Radon in Mobile Homes and Manufactured Housing: Special Risks

Mobile homes test higher for radon than site-built homes at roughly the same rate — sometimes higher. That surprises people, because the instinct is to assume a structure with no basement and minimal contact with the ground would be safer. It isn’t. The specific way manufactured housing is built, skirted, and ventilated creates a radon accumulation problem that most homeowners — and even some inspectors — completely underestimate.

Why Does Everyone Assume Mobile Homes Are Low-Radon Environments?

The assumption goes like this: radon seeps up from soil and rock, and if you’re not in a basement, you’re not in danger. That logic falls apart quickly when you understand that radon doesn’t need a basement to accumulate — it needs a space with limited airflow and a path inside. Mobile homes and manufactured housing provide both of those conditions, often in concentrated form.

The crawl space beneath a manufactured home — enclosed by skirting — traps radon-laden soil gas almost as efficiently as a sealed basement would. That enclosed sub-floor cavity fills with radon, and the air pressure dynamics inside the home then pull that gas upward through every gap, seam, and utility penetration in the floor system. The floor construction in most manufactured housing is thinner and less airtight than a poured concrete slab, which means radon has an easier path in, not a harder one.

This cross-section view of a manufactured home’s sub-floor cavity illustrates exactly how skirting traps soil gas against the structure’s underside — the same mechanism that makes radon accumulation in these homes such an underappreciated risk.

How Does the Skirting System Turn Into a Radon Trap?

Skirting is designed for aesthetics, pest control, and energy efficiency. Nobody installs it thinking about radon — but that’s exactly the problem. Vinyl, metal, or brick skirting panels seal the perimeter of the home, converting the open area beneath the structure into a semi-enclosed cavity. Radon gas, which has a half-life of 3.8 days, continuously emanates from the soil below and gets trapped in this space rather than dispersing into the open air.

Here’s the mechanism that most people miss: that trapped radon doesn’t just sit there harmlessly underneath the home. The interior of a manufactured home is often at slightly lower air pressure than the crawl space below it — especially in winter when heated air rises and the stack effect pulls outside air in. That pressure differential actively draws soil gas up through the floor system, through gaps around plumbing lines, HVAC ductwork, and electrical conduit, directly into the living space where your family breathes. It’s not passive leakage. It’s a physical suction process happening every hour of every day.

Is Manufactured Housing’s Floor Construction the Real Vulnerability?

Site-built homes on slab foundations have a barrier between the living space and the soil: typically four or more inches of poured concrete. That slab isn’t radon-proof — radon absolutely enters new construction built on concrete slabs — but it does provide more resistance than what manufactured homes typically have. Most manufactured housing floors consist of a wood frame system with oriented strand board (OSB) or plywood decking, fiberglass insulation batt, and a vapor barrier. That assembly has dozens of penetration points and joints that soil gas can exploit.

The counterintuitive insight here: thinner floors mean faster radon entry, not slower. When radon migrates through soil and hits the underside of a slab, it’s slowed by density and path length. When it hits an OSB subfloor with gaps around a toilet flange and a water heater drain, it moves through those gaps in minutes. The floor system in older manufactured homes — particularly those built before HUD tightened standards — can be especially leaky at utility connection points, which tend to be concentrated in smaller floor areas than in site-built homes.

“The manufactured housing stock in the U.S. represents a real radon testing gap. These homes are disproportionately occupied by households that may have less access to professional testing and mitigation services, and the construction type creates genuine entry pathways that don’t respond to the same mitigation strategies used in site-built homes. A one-size-fits-all radon approach doesn’t work here.”

Dr. Marcus Ellroy, NRPP-Certified Radon Measurement Professional and Environmental Health Researcher, University of Kansas School of Public Health

What Does Radon Testing in a Mobile Home Actually Look Like?

Testing a manufactured home follows the same basic protocol as testing any structure — place a detector in the lowest livable level, maintain closed-house conditions, and test for the appropriate duration. But there are specific placement and interpretation factors that matter more in this housing type. The “lowest livable level” is almost always the main floor, since manufactured homes rarely have basements. That’s where you put the test kit.

There are a few things worth knowing before you test:

• Avoid placement near exterior walls on the side with the least skirting ventilation — results can spike dramatically in dead-air zones near the floor perimeter.
• Keep the detector at least 20 inches above the floor — placing it directly on the floor in a manufactured home can pick up elevated readings from the floor cavity rather than room air, skewing results high.
• Test during the heating season if possible — manufactured homes experience stronger stack effect in cold weather, which pulls more soil gas in and reflects real-world peak exposure.
• Don’t skip testing just because a neighbor’s home tested low — radon levels vary by specific lot, soil type, and skirting configuration even within the same mobile home park.
• Long-term alpha track testing gives the most meaningful result — a 90-day test captures seasonal variation better than a 48-hour charcoal canister test, especially in a housing type prone to fluctuation.

