Do You Need a Vent Fan And Dehumidifier?

On today’s blog we will discuss when you will need a vent fan and dehumidifier in the crawl space. How we at Crawl Space Ninja determine the best way to install them. Why both are crucial to crawl space encapsulation.


Michael Church’s Crawl Space Repair Myths-Busted book was written to help you Avoid Bad Advice, Bad Decisions, & Bad Repairs.

“This book is the Cadillac for crawl space information and Michael Church is your experienced driver. What can I say? It’s just an amazing read.” 5.0 out of 5 stars The real deal.

Crawl Space Ventilation Fan

The Crawl Space Ninja System includes a vent fan and a dehumidifier, but we do get questions if a vent fan is truly needed if you have a dehumidifier. The answer is yes.

The reason for the vent fan is to move soil gases that might be present in your crawl space out. We are located in East Tennessee where there can be radon present. This doesn’t mean you have a radon problem but other gases can also be present so we like to take precautions.

Crawl Space Dehumidifier

Remember this is only the case when the dehumidifier is a standalone unit within the crawl space. That means that the dehumidifier will not be pulling in air from the living space above.

The only thing you have to keep in mind is that one vent fan can cover up to 5,000 square feet. Make sure crawl space vents are sealed properly. If you have a larger crawl space, multiple units may be needed.

Other Crawl Space Essentials

A properly installed vapor barrier is also needed. Crawl space insulation cannot be overlooked. Standing water from a flooded crawl space can add humidity as well.

Below is a video that talks a little bit more about this same subject, comment and tell us what you think!

Does Hydrostatic Pressure Affect Humidity

What Next?

Do you need help with mold removal, crawl space encapsulation, crawl space insulation, vapor barrier, waterproofing, foundation repair, or controlling humidity in your crawl space and you live in Georgia, Delaware, North Carolina, South Carolina, Tennessee, Ohio, or Kentucky? If so, please contact us to schedule your assessment. Also, let us know in the comments below if you have an idea for a new blog topic.

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36 thoughts on “Do You Need a Vent Fan And Dehumidifier?”

  1. Richard LaCaille

    I installed a sump pump )3ft away from the access door. I’m also going to install (hang with straps)
    the dehumidifier close to the sump pump to gravity feed the condensation tube. Not sure where to install the vent fan. My first thought was to install it in the center of the access door, which is about 4 ft away from dehumidifier. Is that to close?

    1. It may be a good idea to place on the other side of the crawl space so the dry air is pulled further from the dehumidifier. I’m concerned having the AVS too close would suck all your dry air out before having a chance to travel throughout the crawl space. Great question, thank you.

  2. I am in the process of a MASSIVE remediation / renovation of our 100-year-old family farm house. We have had serious mold related illnesses and are, therefore, very concerned about getting it “right” when it comes to our crawl space.

    1. We will be encapsulating with 20-mil plastic.

    2. We will have a dedicated crawl space dehumidifier.

    We have read a great deal about the pros and cons of systems like “EZ Breathe” the pull air from the living structure (via a floor fan) down into the crawl space and then exhaust it outside via another fan in the crawl space. We have read that it can create issues by drawing hot, humid, unconditioned air into the living space via leaks in the walls, etc, and, thereby, create possible condensation within the wall cavities, etc.

    I say that only to ask if the ventilation fan you recommend might not do the same thing, I other words, as it exhausts air from the crawl space, will it not create a negative pressure throughout the entire structure and, therefore, draw air down from the the living space?

    One additional question, also, is this: How likely is a whole house dehumidifier (i.e. in the living quarters with now direct vents, etc., to the crawl space) to effect humidity in the crawl space? An HVAC guy told me that the entire house (including the crawl space) would “equalize.” However, I am skeptical that whole-house dehumidifier in the living space will impact humidity in the crawl space without venting between to the two spaces (something I am not willing to do).

    We really need some guidance and greatly appreciate the assistance we have already received through your videos.

