Crawl Space Encapsulation and Radon: Does It Help, and Is It Enough?

Do not count on encapsulation to lower radon. A liner over the soil blocks some soil gas, but sealing the vents also removes the outdoor air that used to dilute it, and in the best-documented field study the closed crawl spaces measured higher radon than the vented ones. The fix for radon is sub-membrane depressurization (SMD), which pairs naturally with a liner. Test before the work, retest after, and add SMD if the result is at or above the EPA action level.

What Radon Is and Why Crawl Spaces Matter

Radon is a naturally occurring radioactive gas produced by uranium decay in soil and rock. It seeps upward through the soil and enters homes through the path of least resistance - which in a crawl space home is the bare soil floor and the gaps between the crawl space and the living area.

Radon is the second leading cause of lung cancer in the US, responsible for approximately 21,000 deaths annually (EPA estimate). The risk is dose-dependent - the longer you're exposed to elevated radon, the higher the risk. The EPA's radon program sets the action level at 4 pCi/L and recommends that homeowners also consider mitigation between 2 and 4 pCi/L.

How Encapsulation Affects Radon

Encapsulation pushes radon in two directions at once:

1. The liner blocks some soil gas. Radon enters mainly as soil gas from the ground, and a sealed liner over the soil slows it. A liner is not airtight, though: seams, pier wraps and the edges at the walls still leak gas unless the space under it is actively depressurized.
2. Sealed vents remove dilution. Open foundation vents let outdoor air sweep through the crawl space and carry radon out. Once the vents are sealed, whatever gas gets past the liner stays in the crawl space, which is now connected to the house.

Field data says the second effect can win. In Advanced Energy's study of twelve identical houses in Princeville, North Carolina, crawl space radon was higher in the closed crawl spaces in both monitoring periods: an average of 2.9 pCi/L against 0.8 for the vented ones in the first, and 1.1 against 0.5 in the second. In the living spaces, the closed houses averaged 1.9 pCi/L against 0.5 in the first period, with no difference in the second. It was a low-radon site and every reading stayed below 4 pCi/L, but on a high-radon site the same shift would matter. Our before and after guide covers the rest of that study.

So treat encapsulation as a moisture fix that may leave radon unchanged or higher, and decide on radon from your own test results.

When Encapsulation Alone Is Not Enough: Sub-Membrane Depressurization

Sub-membrane depressurization (SMD) is the EPA-certified radon mitigation method for crawl space homes. A pipe is installed through the vapor barrier into the soil, connected to a continuously running fan that draws radon-laden soil gas from beneath the liner and exhausts it outdoors before it can enter the home.

Because the fan keeps the space under the liner at lower pressure than the crawl space, gas that would leak past seams and edges is pulled out instead. It can be installed as part of an encapsulation project or added later. If you know radon is a concern before encapsulating, ask your contractor about integrating an SMD pipe during liner installation - it's significantly cheaper to install during encapsulation than to retrofit afterward.

Testing Protocol

Testing is the only way to know your radon level. Radon varies significantly by location - neighboring homes can have very different levels depending on soil type, geology, and construction details.

• Before encapsulation: Place a short-term test kit (available for $15-$30 at hardware stores or through the state radon program) in the lowest livable level of your home for 48-96 hours. This establishes your baseline.
• After encapsulation: Retest at least 24 hours after the work is complete and the home has been closed up normally. A post-encapsulation test determines whether additional mitigation is needed.
• Long-term monitoring: A long-term test (90 days+) gives a more representative annual average. If your short-term test shows elevated levels, a long-term test before committing to a mitigation system is worthwhile.

High-Radon States in Our Coverage Area

EPA radon zone maps identify the highest-risk areas. Among our covered states, the highest-risk areas (Zone 1, above 4 pCi/L average) include:

• Western Maryland (Garrett, Allegany counties)
• Most of Pennsylvania (entire state is considered elevated risk)
• Most of Ohio (particularly central and SW Ohio)
• Central Kentucky (Bluegrass region)
• Parts of West Virginia (particularly the Appalachian geology areas)

The deep Southeast states (NC, SC, GA, TN, AL, MS) are generally Zone 2-3, meaning radon is present but at lower average levels. Testing is still worthwhile, but acute radon concern is less common than in the Appalachian and Ohio Valley region.

Sources

1. Radon, US Environmental Protection Agency
2. A Field Study Comparison of the Energy and Moisture Performance Characteristics of Ventilated Versus Sealed Crawl Spaces in the South (Final Report, 2005), Advanced Energy for the US Department of Energy (OSTI)