Crawl Space Encapsulation Vent Fan

Are you asking if a vent fan is needed when encapsulating your crawl space? In this article we explain what we recommend per EPA guidelines when it comes to crawl space ventilation.

Foundation Vent Fan

The foundation vent fan is a concept pretty unique when it comes to building practices. Similar to attic ventilation, it is often overlooked and misunderstood. Many homeowners hear conflicting information from pest control and waterproofing companies about crawl space encapsulation vent fans.

Most foundation vent fans are constructed of plastic or metal. Many use computer style fans to move around 100 cubic feet per minute of air. Some place the fan to draw air into the crawl space. Some homeowners have the crawl space air being pushed out.

Whether your fan is pulling in outside air or pushing it out, the effect is pretty much the same in an open or vented crawl space. The make-up air is still being pulled from the air outside.

Vent Fan image

Open Crawl Space Vent Fan

Is having outside air enter your crawl space a bad thing? That depends on the humidity level of the make-up air. If you have an open crawl space in Knoxville, TN or Atlanta, GA chances are you are bringing in hot humid air.

Hot humid air hitting cold, poorly insulated and non-sealed duct-work is a recipe for mold and wood rot fungus. Mold and fungus can affect the sub-floor and cost you money in foundation repairs and floor stabilization.

Perhaps a vented crawl space in Denver, CO or Las Vegas, NV is not that much of an issue in regards to humidity. If the same principle applies with cold ducts in the crawl space, the hot non-humid air can still rob your home of air conditioning.

Using a Vent Fan with Encapsulation

This is where many get confused. For years, crawl space contractors have been sealing all the vents during encapsulation. In fact, many home builders create homes without foundation vents.

Why the drastic change? Research has shown an encapsulated crawl space has less mold, structural issues, and critters than an open crawl space. This doesn't mean all builders are closing crawl spaces but some won't build a vented crawl space.

The problem is soil gases. The crawl space companies and builders that close crawl spaces have created a problem while trying to fix others. When you seal all vents or build with no vents, the soil gases' only route is in your home.

Read our last article:

Crawl Space Encapsulation Dehumidifier

Vented crawl spaces certainly create more mold and structural problems. But, they allow nasty and potentially deadly soil gases to escape.

Installing a vent fan and performing crawl space encapsulation is the best scenario to address both problems. But there is another problem. Many contractors and builders do not include a dehumidifier.

That's like creating a well-insulated home with no heat source or air conditioner. No matter how good the crawl space is encapsulated, humidity, and other moisture problems still find a way to exist.

Crawl Space Encapsulation EPA Guidelines

I actually stumbled across the EPA (Environmental Protection Agency) guidelines by accident. I was researching open vented crawl spaces versus crawl space encapsulation and shocked at what I found.

I find it absolutely appalling that Crawl Space Ninja is one of if not the only company including ventilation and encapsulation.

The EPA says to install a foundation vent fan that will move 1 CFM of air for every 50 square feet of crawl space.

Did you know that? Did the last company that quoted your job tell you that? What about local codes enforcement, do they know what the CFM per square feet rating is?

This is why we look at your crawl space as a whole and try to make it worry-free. We are not just interested in the vapor barrier or a french drain. We don't remove mold without fixing its cause. Insulation and air sealing are not overlooked in crawl space encapsulation either.

When you hire Crawl Space Ninja, we explain how to fix the entire crawl space not just cover up or delay problems. This doesn't mean all homeowners follow or even like our advice. But that's not going to stop us from proposing the worry-free crawl space solution for your family.

I hope you found this information useful. Please comment below if you have any questions or would like to say hello. I am also putting a video for you to watch that could help further explain dehumidification and ventilation.

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About Michael Church

Michael Church has been involved with indoor air quality since 2005 and feels the unhealthy crawl space is one of the major problems causing poor indoor air quality.


  1. John brabo on December 15, 2020 at 7:39 pm

    Can you recommend any contractors in the sacramento region?

    • Michael Church on December 16, 2020 at 10:03 am

      I am so sorry John, we do not know of anyone in that area.

