Radon Mitigation in New Construction: RRNC Explained

The EPA estimates that roughly half of homes built with passive Radon Resistant New Construction (RRNC) features still test above 4 pCi/L after move-in. That single statistic captures the whole topic: RRNC is a powerful head start, not a finished system. Knowing what it actually does, where code requires it, and when to upgrade it to active matters whether you’re a builder confirming requirements, a buyer evaluating a new home, or a homeowner trying to understand why your “radon-resistant” house just tested high.

What Radon-Resistant New Construction Actually Means

RRNC is a set of construction techniques that build a passive radon control system into a home from day one. The system has five core features:

1. A gas-permeable layer of clean aggregate beneath the slab
2. A polyethylene vapor barrier above the aggregate
3. Sealed entry points at all slab penetrations, control joints, and sumps
4. A vent pipe running from beneath the slab up through the home and out through the roof
5. An electrical junction box near the vent stack for adding a fan later

That’s it. RRNC creates a passive system: it gives radon gas an easy path out of the soil and up through the vent pipe, but it relies on natural pressure differences (the stack effect) to do the work. There’s no fan and no active draw.

This matters because passive systems alone don’t always solve the problem. The EPA’s own data suggests roughly 50% of homes built to passive RRNC standards still test above the 4 pCi/L action level after construction. The other half work as designed. Which side of that line your house ends up on depends on soil conditions, weather, how tight the home’s envelope is, and how meticulously the RRNC features were installed.

The fix when a passive system isn’t enough is straightforward: add an inline radon fan to the existing vent stack. Because the junction box is already in place and the pipe is already routed, the upgrade typically costs $300 to $700 and takes a few hours. Our guide on active vs passive radon mitigation covers the upgrade decision in more detail.

Where RRNC Is Required by Code

RRNC requirements are a patchwork. The federal government doesn’t mandate radon control in new construction; it’s a state and municipal matter, layered on top of the building codes communities choose to adopt.

The clearest signal is the EPA Map of Radon Zones. The EPA divides U.S. counties into three zones based on predicted average indoor radon levels:

• Zone 1 (predicted average > 4 pCi/L): the highest-risk band, covering large parts of Colorado, Iowa, North Dakota, Pennsylvania, Ohio, Kentucky, Tennessee, Minnesota, and others
• Zone 2 (2 to 4 pCi/L): moderate risk
• Zone 3 (< 2 pCi/L): lowest risk

Many states and counties in Zone 1 have adopted RRNC code provisions. The most common mechanism is IRC Appendix F, an optional appendix to the International Residential Code that codifies the RRNC techniques. When a state or municipality adopts Appendix F, RRNC becomes mandatory for new homes in covered jurisdictions.

States with state-level RRNC mandates as of 2026 include Illinois, Minnesota, Washington, New Jersey, and others. Several large municipalities (Boulder County CO, Montgomery County PA, parts of Long Island) have local mandates that exceed state requirements. The AARST CCAH (Codes, Construction, And Home) tracker maintains a current map of state and local adoption.

The practical takeaway for buyers: check both your state and your county. RRNC may be required, optional, or completely absent, and the answer matters when you’re deciding whether to trust that the builder is building radon-resistant or whether you need to specify it explicitly in the build contract.

The Five Features of an RRNC System, Explained

The five core RRNC features work together as a system. Skip any one of them and the whole approach degrades.

1. Gas-permeable layer beneath the slab

A 4-inch layer of clean, washed aggregate (typically #57 stone) is placed beneath the slab. The aggregate creates a low-resistance path for soil gas to flow horizontally toward the vent pipe inlet. Without this layer, suction (whether passive or later added by a fan) only pulls from the small area directly beneath the inlet.

The aggregate must be clean. Fines and dust clog the void space and kill the permeability the layer is supposed to provide. Builders cutting corners on stone quality is the single most common RRNC failure mode.

2. Polyethylene vapor barrier above the aggregate

A 6-mil minimum polyethylene sheet is laid over the aggregate before the slab is poured. The barrier serves two purposes: it limits radon migration from soil into the slab itself, and it prevents the slab from sucking moisture (and dissolved radon) up from the ground.

Seams must be lapped and sealed. Penetrations through the barrier (for plumbing, footings) must be sealed. A vapor barrier with torn corners and unsealed seams is decorative, not functional.

3. Sealed entry points

After the slab is poured, every penetration — plumbing, electrical, control joints, sump pit covers, expansion joints — must be sealed with a flexible polyurethane or radon-rated sealant. Slab cracks that develop during cure also need to be sealed.

This is the feature most often skipped or done poorly. Builders sometimes treat sealing as the painter’s job, or assume the slab is “tight enough.” Neither is true. Concrete is porous; uncaulked penetrations are radon entry points.

4. Vent pipe

A 3- or 4-inch Schedule 40 PVC pipe runs from a T-fitting embedded in the aggregate layer, up through the slab, through interior walls or a chase, and out through the roofline. Termination must be at least 12 inches above the roof and at least 10 feet horizontally from any operable window.

The vent pipe must be interior, not exterior. Exterior pipes get cold in winter, condense moisture, and lose draft. They also tend to look ugly enough that homeowners ask to have them moved later — which kills the system.

5. Electrical junction box

A wired junction box is installed near the vent stack (typically in the attic or upper level) so a fan can be added later without rewiring. The box is dead-end electrical: capped wires, no fan, no draw. It exists solely to make the future upgrade a one-hour job rather than a half-day rewiring project.

This is the cheapest feature of the five (typically $50 to $150 added to the build cost) and the single feature that decides whether the passive-to-active upgrade is easy or expensive. Builders who omit it are saving pennies and costing future homeowners hundreds.

