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.

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


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  1. Richard LaCaille on August 19, 2019 at 2:18 pm

    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?

    • Michael Church on August 19, 2019 at 3:37 pm

      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. Bruce Sullivan on September 3, 2019 at 10:04 pm

    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.

    • Michael Church on September 5, 2019 at 10:24 am

      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.

  3. Melvin Davis on August 12, 2020 at 1:43 pm

    How far away from dehumidifier should a vent fan be installed in crawl space?

    • Michael Church on August 14, 2020 at 11:36 am

      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.

  4. mike on September 22, 2020 at 9:06 pm

    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?

    • Michael Church on September 26, 2020 at 2:51 pm

      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.

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