Radon-resistant new construction

If you are building a house, the cheapest time to deal with radon is before the slab is poured. Radon-resistant new construction (RRNC) builds in the parts of a mitigation system from the start.

The basic features

EPA describes a set of techniques that work together:

1. Gas-permeable layer. A 4-inch layer of clean gravel (or an equivalent) under the slab or crawl-space membrane lets soil gas move freely to a collection point.
2. Plastic sheeting. Polyethylene sheeting on top of the gas-permeable layer helps keep soil gas from entering and keeps concrete from clogging the gravel.
3. Sealing and caulking. Openings, cracks, and crevices in the foundation are sealed.
4. Vent pipe. A 3- or 4-inch gas-tight pipe runs from the gravel layer up through the house and out the roof.
5. Junction box. An electrical box in the attic makes it simple to add a vent fan later.

With only the pipe (a passive system), natural air movement pulls some soil gas up and out. If a test after move-in shows elevated radon, adding a fan turns it into an active system, usually at modest cost.

Why it matters where radon is common

EPA originally developed its Map of Radon Zones partly to help building code officials decide where radon-resistant building practices should be a priority. Many jurisdictions in high-radon areas have adopted radon-resistant construction requirements in their building codes. Check with your local building department. You can see your county’s zone with the ZIP lookup.

Test the soil first?

EPA does not recommend testing soil to predict indoor radon. It says soil tests have not proven reliable for predicting what the finished home will measure, because the house itself changes how gas moves. EPA’s advice is to build radon-resistant, then test the home after it is finished.

Test after you move in

Even with RRNC features, test the finished home. If results are 4 pCi/L or higher, a fan can be added to the vent pipe. See how radon mitigation systems work.

Questions for your builder

• Will the house include a gravel layer, sheeting, sealed openings, and a vent pipe to the roof?
• Is there an attic junction box for a future fan?
• Will the builder test the home before closing, or should you arrange it?

What a passive system looks like, piece by piece

A typical radon-resistant new construction (RRNC) package includes:

1. A gas-permeable layer under the slab, usually a 4-inch layer of clean gravel or a soil-gas collection mat. This lets soil gas move freely to a single collection point.
2. A plastic vapor retarder (polyethylene sheeting) on top of the gravel, overlapped and sealed. This slows soil gas from moving up into the house and also helps control moisture.
3. Sealing and caulking of slab openings, joints, and penetrations for plumbing and wiring.
4. A vent pipe, usually 3- or 4-inch PVC, running from the gravel layer up through the conditioned part of the house and out above the roof. As warm air rises through the pipe, it creates a small natural draft.
5. An electrical junction box in the attic, near the pipe, so a fan can be added later if testing shows it's needed.
6. Labeling of the pipe so future owners and contractors know what it is.

EPA explains that radon must be vented above the roof because discharge concentrations can be very high, and they fall to near background within a few feet of the discharge point. Ground-level discharge was disallowed because radon could be drawn back into the house, and because children could be exposed near the outlet.

Activating the system: passive to active

A passive system often lowers radon, but not always enough. If your post-occupancy test is still at or above EPA's action level of 4 pCi/L, or between 2 and 4 pCi/L and you want it lower, a qualified mitigator can install an inline fan in the attic on the existing pipe. That converts the system to active sub-slab depressurization, which is typically far more effective. Because the pipe and junction box are already in place, activation is usually a simpler and less expensive job than retrofitting a system from scratch.

If a fan is added, the installer should also add a pressure gauge (manometer) or alarm so you can see the system is working. See living with a radon mitigation system.

Why RRNC is worth it even if you're not worried

• Cost. Building it in during construction is usually far cheaper than retrofitting.
• Moisture control. The vapor retarder and sealing help keep slabs and basements drier.
• Resale. A labeled RRNC system and a documented low test can reassure future buyers.
• Unpredictability. EPA stresses that radon levels can vary greatly between neighboring homes, so a lot that "should" be low can surprise you.

Testing after you move in

Test as soon as the house is occupied and the HVAC system is running normally. Follow EPA's closed-house conditions for short-term tests, and test in the lowest lived-in level. If the result is at or above 4 pCi/L, follow up with a second test and plan to activate the system. EPA's retest guidance also suggests testing again if you start using a lower level of the home, such as a finished basement, and after major renovations.

Questions to put in writing with your builder

• Will the home include an RRNC system, and which components?
• Where will the vent pipe run, and will it be labeled?
• Is there an attic junction box for a future fan?
• Will the slab and penetrations be sealed?
• Will the builder test after completion, or should you arrange your own test?
• If the test is high, who pays to activate the system?

Get answers in the purchase contract or construction agreement, not just in conversation.

Keep the builder's documentation of the RRNC system with your home records. Future buyers and mitigators will want to see it.