New Construction vs. Existing Homes: Radon Mitigation Differences

Michigan homeowners who are either building a new home or purchasing one built in the past several years sometimes carry an assumption that new construction is automatically safer from a radon perspective than an older home. This assumption is understandable — newer homes are built to higher standards in many respects, with better insulation, more sophisticated HVAC systems, and tighter construction envelopes than their predecessors. But when it comes to radon, the age of a home is less important than what is beneath it and what, if anything, was done during construction to address radon entry.

This post examines the specific differences between radon considerations in new construction and existing homes — how the mitigation approaches differ, what Michigan's building code does and does not require, and what buyers of both new and existing homes should understand about the radon picture they are inheriting.

What Radon-Resistant New Construction Is

Radon-resistant new construction — commonly abbreviated RRNC — is a set of building practices designed to reduce radon entry in new homes and to make future mitigation easier and less expensive if testing after occupancy reveals elevated levels. The EPA has developed specific RRNC guidelines that describe a package of features that, when incorporated into new construction, meaningfully reduce the probability of elevated indoor radon and simplify active mitigation if it becomes necessary.

The core RRNC feature package includes several components that work together. A gas-permeable layer — typically four inches of clean aggregate — is placed beneath the concrete slab to allow soil gases including radon to move freely rather than being forced through the slab itself under pressure. A plastic sheeting vapor barrier is laid over the aggregate before the concrete is poured, providing an additional barrier to soil gas migration into the home. All openings in the concrete floor — around pipes, at penetrations, at construction joints — are sealed with polyurethane caulk or similar materials. And a passive vent pipe system is installed — a vertical PVC pipe that runs from the sub-slab aggregate layer up through the home's conditioned space and exits above the roofline, allowing radon to vent passively by natural convection even before a fan is added.

This passive vent pipe is the feature that makes the RRNC package particularly valuable from a future mitigation standpoint. If post-occupancy testing reveals elevated radon levels in an RRNC home, converting the passive system to an active one requires only the addition of a fan to the existing pipe — a fraction of the cost and complexity of installing a complete mitigation system in an existing home without any pre-installed infrastructure.

What Michigan's Building Code Requires

Michigan has adopted the International Residential Code, which includes an optional radon control appendix — Appendix F — that establishes RRNC standards for new residential construction. The critical word is optional. Because the radon control appendix is structured as an optional addition rather than a mandatory provision, Michigan jurisdictions must explicitly adopt it to make RRNC a requirement rather than a voluntary practice.

Michigan requires RRNC in new construction only in Zone 1 counties — the highest-risk designation. Counties with Zone 1 designation in Michigan, including Kent and Ionia among the West Michigan counties we serve, are subject to the RRNC requirement for new construction under Michigan's current code framework. Zone 2 counties — including Ottawa, Allegan, and Muskegon — are not subject to mandatory RRNC requirements under current state code, meaning new homes built in these counties may have no passive radon-resistant features whatsoever despite carrying meaningful radon risk.

This geographic patchwork has practical implications for buyers of newly constructed homes. A new home built in a Kent County subdivision is more likely to have RRNC features than an identically priced new home built in an Ottawa County subdivision — not because the geology is dramatically different but because the code requirements differ. Buyers of new construction should ask specifically whether RRNC features were incorporated, regardless of which county the home is in, because builder compliance and awareness varies even within counties where requirements apply.

The Passive vs. Active Distinction

Even in new homes built with full RRNC compliance, the passive vent system installed during construction is not a substitute for testing and, if necessary, active mitigation. A passive radon vent relies on natural convection — warm air rising through the pipe and drawing soil gases up with it — rather than mechanical suction from a fan. In some homes the passive system alone reduces radon levels adequately. In others the passive system provides meaningful reduction but not enough to bring levels below the action level. And in others the passive system's effect on radon levels is minimal because the specific sub-slab conditions, pipe routing, or other factors limit its effectiveness.

The only way to know whether a new home's passive RRNC system is performing adequately is to test. The EPA recommends testing new homes after occupancy — after the home has been lived in for at least one heating season — rather than relying on the presence of RRNC features as confirmation of acceptable radon levels. A new home that has never been tested is an unknown radon environment regardless of what was or was not done during construction.

