Post-Mitigation Testing
EPA-protocol verification that your mitigation system is actually working.
A continuous monitor placed on the lowest livable floor under closed-house conditions, 24–48 hours after fan activation. Included with every install we do. Available as a standalone audit if a different installer skipped verification.
Why post-mitigation testing exists
An installed mitigation system is only as good as its post-mitigation test. Without a verification reading, you have no evidence the fan is actually doing what it’s supposed to be doing. The EPA’s mitigation standards (ANSI/AARST SGM-SF) require a post-mitigation test on every active SSD install — it’s not optional, and any installer who skipped it on your system was cutting a corner.
We see three categories of customers for post-mitigation testing: homeowners we just did a mitigation install for (test included), homeowners whose previous installer didn’t test or whose test record was lost, and homeowners who suspect their existing system has degraded. All three get the same protocol — we just bill differently.
How the test runs
Wait period (24–48 hours after fan activation)
If we just activated a new system, we wait 24 hours minimum before deploying the test monitor. For older systems being audited, no wait period is needed.
Closed-house setup
Homeowner closes the house for 12 hours before test start — windows and doors shut except for normal entry. HVAC operates as normal. No whole-house fans, no continuous bath-fan use, no fireplace.
Monitor deployment
We place a calibrated continuous radon monitor on the lowest livable floor — in the basement if the basement is finished/occupied, or on the first floor if the basement is utility-only. The CRM sits 20″ off the ground and 12″ from an exterior wall.
48-hour test run
CRM logs hourly readings, temperature, humidity, and tamper alerts. No homeowner action required.
Pickup & report
We pick up the device, download data, and email a report the same day. Report includes the average reading, the hourly chart, any tamper alerts, and a pass/fail judgment against the 4 pCi/L action level.
What the report tells you
A clean post-mitigation report shows three things: (1) average radon level for the test period, (2) hourly readings that stay flat or fluctuate within a tight band, and (3) no tamper alerts. A reading under 2 pCi/L with flat hourly data is a textbook successful mitigation. Readings that bounce around (say, 1 pCi/L most of the day but spiking to 8 pCi/L during a specific 4-hour window) usually indicate a building-envelope issue — HVAC kicking on hard, a door left open, a basement bath fan running — rather than a mitigation system failure.
Standalone audits on systems we didn’t install
If a previous owner installed a mitigation system, or if your installer didn’t leave you with post-mitigation paperwork, a standalone audit is the right call. The audit is quoted as a flat fee. If the test passes, you get a clean report you can keep with your home records. If it fails, we’ll quote the remediation as a fixed-price job — tuning a borderline system is a smaller scope than a fan replacement or additional suction point, and you’ll see the breakdown before any work begins.
Common findings on standalone audits in Cedar Rapids:
- Fan degraded with age. Radon fans last 10–15 years. A fan running for 12 years on a system that originally tested at 1.2 pCi/L might now be reading 3.8 pCi/L — still under the action level but losing margin. Fan swap is a routine flat-fee replacement.
- New slab cracks. Foundation settling over a decade can open up cracks that didn’t exist at install. Sealing scope depends on how many cracks have opened and where; quoted before work starts.
- Sump pit lid degraded. A sump pit without a sealed lid is a major radon entry point. New lid plus polyurethane seal is a quick fix.
- Manometer no longer accurate. Sometimes the manometer reads “system on” but actual sub-slab vacuum is below spec. Diagnostic test plus tuning is straightforward and quoted up front.
For real-estate transactions
If you’re selling a home with an existing mitigation system and the buyer’s inspection didn’t include a radon test, getting a standalone verification done before listing is a smart move. A current pass report eliminates radon as a negotiation lever. Most agents we work with bundle a fresh post-mitigation test into their pre-listing checklist.
Frequently asked questions
Why does the EPA require waiting 24 hours after fan activation before testing?
When a mitigation fan first turns on, it takes 12–24 hours to establish stable equilibrium in the soil gas under the slab. During that startup period, sub-slab pressure is fluctuating as the soil column de-pressurizes and adjacent air finds new equilibrium. Test results during this window are unreliable — they can read artificially low because radon is being temporarily flushed out. EPA SGM-SF protocol specifies a minimum 24-hour stabilization period, with most certified mitigators waiting closer to 48 hours for a more confident reading. The test itself then runs an additional 48 hours, so total turnaround from system activation to verified result is 72–96 hours.
I had a different company install my system years ago — should I retest?
Yes. The EPA recommends retesting every two years on all mitigation systems, and we see a significant share of older installs that have drifted out of compliance. Three common drift causes: the fan has degraded (loss of static pressure over time), new cracks in the slab have opened up additional radon entry paths, or the homeowner’s HVAC setup has changed in a way that altered the building’s pressure envelope. A standalone post-mitigation test will tell you in 48 hours whether your existing system is still doing its job. If it’s not, we can usually rehabilitate the existing system without a full replacement.
What does it actually mean if my post-mitigation test comes back at 1.5 pCi/L?
It means your mitigation system is working properly. The EPA action level is 4 pCi/L. A well-designed SSD system typically delivers results in the 0.5–2.0 pCi/L range — well below the action level and comparable to typical outdoor air. There’s no functional difference between 0.7 and 1.5 — both are excellent. If a homeowner tells me they’re disappointed with a 1.5 result because they were “hoping for under 1.0,” we usually look at whether anything in the test environment (door left open, HVAC anomaly) might have nudged the reading up, but functionally a 1.5 result is a fully successful mitigation.
Can I do the post-mitigation test myself with a DIY kit?
Technically yes, and a $25 charcoal kit will give you a screening number. But there are two reasons most homeowners pay for a professional CRM test as part of their install: (1) the result becomes part of your home’s permanent radon record, which matters at resale — lenders and inspectors give more weight to a third-party-conducted CRM result than to a homeowner-conducted canister; and (2) if the result is borderline (3.5–4.5 pCi/L), you want hourly data to understand whether the system is right at the edge or whether a specific event during the test pushed the number up. Charcoal kits give you one number for the whole period; CRMs give you 48 hourly readings.
What happens if the post-mitigation test fails — comes back above 4 pCi/L?
On our installs: we return at no charge and tune the system until we hit sub-4. Most common remedies in order of frequency are (1) additional sealing on slab cracks or sump pit, (2) re-balancing a system that’s pulling too hard on one zone and not enough on another, (3) upsizing the fan, or (4) adding a second suction point. Fewer than 5% of our installs need a return visit. On installs we didn’t do, we’ll quote the remediation as a fixed-price job, more if a fan replacement or second suction point is needed.
Call Now
(319) 774-8138
Cedar Rapids radon team. We answer Mon–Sat during business hours. Voicemails returned within 24 hours.
Call (319) 774-8138Have ready when you call:
- Your address (we’ll pull the assessor record)
- Year built and basement type, if you know them
- Any recent radon test result
- Closing date, if it’s a real-estate transaction
Or email info@cedarrapidsradonpros.com.