    1. Hi Bruce, Great questions and congrats on tackling your crawl space. The fan is recommended by EPA in order to move soil gases out when closing the crawl space. The Lomanco and Whisperfan only move 110/120 CFM so not that much air but enough to fall under EPA guidelines. They made a change to building code recently that you don’t need fan if you install one 70 pint dehumidifier every 1,000 square feet. Sounds great but that doesn’t address soil gases. If you are not concerned about gases, you can still do you crawl space to code following the new rule. I personally feel gases should be addressed. We have not experienced any condensation problems with our system as you described and I am not personally real familiar with EZ Breathe other than what I have read online. Hope that helps.

      In regards to dehumidifier in living space, I am not sure it would dehumidifier the crawl space effectively. You may see a few percentage points drop. Stack effect is low to high so it would be more likely that dry crawl space would equal dry living space, not the other way around. Now if your duct work leaks horribly in the crawl, then that could dry more than expected but I’d fix the duct work by air sealing it. Have a great day.

    1. There is not rule or recommendations but I think a minimum of 15 feet is best. Too close and you could be pushing the dry air from dehumidifier out of the crawl space. Hope that helps, thank you.

  3. Hi, I have a dehumidifier in my sealed crawl space and have conditioned air blowing into the crawl space. Do you recommend an exhaust fan in this situation?

    1. Great question, in theory blowing air into crawl should also push air out but EPA says you should push air out and make sure it is going outside not blow air in. You most likely are accomplishing pushing air out but you could be pushing air out of crawl and into living space as well as outside.

      1. Hi. I have an hvac duct blocking part of my crawlspace, I was thinking to create another access point, do you think that having two doors in a crawlspace will be a good idea?

        1. As long as the 2nd door does not compromise the structural integrity and it is and sealed and built with a footer to stop rain water from entering, that should be goo. Thanks. Hope that helps.

  4. I just recently had a French drain system installed, sump pump, 10mil vapor barrier( ground only) vents sealed, and 70 pint dehumidifier installed. Prior to install I had moisture readings in the crawl space of 22-25% and humidity readings of 80-84% with condensation everywhere and standing water in one corner. The dehu has been running non-stop for 9 days straight since install and has not dropped lower than 58%RH but stays at 60% most of the time. Without doing a full encapsulation, will a vent fan being installed help further reduce the RH in the crawl space or do I need to consider completing the encapsulation? I need the fan anyway as code from research calls for a fan to exhaust soil gases from my understanding. My only other option being I’m in a humid area would be adjust dehu to 60% so it doesn’t run constantly. The crawl space is just over 2000 square feet.

    1. A complete encapsulation is the best way to control humidity. Adding a fan is designed to address soil gases so I don’t think it will help lower humidity. Keep in mind too that your crawl space can take several weeks to dry out so the added RH could be coming from the wood releasing moisture. Adjusting to 60% could allow humidity to get in mold growing levels. Hope that helps. Thank you

  5. I was told when I install a crawlspace dehumidifier that I need to have one vent open for air for my furnace, which is installed in the crawlspace. If so how large should that vent be an I am assuming that it should be a good distance away from the vent fan. The crawlspace is about 1300 sq ft.

    1. Hi Bruce, I apologize but I would recommend you contact your HVAC contractor for the answer. We only deal with ventilation for soil gases which is 1 cubic feet per 50 square feet of encapsulated or sealed crawl space. Local codes for furnaces differ from county to county but your HVAC contractor should know. Hope that helps.

  6. When all vents are closed and a humidifier is installed does the vent fan exhaust to outdoors?
    Is this the only type of ventilation that is needed. Do you have a video showing this set up?
    Where does fresh air come from?

  7. Hi Michael, I have a 100 year old house located in the Southeast. A lot of the issues you talk about, we experience in our home due to the high humidity during summer months. Crawl space humidity can easily get over 80%. That led to a musty smell and some minor mold growth.

    We recently had some work done to our crawl space – we did NOT do a full encapsulation, but cleaned up the mold, replaced the vapor barrier (the old one had exposed dirt and was poorly laid), and installed a dehumidifier.

    When they installed the dehumidifier, they sealed our vents with foam board. I struggle with that because of the existing musty smell and the fact that we don’t have a full encapsulation. I’m worried that the air has to go somewhere and that smell is going to go up!

    Is it proper to seal the vents if you don’t have a full encapsulation? I’m thinking we need to cycle air out of the space to provide some relief of the “musty” air to prevent it from just going up.

