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Following up on the musty smell in our crawl space

[email protected] | Posted in General Questions on

Editor’s note: This is a follow-up to a previous thread on GBA. The previous thread was Why do I have a musty mildew type smell in my conditioned crawl space?

Yes we installed a dehumidifier not a humidifier. We had a guy check to see if there is moisture down there anyway and he says no. We have a gas furnace down there and a heat pump/ac unit. We had turned off the heat pump last year and just use the gas furnace b/c it’s cheaper heats very well. We were wondering if that is what caused the mildew smell. My Ac guy says no. He remeoved the expensive air filter that made our house smell worst.
We have installed the dehumidifier and he has shut off the vents that circulate air down there. So the smell has gone down quite a bit.
You had said to install a fan. Did you mean only one fan sucking air out? We have a vent on the first floor like a register from which the air flows down to the a/c etc I guess it is called an exchanger.
But the vents down in the crawl space are closed so we are not heating or cooling the crawl space any more.
Why should we install a fan? And where exactly? By cutting off the vents in the crawl space are we doing something wrong?
Thanks for answering we have been away and just got back.

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  1. Expert Member
    Dana Dorsett | | #1

    With no active air exchange in the crawlspace it's pretty easy for moisture to build up down there. It's better to insulate & air seal the crawlspace walls, then actively condition that air with a bit of heating/cooling system air flow, which prevents stagnation build up.

    If it's cool & damp enough down there it's pretty easy for mold to start growing on heating-only system filters during the cooling season.

    Putting a link to your prior thread might offer more clues as to how your house is configured, your climate zone, etc..

  2. whitenack | | #2

    Linda, this may be a silly question, but are you sure the smell is coming from the crawl space? Could you have a moisture problem somewhere else like a leaky window or roof that is causing the smell? Does it smell worse after it rains?

  3. user-2310254 | | #3

    Dana. Linda has a conditioned crawlspace, as I recall. There is a vent between the first floor and crawlspace, but I don't imagine it is doing much if the heat pump is turned off.

    Here is the thread:

  4. GBA Editor
    Martin Holladay | | #4

    I urge you again to read this article: Building an Unvented Crawl Space.

    In that article, I explain that building codes require (and building scientists advise) that a sealed crawl space like yours be conditioned. There are two ways to do that; if you are worried about crawl space odors, only one of the two methods makes sense.

    Here are the steps:

    1. You don't want to provide any direct heating or cooling with forced-air registers. It sounds like you were doing that formerly, but that these registers have been closed and sealed. So far, so good.

    2. You need to install an exhaust fan in your rim joist or a crawl space wall, so that the fan removes crawl space air and sends the air outdoors. This fan needs to be carefully sized; it needs to be rated at 1 cfm for each 50 square feet of crawl space floor area.

    3. Your house has a passive floor register that allows upstairs air to enter the crawl space. You need to leave this register open. I'm guessing that this is the register that you describe this way: "We have a vent on the first floor like a register from which the air flows down."

    Here's how the system works: the fan removes a little bit of air from your crawl space, and the floor register allows upstairs air (which is usually dry) to enter the crawl space. This system lowers the humidity level in your crawl space while preventing odors from entering you house.

  5. whitenack | | #5

    Martin, just a quick side question: If you have a sealed crawlspace that opens into a much larger basement through an access door, do you still need a vent to the outside, or would a hole in the access door (or no access door at all) provide enough air to circulate into the basement.

  6. GBA Editor
    Martin Holladay | | #6

    If there are no odors or humidity problems, I would say there is no reason to worry or to do anything.

    If you notice odors or humidity problems, you could install an exhaust fan as I described.

  7. The Real Crawl Space Science Expert | | #7


    its truly amazing some of the things I see out here. There is such a vast point of view on how to handle crawl space problems. I feel sorry for consumers because there is so much bad information out there. You have my apologies on that. The good news is that I will help you sort all of this out and get you on the right path. Based on your last response, it sounds like you have a sealed crawlspace Crawlspace with a dehumidifier running to reduce excess moisture buildup. I am assuming that you have a good vapor barrier installed. Should have 100% soil coverage to slow down the moisture release coming up into the space. It also sounds like you have a vent cut into the floor to introduce air from the first floor into the crawlspace. Okay. First, you are correct if you are concerned about cutting off the outside air completely from getting inside the space. The air will get stale over time. You are basically encapsulating all of the odors and smells inside the crawlspace. Construction materials, soils, plastics, insulation, biofilm contaminants etc. all off gas odors and make their way up into the living areas. Houses are designed to ventilate upwards into the attic areas. Attempting to vent air from the first floor down to the crawlspace or using forced air from your exchanger is problematic. Changing the temperature inside your crawl space does not remove moisture. It may change the dewpoint, but the moisture remains. Only when you run your air conditioning or the dehumidifier do you get moisture reduction. What do you do in the spring, fall, and winter when you are not running the A/C? You dont. Second, dehumidifiers are expensive and they are energy hogs. They are also extremely inefficient inside crawlspaces. Since they are relative humidity control devices, they aren't smart enough to know when to turn on off especially in cooler temperatures. During the winter time your dehumidifier may run 12 to 14 hours a day to reduce a small amount of moisture when the dew point is low and there is no need for the dehumidifier at all. They run excessively, overheat the compressor, and they burn up. When this happens you have to replace the unit or have the old one repaired. Commercial dehumidifiers run $1500 or so, so you can see where your long-term expense on unit replacement and electrical usage can be. THERE IS A BETTER WAY!

