the wall.

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  1. Expert Member
    Peter Yost | | #1

    Hi RH68 (be great to have a name for the GBA Q&A community to use) -

    I wish I had a ready answer rather than questions but:

    1. is your clothes dryer in this space? If so, what happens to the odor when you are done a load of clothes--does the odor get better or worse or stay unchanged?

    The reason I ask this: clothes dryers exhaust about 200 cfm when they are running. If your odor went down or away, that means that pulling in outside air is diluting the odor source. If the smell gets worse, than it means that some pathway related to the odor is pulling in more of the odor OR it means that the odor is related to your dryer exhaust leaking into your basement, and some thing crawled into the vent or the dryer during the really cold weather.

    2. sometimes during really long extended freezes poorly or uninsulated basements foundations get a "head" of melted water in a column right next to the foundation and that creates a leak at the cold joint of your foundation wall and footing, which results in something getting wet that normally stays dry.

    Those are my two best shots but with this response, your question will get elevated for a short time to the home page and may attract the attention of others to respond!


  2. Expert Member
    Zephyr7 | | #2

    There is always the chance a small animal burrowed down along the foundation somewhere, or into a perimeter drain, and then died there. If the animal has been dead for a while but conditions weren’t right for the smell to come in, it might not be the smell you’re expecting. I’m not saying for sure there is a dead critter stuck somewhere, but it’s something to check.

    I’d check any vents/pipes first, so sump pump, dryer vent, and any potentially open sewer pipes (do you have a basement drain or sink that isn’t often used? You might have a dry trap letting sewer gas seep in, just pour some water in a dry trap to fix this). Check along the rim joist perimeter, especially around corners and other areas where different framing members come together and might have gaps. Don’t forget about any electrical conduits that pass through the exterior wall. Stuffing backer rod or mineral wool in these can seal around the wires inside if needed.

    The last thing I can think of is to see if you have any old containers in the basement that have rotted out and are leaking their contents. I had some old containers of some kind of cleaner where the bottom rotted out and the contents deeper into the concrete and smelled for quite a while.


  3. Tom May | | #3

    Stagnant water in the drains leading to the sump? From possible ice dam? Try flushing it out and cleaning the sump pit.

  4. Expert Member
    Michael Maines | | #4

    What is your heating fuel? How is your boiler vented? Exhaust gasses have an acidic and sulfurous smell that is comparable to cheese, melted butter and sweaty socks.

  5. RH68 | | #5

    Hey everyone, thanks so much for the thoughtful responses. I'm still trying to find the source of the smell and will take all this into consideration. Here are some answers to the questions that came up...

    No laundry washer or dryer in the basement.

    We do have a floor drain, but I put my head down there a while back, and it didn't have any odor other than "old". I'll pour some water down it just to be sure the line hasn't dried out in a weird way.

    The idea of wet soil and clay at the foundation and floor had crossed my mind originally. A friend of mine in the medical profession said that bacteria and fungus can smell like all sorts of crazy things. The internet has certainly reinforced that with some interesting, and sometimes plain gross, tales in that respect.

    I haven't heard the sump pump kick on in quite a while, but I figured it was because everything outside was frozen. The idea that there could be stagnant water in the drain lines was something I hadn't considered at all. I'll check that out somehow.

    We don't keep many chemicals or liquid supplies in the basement, but what I checked was fine at the time.

    Heating fuel is natural gas for our boiler and water heater. The flues run up the chimney and out the roof. I didn't smell anything weird around the chimney or at other floors of the house.

  6. Expert Member
    Zephyr7 | | #6

    Keep in mind that noses get desensitized to smells pretty fast, so you have only a limited amount of time to “sniff things out” before your nose will no longer be very sensitive and will need a break.

    If stuff is stagnant in your footing drain, and you can get to it from your sump pump, it’s sometimes possible to poke a garden hose down the drain (usually terra cotta or corrugated polyethylene) and then run some water to flush things out. Don’t blast it though, sometimes that can cause problems. You want a good flow but not a nozzle that can damage things.


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