    1. Hi Jack, sealing the vents is designed to slow humidity so the dehumidifier can work more efficiently. That being said, we never recommend sealing all vents. We seal all but one and install a foundation vent fan to blow air out of the crawl space per EPA Indoor AirPlus guidelines of 1cfm/50sq feet of crawl space to help move out soil gases. If the mold was addressed properly, I am not sure why you are still getting a musty smell. Hope that helps.

      1. I am in the same situation. Although my house is only 47 years old, it also has that musty smell in the crawl space. Insulation was originally installed under the floor. That was removed when I decided to have the vents sealed and install a dehumidifier. Unfortunately, the musty smell stayed. I had a ventilation fan installed in one of the vents. It may help a little, but not enough. I don’t feel like there is a fix for this, other than reopening the crawl space. If their is a fix, I have yet to find it while searching the internet.

        1. Did you treat the crawl space for mold (have it removed/apply a cleaner/preventative)? Many times just drying the crawl space will not remove odors if the source of the odor is still present, like mold.

  8. I was told that there was not enough mold to require treatment and that it would go dormant once the crawl space was dry. They did treat it with an odor remover. That didn’t seem to help.

  9. Hello Mike,

    My house is bayfront, 75’ from the water. The house was built in 2004 and is on piles. The house is elevated 3’ above the grade, So my crawl space floor is on grade. The crawl space is I closed by framing and 1/2” composite board. The walls have flood vents and conventional vents. There is a rat slab on grade. In between the floor joists is spray foam and then Kraft faced insulation. The HVAC is forced hot/cold air with no duct work or equipment in the crawl space. I had to run a cable wire in the crawl space the other day and noticed what appears to be a lot water droplets on the Kraft face insulation. In another part of the crawl space I noticed the water droplets on the bottom side of a pvc waste line. I do not think there is a leak but not 100%. I’m assuming that it’s condensation. The temperature outside was 84 degrees and the humidity inside the first floor of the house was 58%. What are your Thoughts and how should I handle it? Thanks

      1. James Mordaga

        Should I install a vapor barrier on top of the rate Slab. Should o seal the air vents. Which model would you recommend, the crawl space is 1400 sq ft 3’ high

  10. I will be in capsulating our crawl space approximately 880 ft. I purchased a fan with a humidistat. Is it okay to seal all vents and put the fan in one of the vents without any cross ventilation? Will that remove the gase

    1. That is a good start. Makeup air will be coming from sill plate and other openings. But keep in mind fans are not a one size fits all. If your home has high levels of radon, soil gases. The EPA recommended 1cfm/50sq feet will not be enough. Testing for radon may be required even if you install a fan.

  11. Hi Mike,

    We recently purchased a home that has a fully sealed (no vents) crawl space and had a poorly laid vapor barrier. There was significant wood rot, some minor mold, and bad odors in the crawl space that we’re looking to remediate. The humidity was also very high due to a leaking bathroom and poorly graded slope near the foundation. A vendor recommended that we install a French drain, sump pumps, replace the vapor barrier, add dehumidifiers, and also install fans with intake from outside the attic and exhaust to the outside. Do you recommend pulling air from the attic into the crawl space? Do you need the fans with both intake and exhaust if we also have a vapor barrier and dehumidifiers?

    Thank you

    1. Everything to me sounded great until that pull air from attic statement. We install fans to push air from crawl space straight to outside, not draw air from other parts of the home. I am not sure what would be accomplished by doing that. Typically makeup air intake is not needed due to all the air gaps in the foundation already. Hope that helps.

  12. I have a 1000 square foot house built in 1959 with 10-mil plastic, but not encapsulated. No insulation (energy bill only goes over $100 maybe 2-3 winter months and 2-3 summer months).

    I’ve had the house since 2019. Before I got the 10 mil plastic, living space relative humidity could reach 80% sometimes even with the A/C running in much of summer.

    No, humidity inside the house is inching back up slowly each year. And it’s worst in autumn as temperatures drop but humidity is still high (North Carolina). The past few weeks, living space relative humidity is topping 70% fairly often and calendars are curling.

    So I figure dehumidifier is the next way to go (we installed the electrical just in case it would be needed, when I got the 10mil plastic and sump pump).

    Do you close all the crawlspace vents? Leave one open? Get this vent fan thing?

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