Radon in manufactured and mobile homes: testing, skirting, crawl spaces and fixes

It's often assumed that manufactured and mobile homes don't have radon problems because they sit above the ground. Elevated floors and ventilated underfloor spaces do tend to lower radon compared with basements, but they don't guarantee low levels. Skirting, enclosed crawl spaces, ductwork under the floor, and the home's own pressure can all draw soil gas inside. The only way to know is to test.

How radon gets into a home that isn't on a slab

EPA explains that a house's indoor air pressure is usually lower than the pressure in the soil around and beneath it, so soil gas is drawn in through any openings. In a manufactured home, those openings include:

• holes for plumbing, drains, and wiring in the floor;
• gaps around the HVAC duct system, which often runs under the floor in a belly board;
• seams in the underbelly or bottom board;
• an enclosed crawl space created by skirting, which can trap soil gas beneath the home.

The more the space under the home is enclosed and unventilated, the more soil gas can accumulate there.

Skirting and ventilation

Skirting protects pipes from freezing and keeps animals out, but it also changes airflow under the home. Many skirting installations include vents. If vents are blocked, closed for winter, or never installed, radon can build up beneath the floor. Ventilation under the home dilutes soil gas and reduces the home's suction on the soil. EPA's guide notes crawl space ventilation can lower indoor radon, but its effect varies, with EPA's table listing natural crawl space ventilation at 0 to 50 percent reduction. In cold climates, more ventilation can mean frozen pipes, so pipes and ducts may need insulation.

Testing a manufactured home

• Use a short-term test kit and follow closed-house conditions: windows and exterior doors closed, except for normal entry and exit, for 12 hours before and during the test.
• Place the device in a main living area or bedroom, about 20 inches or more above the floor, away from exterior walls, vents, and heat sources.
• Avoid kitchens, bathrooms, and laundry areas.
• If the result is 4 pCi/L or higher, confirm it with a second test.

For context, EPA reports the average indoor radon level in U.S. homes is about 1.3 pCi/L, and the average outdoor level about 0.4 pCi/L.

Mitigation options

Because manufactured homes usually sit over a crawl-space-like area, the methods are similar to those EPA describes for crawl space homes:

Submembrane suction. EPA calls this the most effective way to reduce radon in crawl space homes when properly applied. A high-density plastic sheet covers the soil under the home, and a vent pipe and fan draw radon from beneath the sheet and vent it outdoors. EPA's table lists it at 50 to 99 percent reduction. In a manufactured home, the pipe and fan are typically mounted on the exterior, with the discharge routed above the roofline.

Improving ventilation under the home. Unblocking or adding skirting vents, or installing a fan to move air through the underfloor space, can lower radon. It's simpler and cheaper, but less reliable and less effective for high levels.

Sealing the floor. Sealing plumbing and duct penetrations and repairing the bottom board reduces entry paths. EPA notes sealing supports other methods but isn't recommended as a fix on its own.

Duct repairs. Leaky under-floor ducts can pull in air from beneath the home. Sealing them can reduce radon and improve heating and cooling efficiency.

Practical considerations

• Clearances are tight. Working under a manufactured home is cramped, which can affect cost and how the membrane is installed.
• Tie-downs and piers must not be disturbed. A qualified mitigator will work around them.
• Park rules. If you're in a manufactured home community, check rules on exterior changes before installing a pipe and fan.
• Licensing. Ask your state radon office whether mitigators must be licensed in your state.

Why it's worth testing

EPA's risk tables estimate that at 4 pCi/L, about 7 of 1,000 people who never smoked, and about 62 of 1,000 smokers, could get lung cancer from a lifetime of exposure. Those numbers don't depend on the type of house, only on the radon level. A test kit costs little and takes a few days.

If you rent the lot or the home

If you own the home but rent the lot, you'll usually need the community's permission for exterior work. If you rent the home, share your test results with the owner in writing. See radon in rentals and apartments.

Frequently asked questions

My home is on wheels and piers. Can radon really get in? Yes. Radon can collect in the space under the home, especially behind skirting, and be drawn in through floor penetrations and duct leaks. Testing is the only way to know.

Will adding more skirting vents fix it? It may help at modest levels. EPA lists natural crawl space ventilation at 0 to 50 percent reduction. For higher levels, submembrane suction is more reliable.

Does the HVAC system matter? Yes. Under-floor ducts that leak can pull air from beneath the home into the living space. Having ducts inspected and sealed is good for radon and energy bills.

Should I test in winter? Winter often shows higher levels because of the stack effect and closed windows, which makes it a good time for a conservative test.

Quick checklist

• Test, even if the home is raised.
• Check that skirting vents are open and working.
• Seal floor penetrations and repair the bottom board.
• If confirmed at 4 pCi/L or higher, get a submembrane suction estimate.
• Retest after any fix, and periodically after that.

Key takeaways

• Raised floors don't guarantee low radon. Test every home.
• Skirting can trap soil gas, so keep vents open and working.
• Sealing floor penetrations and duct leaks helps, but isn't a fix on its own.
• Submembrane suction is the most effective option for crawl-space-style foundations.
• Check community rules before installing exterior pipes or fans.