Misconception: Seal All Vents of Encapsulated Crawl Space

Is it a good idea to seal all the vents of an encapsulated crawl space? Sealing all vents is a great way to control humidity and moisture, but don’t overlook soil gases.

Controlling moisture while creating a soil gas or crawl space odor issue is not healthy for your home.

What is a Sealed Crawl Space?

A sealed crawl space, or encapsulated crawl space, is a term given to sealing all vents and installing a crawl space vapor barrier on the walls, floors, and pillars. This design is one of the best ways to create a healthy crawl space according to many experts and engineers.

Concerns with Soil Gases and Humidity

Many homes have radon and other soil gases in the crawl space. If the crawl space is not properly ventilated during encapsulation, soil gases can become more potent in the living space, just like how homes can become very humid when a properly sized crawl space dehumidifier is not included during encapsulation and sealing the vents.

The Importance of Proper Water Management

Still, other homes may flood and float the plastic if necessary water management or crawl space waterproofing is not installed prior to encapsulation. We encourage you to consider all these options when encapsulating your crawl space to ensure it is worry-free for years to come.

Why Sealing the Vents is Important

Sealing the crawl space vents is needed during crawl space encapsulation to help control humidity. Proper ventilation is just as important in crawl space encapsulation as humidity control. Ventilation will move soil gases and odors from the crawl space before they enter your living space.

EPA’s Take on Crawl Space Ventilation

I first heard of crawl space ventilation with encapsulation from the EPA’s website. I was studying radon and came across a picture that showed one crawl space vent fan blowing air out of an encapsulated crawl space. All the other vents were sealed and a dehumidifier was installed. There was vapor barrier and crawl space insulation properly installed as well.

Balancing Encapsulation with Ventilation

It made sense to me that completely sealing the crawl space without proper ventilation would cause soil gases and odors to enter the living space. Ever since I read the article I have been a believer in proper crawl space ventilation with encapsulation. This is why Crawl Space Ninja installs a crawl space foundation fan in all encapsulated crawl spaces.

What Next? Contact us about your Crawl Space problem

Contact us if you need help fixing your crawl space or yard drainage by clicking here.

Do you need help with controlling humidity in your home and you live in Alabama, Georgia, Delaware, North Carolina, South Carolina, Tennessee, or Kentucky? If so, please contact us to schedule your assessment. Also, let us know in the comments below if you’d like to suggest a future blog post.

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22 thoughts on “Misconception: Seal All Vents of Encapsulated Crawl Space”

  1. Can you please provide the code in the EPA standards proving that you have to have a ventilation system in an encapsulated and dehumidified crawlspace?

    1. 1.4 Basement and Crawlspace Insulation and Conditioned Air
      Indoor airPLUS Requirements:
      Seal crawlspace and basement perimeter walls to prevent
      outside air infiltration.
      Insulate crawlspace and basement perimeter walls according
      to the prescriptive values determined by local code or R-5,
      whichever is greater.
      Provide conditioned air at a rate not less than 1 cfm per 50 sq.
      ft. of horizontal floor area. This can be achieved by a dedicated
      supply (2015 IRC section R408.3.2.2) or through crawl-space
      exhaust (2015 IRC section R408.3.2.1). However, if radon resistant features are required (see Specification 2.1), do not
      use the crawlspace exhaust method.

      1. Hello Michael,

        First, I would like to say how grateful I am for you sharing your knowledge with everyone, I have learned so much.

        I am a licensed mechanical contractor and I’m encapsulating my own crawl at the moment. After reading the above link to the epa attachment, it looks like they give me three options in section 1.4.

        1) Use the HVAC to supply the conditioned air
        2) Exhaust the air out of the crawl (Clearly states not to use if radon is an issue)
        3) Use a dehumidifier

        I’m not seeing where I’m required at all to exhaust if I’m using a dehumidifier. I definitely want to do this right, can you please comment on my way of thinking?

        Also I can’t wrap my mind around if I seal my crawl from the ground, why would experience any soil gas intruded into my home to begin with? If it’s sealed it should have no way up right? Would really appreciate being educated on that as well if you have the time.

