Why the Post-Mitigation Test Matters
A radon system is not complete just because the fan is running.
It is only finished when a post mitigation radon test proves the home is safe. From what we have seen, this clearance test is the crucial proof that buyers, sellers, and lenders demand before closing a sale.
You need this data to confirm your indoor air quality meets the Environmental Protection Agency (EPA) action level of 4.0 pCi/L. Our post-mitigation testing service provides this required documentation quickly.
The peace of mind is entirely worth the small extra step.
When to Test
You should start a radon clearance test at least 24 hours after system installation, but no later than 30 days. This timing ensures the vacuum under your foundation has stabilized completely.
Give the system time to settle
The system needs at least 24 hours to draw out existing soil gas before testing begins. When the newly installed fan turns on, it immediately starts pulling radioactive particles from under the slab.
Levels inside the house drop rapidly within the first few hours. We follow the latest 2026 National Radon Proficiency Program (NRPP) guidelines, which require this full day of operation to reach a stable baseline. Starting a continuous radon monitor (CRM) too early usually results in an artificially high reading.
Follow closed-house conditions
Keep all windows and exterior doors shut for exactly 12 hours before and during the entire testing period. You can still use doors for normal, quick entry and exit routines.
Our technicians constantly remind clients to avoid running whole-house fans or wood fireplaces during this window, as they alter indoor air pressure. A major pro-tip is to reschedule if the US National Weather Service predicts a severe storm or high winds.
Extreme barometric pressure drops can pull excess radon indoors and easily ruin an otherwise passing test.
Test the same level
Place the testing device in the lowest livable area of your home, ideally right where the initial baseline test sat. An apples-to-apples comparison gives you the most accurate picture of your total reduction.
We always position our RadStar or SunRADON continuous monitors at least 20 inches off the floor to meet federal measurement protocols. Avoid placing devices in humid spaces like bathrooms, laundry rooms, or dirt crawlspaces.
| Timing step | What happens |
|---|---|
| Install day | System is installed, sealed, and labeled |
| At least 24 hours later | Foundation vacuum stabilizes; test can safely begin |
| 48 hours minimum | Professional CRM device records hourly variations |
| Within 30 days of install | Clearance testing process fully completed |
| Every two years after | Routine maintenance retest |
Independent vs. Installer Testing
You can hire the original installer to perform the clearance test, or you can hire an independent, third-party measurement professional. Both options are completely valid as long as the technician follows state and national protocols.
Our clients often ask which route is better for a fast-moving real estate transaction. The choice usually depends on your buyer’s specific lender requirements, with standard professional tests typically costing between $150 and $250 nationwide.
- Installer testing. The company that installed the system uses a credentialed technician to run the test. This route is typically faster and cheaper since they already know the property layout.
- Independent testing. A completely separate credentialed tester handles the measurement. Many US real estate agents prefer this third-party verification to avoid any perceived conflict of interest.
Every tester must hold active credentials through programs like the Iowa HHS radon program or the NRPP. Check your purchase agreement carefully, and ask your realtor which specific approach the contract demands.
Reading Your Result
Your retest after radon mitigation will show a clear number indicating success, ideally well below 2.0 pCi/L. A passing result means the system is effectively venting the dangerous soil gas outside.
Our technicians look closely at how the new numbers compare to your home’s original baseline. You want to see a dramatic drop, but understanding the specific thresholds helps you make informed health decisions. The World Health Organization (WHO) actually recommends a tighter limit of 2.7 pCi/L for long-term safety.
- Below 1.0 pCi/L: Excellent performance. This is the ideal range we routinely see on homes that originally started way above 4.0.
- 1.0 to 1.9 pCi/L: A strong, safe result. You are sitting comfortably under both the US EPA and WHO safety thresholds.
- 2.0 to 3.9 pCi/L: Below the mandatory action level, but still a bit elevated. Depending on your starting point, the technician might upgrade the fan size to drive this number lower.
- 4.0 pCi/L or above: The system failed the clearance test and needs immediate adjustment.
Compare your final clearance number directly to your starting level to calculate the reduction percentage. A property that dropped from 12.0 down to 0.8 pCi/L achieved a massive 93% reduction in cancer-causing particles. We have successfully achieved up to 98.9% reduction using high-suction fans on dense soil. Check out how well systems work for more detailed performance expectations.
Keep your report
Save your post-mitigation test report with your system paperwork in a safe folder. This official document is incredibly useful for future home sales, 5-year fan warranty questions, and comparing future retests.
If the Test Fails
A failed radon test after system install usually means the fan suction is not reaching every part of your foundation. The solution requires a technician to diagnose the air flow issue and adjust the setup.
A high post-mitigation reading is uncommon with a well-designed layout, but it occasionally happens in older properties. Our field team usually traces these failures back to a few specific structural challenges. You will likely need additional suction points or better sealing.
Common reasons for a failed test include:
- A dense slab section blocking suction, especially in additions or split-level foundations.
- An unsealed sump pump pit allowing indoor air to break the vacuum.
- An exposed dirt crawlspace area that installers did not encapsulate.
- A fan unit that is severely undersized for the specific clay or gravel soil type.
- A significant foundation crack that missed the initial urethane caulk sealing process.
The standard fix is to diagnose the pressure field, adjust the hardware, and then run another 48-hour test. Every custom installation we perform carries a rock-solid 5-year warranty on the fan unit. If your post-mitigation results miss the safety target, we will add suction points or upgrade the PVC pipe size at no extra cost.
After the Clearance Test
Once your system passes the clearance test, you just need to check the manometer gauge monthly and schedule a routine retest every two years. The system runs automatically in the background and requires very little daily maintenance.
Our installation crews leave a clearly marked U-tube gauge on the pipe to make monitoring incredibly easy for you. Regular checks ensure the fan keeps creating the necessary vacuum under your house. You must stay proactive to keep your indoor air safe year after year.
- Monthly: Glance at the U-tube manometer gauge on the PVC pipe. Uneven liquid indicates the fan is pulling suction correctly. Perfectly level liquid means the fan lost power or died, so call us for a warranty replacement immediately.
- Every two years: Run a standard 48-hour continuous monitor test. Check out our guide on how often to test for radon for specific seasonal advice.
- After big changes: Always retest after finishing a basement, installing a new HVAC system, or adding interior French drains.
Perhaps another company installed your fan years ago, but you never actually received a final clearance report. It is never too late to verify your family’s safety. Schedule a modern digital test this week to find out exactly where your exposure levels sit right now.