  2. Bryan on December 31, 2020 at 11:11 pm

    I’m a home builder in Tennesse area which we encapsulate all of our crawlspaces for all the reasons we agree with you on. We also put in the radon mitigation piping that’s under the crawlspace poly and exits through the roof. We are wanting to follow epa exhaust guidelines on encapsulated crawlspaces and wanted to know if installing a radon fan would be sufficient in meeting the 1 cfm for 50’ of crawlspace. . This assumes all calculations match up. In the end, just looking to install one fan and resolve two issues with soil gases and radon prevention. Thanks and really enjoy your detailed information

    • Michael Church on January 4, 2021 at 2:01 pm

      Hi Bryan, great question and from what I understand about how they write the requirement, your way of installing a sub-membrane depressurization system is sufficient and should not require the fan in the foundation vent. Thank you, hope that helps.

  3. Ralph R. on February 8, 2021 at 8:56 pm

    I just had a full encapsulation, but another company with similar products, since Crawl Space Ninja doesn’t service me area. Well when I asked about the vent fan, they indicated that it was not needed nor a service they provide. I didn’t take measurements before hand, but my radon level is hitting around a 4.0 pCi/l peak with 3.5 48 hour average. Is this an crawl space vent issue or a radon mitigation service issue? I am think I will have to DIY install the vent fan to pull the air out and monitor for a few days.

  4. Ali Cameron on April 18, 2021 at 11:30 am

    We just had our crawlspace encapsulated with 12 mil poly. It goes up to the sill plate and the exterior walls are insulated with 12 R value foam board. The only 4 vents we had in the foundation walls were covered. We installed a sump pump and a Moisture Medic 90 (Aprilaire) dehumidifier. The crawlspace is approximately 1325 square feet. It’s one big open space with concrete block footings down the center of the home. My question is do we need to have an exhaust fan venting air out of the crawlspace through the foundation and a vent from HVAC system installed pulling air through the crawlspace or is the air circulating from the air movement of the dehumidifier enough? Someone suggested putting a box fan down there on a timer to circulate the air around a couple times a day. It just seems odd to be all sealed and virtually no air movement or a way for the seemingly “trapped air” to escape.
    All HVAC ductwork was insulated too at the time this was done. This was all done after mold remediation took place.

    • Michael Church on April 26, 2021 at 12:59 pm

      According to EPA guidelines you should exhaust 1CFM of air for every 50sq feet of crawl space to the outside air. Normally a Lomanco PCV1 fan is enough to accomplish this. Here is a link: Hope that helps.

  5. Chuck Davis on April 18, 2021 at 11:03 pm

    Good information and video. Thanks. Exhausting air from an encapsulated crawlspace does seem to be something that has been overlooked.

    We had our crawlspace in Durham, NC encapsulated with spray foam. Ducts and registers were sealed. The crawlspace is *really* tight.

    A 400 CFM inducer was installed in the HVAC. No dehumidifier. RH peaks at around 70% in summer, drops to 45%-55% in fall/winter/spring.

    There is a 4″ drain that connects through the foundation to a 6″ collector on the footer with about a 75 foot run to an outfall, but I’m not sure how much air flow/pressure relief this provides. (I could probably run a flame test.)

    What would be your thoughts on a passive exhaust vent in the crawlspace door vs. dehumidifier vs. leaving as is?


    • Michael Church on April 26, 2021 at 1:02 pm

      A dehumidifier is the best way to control humidity in our experience. Exhaust fans in humid environments do little to control humidity. If your humidity spikes above 55% I would not leave as is. Thank you and hope that helps.

  6. Aaron Buchanan on August 11, 2021 at 10:38 pm

    So do these foundation vents fans run 24/7 or do they cycle on? Do they make the dehumidifier work harder since now longer sealed from the outside environment, I guess if they run 24/7 blowing air out that wouldn’t matter but if they cycle on I would imagine during the phase they are off would cause a moisture gradient in which the RH in the crawlspace would spike. Are they energy intensive or have a low energy consumption profile?

