Radon in Basements vs. Upper Floors: Where to Test

Why Radon Concentrations Are Higher Near the Ground

Radon enters a home from the soil beneath and around the foundation. Because the gas rises from underground, the levels it reaches are not distributed evenly from floor to floor — they are highest at the point of entry and dilute as air circulates upward. This is why testing protocol consistently focuses on the lowest livable level of a home: that is where occupants are most likely to encounter the highest concentrations, and where any health risk from long-term exposure is most significant.

The physics behind this pattern are straightforward. Radon migrates through soil under a pressure gradient — the interior of a home is typically at slightly lower pressure than the surrounding soil, which draws soil air inward through any available pathway. Cracks in a slab, gaps around utility pipes, mortar joints in block-wall foundations, and exposed soil in crawl spaces all serve as entry routes. Once inside, radon mixes with the interior air. On the floor where it enters, that concentration is freshest and highest. As interior air circulates upward through the home — carried by thermal convection, HVAC distribution, or simply opening doors — radon diffuses into greater volumes of air, reducing its concentration with every floor gained.

This dilution effect is real, but it does not eliminate radon risk on upper floors — it reduces it relative to the basement or lowest level. In a home with very high radon at the lowest level, concentrations on the first floor above grade can still be elevated enough to warrant attention. The upper floors are genuinely safer in a relative sense, but "safer than the basement" does not necessarily mean safe in an absolute sense. Testing at the point of highest concentration gives you the number that matters most.

Basements: The Priority Testing Location

A full basement is the area of a home where radon accumulates most directly. The basement floor and walls are in contact with the soil on three or four sides, and the concrete or masonry that separates the living space from the earth is porous at a microscopic level — even without visible cracks. Pressure differentials pull soil air through those surfaces continuously. In a finished basement used as a bedroom, family room, home office, or any regularly occupied space, the occupants are spending significant time in the area where radon is most concentrated.

Finished basements raise the risk profile considerably because the assumption that "the basement is just storage" no longer applies. A person who sleeps in a basement bedroom eight hours a night is accumulating radon exposure at the highest-concentration point in the home, night after night. The EPA's guidance on where to test is explicit: testing should happen in the lowest level of the home that is regularly used, or the lowest level that could be used in the future. If a basement is finished or finishing is planned, that is your test location.

Unfinished basements matter too, even if they are used only occasionally for laundry or mechanical access. An unfinished basement that connects through ductwork, stairwells, or shared air handling to the floors above can distribute radon throughout the home. The basement itself may see the highest concentrations, but the air it contributes to the upper floors raises those levels as well. Testing the basement establishes the source point for understanding the whole home's radon profile.

Slab and Crawl Space Homes: Where Ground Level Is the Risk Zone

Not every home in Kannapolis, Concord, or Landis has a basement. A significant portion of the housing stock in this area consists of slab-on-grade and crawl space construction — and for these homes, the radon entry dynamics are different but the principle remains the same: test at the lowest livable level where radon enters and concentrates.

In a slab-on-grade home, the ground floor is simultaneously the entry point and the living space. The concrete slab sits directly on the soil, and radon migrates through the slab and through any penetrations around pipes, conduit, and utility entries. There is no basement buffer below the living area — the occupants are already on the lowest level. Testing in the ground-floor room where the family spends the most time — a living room, a bedroom, a frequently used home office — captures the most relevant concentration for long-term health assessment.

Crawl space foundations introduce a layer between the soil and the living area, but that layer is not a barrier to radon movement unless it has been properly sealed and depressurized. An open crawl space with exposed soil is essentially a radon reservoir directly beneath the floor assembly. The gas migrates upward through gaps in the subfloor, around floor joists, and through utility penetrations into the living area above. Even an encapsulated crawl space with a sealed vapor barrier reduces — but does not necessarily eliminate — radon entry. Testing the lowest livable level of a crawl space home establishes whether the space and its barrier are performing as intended.

Upper Floors: Lower Risk, But Not Zero Risk

The second floor of a typical home, and any floor above it, generally shows lower radon concentrations than the level where the gas enters. Air exchange between floors, the volume of air radon must diffuse into, and the simple distance from the entry point all contribute to lower readings at higher elevations. Bedrooms on the second floor of a two-story home are genuinely less exposed to radon than a finished basement bedroom would be in the same house.

This relative safety is not a reason to skip testing the lowest level. The point of a certified radon test is to measure at the location of greatest risk — the lowest livable area — and use that result to make an informed decision about the whole home. A low result at the lowest level tells you the upper floors are even lower. A high result at the lowest level tells you action is warranted, and that the upper floors, while lower, may still be elevated.

There are circumstances where upper-floor testing becomes relevant as a follow-up. If a mitigation system has been installed and you want to verify that it is working throughout the home, testing at multiple levels gives you a more complete picture. If a home has an unusual HVAC configuration that draws basement air directly into upper-floor supply, a follow-up test upstairs after a basement result confirms how that air movement affects upper-level concentrations. These are follow-up questions — the initial test always starts at the lowest livable level.

Choosing the Right Room at the Right Level

Once you have identified the lowest livable level of your home, not every room at that level is an equally good test location. The EPA protocol for short-term testing calls for placement in a regularly occupied room — a room where someone spends four or more hours per day — that is not a kitchen or bathroom. Kitchens and bathrooms are excluded because their ventilation patterns (range hoods, exhaust fans, windows opened for humidity control) can artificially lower radon readings relative to the rooms where people actually spend their time.

In a finished basement, a bedroom or family room is the right choice. In a slab-on-grade home, a living room or regularly used bedroom on the ground floor is the standard location. The test device — a continuous monitor or passive measurement kit — is placed at breathing height, away from drafts, exterior walls, and openings. These placement standards ensure that the reading reflects what a person living and breathing in that space would actually be exposed to, rather than an anomalous high or low caused by proximity to an entry crack or an exhaust pathway.

If a home has a basement but the family uses it rarely, it is still worth testing there — because the question is not only about current occupancy but about the radon source that affects the whole home. Even if the basement result is the only test, it gives you the worst-case exposure scenario for the structure, which is the most useful single data point for making mitigation decisions.

What a Test Result Means for Your Home

The EPA's action level is 4 picocuries per liter of air (pCi/L). A result at or above that level is a signal that radon mitigation — typically a sub-slab depressurization system — is worth pursuing. Results between 2 and 4 pCi/L are below the action level but above the U.S. average, and many homeowners choose to mitigate at those levels as well. Results below 2 pCi/L are genuinely reassuring.

Whatever the result, a certified professional test gives you a documented, defensible number — the kind of result that holds up in a real estate transaction, satisfies a lender's request, and gives you a real baseline to compare against a future test. A standard residential radon test from Wightman Inspection Group runs $175–$225 and covers the full protocol: placement in the appropriate room, calibrated measurement equipment, and a written report you can keep, reference, and act on.

For homeowners and buyers in Kannapolis, Concord, and Landis, the answer to "where should I test?" is consistent: the lowest livable level, in a regularly occupied room. That is where radon poses the greatest risk, and that is where a certified test delivers the most actionable information about your home.