Does Radon Come Back After Mitigation?

Short answer: no — radon does not "come back" if your mitigation system is working correctly. A sub-slab depressurization system continuously vents radon from beneath your foundation. As long as the fan is running and the piping is intact, radon stays low. But a few specific situations can cause levels to rise after mitigation.

Why Radon Stays Low With a Working System

Sub-slab depressurization creates negative pressure beneath your foundation. The fan draws radon-laden soil gas up through a pipe and exhausts it above the roofline before it enters living space. This is continuous — not a one-time treatment that fades over time.

Reasons Radon Can Rise After Mitigation

Fan failure is the most common cause. Fans last 10–20 years but can fail earlier. If the fan stops, radon returns to pre-mitigation levels within days. Check the manometer (U-tube gauge on the pipe) monthly — if both sides of the fluid are level, the fan is not creating suction. Fan replacement costs $150–$300 including labor.

Pressure dynamic changes from HVAC upgrades, basement finishing, or major renovations can shift how air moves through your home and undercut a previously adequate system. Retest after any significant renovation.

Sealant degradation around floor drains, utility penetrations, and control joints can open new radon entry points over years of thermal expansion. Less common than fan failure, but worth checking during annual system inspections.

Undersized original system — if post-mitigation results were borderline (2–3.9 pCi/L) rather than clearly below 2 pCi/L, any small change can push you back above 4 pCi/L.

How to Know Your System Is Still Working

Monthly visual check (2 minutes): Confirm the manometer shows suction, the fan is audible, and the exterior vent exhausts airflow.

Retest every 2 years: The EPA recommends this even with an active system. A continuous monitor (Airthings Wave Plus) provides early warning between tests.

A properly functioning system should achieve below 2 pCi/L. If you retest and find levels above 4 pCi/L with the fan running, contact the installing contractor — reputable mitigators guarantee their work.

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