Practical advice: Test before you finish a basement. Mitigating first is usually cleaner and cheaper than fishing pipes through new ceilings later.
Why lower levels run higher
Soil gas enters at the foundation. Upper floors dilute that air with outdoor leakage and with air already mixed through the house. Bedrooms upstairs can still be elevated if the stack effect is strong—but the lowest lived-in level remains the standard test location.
Finished vs unfinished basements
Finishing does not create radon, but it changes how you use the space. A storage basement becomes a media room with eight-hour evenings. Exposure hours matter as much as the pCi/L number. Carpet, walls, and ceilings also complicate later pipe routing.
Crawl-space specifics
Ventilated crawls can still contribute radon to floor cavities above. Encapsulated crawls with sealed liners and dehumidifiers need intentional soil-gas control—often ASD under the liner. Do not assume encapsulation alone solved radon without a test.
Sumps and floor drains
Open sumps are classic entry points. A sealed, gasketed lid with a clear view of the pit is standard on mitigated homes. Floor drains tied to soil or gravel beds deserve attention during a visual inspection.
Next steps
Start with testing. If high, review how mitigation works.
Need a local next step? Call (605) 600-6282 to talk through testing, real estate timing, or mitigation for your Rapid City–area home.