Where Should a Radon Test Be Placed in an Illinois Home?

Choosing the correct location for a radon monitor is an important part of obtaining meaningful results. Illinois rules require more than simply placing one device somewhere in the basement. Depending on the home’s size, layout and foundation design, testing may be required in multiple areas at the same time.

This is particularly relevant in Chicago-area homes, where additions, split-level construction, crawlspaces, basements and mixed foundation types are common. Understanding where a radon test should be placed helps buyers and homeowners know why one property may require a single monitor while another needs two or more.

Illinois Radon Test Placement Requirements

Illinois measurement protocol requires radon testing in each lowest structural area suitable for occupancy. When a home contains more than one foundation type, each qualifying area must be evaluated separately.

For example, consider a home with:

• A basement beneath the original structure

• A slab-on-grade family-room addition

• A bedroom located over a crawlspace

These represent three different structural areas in contact with the ground. Illinois protocol may require simultaneous measurements in the basement, the slab-on-grade room and an occupiable room over the crawlspace.

A single monitor in the basement may not accurately represent the other foundation areas. Soil conditions, air movement, foundation openings and pressure differences can vary throughout the property.

What Does “Suitable for Occupancy” Mean?

A suitable area is not limited to a finished bedroom or family room. Under Illinois regulations, it can include an area that is currently occupied or one that could be regularly used without renovations.

Common examples include:

• Family rooms

• Living rooms

• Bedrooms

• Home offices

• Playrooms

• Exercise rooms

• Workshops

• Unfinished basements that can already be used regularly

An unfinished basement does not have to contain finished walls and carpeting to qualify. If someone could reasonably use the area as a workshop, exercise space, recreation area or similar occupied space without renovating it first, the basement may be suitable for occupancy.

A crawlspace itself is not an appropriate representative testing location. When a portion of the home is constructed over a crawlspace, the monitor is normally placed in a suitable room above that crawlspace.

Why Different Foundation Areas May Need Separate Monitors

Radon forms naturally in soil and rock and can enter through openings where a building contacts the ground. Common entry pathways include cracks in concrete, floor joints, sump pits, gaps around pipes and utility penetrations.

Different foundation sections do not necessarily interact with the soil or indoor air in the same way. A basement beneath the original home may have different radon conditions than a slab addition built years later. A room over a crawlspace may also be affected by different ventilation and foundation conditions.

Testing each qualifying foundation area provides a more complete assessment than assuming one reading represents the entire building.

This is why split-level homes, homes with additions and buildings with basement-and-crawlspace combinations frequently require additional evaluation.

When Is an Additional Radon Monitor Required?

Multiple monitors may be necessary for two primary reasons:

Multiple foundation types

Each lowest structural area suitable for occupancy may require its own measurement. A home containing a basement, slab-on-grade addition and room over a crawlspace may therefore need more than one monitor.

The size of the tested area

Illinois protocol also requires additional test locations when the area being tested exceeds 2,000 square feet. Large basements, expansive lower levels and larger residential buildings may require multiple monitors even when the property has only one primary foundation type.

The number of monitors should be determined from the property’s actual layout rather than its total advertised square footage alone. Before testing begins, the measurement professional should evaluate the foundation configuration, occupiable areas and size of each area being tested.

Attentive Home Inspection charges $50 for an additional radon monitor when the property’s size or configuration requires one. The additional device is used to test the home appropriately—not simply as an optional upgrade.

Where Should the Monitor Be Positioned Within the Room?

After the appropriate structural areas are identified, each monitor must be positioned correctly within a representative room.

Illinois placement standards generally require the device to be:

• Between 20 inches and 6 feet above the floor

• At least 3 feet from exterior doors and windows

• At least 3 feet from ventilation ducts

• Outside the direct flow of air from a supply or return vent

• At least 1 foot from an exterior wall

• At least 4 feet from fireplaces, furnaces and other heat sources

• Away from direct sunlight

• At least 7 feet from a sump pit

• At least 4 inches from nearby objects

• Left undisturbed throughout the measurement period

The monitor should not be placed wherever it is simply easiest to reach. Airflow, heat, humidity and proximity to potential radon entry points can affect whether the location provides a representative measurement.

Closets, cupboards, crawlspaces, sumps and small foundation nooks should not be used as representative measurement locations. Kitchens, bathrooms, laundry rooms and other high-humidity areas can also be inappropriate for certain testing devices.

Finished Versus Unfinished Basements

A finished basement used as a bedroom, office, family room or home gym clearly qualifies as occupied living space. The monitor should be placed in a representative room where people regularly spend time while still following the required distance and placement rules.

An unfinished basement requires a closer look. If it can currently be used as a workshop, exercise space, play area or recreation room without renovations, it may still be suitable for occupancy. If the basement is only a cramped utility or storage area that cannot reasonably be occupied without significant work, the correct testing approach may be different.

The determination should be based on the condition and potential use of the space at the time of testing—not merely whether the basement has finished walls.

Radon Testing in Split-Level Homes and Additions

Split-level homes deserve particular attention because several sections of the building may contact the ground at different elevations.

A lower-level family room may sit on a slab, while another portion of the house has a basement. An addition may have been constructed over a crawlspace or separate slab. Even though all these rooms belong to one home, they can represent different lowest structural areas.

A measurement professional should examine the entire footprint of the building before placing the equipment. This prevents a separate foundation area from being overlooked and helps ensure the results represent every qualifying portion of the property.

Closed-Building Conditions Still Matter

Correct placement is only one part of a reliable short-term radon test. Closed-building conditions must also be maintained for at least 12 hours before a short-term test begins and throughout the testing period.

Windows should remain closed, and exterior doors should be opened only for normal entry and exit. Heating and air-conditioning systems can operate normally, but equipment that intentionally brings substantial outdoor air into the home should not be used in a way that disrupts testing conditions.

The devices should not be moved, covered, unplugged or disturbed. If unusual conditions occur during the test, they should be reported to the measurement professional.

For more preparation details, read How to Prepare for a Radon Test at Home.

Proper Placement Produces More Useful Results

Radon testing is not simply a matter of placing one monitor in the lowest-looking room. Illinois protocol considers foundation types, occupiable areas, square footage and the exact position of each device.

A properly planned test may require one monitor in a straightforward home or several monitors in a larger property with mixed foundation construction. Evaluating the layout before testing helps prevent important areas from being overlooked and gives buyers and homeowners more dependable information.

Attentive Home Inspection provides professional radon testing in Chicago, Cook County, DuPage County and Lake County using continuous radon monitoring equipment. Testing includes proper placement, a minimum 48-hour measurement period and a written report explaining the results.

Schedule your radon test before closing or whenever you need a clearer understanding of your home’s indoor radon level.