Crawl Space Ventilation – Myth #4

In this crawl space myth-busting video, we’re going to talk about

“Do you need ventilation or not in your crawl space?”

Most companies say you don’t need ventilation in an encapsulated crawl space. Why do crawl space contractors say that installing a dehumidifier to address high humidity eliminates the need for crawl space ventilation?

Soil Gases in Crawl Spaces

Imagine that your crawl space was open and ventilated, so you had all this air moving through these open vents of the crawl space. Then you go along and you seal up all the vents and you put in a dehumidifier. Well, if you’re in a place like here in Knoxville, Tennessee or in Charlotte, North Carolina or Nashville where you have radon and soil gases in the crawl space, sealing up all those vents only causes that radon and soil gas to go up into your home.

Does Fixing Humidity Problem Increases Soil Gases

You certainly don’t want to fix a humidity problem and cause a soil gas problem. What the EPA recommends is that you put one fan, that’s one CFM of air for every 50 square feet in the crawl space to push the air out. Now, that make up air can come from different places and we’ve done a video about that if you want to check out the make up air video.

Crawl Space Encapsulation

If you’re going to perform crawl space encapsulation, make sure you install a dehumidifier and ventilation along with proper insulation. Also make sure you have a no cat pee guarantee vapor barrier installed. Reinforced vapor barriers can give off cat urine odors as they get wet and separate.

Crawl Space Vapor Barrier

Yes, you read correctly. There are vapor barriers, vapor retarders, crawl space plastic; whatever you want to call it that is being sold and installed by crawl space repair contractors that gives off an odor similar to cat urine. There are crawl space companies scrambling because they refuse to admit it’s the plastic causing the problem but watch this video and make up your own mind.

6 thoughts on “Crawl Space Ventilation – Myth #4”

  1. Hey Michael,

    Great videos – learned a ton!

    I’ve been looking at encapsulating but I’ve seen some studies suggesting that show closed crawlspaces could actually INCREASE radon.

    Studies here:
    Page 108:
    Page 3:

    Specifically from the second study, page : “This benefit is dependent on continuously operating a
    crawlspace exhaust fan, and is lost if the fan fails. Without power venting, there is a concern about increased pollutant concentrations because of reduced dilution. Radon levels in the conditioned marine crawlspaces averaged 12.9 pCi/L. This level is almost nine times the 1.45 pCi/L average in the vented crawlspaces, and more than three times the EPA action level for exposure in homes.” I do find it interesting the homes had both passive radon mitigation (Washington code) and a powered fan (EPA). I would think these would work against each other.

    I don’t have a radon problem with our vented crawl but would not want to create one by switching to encapsulation. Can you comment?

    1. Hi Tim, our Ninja certified crawl space encapsulation is designed to always supply the minimum EPA recommendation of 1 cfm of ventilation blowing out for every 50 square feet of crawl space. We find some situations this may not be enough. If that is the case we could add more fans or we install a sub-membrane depressurization system. We do this because we understand there is no one size fits all for every crawl space and region of the country. Hope that helps. Thanks.

  2. Here’s a head scratcher….I recently had a sub membrane depressurization installed in my crawlspace. A Powered fan draws from piping laid under vapor barrier and blows the air out a pipe above my roof. They laid all new 10mil vapor barrier on top of my 6 mil that was old and didn’t have great coverage. Not sure if that’s how I would’ve done it, but it works. Radon levels instantly dropped from the 40’s-50’s to around 3 to 4 pci/L. With the passive crawlspace vents OPEN.

    After about 2 weeks, I closed the vents, thinking that the number would climb slightly, but I wanted to see if it could be done. The levels dropped a lot to a range of 0.5 to 1 pci/L. Isn’t that the OPPOSITE of what you’d expect? So I tried opening the vents after 2 weeks and sure enough the levels went instantly back up to around 3 pci/L.

    Any ideas? I still don’t know whether to leave them open or closed. Humidity is around 50 to 55 down there…a tiny bit of water comes in from the back yard when it rains really hard. The fan installers said the fan should help dry it out.

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