Understanding Radon and Its Health Risks

Radon is a colorless, odorless, and tasteless radioactive gas that forms naturally when uranium in soil, rock, and water decays. It is the leading cause of lung cancer among nonsmokers and the second overall cause of lung cancer in the United States, responsible for an estimated 21,000 deaths annually, according to the Environmental Protection Agency. The gas enters buildings through cracks in concrete slabs, gaps around service pipes, floor drains, construction joints, and even through porous cinder block walls. During renovations and additions, these pathways can be disturbed or newly created, making it critical to address radon proactively.

The health risk from radon is cumulative. Inhalation of radon decay products can damage lung tissue and lead to cancer over many years of exposure. The EPA recommends taking action to reduce radon levels when the annual average concentration exceeds 4 picocuries per liter (pCi/L) of air. However, even levels between 2 and 4 pCi/L can pose risks, and many experts suggest mitigation at any level above 2 pCi/L. Because renovations often involve excavation, foundation work, and changes to building envelope pressure, they can significantly increase indoor radon levels if proper precautions are not taken.

Pre-Renovation Radon Testing

Testing your home for radon before any renovation or addition work begins is essential. A baseline measurement tells you whether existing radon levels are elevated and helps guide the scope of mitigation needed. It also ensures that any changes you make during construction don't inadvertently worsen the problem.

Types of Radon Tests

Radon testing methods fall into two categories: short-term and long-term. Short-term tests (2 to 90 days) use charcoal canisters, alpha-track detectors, or continuous monitors. They provide a quick snapshot but are less reliable for making permanent decisions. Long-term tests, typically lasting more than 90 days, use alpha-track detectors to measure average radon levels over a full seasonal cycle, offering a more accurate picture of year-round exposure. For pre-renovation purposes, a short-term test can give you a reasonable indication, provided you follow the test instructions carefully—keep windows and doors closed for at least 12 hours before and during the test, and avoid operating whole-house fans or unsealed window air conditioners.

When to Test

Test during the cooler months if possible, because closed-house conditions produce higher radon readings. If your current test shows levels at or above 2 pCi/L, include radon mitigation in your renovation plans. If levels are below 2 pCi/L, consider retesting after construction to confirm no new pathways have been created. You can purchase a test kit from hardware stores, online retailers, or hire a certified radon measurement professional. The National Radon Program at Kansas State University offers discounted test kits and can direct you to state-certified testers.

Learn more about EPA's radon testing recommendations.

Radon Mitigation Strategies During Renovations

Integrating radon mitigation during a renovation is far more cost-effective and less disruptive than retrofitting later. The following strategies can be applied depending on your home's foundation type and the scope of the project.

Sealing Entry Points

Sealing cracks and openings is the first line of defense but is rarely sufficient alone. Use polyurethane caulk or hydraulic cement to seal gaps in concrete floors and walls, especially around utility pipes, sump pits, and floor drains. For block walls, seal the top row of blocks where they meet the sill plate. During a renovation, you can also install a vapor barrier beneath a new slab or over a crawl space floor, securely taped at seams and sealed against the foundation walls. This prevents radon-laden soil gas from entering the living space.

Sub-Slab Depressurization (SSD) Systems

The most common and effective radon reduction method for homes with a slab-on-grade or basement foundation is active sub-slab depressurization. This system uses a vent pipe inserted through the slab into the gravel layer beneath, where a fan draws soil gas out and exhausts it above the roofline or away from windows and vents. During a renovation, it is straightforward to install the pipe and fan before pouring a new slab or while existing concrete is exposed. For additions, you can design the SSD system from the ground up, ensuring the vent pipe routes easily to the exterior. Passive systems (without a fan) can reduce radon by up to 50% but may not bring levels below the action threshold; a fan can be added later if needed.

Crawl Space Mitigation

For homes with crawl spaces, install a thick (6-mil or heavier) polyethylene sheet over the dirt floor, sealing it against the foundation walls and piers. Place a vent pipe through the sheeting to a fan that draws gas from beneath the liner and exhausts it outdoors. This technique, called sub-membrane depressurization, is highly effective. If the crawl space is enclosed, make sure vents are properly positioned and sealed to avoid depressurizing the home.

