I had my crawl space encapsulated and outside vents sealed. Why have my radon levels gone up?

Sealing a crawl space and closing the vents traps the soil gas that used to drift out through those openings, so radon can climb even though the space looks cleaner and drier. Encapsulation fixes moisture. It does not pull radon out of the ground. You need a radon mitigation system for that, and on an encapsulated dirt or fieldstone crawl that means sub-membrane depressurization, which runs $900 to $6,000, and most homes land around $1,950 to $2,250.

Here is what is going on under your house. Radon comes up out of the soil. Before you sealed the crawl, some of that gas leaked out the open vents and never reached your living space. Once you encapsulate and seal those vents, you close that escape route. Good for energy. The DOE notes that up to about a third of a home’s heat loss can come from an uninsulated foundation or crawl, so sealing is the right call. But the radon that used to vent out now has fewer ways to leave, and a sealed vapor barrier on its own does not stop it from finding the seams and rising into the rooms above.

That is normal. It catches a lot of homeowners off guard, and it is not a sign the encapsulation was done wrong. The two jobs are separate. One controls water and humidity. The other controls a gas.

The fix is a mitigation system built for a membrane crawl. Our radon lead, Branden (we call him B-Radon), installs sub-membrane depressurization on dirt and fieldstone crawls. A pipe runs under the vapor barrier, a quiet fan pulls the soil gas out from beneath it, and that gas vents up and out above the roofline before it can enter your home. The encapsulation you already paid for makes this work better. The barrier becomes the seal the fan draws against. So the two systems work together instead of fighting each other.

The EPA action level is 4.0 pCi/L. At that number or above, fix it. Between 2 and 4 pCi/L the EPA says to consider fixing, and most New England homeowners do, because our granite-heavy soil tends to run high. NH DHHS tracks radon across the state and recommends every home get tested. If you sealed your crawl and your number jumped past 4.0, that is your cue to call.

You do not need to tear out the encapsulation. You do not need to open the vents back up either, which would undo the moisture work and the energy savings. The membrane stays. We add the depressurization to it. A radon test runs $50, credited toward the job if you move ahead if you want a fresh reading before we scope the system, though many homeowners already have a recent result from the test that flagged the problem.

What drives the cost of the system is the size and layout of the crawl, how many suction points the soil needs, and the fan run to get above the roof. We come look first. The inspection is free, the estimate is free, and we get you a quote within 24 hours. Radon mitigation carries a 10 year warranty on our work. We do not guarantee a specific radon level unless we put that in writing, because the number depends on your soil and how the house breathes, and no one can promise a reading they do not control.

One more thing while we are under there. If the crawl is damp or the floor wall joint is wet, that is a different problem from radon, and you may want our crew to look at the drainage and dehumidifier too. But you may not need any of that. If the encapsulation is dry and doing its job, leave it be. The only thing you are missing is the radon piece.

We serve New Hampshire, Maine, and Massachusetts, and we have done this work for more than 5,000 New England homeowners. We are state radon certified (RMS-113966) and OSHA certified, BBB A+ accredited, and rated 4.9 across 250 Google reviews.

Read more on our radon mitigation page and our crawl space services, and see what we stand behind on the 603 guarantee. For the science and the action level, the EPA radon program and NH DHHS are the authorities to read.

Call us at 603-610-1770 or book your free inspection and we will figure out the radon piece with you.

Related questions

Did encapsulating my crawl space cause the radon, or was it always there? The radon was always coming up from your soil. Sealing the vents just kept more of it inside instead of letting it drift out, so your reading went up. The gas is not new. The escape route changed.

Can I just reopen the crawl space vents to lower the radon? You could, but you would lose the moisture control and the energy savings you paid for, and open vents are an unreliable way to manage radon anyway. A sub-membrane depressurization system lets you keep the sealed crawl and still vent the gas. That runs $900 to $6,000, and most homes land around $1,950 to $2,250.

Do I need a brand new system or can you add radon to my existing encapsulation? We add to it. The vapor barrier you already have becomes the seal our fan draws against, so the depressurization piping tucks under it without tearing anything out. The inspection and estimate are free.

What radon number means I have to act? The EPA action level is 4.0 pCi/L. At or above that, fix it. Between 2 and 4 pCi/L the EPA says to consider fixing, and given New England’s granite soil, most homeowners here do.