Converting a passive RRNC system to an active one when testing reveals elevated levels is straightforward. A fan is added to the existing pipe, electrical service is connected, and the system begins operating. The diagnostic work required in an existing home without sub-slab infrastructure — drilling through the slab, creating a suction pit, verifying sub-slab communication — is already done by virtue of the passive system's installation. The cost of activating an RRNC system is substantially lower than a full installation in an existing home.

Radon Mitigation in Existing Homes: The Full Process

For existing homes without any RRNC infrastructure — the situation for virtually all Michigan homes built before the RRNC requirements took effect and for many newer homes in non-mandatory counties — radon mitigation requires the full installation process that we covered in depth in our posts on how mitigation systems work and sub-slab depressurization.

This full process involves core drilling through the existing concrete slab, creating a suction pit in the aggregate beneath, routing new PVC pipe from that suction point to above the roofline, installing a fan, sealing the suction point, and installing the manometer indicator. In homes with good sub-slab communication and straightforward pipe routing, this is a one-day installation that produces excellent results. In homes with more complex conditions — multiple foundation types, poor sub-slab communication, finished basements with complex pipe routing requirements — the process requires more diagnostic work and installation effort.

The diagnostic assessment that precedes installation in an existing home is therefore doing work that the RRNC installation process did automatically during construction. The sub-slab communication test, the suction point location decision, the fan selection based on measured resistance — all of these are steps that an RRNC installation incorporates during construction and that a retrofit installation must accomplish through professional assessment.

Cost Differences Between New Construction and Retrofit

The cost difference between activating a passive RRNC system and installing a complete mitigation system in an existing home is meaningful. Activating a passive system — adding a fan to an existing pipe — typically costs several hundred dollars, a fraction of the $800 to $1,500 range for a standard existing-home installation that we covered in our post on mitigation costs in Michigan.

This cost difference reinforces the value of RRNC from a long-term perspective. The marginal cost of incorporating RRNC features during new construction — the aggregate, the vapor barrier, the sealants, and the passive pipe — is estimated by the EPA at approximately $350 to $500 per home when done during original construction.

This is a small fraction of the cost of a full retrofit installation if elevated radon is discovered after the home is built. For builders and for buyers evaluating new construction options, the economics of RRNC clearly favor inclusion even in counties where it is not mandated.

What Buyers of New Construction Should Ask

Buyers of newly constructed homes in Michigan should ask specifically and directly about RRNC features during the purchase process rather than assuming that a new home has been built with radon in mind. Specifically worth asking: was an EPA-compliant RRNC feature package installed, including aggregate layer, vapor barrier, floor and penetration sealing, and passive vent pipe? If a passive vent pipe is present, where does it terminate and is it accessible for fan installation? Has the home been tested for radon after at least one heating season of occupancy?

Builders who have incorporated full RRNC features will be able to answer these questions specifically. Builders who have not incorporated RRNC — even in counties where it is required — may be less forthcoming, which is itself informative.

For buyers of new construction in Zone 2 counties like Ottawa, Allegan, and Muskegon where RRNC is not mandated, the same questions are worth asking. A builder who voluntarily incorporates RRNC in a non-mandatory county is demonstrating a higher standard of practice that reflects well on the overall quality of the construction.

The Bottom Line for Michigan Homeowners

Whether you are moving into a brand-new home or a century-old farmhouse, the radon question deserves the same response: test the home, understand the result, and act accordingly. New construction with full RRNC compliance makes the mitigation step easier and less expensive if it becomes necessary. But it does not eliminate the need to test, and it does not guarantee acceptable radon levels without verification.

Existing homes without RRNC features have a straightforward mitigation path available through professional sub-slab depressurization, and the age of the home does not prevent that path from achieving excellent results. The goal — a tested, verified, safe indoor radon environment — is achievable in new and existing homes alike.

At All Michigan Mitigation, we work with homeowners across West Michigan in homes of every age and construction type — from newly built subdivisions to century-old farmhouses — and we bring the diagnostic expertise and installation quality that produce confirmed results regardless of what the construction history of the home looks like.

Visit us at allmichiganmitigation.com to schedule a test for your new or existing home or to discuss what mitigation would involve for your specific construction type.

All Michigan Mitigation is a West Michigan radon testing and mitigation company dedicated exclusively to helping homeowners understand and eliminate radon risk in their homes.