Basement Renovations

If you are finishing a basement, take extra precautions. Before framing, install an SSD system if one does not exist. Use concrete caulk to seal the perimeter joint between floor and wall, and any other potential entry points. Choose moisture-resistant materials like closed-cell spray foam for insulation, which also helps create an air barrier. Consider installing a sump pit cover that is airtight and connects to the SSD system to prevent radon from being drawn into the basement from the sump.

Radon-Resistant Construction Techniques for Additions

When building a home addition, incorporate radon-resistant features from the foundation up. These methods are mandated in some states and are cost-effective when done during initial construction.

Installing a Gas-Permeable Layer

Under the concrete slab or floor, place a 4-inch-thick layer of clean gravel or stone (typically <1-inch diameter). This layer allows soil gases like radon to move horizontally to the vent pipe rather than building up under the slab.

Laying Polyethylene Sheeting

On top of the gravel, install a heavy-duty (6-mil or thicker) sheet of polyethylene plastic that fully covers the entire slab area. Overlap seams by at least 12 inches and seal with tape. The sheeting should extend up the foundation walls and be attached to prevent gas from bypassing the vapor barrier. If necessary, use two layers for added protection.

Sealing and Curing the Concrete Slab

Pour a concrete slab of at least 4 inches thickness, using a mix with minimal shrinkage and low permeability. Ensure the slab is properly cured to reduce cracking. All joints, pipe penetrations, and control joints should be sealed with a flexible caulk. Avoid using cold joints near the perimeter of the addition.

Vent Pipe and Fan Installation

Install a 3- or 4-inch PVC vent pipe from the gas-permeable layer through the slab and extend it vertically through the roof or an exterior wall. In radon-prone areas, the pipe should be run inside the conditioned space to help keep it warm and reduce condensation, then exit above the roofline. Include an electrical junction box near the location of a future fan. Initially, the system can be left passive; if test results after construction show elevated radon, you can easily add an in-line fan. Even if you don't need the fan immediately, having the pipe in place saves significant cost compared to retrofitting.

EPA Radon-Resistant New Construction resources

Post-Renovation Testing and System Maintenance

Once the renovation or addition is complete and any mitigation system is operational, wait at least 24 hours before testing for radon again—longer if a new slab needs curing time. Use a long-term test (90 days or more) to verify that average radon levels remain below 2 pCi/L. If levels are still above 2, you may need to upgrade the fan or adjust venting.

Maintain your radon mitigation system by:

  • Checking the manometer or u-tube gauge monthly to confirm the fan is pulling negative pressure under the slab.
  • Inspecting the exterior vent pipe for blockages, nests, or damage.
  • Replacing the fan every 5 to 10 years as needed. Fans last about 5 years in continuous operation.
  • Retesting at least every two years, or after any major structural change, such as a new roof, HVAC system, or window replacement that could alter air pressure.

Long-Term Radon Safety Tips

Beyond mitigation systems, maintain a radon-aware home by:

  • Keeping basement and crawl space vents open in summer to promote natural dilution.
  • Using an energy recovery ventilator (ERV) or heat recovery ventilator (HRV) to exchange indoor and outdoor air without excessive energy loss.
  • Never operating a whole-house attic fan when windows are closed, as it can depressurize the home and draw radon in from the soil.
  • Checking sump pump lids for airtight seals and connecting them to the radon vent system if possible.
  • Educating family members about radon so they understand the importance of avoiding open windows and doors in crawl spaces or basements that might break the venting system's pressure balance.

Finally, consider having your well water tested if you use a private well; radon can dissolve into groundwater and be released into the air through showering, dishwashing, and laundry. A point-of-entry aeration system can reduce radon in water by up to 99%.

Find a certified radon mitigation professional through the National Radon Proficiency Program (NRPP).

Additional radon health guidance from the CDC.