Radon in New Construction Homes: What Buyers Need to Know

The Myth: "New Homes Don't Have Radon"

It's one of the most common misconceptions about radon. The logic seems intuitive: new homes are better sealed, use modern materials, and were built with current codes. They must be safer.

The problem: radon comes from the soil and geology beneath the home, not from the building itself. A brand-new house on uranium-bearing granite in Colorado will test just as high as a 1960s ranch on the same lot. In some cases, it tests higher — because modern energy-efficient construction creates tighter envelopes that trap radon more effectively than the leaky older homes they replace.

New construction + tight building envelope = potentially higher radon than an older home in the same location.

What Is Radon-Resistant New Construction (RRNC)?

RRNC is an EPA-recommended set of building practices designed to reduce radon entry during construction. Many states and some building codes require or strongly encourage RRNC in high-radon areas. The components:

1. Gas-permeable sub-slab layer

4" of clean aggregate (gravel) placed under the slab before pouring. This creates a free-air space where radon can move laterally to the suction point rather than accumulating and being forced upward.

2. Plastic sheeting

6-mil polyethylene sheeting placed over the aggregate layer before the slab is poured. Reduces direct soil gas contact with the slab.

3. Slab sealing

All penetrations (plumbing, conduit, control joints) sealed with appropriate materials during construction.

4. Passive vent stack

A 3"–4" PVC pipe running from the sub-slab aggregate layer, through the conditioned space, and out above the roofline. In passive mode (no fan), this provides some natural ventilation of sub-slab gases via stack effect.

5. Electrical junction box (optional in passive, required for "active-ready")

An electrical outlet near the fan location in the attic or exterior wall, ready for a fan to be added later if passive venting proves insufficient.

Does RRNC Eliminate the Need to Test?

No. RRNC reduces radon entry — it does not eliminate it. EPA data shows RRNC homes average 30–50% lower radon levels than conventional construction, but:

• In high-radon geology, a 50% reduction may still leave levels above 4.0 pCi/L
• Passive vent stack effectiveness varies with stack effect, weather, and building orientation
• The passive system may work fine for years, then stop working adequately as the building tightens

Test a new RRNC home within the first year — ideally in the first heating season. If levels are above 4.0 pCi/L, adding a fan to the existing passive vent stack typically costs $300–$600 (just the fan and electrical connection, no pipe work needed).

What to Ask Your Builder

If you're buying new construction in a high-radon state:

1. Is the home built to RRNC standards? Get this in writing. Ask specifically about the sub-slab gravel layer, plastic sheeting, passive vent pipe, and junction box location.
2. Where is the passive vent stack terminated? It should exit above the roofline, not into an attic or garage.
3. Is there a junction box for future fan installation? This makes future activation inexpensive.
4. Has the home been tested? Some builders in high-radon areas test before closing — ask for the result.

After Moving In

Test within the first 6–12 months, during the heating season if possible. If passive RRNC isn't adequate:

• Fan activation: If the passive stack is already installed, a contractor adds a fan and electrical connection. This typically costs $300–$700 — far less than a full mitigation installation from scratch.
• Full installation: If no RRNC features were included, treat it like any other home needing mitigation: $1,000–$2,500 depending on foundation type and complexity.

Don't assume RRNC means you're safe. Test, confirm, then maintain.

Shop radon test kits → | Find a certified mitigator →