Where Should a Radon Test Be Placed in Your Home?

Test placement is one of the most consequential and least discussed aspects of radon testing. The device itself — whether a charcoal canister or an electret ion chamber — is only as useful as the accuracy of the measurement it produces, and accurate measurement depends heavily on where the device is placed within the home. A test placed incorrectly can understate or overstate actual radon levels in the spaces where people spend their time, producing a number that is technically a measurement but not a reliable representation of your family's actual exposure.

This post is specifically about placement — where to put the test device, why those guidelines exist, and what to avoid. The testing process overall is covered in our post on what happens during a professional radon test. The question of which type of test to use is covered in our post on short-term versus long-term testing. This post owns the placement question specifically, because it is detailed enough and consequential enough to deserve dedicated treatment.

The Governing Principle: Measure Where People Live

The purpose of a radon test is to measure the concentration of radon in the air your family breathes during the time they spend in the home. That purpose defines the governing principle for placement: the test device should be located in the area of the home where people spend the most time, under conditions that reflect how that space is actually used during normal occupancy.

This sounds straightforward, but it has several specific implications that are worth unpacking carefully because they run counter to some homeowners' instincts about where testing should happen.

Floor Level: Always the Lowest Livable Area

The most important placement rule is that the test device goes in the lowest livable level of the home. Radon enters through the foundation and concentrates most heavily in the spaces closest to the ground. Testing on the main floor or upper floors of a home while there is an occupy able basement level will systematically understate the radon concentrations in the area of highest concern and produce a result that does not reflect the worst-case exposure in the home.

Lowest livable level means the lowest floor of the home that is currently used or could reasonably be used for regular human occupancy. An unfinished basement used only for storage and laundry qualifies as a livable level for testing purposes — it is a space that people enter and occupy, even if not for extended periods, and it is the space where radon levels are highest. A finished basement used as a family room or bedroom is obviously a livable level. A crawlspace is not typically considered a livable level, but a home built on a slab with no basement is tested on the ground floor.

The practical implication is that if your home has a basement — finished or unfinished — that is where the test device belongs. Testing only the main floor of a home with a basement produces a result that is genuinely less useful than a basement-level test for understanding your home's radon profile.

Height Above the Floor

Within the lowest livable level, the test device should be placed at a height between twenty inches and approximately seven feet above the floor — breathing zone height, essentially. Placement directly on the floor or within a few inches of it is not representative of the air people actually breathe while standing, sitting, or sleeping in the space. Placement near the ceiling does not reflect typical breathing zone conditions either.

For most practical purposes, placing the device on a table, shelf, or counter at roughly desk or workbench height — two to four feet above the floor — satisfies the height requirement and keeps the device away from both the floor-level concentration gradient and ceiling-level air movement patterns.

Distance From Exterior Walls and Windows

The test device should be placed away from exterior walls — at least twelve inches, and ideally further. Exterior walls are the boundary between the conditioned interior and the outdoor environment, and air movement near exterior walls is not representative of conditions in the central air mass of the space. Similarly, the device should be kept away from windows and exterior doors, where drafts and air infiltration can dilute the radon concentration in the immediate vicinity of the device without affecting the overall room concentration.

Drafts from windows, doors, and any other exterior opening affect the local air conditions near the opening without necessarily representing the air conditions throughout the space. A device placed near a drafty window may record a lower radon level than the room's average simply because outdoor air dilution is higher at that location.

Rooms and Spaces to Avoid

Certain room types should be avoided for test placement regardless of their floor level. Kitchens and bathrooms are the most important exclusions. Both rooms have exhaust ventilation — range hoods, bathroom exhaust fans — that creates localized air movement and pressure conditions not representative of the home's overall radon environment. The humidity levels in these spaces can also affect some test device types. Radon measurements in kitchens and bathrooms reflect the specific conditions of those spaces rather than the lived environment of the home.

