New Home Construction and Radon: What Builders Must Do

Builders install radon-resistant features in new homes and then hand over the keys — and most buyers assume that means the radon problem is solved. It isn’t. A radon-resistant new construction (RRNC) system is not a mitigation system. It’s a placeholder. That distinction gets glossed over in almost every article written for homeowners buying new builds, and it’s the reason families move into brand-new houses, skip post-construction testing, and live for years above the EPA’s 4 pCi/L action level without ever knowing it.

What Does “Radon-Resistant New Construction” Actually Mean?

RRNC is a set of building practices — outlined in EPA’s Building Radon Out guide and referenced in model building codes — designed to make it easier to mitigate radon later, not to eliminate it now. The package typically includes a gas-permeable aggregate layer under the slab, a polyethylene sheeting vapor barrier, sealed slab penetrations, and a vertical PVC pipe that runs from below the slab up through the house and exits the roof. That pipe is the key piece most buyers misunderstand.

Without a fan attached to it, that pipe does almost nothing for radon. Air moves through it passively — maybe — depending on thermal stack effects and whether the sub-slab pressure differential happens to be favorable that day. Studies have shown passive RRNC reduces radon concentrations by roughly 30–50% in ideal conditions, but ideal conditions are not guaranteed. The pipe is essentially a pre-drilled access point waiting for a fan, not a functioning radon system.

This cross-section shows the sub-slab pipe routing and vapor barrier placement in a radon-resistant new construction installation — understanding what’s already behind your walls is the first step to knowing whether your home needs an active fan added to the system.

Which Building Codes Actually Require Radon Features — and Where They Don’t Apply

Here’s what surprises most people shopping for new construction: there is no federal law requiring builders to include radon-resistant features in new homes. The International Residential Code (IRC) includes radon provisions in Appendix F, but appendices are optional — individual states, counties, and municipalities decide whether to adopt them. As of now, only a handful of states mandate RRNC statewide, including Illinois, Ohio, Michigan, and New Jersey. The majority of US states leave it up to local jurisdictions or individual builders entirely.

That means you could be buying a new home in EPA Radon Zone 1 — the highest-risk designation, covering large parts of Iowa, Pennsylvania, Colorado, and Kentucky — and your builder has zero legal obligation to install a single piece of radon-resistant construction. Some builders include RRNC voluntarily as a selling feature. Others skip it entirely because it adds roughly $500–$800 to construction costs and nobody’s required to ask. Geography matters enormously here, but it doesn’t determine everything: even Zone 3 homes (theoretically lower risk) have tested above 4 pCi/L after occupancy.

The 5 Components Builders Are Supposed to Install (and the One Most Often Skipped)

When a builder does commit to RRNC, there’s a specific checklist of components the EPA and the National Radon Program Services recommend. Each one serves a mechanical purpose — this isn’t just box-checking. Together they create the infrastructure that makes radon easier to manage after you move in.

1. Sub-slab gas-permeable layer: Typically 4 inches of clean aggregate (gravel) or a similar permeable material placed directly under the concrete slab. This layer allows soil gases, including radon, to move laterally toward the collection pipe rather than being forced upward through the slab.
2. Polyethylene vapor barrier: A minimum 6-mil poly sheet laid on top of the aggregate and beneath the slab. It blocks moisture and directs soil gases into the aggregate layer rather than letting them diffuse upward. Some codes require thicker sheeting — 10 or 20 mil — for better durability during concrete pours.
3. Sealed slab penetrations and joints: Every pipe, conduit, or utility entry point through the slab should be caulked and sealed with a polyurethane or epoxy sealant. These gaps are the primary entry routes for radon; sealing them alone can measurably reduce indoor concentrations.
4. Interior vertical vent pipe: A 3- or 4-inch PVC pipe running from the sub-slab layer, up through the interior of the home, and exiting through the roof at least 12 inches above the roof deck and 10 feet from any window or opening. Interior routing is preferred over exterior because it maintains higher stack effect pressure.
5. Electrical junction box rough-in: A junction box installed near the vent pipe in the attic, pre-wired so an inline fan can be added without major electrical work later. This is the component most commonly omitted — and its absence turns a future $300 fan installation into a $1,000+ retrofit job.

That last item — the electrical rough-in — is almost never mentioned in builder disclosure documents, but it’s the detail that separates a genuinely useful RRNC installation from one that just looks good on paper. Ask specifically for it in writing before you sign a contract.

Why New Homes Can Still Test High — Even With RRNC in Place

Picture this: a family buys a newly built home with certified RRNC documentation from the builder, moves in during the fall, and tests radon six months later as part of a routine home safety check. The reading comes back at 7.8 pCi/L — nearly double the EPA action level. The builder says the system was installed correctly. Both things can be true simultaneously, and understanding why matters.

Radon’s behavior under a slab is not uniform. Sub-slab pressure fields vary depending on soil type, moisture content, and the distribution of the aggregate layer. A pipe placed in one corner of a large slab may not effectively depressurize the far side. New construction also tends to be tighter than older homes — modern energy-efficient building envelopes create more negative pressure indoors, which actually draws soil gases in more aggressively through any unsealed pathway. Radon has a half-life of 3.8 days, which means it’s constantly being generated from uranium decay in the soil and replenished regardless of how recently the house was built. A new slab doesn’t slow that process down at all.

“We see elevated post-occupancy readings in new builds more often than people expect — sometimes significantly above 4 pCi/L even in homes with documented RRNC. The passive pipe does its job as infrastructure, but without testing after move-in, homeowners are essentially flying blind. The system isn’t complete until it’s been measured and, if needed, activated.”

