Most buyers walking through a Naperville home treat the radon test like a coin flip they're hoping to win. It isn't much of a flip. Given what sits under this part of DuPage County, the coin is weighted, and it's weighted toward elevated.
Local licensed mitigation contractors report that more than half of the homes tested in Naperville come back at or above the EPA's action level of 4.0 pCi/L, and DuPage County's average indoor reading already sits close to that same line before anyone tests a specific address. An elevated number here isn't a red flag pointing at one unlucky house. It's closer to the local starting point.
That changes the question worth asking before you go under contract. It isn't "what if my test comes back high." It's "do we have enough days left in this contract to handle it if it does."
What the Ground Under Naperville Actually Does
Radon rises out of soil and collects in basements and lower levels. Naperville's housing stock, from decades-old ranches to new construction with deep basements on former farmland, gives it plenty of ways in. The geology driving this is glacial till deposits and dolomite limestone bedrock common throughout DuPage County, which create natural pathways for the gas to migrate upward through the ground and into a foundation.
A quick read on where a typical Naperville home lands against the benchmark:
| Reference point | Reading |
|---|---|
| EPA action level | 4.0 pCi/L |
| DuPage County average indoor reading | approximately 4.3 pCi/L |
| Share of Naperville homes tested at or above the action level | over 50% |
The interpretation matters more than the numbers themselves. If you're touring listings this fall, the safer assumption walking in is that a test will come back at or above 4.0 pCi/L, not below it. And a fresh build doesn't exempt you. New construction sits on the same bedrock as a 1970s split-level three doors down. The geology doesn't check the permit date.
What Illinois Law Actually Requires
Two statutes govern this in Illinois: the Radon Awareness Act and the Residential Real Property Disclosure Act. Together they require a seller to hand the buyer an IEMA pamphlet on radon testing guidelines and a formal disclosure form before the buyer becomes bound to the purchase contract. If the seller already has test results, those get disclosed too.
Here's what those laws don't do. Neither one requires the seller to test the home, and neither requires mitigation. The obligation is honesty about what you already know, not a guaranteed outcome. A lot of out-of-town buyers arrive assuming Illinois mandates a clean radon result before closing. It doesn't. It mandates disclosure, and a seller can't sidestep that by agreeing to sell "as is." The statute itself makes that obligation stick regardless of what the contract says about condition.
If you're one of the relocation or rental clients weighing a lease instead of a purchase, the same disclosure logic has followed you into that market too. Illinois expanded radon disclosure requirements to cover most residential leases starting in 2024, so tenants now see similar paperwork before signing.
The Part That Actually Catches People Off Guard
Here's where the real risk lives, and it isn't the number.
Chicagoland contracts typically run an attorney review and inspection period of roughly five business days. Radon testing has to happen inside that window, and by regulation it has to run a minimum of 48 hours, performed by an IEMA-licensed measurement professional. A hardware store kit doesn't count for a transaction.
Walk the math forward. Two days for the test to run. A day to get results back. If the number comes in elevated, a licensed contractor can often quote the fix from the report within a day or two. If the seller agrees to install a system before closing, that installation is usually a single visit of four to eight hours, but confirming it worked requires a follow-up test another 24 to 48 hours after the system goes in.
Stack all of that against a five business day window and there's very little room left. Sellers who wait for the buyer's test to learn their own number are negotiating against a clock that started before they had any information. Buyers who assume a verified, retested system will be done before their own deadline sometimes find out the schedule simply doesn't fit, which is usually the moment a credit or price adjustment becomes the practical answer instead of the fully verified install everyone would prefer.
An elevated reading here says more about the ground than about the house.
How Naperville Deals Actually Resolve an Elevated Number
When a test comes back high, buyers and sellers here tend to land on one of three paths:
- Seller installs, buyer gets a verified retest before closing. This gives the buyer certainty and the seller control over which contractor does the work, but it only works if the calendar leaves room for the retest to finish in time.
- Closing credit. The seller credits an agreed dollar amount toward the buyer's closing costs, and no work happens before keys change hands. This is the faster option when the review window is already tight, and it lets the buyer pick their own contractor after closing.
- Price adjustment. Less common locally, but functionally similar to a credit, folded into the sale price instead of handled at the closing table.
None of these require a lawsuit or a dead deal. They're negotiated the same way a cracked window or an aging water heater gets negotiated, as a line item against the inspection report.
On cost, a standard sub-slab depressurization system, the fix used for most homes in this area, typically runs $1,100 to $2,000 in the Naperville market as of early 2026, a bit below the broader Chicago metro average of roughly $1,300 to $1,500. When a radon test is bundled into a standard home inspection locally, pricing commonly starts around $220. Naperville-based mitigation firms like Guardian Radon Mitigation, operating in the area since 2008, and Clean Air Radon Systems, a DuPage County crew working since 2017, both handle this work regularly enough that quoting from an inspection report the same day it lands isn't unusual.
Homes with finished basements or crawl spaces without a clear capillary layer can complicate diagnosis and placement, so it's worth asking any contractor to walk through where the vent pipe will run before work starts, particularly if a sump pump already occupies that corner of the basement.
What This Means If You're Listing or Buying This Fall
If you're selling, order your own radon test before you list rather than waiting to react to the buyer's. Knowing your number in advance means you decide whether to mitigate ahead of time, price it in, or handle it case by case, instead of making that call on day three of someone else's five day clock.
If you're buying, request the test the day attorney review opens, not partway through it. Ask directly which licensed contractor the seller would use if a system is needed, and do the math on whether a verified retest can realistically finish before your deadline. If it can't, a credit keeps the transaction moving without leaving you to carry the scheduling risk. Confirm any tester or installer holds current IEMA licensing, since an unlicensed test can be rejected outright and cost you days you don't have.
A Short FAQ
Does an elevated radon test mean the sale falls apart? Rarely on its own. It typically becomes a negotiated item resolved through a credit, a price adjustment, or a seller-installed system, the same way most other inspection findings get handled.
Can a seller just refuse to test? Yes. Illinois law requires disclosure of known results, not testing itself. In practice, most buyers request their own test during the inspection period regardless of what the seller has or hasn't done.
Does a home with a mitigation system already installed sell for less? A documented, working system generally reads as a maintained detail rather than a problem. The bigger risk to value is an elevated number that nobody addressed and everyone now has to disclose.
What about new construction? New construction sits on the same DuPage County ground as resale homes, so the underlying geology hasn't changed. Some builders include a passive system during framing, some don't. Ask directly, and plan to test once the home is fully enclosed.
If you're weighing a purchase or a listing in Naperville this fall and want help planning the radon step so it doesn't collide with your closing date, Contact Carrie — your trusted real estate partner.