Laundry rooms, utility rooms, and mechanical spaces should be avoided for similar reasons — the presence of active equipment, exhaust pathways, and atypical air movement makes them poor representatives of the home's general radon environment.

Closets and enclosed storage areas are not appropriate test locations. These spaces have minimal air exchange with the rest of the home and can produce anomalous readings — either higher or lower than ambient levels — that do not reflect conditions in the occupied areas.

Spaces with sump pumps or floor drains require specific attention. These features are among the most significant radon entry points in a home, as we covered in our post on how radon enters a home, and placing a test device directly adjacent to an uncovered sump pit would produce an artificially elevated reading that overstates the concentration throughout the living space. Reasonable distance from sump openings — several feet at minimum — produces a more representative result.

Avoiding High-Movement and High-Humidity Areas

Beyond specific room types, any location with atypical air movement or humidity should be avoided. High-velocity air movement near HVAC supply or return registers dilutes radon in the immediate vicinity and produces a locally lower measurement that does not represent the overall space. The test device should be placed away from supply registers, return air grilles, and any fans or air-moving equipment.

Unusually high humidity — not just bathroom humidity but basement humidity in homes with water infiltration issues or condensation problems — can affect the performance of some device types. If your basement has chronic moisture issues, noting this when submitting a test for laboratory analysis is worth doing, and consulting with a professional about whether specific device types are better suited to high-humidity environments is a reasonable step.

Direct Sunlight Exposure

The test device should not be placed in direct sunlight. Solar radiation and the heat it generates can affect device performance and produce readings that are not representative of normal air conditions. In a basement this is rarely a concern, but in above-grade spaces with significant sun exposure — south-facing rooms with large windows — it is worth checking that the selected placement location is shaded from direct sun during the test period.

Multiple Testing Locations

For many homes, a single test device placed according to the guidelines above is sufficient to characterize the home's radon profile meaningfully. But certain situations warrant testing in multiple locations simultaneously.

Homes with multiple distinct occupied areas on the lowest level — a finished basement with both a family room and a bedroom, for example — benefit from devices in both spaces to capture any variation in radon concentration across the floor. Homes where the main floor is also used as primary living space, with limited basement use, may warrant testing both levels to understand the full exposure profile.

Homes with additions built on separate foundations may have different sub-slab conditions beneath different sections — the original footprint and the addition may have meaningfully different radon levels. Testing both sections provides a more complete picture than a single device in one area.

Commercial buildings, as noted in our post on commercial radon testing, routinely require multiple devices due to the larger footprints and multiple occupied zones involved.

The Professional Advantage in Placement Decisions

The placement guidelines described in this post are the same guidelines certified professionals follow — they are drawn from AARST and EPA standards for radon measurement. The difference between a professional and a homeowner working with a DIY kit is not in the guidelines themselves but in the experience and judgment applied to specific situations that do not fit neatly into the standard case.

A basement with an uncovered sump pit in the center of the room, exhaust fan ductwork running along one wall, and limited floor area requires placement judgment that goes beyond following a checklist. A finished basement with multiple rooms requires a decision about which room or rooms are most representative of the household's actual exposure. A home where the lowest livable level is technically an unfinished basement used entirely for mechanical equipment requires a judgment about whether the main floor is the more relevant testing location for the family's actual exposure profile.

These are the kinds of decisions that professional experience informs, and they are among the practical reasons why professional testing produces more reliable results than DIY testing in situations that involve any complexity — as we discussed in our post on DIY versus professional testing.

At All Michigan Mitigation, proper test placement is foundational to the professional testing service we provide. Every device we place follows established protocols and reflects the specific conditions of the home we are working in. The result you receive from a professionally placed test is a result you can act on with confidence.

Visit us at allmichiganmitigation.com to schedule a professional radon test or to reach out with any questions about what the testing process involves for your specific home.

All Michigan Mitigation is a West Michigan radon testing and mitigation company dedicated exclusively to helping homeowners understand and eliminate radon risk in their homes.