        1. Hi Tyler, we find completely sealing the crawl space is difficult. Vapor barrier touching concrete with adhesives can eventually fail and leave it unsealed over time. I know it may defy logic but soil gases can penetrate vapor barriers because it is designed as a vapor retarder not an air barrier. Barrier is actually the wrong term, retarder is more accurate because even some vapor gets through the plastic. Using a supply/return can pull crawl space air into the home and add unnecessary load to the HVAC. It has been our recommendation to install a dehumidifier and foundation vent fan to help prevent smells and soil gases from entering the living space via the stack effect per EPA guidelines. BTW, you are correct on using the word required. For us it is a best practices based on our experience. Hope that helps.

    2. My crawlspace is encapsulated with 2 Dehumidifiers (roughly 3k sq ft). Our house sits like a U, the Dehumidifiers are placed near the entrance of each top of the U (hope that makes sense).
      We have 45-55% throughout the whole crawlspace except 1 of the top of the U(bottom of same side which is all open together is 55%)
      That area fluctuates 57-67% , started doing this just a couple months ago whereas not prior. No leaks in crawlspace and we have been very very dry for this time of year.

  2. Hi Micha

    I live in western NC and have a professionally sealed (pillars sealed, white thick plastic, the whole nine yards) fully encapsulated crawl space with a Santa Fe compact 70 dehumidifier which should completely handle the crawl space size. Also a radon system was installed to mitigate radon buildup, and the vents were sealed with foam board. I recently installed a sensor push humidity detector and most of the year, humidity is down to 50% or 60%. However, in the summer, depending on the day, I see humidity levels of 70% or even 80%. The wood joists look great, like they were just installed and this house was built in 1988. Is there anything else I should do to get humidity down, or is it acceptable under the current conditions?

    1. Hi Scott, I recommend the humidity between 45% to 55%. The winter months tend to be drier anyway and some crawl spaces can even dry out during that time with no dehumidifier. Check and make sure the dehumidifier is operating properly. You can get a humidity reader and place on the exhaust air side of the dehumidifier. Dry air exiting the dehumidifier should be 40% or less. If it is not, change the filter and make sure the condensation discharge line is not clogged. Also check the coils behind the filter. If they are iced over, call Santa Fe for repair advice. Hope that helps.

  3. Ignacio Zambrano

    Hi Michael,

    I also wanted to say thank you for sharing your knowledge throughout the years. I have been an active reader of the information you provide since I started thinking about encapsulation. I am a home owner and encapsulated my crawlspace with a santa dehumidifier and sum pump a couple years ago. I added two crawlspace vent exhaust fans against the encapsulation company recommendations, based on your recommendations. They wanted me to seal all vents. The crawlspace looks great but two minor issues i have noticed:
    1) Lots of crickets/spiders living in the areas surrounding the exhaust vents. Is this a common theme with encapsulation with an exhaust fan?

    2) When I return home after leaving a few days on vacation, i notice the presence of a musty smells in the house that goes away once we start doing common daily activities (cooking, running the ac/furnace, etc.) i know this is coming from the crawl because thats what my crawlspace smells like. The humidity level in my crawlspace is between 40-55% throughout the year. Is this smell after returning home normal with encapsulated crawlspaces?

    1. Hi Ignacio, thank you so much for reading our articles. When the crawl space was encapsulated was there mold present prior and if so was it removed and disinfected? This article states cave crickets eat mold/mildew. “Since these crickets are native in deep forests, caves, and damp, dark places, they are called cave crickets. Usually, they eat mold and mildew. Damp crawlspaces and basements often turn out to be their breeding grounds.” https://pestcontrolzone.com/crickets/. Other articles state they like rotting wood. Spiders can be in the crawl space feeding on the cave crickets. I am not saying an encapsulated crawl space will be pest free but if the environment is less inviting, they should be minimized. Also, I have seen properly encapsulated crawl spaces with subfloor insulation that has trapped a plumbing leak and cause crickets, silverfish, roaches and other pests to move in. Another reason I don’t like subfloor insulation. Hope that helps.

  4. What if I put a pan pipe through the 10 mill plastic into my sump pump basin and run that vent pipe right up out my roof.
    I mean I have a French drain system going around the entire water on the inside of the crawl area.
    Shouldn’t the sump pump pipe work as a ventilating pipe and if I put an additional pipe going up through the roof that should be sufficient for any gas is under that encapsulation to escape.” Right?

