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Community and Q&A

Spray Foam Smell

patch78 | Posted in General Questions on


I’m looking at a house that had closed cell (Foam Lok 2000) spray foam insulation installed in the basement rim joists and an adjacent crawlspace. When viewing the home, I immediately noticed a “sweet” smell upon opening the door to the basement. It seems to be stronger on hotter days.

I’ve since read the horror stories about bad spray foam jobs, but here’s the thing: the smell is pronounced, but not unpleasant or overpowering. I spent a significant amount of time in the basement with no issues. And the foam looks good…no dark, “fried” spots indicative of iso-rich foam (I’m assuming a sweet smell would point towards uncured MDI). I don’t think the odor is just from hidden bad spots, either. I can smell it directly from a piece of nice looking foam. I considered poor ventilation…but surely even a poorly ventilated space that’s opened now and then would be free of foam odor after 6 years?

Everything I read says a good foam job should be odorless after some time. Is it truly odorless…or is that just in comparison to the strong odors right after installation? Just trying to see if even a good foam job could still smell. Like I said, the smell doesn’t bother me (as long as it stays in the basement), but I worry about health effects from long term exposure.

I feel like if I already owned this house, I’d just air out the basement now and then and be done with it, but I’m not sure that’s good enough to consider buying it. I’m really on the fence about this, so would appreciate anyone’s advice or opinion.

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  1. Expert Member
    BILL WICHERS | | #1

    I’d consider this to be like the “fresh paint” smell. In a closed space, it will take lots longer for any smell to dissipate because stuff diffuses out of any material based on relative concentrations of whatever is diffusing out. As more of that stuff diffuses out, less is able to if the concentration builds up on the “other side”.

    My guess is airing out the basement for a week or two would make a big difference, but it’s difficult to be sure. I wouldn’t think you’d smell anything after 6 years, even with just normal air leakage in a space, but again, it’s hard to be sure.

    Are you sure it’s the spray foam you’re smelling? Usually “bad spray foam smell” is pretty nasty, not a “sweet” smell. It’s difficult to describe smells with words though!


    1. patch78 | | #2

      Thanks. We tried putting a piece in a ziploc bag for a while, and upon opening we can detect the same smell briefly, so it really does appear to be the foam.

      I've considered the ventilation issue a lot. The vast majority of the foam is in the crawlspace, which like I said, vents only to the basement through two small basement windows (the crawlspace is under an addition, so these are windows that used to go to the outside) and the basement is not ventilated at all, but it is 100 years old, so probably not leak tight. Don't know how often the owners opened the door - but like you said, it seems unusual to still have an odor after 6 years.

      The crawlspace is under the living room, with nothing to stop vapors from diffusing up into that space, so that is a concern if there is an issue with the foam. I would bet you're right and a good airing out would help. Just not sure I'd bet a mortgage and my family's health on it unless I can understand what's going on better.

      1. Expert Member
        BILL WICHERS | | #4

        You could ask the current owners if they'd be willing to let you run a simple test:
        Open both windows, put a box fan in one blowing OUT. Let that run for several days, then see if the smell improves. If it has, close both windows and wait a few more days. Is the smell STILL improved, or did it get worse again?

        If the smell STAYS better after running the fan test, then you CAN air out the space to SOLVE the problem. This means there is a finite amount of smelly stuff that needs to be ventilated, but once it's out, nothing replaces it (i.e. no continuous offgassing is going on), so you just need to keep ventilating for a while until everything is gone. If the smell CAME BACK after the fan test, then you might NOT be able to SOLVE the problem. In this case, the spray foam might still be offgassing which could potentially be something that would not be rectified even with long periods of ventilation.

        From there, you have some real data to help you make a good decision before committing to a mortgage on the property.


  2. Expert Member
    Peter Engle | | #3

    Even if the smell was strong/noxious, you can mitigate it by running the crawl space at a negative pressure. A very small (25 cfm) fan exhausting to the exterior from the crawl would keep it at just a slightly negative pressure and vent the odor to the exterior. Added bonus is that the crawl space also stays dryer with conditioned indoor air flowing through it. I would close one of the windows to the basement and install the fan in the opposite corner of the crawl to induce crossflow.

  3. user-2310254 | | #5

    I had open-cell foam in my last house and have it in my current house. In both cases, there was a residual smell wherever the foam was not sealed behind drywall.

  4. Expert Member
    Akos | | #6

    Fixing SPF smell is a very expensive proposition. My own home has closed cell SPF in the ceiling and it has absolutely no smell, I would not buy someone else's problem.

    1. Expert Member
      Peter Engle | | #7

      Good point. The OP doesn't own the house or the problem, whether it is a big problem or not. There are other houses out there, so why buy one with a potential problem? If you can smell something coming from the SPF, that something is probably not something you want to breathe for a lifetime. Whether it smells sweet or nasty, there's still something organic offgassing after 6 years and it's probably not good for you. Why take any chances?

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