Is Radon Only in Basements?

Radon is an invisible and odorless radioactive gas that originates from the natural decay of uranium in soil and rock. Because it is often found in basements, some people mistakenly believe that it only occurs in basements. However, the primary risk to homeowners comes from its accumulation inside structures, regardless of foundation type.

Radon Entry Points and Foundation Types

• Basements.
  Homes with basements are the most common suspects for having higher radon levels. The fact that porous foundation walls allow radon gas to enter a home is no secret and with most basements containing a sump well, there are many access points from which radon can enter a home.

• Crawl spaces.
  Homes above a crawl space can suffer the same fate as homes with basements. Whether your crawl space is a dirt crawl (unfinished), poured concrete (finished), or even lined with a protective barrier, radon can still enter your home. Additionally, with its closer proximity to the ground, the living areas of the home may be more susceptible to radon as it does not have the same amount of time to disperse in a full basement prior to entering living areas.

• Slab on-grade foundations.
  Homes built on a slab are also at risk for high radon levels. As a matter of fact, homes built on a slab do not have the buffer of a basement or crawl space to absorb and dissipate radon gas before it enters the living area of a home. There is no single style of home that is not prone to radon gas and potentially highly dangerous levels.

Once radon gas enters a structure, it doesn’t stay limited to one area. Homeowners often ask, “Is radon only in basements?” The answer is no. While basements typically show the highest readings, radon can easily travel upward and accumulate throughout the home.

How Radon Spreads Indoors

Because radon comes from the natural decay of uranium in rocks and soil, there is always some level present in the air. The risk comes from radon accumulation in homes, which happens when gas seeps in faster than it escapes. This can occur with poor ventilation in the home or when the home is energy-efficient and airtight which traps radon within the home. High concentrations can elevate a homeowner’s radon risk, increasing the chance of developing lung cancer after long-term exposure.

• Stack effect.
  The upward movement of radon through a home occurs due to what’s known as the stack effect. As warm indoor air rises, it escapes through upper levels and pulls air from the lower levels to replace it. That airflow can draw radon from the soil beneath the foundation, through tiny cracks in the walls or floor, and into the main living area. Even though radon is heavier than air, once inside it mixes and spreads.

• HVAC circulation.
  Ventilation and HVAC systems can intensify this process. Furnaces, air conditioners, and ductwork continuously circulate air throughout the house, moving radon from the basement into bedrooms, hallways, and kitchens.

• Unbalanced airflow.
  When airflow is unbalanced – such as when exhaust fans or heating systems create negative pressure – radon can be drawn into areas far from the original entry point.

Radon in Finished vs. Unfinished Basements

In homes with finished basements, occupants may spend more time near radon entry points. Studies have found unsafe readings of radon in finished basements, where wall paneling and insulation trap air, and in unfinished basements, where exposed concrete allows easier seepage. Even upper floors are not immune. Radon in multi-level homes often appears at measurable levels due to air circulation and temperature differences between floors.

The takeaway: radon moves silently and unpredictably once it enters. Testing every level of the home gives a clearer understanding of the radon level and helps homeowners take steps to reduce exposure before problems arise.

Where to Test for Radon in a Home

After confirming that you have a radon detector in frequently used rooms, it’s important to know where radon testing should take place for accurate readings. Homeowners often believe a single test in the basement is enough, but that approach may overlook exposure in other rooms. Testing both the lowest occupied level and upper living areas offers a more complete picture of your home’s air quality. Testing bedrooms and family rooms offers a more complete picture of exposure because these are spaces where people spend the most time.

• Houses with crawl spaces.
  Testing should occur directly above or near the exposed ground. Even if plastic barriers seal the area, radon in crawl spaces remains a concern because small gaps or punctures in the material let gas rise freely. Testing these zones helps identify hidden leaks contributing to a high radon level elsewhere in the home.

• Homes without basements require testing at ground level, as gas can enter through radon sources in homes like sump pits, floor drains, and plumbing penetrations. These are small but steady radon entry points. Even slab-on-grade foundations can draw gases upward when indoor air pressure is lower than the surrounding ground.

• Upper floors of the home.
  The stack effect pulls replacement air upward from the basement, spreading the gas through stairwells, vents, and ductwork. This is particularly relevant in multi-story homes where HVAC systems constantly circulate air between levels.

If results indicate elevated readings, professional mitigation may involve sealing cracks, installing sub-slab ventilation, or modifying the air exchange rate. Every building interacts differently with the soil, so personalized testing across multiple areas provides the most reliable understanding of your indoor air.

Short-term radon test kits are useful for quick checks, like after home repairs, remodels, or HVAC adjustments. They measure for a few days and provide fast insight into current exposure. For the most accurate picture, a long-term radon test lasting several months captures seasonal fluctuations and day-to-day air pressure changes. Homes with uneven heating or cooling may see shifting results because radon in foundation cracks expands and contracts with temperature and moisture.

Even if you've already tested your home for radon or have a radon mitigation system, retesting should be done after renovations, duct cleaning, or appliance upgrades. New HVAC systems or ventilation changes can alter air circulation and unintentionally spread radon through ductwork. Retesting confirms that mitigation efforts continue to perform effectively over time.

What About New Homes Built with Radon-resistant Construction Techniques?

As early as 1990 some homes have been built using radon-resistant materials and with pre-piped radon mitigation systems installed. Clean gravel “airflow layers”, vapor barriers, additional polyurethane caulking, and vent pipes are all widely used. However, as the name states, these techniques are radon “resistant” and cannot fully remove the risk of a high radon level. Testing should still be performed to determine a home's radon level and in instances when a high radon level is present, some of the installed vent pipes used for passive radon reduction can easily be converted to an active system by a professional radon mitigation specialist.