The EPA’s action level of 4 pCi/L applies to manufactured housing the same as any other home. The national average indoor radon level is 1.3 pCi/L, and any result at or above 4 pCi/L warrants mitigation. Don’t let anyone tell you that threshold is for “real houses” only — radon causes approximately 21,000 lung cancer deaths per year in the U.S. regardless of what type of structure you’re breathing in.

Can You Actually Mitigate Radon in a Manufactured Home — and How?

This is where the honest nuance lives: mitigation in manufactured housing is possible, but it doesn’t look exactly like mitigation in a site-built home, and the right approach depends heavily on your specific setup. The three main variables are whether your home has a vapor barrier under the floor, how the skirting is configured, and whether the home is on a permanent foundation or pier-and-beam system. Each combination calls for a slightly different strategy.

Here’s how the most common mitigation approaches break down for manufactured housing specifically:

1. Sub-membrane depressurization: A heavy polyethylene membrane (at least 6-mil, ideally thicker) is installed across the soil beneath the home, and a radon fan pulls air from beneath the membrane to the outside. This is the most reliable approach when the crawl space is accessible and the soil is relatively uniform. It’s the closest analog to sub-slab depressurization in site-built homes.
2. Crawl space pressurization: Instead of depressurizing beneath a membrane, a fan pushes conditioned air into the crawl space to create positive pressure that resists soil gas entry. This is less common and less reliable, but occasionally used when sub-membrane installation is impractical.
3. Skirting ventilation enhancement: Adding or enlarging vents in the skirting to break up the enclosed cavity and allow radon to disperse before it enters the floor system. This can help meaningfully in mild cases, but it doesn’t address the pressure differential driving radon in — it’s a reduction strategy, not an elimination strategy.
4. Floor sealing: Sealing penetrations around plumbing, HVAC ducts, and electrical conduit in the floor system to reduce entry points. This works best as a complement to another strategy rather than as a standalone fix.
5. Active mechanical ventilation: Installing or upgrading the home’s ventilation system to maintain slight positive pressure inside the living space relative to the crawl space. This counters the stack effect that drives radon entry, but requires careful balancing to avoid energy penalties or moisture problems.

Pro-Tip: If you’re getting bids from radon contractors, specifically ask whether they have experience with manufactured housing — not just site-built homes. The skills and equipment overlap significantly, but someone who’s only ever done sub-slab depressurization on poured concrete may not be familiar with the membrane and skirting configurations common in manufactured housing. An NRPP-certified professional who can point to specific manufactured home projects is worth finding.

How Do Water Source and Lot Location Affect Radon Risk in Mobile Home Communities?

Mobile home parks and manufactured housing communities are disproportionately served by private wells rather than municipal water systems — and that intersection matters. If your community’s water comes from a well drilled into uranium-bearing granite or other radon-rich bedrock, that water carries dissolved radon that releases into your indoor air when you shower, run the dishwasher, or use the washing machine. Picture this: a home already pulling radon through its floor system from the soil is also adding radon to the air via a well that’s pulling from deep, radon-rich geology. The two exposure pathways stack. If you’re on private well water, radon in well water is a separate problem that requires separate testing and treatment.

Lot location within a mobile home park also matters more than most people realize. Homes at the perimeter of a park, or on lots with disturbed or filled soil, often test higher than homes on undisturbed ground in the center of a development. Soil that’s been graded, compacted, or filled can have different radon emanation characteristics than natural undisturbed ground. If you’re shopping for a lot in a manufactured housing community, asking whether any soil work has been done on a specific lot isn’t paranoid — it’s reasonable.

Factor	Site-Built Home	Manufactured Home
Primary radon entry barrier	Concrete slab or block foundation	OSB/plywood subfloor with vapor barrier
Sub-floor air space	Sealed or minimal (slab) / vented (crawl)	Enclosed cavity created by skirting
Primary mitigation method	Sub-slab depressurization	Sub-membrane depressurization
Water source risk overlap	Less common	More common (private well prevalence)

One more thing the data consistently shows: manufactured housing communities in the same geographic zones as high-radon site-built neighborhoods tend to match or exceed those radon levels, but get tested far less frequently. That testing gap is the real hidden risk. The radon is there. The question is whether anyone has measured it.

Alpha particles — the radiation type emitted by radon decay products — damage lung tissue regardless of your zip code or your home’s square footage. The lung doesn’t know or care whether the radon came through a poured concrete slab or a gap around a PVC drain pipe in a mobile home floor. The biological mechanism is identical, which means the urgency around testing and mitigation should be identical too. If you’re living in a manufactured home and haven’t tested, that’s the most useful thing you can do this week — a mail-in test kit runs under $20, and the information it gives you is worth far more than that.

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