Passive vs Active in a New Build

Most RRNC systems are installed passive. The reason is cost: the builder saves $200 to $400 on the fan, wiring run, and labor. For Zone 2 and Zone 3 properties this is often a defensible choice. For Zone 1 it usually isn’t.

The right rule of thumb for new construction:

• Zone 3: passive RRNC is reasonable; test post-occupancy and upgrade only if needed
• Zone 2: passive RRNC with junction box pre-wired; test and upgrade if needed
• Zone 1: install active from the start. The fan and wiring are cheap during construction; you almost certainly need them; the post-occupancy test becomes a confirmation rather than a deciding factor

Tight, energy-efficient builds shift the calculus further toward active. A modern home with continuous air barriers, low infiltration rates, and a tight envelope loses the stack effect that passive systems rely on. The same RRNC rough-in that would have worked in a 1990s leaky build often falls short in a 2026 net-zero build.

The “test before you decide” rule applies regardless: run a short-term radon test in the first occupied month, ideally during winter when levels tend to be highest. If the result is at or above 2 pCi/L, the fan upgrade is the right call. The 2 pCi/L floor (rather than the 4 pCi/L action level) accounts for the fact that short-term tests under-represent year-round exposure and that fan upgrades during construction are cheaper than retrofits.

What Buyers Should Ask Their Builder

If you’re buying a new home or building one, get these questions answered in writing before the slab is poured. Once concrete sets, retrofitting any of these features is dramatically more expensive.

1. Is the home being built to RRNC standards? If yes, ask specifically whether they follow IRC Appendix F or ASTM E1465. “Radon-resistant” without a referenced standard is marketing language.
2. Does the vent stack terminate above the roofline, not into the attic? Attic terminations are non-compliant and create attic radon problems instead of solving home radon problems.
3. Is the junction box pre-wired and accessible? Confirm location and ask to see it during the framing walk-through.
4. Will you provide post-occupancy testing or a credit for one? Some builders include this; most don’t. Push for it.
5. If active: who installed the fan, what’s the warranty, and where is the manometer? The U-tube manometer should be visible at the vent pipe, not hidden in the attic where you’ll never see a failure.

A builder who answers all five questions clearly and in writing is taking RRNC seriously. A builder who waves off any of these is signaling the rough-in is decorative.

What This Costs at the Construction Phase

The economics favor doing the work during construction. Approximate costs for a typical single-family home:

• Passive RRNC rough-in only: $250 to $800 added to the build (aggregate is often already there for drainage; the marginal cost is the vapor barrier, vent pipe, junction box, and sealing)
• Active RRNC (passive + fan + wiring): add another $300 to $700, for a total of $550 to $1,500
• Retrofit the same system after construction: $1,500 to $3,000

The retrofit penalty is real. Without the gas-permeable layer beneath the slab, the contractor has to drill multiple suction points and hope each one finds enough permeability to work. Without the pre-routed vent pipe, the install has to either route a new pipe through the home (invasive) or run an exterior stack (less effective and ugly). The junction box being absent means another half day of wiring work.

For full retrofit pricing context, see our radon mitigation cost guide.

If the as-built drawings reveal foundation issues — settlement cracks, slab heave, or perimeter drainage problems — address those before retrofitting the radon system. Mitigation works on a stable foundation; piling it onto active foundation movement creates a system that fails when the slab shifts. See our foundation repair directory for vetted contractors.

If You Bought a New Home Without RRNC

Plenty of new homes get built outside RRNC jurisdictions with no radon features at all. If yours is one of them:

1. Get the as-built drawings. They’ll show the foundation construction, slab penetrations, and whether any radon features were installed. If your builder is unwilling to share them, a structural inspector can review what’s visible.
2. Test first, mitigate only if needed. Run a long-term test (90-day or 12-month alpha-track) before assuming you need a system. The county may be Zone 2, the soil under your specific lot may be lower-permeability than the neighborhood average, and the result may genuinely be low.
3. If the test confirms elevation, retrofit. The retrofit is more invasive than the rough-in would have been, but it’s a well-understood project and most certified contractors handle one or two of these per week.

Frequently Asked Questions

Are all new homes required to have radon mitigation? No. RRNC requirements are state and local. Many new homes in Zone 1 counties are mandated to be built radon-resistant; many in Zone 3 are not. Check your state code and local building department.

How can I tell if my new house has a passive radon system? Look for a 3- or 4-inch PVC pipe running from a basement or slab penetration up through the home and out through the roof. Look for a junction box near the top of the pipe (often in the attic). Check the as-built drawings for “RRNC,” “Appendix F,” or “ASTM E1465” references. If the home has all three indicators, you have a passive system.

Do I still need to test for radon if my home was built to RRNC? Yes. The EPA estimates that roughly half of homes built to passive RRNC standards still test above 4 pCi/L. Testing is the only way to know whether your specific home’s system is doing its job or whether it needs a fan upgrade.

Can a builder refuse to install RRNC? In jurisdictions where it’s required by code, no — refusing would be a building code violation. In jurisdictions where it’s optional, a builder can refuse, but most will install it if you specify it in the build contract and pay the modest added cost.

Does an RRNC system void if I finish my basement later? Not if the finishing is done correctly. The risk is sealing or covering one of the system’s working components: the vent pipe must remain accessible at the top (for fan installation), the manometer must remain visible, and slab penetrations must not be re-opened during the finishing work. A contractor who finishes basements should be familiar with RRNC system preservation; ask before signing.

Find a radon mitigation contractor who handles new construction, RRNC retrofits, and post-occupancy fan upgrades through our directory.