Where radon enters a home

Updated September 24, 2026 · Rapid City, South Dakota

Highest-frequency entry points

• Cracks in concrete slabs and floor–wall cold joints
• Open or poorly sealed sump pits
• Floor drains tied into gravel beds
• Pipe penetrations (water, sewer, gas, electrical)
• Hollow block walls and unsealed top courses
• Crawl-space soil and torn vapor barriers
• Crawl-to-basement doorways and chases

What sealing can and cannot do

Sealing visible gaps is good craftsmanship and helps mitigation systems. Sealing alone seldom replaces active depressurization when indoor levels are well above 4 pCi/L—soil gas finds microscopic paths.

Foundation Pressure Dynamics in the Black Hills

Air pressure differentials are the primary engine driving soil gas indoors. Rapid City homes experience significant seasonal pressure swings. During cold northern Great Plains winters, heated indoor air rises and escapes through attic bypasses and upper windows (the stack effect). This creates sustained negative pressure in basements and crawl spaces, essentially pulling soil gas out of permeable bedrock and aggregate fills like a syringe.

Furthermore, local mechanical systems—such as bathroom exhaust fans, clothes dryers, and atmospheric combustion furnaces—compound this depressurization. Sealing perimeter joints and foundation penetrations reduces parasitic air loss, but creating negative sub-slab pressure via active mitigation remains the definitive solution.

Diagnosis vs guesswork

Visual inspections guide system design. Air testing confirms whether the house needs work. Start with testing fundamentals.

Remediation Strategies: Sealing vs. Active Sub-Slab Suction

While homeowners often begin by purchasing polyurethane caulking to seal visible slab fissures and perimeter expansion joints, chemical sealing alone cannot withstand persistent sub-slab pressure differentials. Soil gas molecules (radon atoms) easily penetrate porous concrete aggregate, hollow masonry cinder blocks, and hairline fractures invisible to the human eye.

Sealing serves as an essential supporting component of a radon reduction system by preventing conditioned indoor room air from being drawn into the sub-slab extraction pipe. However, achieving sustained, long-term radon reduction below the EPA 4.0 pCi/L threshold almost universally requires installing active soil depressurization (ASD), where an inline mechanical fan continuously maintains a negative pressure envelope beneath the foundation.

Sources

1. U.S. EPA. Consumer’s Guide to Radon Reduction: How to Fix Your Home (EPA 402/K-10/005). epa.gov/radon/consumers-guide-radon-reduction
2. ANSI/AARST. Soil Gas Mitigation Standards for Existing Homes (SGM-SF-2023). standards.aarst.org
3. Building Science Corporation. Sub-Slab Depressurization & Soil Gas Control Guide. buildingscience.com
4. South Dakota Department of Health. Radon Foundation & Entry Point Guidance. doh.sd.gov/programs/radon