    I am now going to introduce you to a real crawlspace science product. An engineered system developed by engineers to solve your crawl space moisture problem without the dependence on forced air or an expensive dehumidifier.

    We need to retrofit your crawlspace incorporate an Atmox controlled ventilation system into the equation. This is a dewpoint controlled fan system that will allow us to introduce outside air into the crawlspace when it is dry enough to use. The Atmox controller has engineered software and interior and exterior sensors sending data back to the controller. If the air is too hot or too cold, it wont use it. If it is too wet outside in the dewpoint is too high, it will not use the outside air. However, believe it or not even in the summer time there are periods of the day even at night when the dew points can drop to the upper 50s and low 60s and we can use that outside air because it is drier than the air that is under the house. This technology will seem foreign to you, so it is important that you educate yourself on how a dewpoint controlled system works. We can also take your existing dehumidifier, put a relay kit on it, and tie the unit into the Atmox controller as a backup system. In essence, we turn your dehumidifier into a dewpoint controlled mechanism so it runs more efficiently. Once I put your dehumidifier on the controls, I can extend its lifespan by 15 to 20 years because it literally becomes a backup system. Once the initial moisture buildup is removed, the fans take over, and you now have fresh air in the whole house envelope that will improve air quality as well as getting rid of your moisture problems with a guarantee. You can stop using forced air begin to enjoy the benefits of using your crawlspace vents in a controlled way as they were intended. The most important advantage the system is that it runs on low voltage. When the system is running it is using the equivalent of a 25 W lightbulb. This will save you hundreds of dollars in operating costs over the next 10 years. I will add a couple links here so that you can find us. My name is Michael. You may call me at 704-787-6972 if you would like to learn more about dewpoint technology. Good luck!

  8. user-2310254 | | #8

    There was a post promoting Atmox in 2009. Here is what GBI guest blogger Allison Bailes had to say in response:

    "Crawl spaces communicate pretty darn well with the houses above them before they're encapsulated, and sealing them up doesn't necessarily make that any worse. If you follow Advanced Energy's recommendation to air seal the floor to isolate the crawl space from the house, it communicates less after encapsulation.

    I've encapsulated a lot of crawl spaces in the past 5 years and my experience, confirmed by the homeowners' feedback, is that the musty smell goes away when it's done correctly. I'm not a mold expert, but I know that mold needs moisture to grow. Remove that, and it's not a problem anymore.

    If venting the crawl space with lots of outside air works for you, that's great. I distrust that method, however, because it requires conditions that don't often exist in much of the South during the summer. Which part of North Carolina are you in?

    It also depends on a properly functioning control system to find the dew point and turn the fans on or off. Since that system is in the crawl space, how will the homeowners know if it malfunctions and brings in air with a dew point of 75 degrees every day for a couple of months?

    In the end, I believe a properly encapsulated crawl space will outperform a mechanically-vented crawl space across most of the southeastern US because the dew point just doesn't go low enough for ventilation to help."

  9. GBA Editor
    Martin Holladay | | #9

    Michael Masserang,
    GBA message boards are not intended as sites where manufacturers can post free ads. You have been warned. Please don't try to do it again.

  10. The Real Crawl Space Science Expert | | #10

    Steve Knapp,

    I can tell from your post that you do not understand how dew point technology works. Until you study and understand how the controller functions your comments are unqualified. I will not be able to answer your questions until you do so. Based on the comments made by Martin Holladay my insight on helping people solving crawl space problems is not welcome here. I am an Independent Crawl Space Science Expert. I do not work for a manufacturer thank you. I dont appreciate being threatened. You can be assured I wont visit this site again and I will report my negative experience to other industry professionals that I associate with in the industry. Good day.

  11. Expert Member
    MALCOLM TAYLOR | | #11

    Michael wrote: "I do not work for a manufacturer"

    You should probably edit your Linkedin profile then.

  12. Jon_R | | #12

    Linda, during mild weather with no AC use, pulling interior air into the crawlspace will often create conditions that will support mold growth. Diverting air from a ducted AC system doesn't work either (it's not running). I'd stick with your dehumidifier/humidistat (+ monitor) in a fully sealed crawlspace (no crawlspace connected vents or fans to anywhere else).

    As you noticed, this doesn't completely eliminate mold odor. Probably because mold is growing behind vapor barriers/retarders and air sealing of these barriers is never perfect. So there is some mold smell in the crawlspace and some of this can leak into the house interior.

    Another source of mold odor is caused by hot, humid air being pulled into the house interior walls and then cooled down. Using a crawlspace exhaust fan and passive vent to pull air from the interior of the house to partially condition a crawlspace makes this worse.

    Solution: run the entire house at a slight positive pressure during the non-heating season. Ie, a fan to the house interior directly from the outside. No odors will drift in from the crawlspace and you won't be encouraging moisture/mold in the walls or the crawlspace.

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