Crawlspace Radon Mitigation

What the problem is

In a slab home, radon has to find cracks. In a crawlspace home, there is often nothing between soil and floor framing but air. Soil gas rises into the crawlspace, and the stack effect draws it upward through floor penetrations, duct chases, and the rim joist into the rooms above. Crawlspaces frequently produce the highest radon entry rates of any foundation type.

How to know if this applies to you

• Your home has a crawlspace with exposed dirt, gravel, or a torn plastic sheet.
• A radon test on the lowest lived-in level came back at or above 4.0 pCi/L.
• The crawlspace smells earthy or musty, or has visible moisture.
• Floors above the crawlspace feel cold and drafty in winter.
• Part of the home is slab and part is crawlspace — a very common pattern in additions.

How the crawlspace is diagnosed

1. 1Access and clearance are checked — height determines how the work is performed and priced.
2. 2Existing vapor barrier condition, seams, and coverage are assessed.
3. 3Soil moisture and any standing water are noted; water intrusion is corrected before encapsulation.
4. 4Foundation vents, rim joist condition, and duct penetrations are surveyed.
5. 5Communication under any adjacent slab section is measured, since mixed foundations often need two approaches tied into one system.

What the mitigation process looks like

1. 1Debris is cleared and the soil surface is graded smooth.
2. 2A heavy polyethylene membrane (commonly 6–20 mil) is laid across the entire soil floor.
3. 3Seams are overlapped and sealed, and the membrane is mechanically fastened and sealed to foundation walls and piers.
4. 4A perforated suction pipe is set beneath the membrane, sometimes in a gravel bed, so vacuum spreads across the whole area.
5. 5Pipe is routed to an inline fan located outside conditioned space and exhausted above the roofline.
6. 6Rim joist gaps, duct penetrations, and the crawlspace access hatch are sealed.
7. 7A manometer is installed and the system is labeled.

Expected timeline

What affects the cost

• Square footage of soil to be covered.
• Clearance height — low crawlspaces are slow, physical work.
• Membrane thickness specified.
• Debris removal, moisture correction, or grading needed first.
• Whether an adjacent slab section must be tied into the same system.

What happens after installation

Verification testing follows the same protocol as any other system. Beyond the radon result, an encapsulated crawlspace usually runs drier, which reduces musty odors and moisture-driven wood problems, and floors above it often feel noticeably warmer in winter. Inspect the membrane after any plumbing or HVAC work under the house — trades walking through it can tear seams.