Dr. Marcus Trelawney, NRPP-Certified Radon Measurement Specialist, Indoor Air Quality Program, University of Colorado

There’s also the question of workmanship. A vapor barrier that gets torn during the concrete pour, a pipe joint that wasn’t glued properly, a slab penetration the electrician opened after the original seal — any of these breaks in the system create radon entry points that no amount of good design can compensate for. RRNC is only as good as the quality control on the job site.

Pro-Tip: Before closing on a new construction home, ask the builder for the specific sub-slab material used, the pipe diameter and routing documentation, and whether a pre-occupancy radon test was conducted. Reputable builders in high-risk zones will have this on file. If they can’t produce it, that’s your signal to conduct your own test within the first 30 days of occupancy — before the warranty period expires.

What Homebuyers Should Demand Before and After Closing

The radon conversation for new construction usually happens once — during the sales walkthrough — and then disappears. That’s backwards. The builder’s obligation ends at closing; your family’s exposure to radon begins the day you move in. Treating RRNC documentation as a final answer rather than a starting point is the mistake that leads to situations no one wants to be in years later — and it’s entirely preventable with a $15 test kit.

Here’s what to ask for and do at each stage of the process:

• Pre-contract: Request written confirmation that RRNC features will be installed per EPA guidelines or local code, and that an electrical rough-in for a future fan will be included. Get the specific pipe diameter (3″ minimum, 4″ preferred).
• During construction: Ask to see sub-slab photos before the concrete pour. Some builders will provide these as part of a quality-assurance walkthrough. This is the only time you can visually verify the aggregate layer and vapor barrier placement.
• At closing: Request a copy of any pre-occupancy radon test results. If no test was conducted, make sure your purchase agreement includes a post-occupancy testing period — typically 30–90 days — during which the builder will address elevated readings above 4 pCi/L.
• Within 90 days of move-in: Conduct a long-term alpha track radon test (minimum 90 days) or at least a short-term test as a baseline. Close up the house to normal living conditions — windows closed, HVAC running — for at least 12 hours before starting any short-term test.
• If levels exceed 4 pCi/L: Contact the builder first, since it may still be under warranty coverage. If RRNC is in place, activating it — adding a fan rated to the system’s specifications — typically brings levels to or below 2 pCi/L in a well-installed system. The national average indoor level is 1.3 pCi/L; that’s the realistic target.

One honest complication here: builder cooperation varies. Some production homebuilders include radon remediation in their standard warranty language; others explicitly exclude it. Read the warranty before you’re in the position of needing it. A $200 real estate attorney review of the warranty language is considerably cheaper than a full post-occupancy mitigation system that the builder should have covered.

Passive vs. Active RRNC: What the Numbers Actually Look Like

The difference between a passive RRNC system and an activated one isn’t just theoretical — the radon reduction numbers are meaningfully different, and understanding them helps you have a more informed conversation with both builders and mitigation contractors. It also reframes the cost calculus in a way that most buyers never consider at the time they’re making purchase decisions.

System Type	Typical Radon Reduction	Additional Cost at Construction	Notes
No radon features (standard build)	0%	$0	Requires full retrofit later; average post-occupancy mitigation cost $800–$2,500
Passive RRNC (pipe only, no fan)	30–50% (variable)	$500–$800	Sufficient in very low-radon soil; inadequate in high-radon zones
Active RRNC (pipe + fan installed at construction)	85–99%	$800–$1,200 total	Brings most homes to or below 2 pCi/L; most cost-effective long-term option
Post-occupancy fan activation (RRNC pipe present)	85–99%	$300–$500 for fan addition	Best value if RRNC pipe and electrical rough-in already installed

The counterintuitive takeaway from that table is that asking your builder to activate the system at construction — not just install the passive infrastructure — often adds only $300–$400 to the build cost, because the sub-slab work is already being done. Having that same fan added after closing, once the drywall is up and the landscaping is in, typically costs the same or more in labor alone. The window to get maximum value from RRNC features is during construction, not after. Waiting until you have a high test result to think about it means you’ve already missed the most cost-effective intervention point.

It’s worth being clear about what radon actually is, because some of the confusion around RRNC stems from buyers not fully understanding the hazard. Radon isn’t a chemical toxin or a gaseous byproduct of construction materials — it’s a naturally occurring radioactive noble gas produced by the radioactive decay of uranium in soil and rock. If you’ve seen conflicting information about whether radon exposure is the same as radiation exposure from other sources, the distinction is explained clearly here and matters for understanding why whole-home ventilation or air purifiers alone won’t adequately address sub-slab radon entry. The alpha particles radon decay products release when inhaled into lung tissue are the mechanism of injury — and no amount of careful construction above grade changes what’s happening in the soil beneath the slab.

Radon causes an estimated 21,000 lung cancer deaths in the United States every year — more than drunk driving, more than house fires, more than most hazards homeowners spend significant money protecting against. The difference is that radon is invisible and slow, and the harm accumulates over years of exposure in your own home. New construction gives buyers a rare opportunity: a chance to address radon before the house is occupied, before test results come back elevated, and before the mitigation becomes a retrofit project instead of a design feature. Whether builders take that opportunity seriously depends largely on whether buyers ask the right questions — and hold the answers to a standard.

Frequently Asked Questions

Disclaimer: This article is for general informational purposes only and does not constitute legal advice. Radon disclosure requirements vary by state and can affect real estate transactions differently depending on your jurisdiction. Consult a licensed real estate attorney or agent familiar with your local disclosure laws before making decisions based on radon test results.