    1. Hi Richard, I have seen sump pump basins and waterproofing systems used in conjunction with soil gas mitigation. These were systems professionally installed by radon mitigation companies.

  5. Hi Mike I live in New York just east of Buffalo.
    We have snowy winters and the months are colder in November December January February March April.
    I just encapsulated my crawl area I install the French drain system around the interior of the footer before I encapsulated it along with the sump pump and basin.
    I don’t want to put a foundation van in because I feel the sump pump pipe going through that foundation is the vent.
    I mean I’m earth gases are going to get Internet pipe and they’re gonna Xscape right out that sump pump pipe.
    And if that’s not enough I can always run another pipe from the sump pump right up through the roof.
    That should be sufficient for any earth gases to escape.
    Correct me if I’m wrong.

    1. Hi Richard, I have seen sump pump basins and waterproofing systems used in conjunction with soil gas mitigation. These were systems professionally installed by radon mitigation companies.

  6. Hello Michael,

    Regrettably, I can’t use a dehumidifier for my project. Barring that, I am considering using one foundation vent of two within an encapsulated area for a 12v fan mounted on the ~outside~ of the vent, to exhaust the stale air that would accumulate; and with the other vent open only to the soil underneath to perhaps prevent ballooning of the polyethylene sheet. Both vents have been closed year round. The fan would not pull a lot of air; rather, just enough to keep the space “freshened”, for the lack of a better word. Air intake would be accomplished through the structure itself. Or, should both vents be sealed, the space like a closed jar? Which would be best? Incidentally, I can’t help but think what had been done in medieval times; a dirt floor, only? Also, once the floor goes down over said space there will be no future access; that is, without ripping up the floor. Radon is not an issue, I don’t believe, here just south of the Memphis area. The encapsulated space is under a bedroom and bathroom, only, and to be isolated from that of the rest of the house. The floor of the rooms are only about two feet above the soil. Thanks.

  7. Hey there,

    I’ve just finished cleaning out my crawl space and am now about to start sealing off the crawl space vents and I’ve run into a problem. The crawl space vents on the outside have there top edge flush with the wood panel siding of the house. The vent covers i bought take up about 2 inches of room around the vent. This makes it so I cant use the vent covers i bought without taking up some of the wood siding . Do you have any ideas on other things i could use to seal the crawl space vents?


  8. High Michael,
    Thank you for all your advice. I encapsulated my crawlspace, and now I am dealing with something many people are not considering. I have a mechanical room on the first floor of my house. It has a gas furnace and gas hw heater. The combustion intake air for this room were from a duct to the attic and two vent holes in the floor. Since the crawlspace was typically ventilated to OA this was ok to pull the combustion air intake from the crawlspace. But now since I have encapsulated the crawlspace there is no direct ventilation to the OA and this is causing me concerns. What would be the issues if I left one vent in the encapsulated crawlspace open for combustion air intake?

  9. Hi Michael, you’re the best and I’d greatly appreciate your advice on my scenario. We are installing thick drainage matting in our crawlspace together with a sump pump and a very nice dehu. We’re told this will basically act like an 85% encapsulation, but it’s not all the way. Matting will be wall to wall, taped, with wrapped piers. Home was built in 1960 (Texas). Our joists unfortunately run directly in front of our vents, so the crawlspace has zero air and humidity is out of control (hence the work). My question is.. while the dehu will help reduce humidity and filter the air, shouldn’t we have fresh air circulate in the crawlspace at least from time to time? Otherwise, I presume the air has nowhere to go but directly into our older house, and filtered or not, I don’t want crawlspace air inside my home. We were told to cover our vents and I’m just puzzled on next steps. Thank you!

    1. Michael Church

      Hi Jordan, yes it would be advantageous to have negative air in the crawl space to draw soil gases to the outside versus into your living space. When we have issues with no vents or not accessible, we still seal them, even if it is from the outside to help control humidity with a dehumidifier. Then we use what we call a whisper fan. This fan is mounted in the crawl space and is ducted through a 4″ hole we drill into the foundation and we cap the duct pipe off with a dryer vent on the exterior to keep pests out. Here are links to what I am describing: https://diy.crawlspaceninja.com/s-p-in-line-whisper-fan-td100x1/ and https://diy.crawlspaceninja.com/vent-fan-install-kit-for-whisperfan/ – I hope that helps.

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