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Why do I have a musty mildew type smell in my conditioned crawl space?

[email protected] | Posted in General Questions on

The house we moved into is now 11 years old. We have been here for 2 years. Just this year we started having a mildew basement type smell coming into our first floor when we run the ac. It is summer and hot. We have a conditioned craw space that was put in when the house was built. There is not junk down there. We have a heat pump and ac unit and a gas furnace in the craw. Everything was fine until this summer when our first floor began to smell like a musty basement.
We call a couple heating cooling guys. One suggested we put in an air filter in the duct in the craw that would supposedly kill all organism etc. We have done that and I dont think the mildew type smell has gone away maybe a bit reduced. The problem is that the filter itself gives off and odor. So now I smell the mildew plus some other odor. It is supposed to smell like bleach and be some kind of ozone thing. I am not sure what it smells like but my first floor definitely has an odor. We paid $900.00 for the filter and installation plus he installed a dehumidifier. He also closed off the vents to that circulate air from the crawl into our house . This was to prevent the musty smell in the crawl from getting in our house. Any how we still have an odor( musty , mildew and something else). We called a duct cleaning guy who will charge us $800.00 to clean the ducts and the coils of the ac unit.
The question is why is there a musty smell in the crawl in the first place when it was never there before? Our crawl is pretty new and nicely conditioned. So how do I find out why there is a musty smell in the craw? Also I have asked the fellow to remove the filter if things don’t improve in a few weeks. Any suggestions and thanks so much.

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  1. user-2310254 | | #1

    If you have a musty smell, you have a moisture issue.

    Have you inspected the crawl space to see if there are any areas that look wet? Where are you located? What setting are you using on the dehumidifier? How long does it run?

    You say the crawlspace is conditioned. How was that done exactly? Please describe how the space is insulated and whether you have a concrete slab or a liner covering the soil.

    I wouldn't spend any more money until you have more information. Putting in a filter is not addressing the root of the problem, for example.

  2. GBA Editor
    Martin Holladay | | #2

    There are two ways to condition a crawl space.

    1. One method introduces supply air from your forced-air system (your furnace and air conditioner) into the crawl space. There should be a floor register that allows crawl space air to enter the home above (thereby encouraging circulation). This approach can allow smells to migrate from the crawl space to your home.

    2. A better method requires the installation of a small exhaust fan in a rim joist of your crawl space. There should also be a floor register to allow conditioned air from upstairs to enter your crawl space. This approach is less likely to allow smells to migrate to your home.

    You might want to consider reconfiguring the way your crawl space is conditioned.

    For more information, see Building an Unvented Crawl Space.

  3. [email protected] | | #3

    Thank you. We have gravel covered by a plastic material. Its a new house only 11 years old and when we bought it the inspector said that the crawl space was great. We have a heat/cooling pump and last year we turn off the heat pump to use the gas furnace. I wonder if turning off the heat pump made the crawl space more moist. But just a week ago we installed a humidifier. Yes I finally realized that we need to address the root problem. But what it is I don't know. We are going to go down and look for any leaks.
    It was supposed to be a great looking and insulated space in a practically new house. We live above a river though. We are up high there is a street and steep and long embankment so we are not right on the river but it must be a moist area.
    The humidifier is running 24/7 I don't know the setting.
    We had vents that allowed an exchange of air into the upstairs but closed them off last week when we put in the filter and the humidifier . We didn't want the musty air coming up into the house. WE never had this problem in our old house where we had a regular crawl space.
    What would the small exhaust fan do? I guess we just have to figure out why the crawl space has become musty. Thanks.

  4. GBA Editor
    Martin Holladay | | #4

    In your recent comments, you mentioned a "humidifier." I hope that you meant to write "dehumidifier."

    Operating a dehumidifier (at least on a temporary basis) makes sense.

    If you want to reconfigure your method of conditioning your crawl space, here's what you would need to do:

    1. Install a small exhaust fan (rated at 1 cfm per 50 square feet of crawl space) in a rim joist to exhaust air continuously from your crawl space. This fan should not be oversized.

    2. Seal any forced-air registers (that is, registers connected to ductwork) in your crawl space.

    3. Make sure that the floor register that connects your upstairs with the crawl space (the passive air transfer path) is left open.

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