    • Michael Church on August 12, 2021 at 10:31 am

      The EPA recommends running the fan 24/7. I am sure they do affect humidity some but the power of our dehumidifiers takes care pf that. The Aprilaire E Series dehumidifiers and our foundation vent fans are some of the lowest energy users on the market. I believe the fan uses around 1/2 an amp and the dehumidifier varies by model. Hope that helps. Thanks.

  7. Mike Mantino on August 25, 2021 at 5:19 am

    How is the air made up if a vent fan is taking it out? The space being ‘sealed’ and encapsulated suggests that there is no way for air to get into the spade, but the vent fan will be taking air out. Where is the new air coming from? Are you relying on leakage from penetrations? Or is air brought in from the home through the a/c ducts?

    • Michael Church on August 30, 2021 at 1:12 pm

      Great question Mike, most crawl spaces have leaks around the perimeter or living space for the make-up air to enter. We are only using 1 cfm per 50 sq feet calculation so it is not difficult for the crawl space to have leaks to provide that low of air flow. Hope that helps. Thanks.

  8. Craig Bowman on August 25, 2021 at 4:36 pm

    Hi Michael. I live in Western North Carolina and I have a small 8′ X 24′ (192 SF) crawl space under my utility room. It is slightly damp on one side (with a ton of crickets!) and causes the utility room to smell bad all the time. I am in the process of encapsulating it now. It has 3 foundation vents to the outside. I am assuming I will still need to vent the space regardless of the size? I had originally thought I would have to plug off 2 of the vents and install a vent fan in the 3rd vent, but then wondered where will the air come in from for the fan to exhaust the space? A dehumidifier would be way overkill for a space that small, wouldn’t it? I’m not even sure how I would put one down there (or if they even make one that small). There’s not much room between the crushed stone ground cover and the floor joists. There is very little if any mold down there now, so I don’t want to create an atmosphere for any to start growing. Thank you for all of your videos and taking the time to answer questions.

    • Michael Church on August 30, 2021 at 1:20 pm

      Hi Craig, thanks for watching our videos and reading our articles. Small crawl spaces with little room for dehumidifiers may still require ducted dry air into the space from another part of the home. This could be done from the living space but the severity of your crawl space issue and the expense to do that would be for you to decide. If you don’t notice mold I would still test humidity and wood moisture levels to ensure the wood is dry. I assume in West NC your outside is humid like ours so fans alone will most likely only draw in more humid air. Normally there are enough air leaks around the perimeter of the foundation for the make-up air. This is why a dehumidifier would be needed if you decide to ventilate to dry that make-up air entering the crawl space. There is a chance even more humid air will enter the space if fans are installed and no dehumidifier which could make the problem worse.

  9. Amanda on September 12, 2021 at 8:41 am

    Hi Michael,

    I have a fully encapsulated crawl space. The installers put a vent grill from my HVAC right into the crawl space. The humidity is too high. Should I close that vent OFF and add a dehumidifier and exhaust fan?? OR do I leave that HVAC conditioned air and add a dehumidifier and exhaust fan?


    • Michael Church on September 13, 2021 at 10:51 am

      Hi Amanda, every home is different but we don’t usually recommend HVAC to control crawl space humidity. Dehumidifier plus foundation vent fan is recommended. Here is a video that I hope helps. Thank you.

  10. David Thibodeau on September 24, 2021 at 2:11 pm

    Hi Ninjas!

    Ok so I’ve getting great info (via your You Tubes) about encapsulated crawl spaces and ventilation.

    However I still have not found any clips addressing my particular concern about ventilation. I get that the EPA requires an exhaust vent even with a closed crawl space because of potential build up of soil gasses.
    But if the area is encapsulated then would there be any threat of soil gasses?

    • Michael Church on September 25, 2021 at 2:36 pm

      Hi David, for us a closed and encapsulated crawl space are the same thing so yes, if you close the vents, regardless of how you install the plastic, the EPA recommends a vent fan to address soil gases. Hope that helps. If I misunderstood your question, please let me know.

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