Radon Test Placement by Home Layout: Where to Put a Test for Accurate Results
Radon testing is easy to buy and easy to run, but placement is where most people accidentally ruin their results. A radon test placed in the wrong spot can come back misleadingly low, misleadingly high, or simply not representative of how your family actually lives in the home.
This guide is a practical, layout-based way to choose the right test location. It covers common home types (basements, slab-on-grade, crawlspaces, split-levels, finished basements, and multi-story homes), explains what “lowest lived-in level” really means, and gives placement guidance that works for both mail-in tests and continuous monitors.
Key reference points: EPA’s Citizen’s Guide to Radon explains that you should test the lowest level of the home that you use regularly, avoid kitchens and bathrooms, and follow closed-house conditions for short-term tests. EPA: A Citizen’s Guide to Radon (PDF). If your result is 4.0 pCi/L or higher, EPA recommends fixing the home, and it also recommends considering action between 2.0 and 4.0 pCi/L. EPA action level guidance.
Table of Contents
- Quick rules that apply to every layout
- What “lowest lived-in level” means (and where people get it wrong)
- General placement rules (height, distance, airflow)
- Basement homes: unfinished vs finished basements
- Slab-on-grade homes
- Crawlspace homes
- Split-level and tri-level homes
- Two-story and multi-story homes
- Walkout basements and daylight basements
- Additions, sunrooms, and partial slabs
- Apartments and condos (ground floor vs upper floor)
- Real estate testing placement notes
- Common placement mistakes
- Do you need more than one test location?
- After placement: closed-house conditions and timing
- FAQs
- Sources
Quick rules that apply to every layout
If you remember nothing else, remember these five rules. They work for almost every home.
Rule 1: Test where you live, not where you store boxes. EPA guidance focuses on testing the lowest level of the home that you use regularly. EPA Citizen’s Guide (PDF)
Rule 2: One test does not represent the whole house. You are not measuring “the home.” You are measuring one room on one level during one time window. That is why placement and follow-up matter.
Rule 3: Avoid kitchens, bathrooms, and laundry rooms. These rooms have ventilation, humidity, and airflow patterns that can distort results. EPA specifically recommends avoiding kitchens and bathrooms. EPA Citizen’s Guide (PDF)
Rule 4: Stay away from drafts, windows, exterior doors, and supply vents. Drafts can skew results. Place the test in a calm air area.
Rule 5: If you will finish the basement later, test it now. Basement use changes exposure. A basement that becomes a living space should be treated as a living level for radon decisions.
What “lowest lived-in level” means (and where people get it wrong)
EPA uses the concept of the “lowest lived-in level.” This does not mean “the lowest level that exists.” It means the lowest level you spend time in regularly. If your basement is unfinished and you only go down there to swap laundry or grab supplies, the basement might not be your lived-in level. If you have a finished basement family room where your kids hang out nightly, the basement absolutely becomes your lived-in level.
The most common mistake is testing a first-floor living room when a basement is finished or used daily. That can produce a “comforting” number that does not represent real exposure. The opposite mistake also happens: testing an unfinished storage basement and assuming that number represents exposure if the family never spends time down there.
EPA’s Citizen’s Guide helps resolve this by focusing on the lowest level used regularly. EPA Citizen’s Guide (PDF)
General placement rules (height, distance, airflow)
Once you pick the right level and the right room, placement within the room still matters. Most kit instructions will cover this, but these general rules are consistent with the intent of EPA’s guidance.
- Height: Place the test in the breathing zone, not on the floor and not up near the ceiling. Many test kits recommend a few feet above the floor, on a table or shelf.
- Distance from drafts: Keep it away from windows, exterior doors, fireplace drafts, and supply vents.
- Avoid humidity extremes: Avoid bathrooms, damp utility corners, and areas with frequent steam or moisture spikes.
- Normal living conditions: For short-term tests, follow closed-house conditions guidance during the test window (more on that later).
EPA’s Citizen’s Guide includes a practical list of where and how to place a test and emphasizes following kit instructions. EPA Citizen’s Guide (PDF)
Basement homes: unfinished vs finished basements
Basements are where most radon problems show up because they are closest to the soil and often have more openings and pathways. The key placement question is whether the basement is a lived-in level.
If the basement is finished or used daily: Test in the basement in a regularly used room, such as the family room, office, or basement bedroom. Avoid the utility room and avoid the laundry area if possible.
If the basement is unfinished and rarely used: EPA’s concept of lowest lived-in level points you toward testing the first floor if that is the lowest level you use regularly. But if you plan to finish the basement, test it now anyway. A finished basement changes exposure dramatically because time spent there increases.
If kids sleep in the basement: Treat that as a high-priority placement. Sleeping is long duration exposure.
If your basement has a sump pit: You still test the room air, not the sump pit itself. But sump areas can influence radon entry, which is another reason basement placement matters when it is lived-in.
Slab-on-grade homes
Slab-on-grade homes can still have elevated radon. Radon can enter through cracks in the slab, expansion joints, plumbing penetrations, and gaps along the slab edge. In a slab home, the “lowest lived-in level” is typically the main floor.
In most slab homes, place the test in a regularly used room on the main floor, away from exterior doors and large windows. A living room, family room, or bedroom can work as long as it reflects normal occupancy and is not a kitchen or bathroom.
If your slab home has an attached garage, avoid placing the test near the garage entry door or in areas where garage-to-house pressure and airflow might create unusual conditions.
Crawlspace homes
With crawlspaces, the mistake homeowners make is trying to test inside the crawlspace. For most residential decisions, you care about radon in the air where people live. The crawlspace is not a lived-in area, and conditions in a crawlspace are often not representative of indoor air in the home.
In a crawlspace home, you usually test on the lowest lived-in level above the crawlspace (typically the first floor). Choose a regularly used room, avoid kitchens and bathrooms, and follow the general placement rules.
If the crawlspace is sealed/encapsulated or has a vapor barrier, you still test indoor air in the living space. If you later decide to mitigate, crawlspace systems may use sub-membrane depressurization or related methods. The test placement does not change just because the crawlspace exists.
Split-level and tri-level homes
Split-level homes confuse people because there is no single “lowest level.” Often there is a lower-level family room that is partly below grade, and then a basement or crawlspace below that, plus an upper level. The correct test level depends on which of those levels is the lowest level used regularly.
A practical approach is:
If the lowest level used daily is the lower-level family room: Test there. That room is often partially below grade, which can be a meaningful radon exposure zone.
If the home also has a basement below the lower-level family room and that basement is used regularly: Test the basement instead. Basements generally have higher potential.
If the basement exists but is not used: Testing the lower-level family room may be more representative of real exposure if that is where the family spends time.
In many split-level homes, it is reasonable to run a second test later in the next lowest lived-in level if the first result is borderline and you want clarity. If your lower-level is used heavily, treat it as your priority.
Two-story and multi-story homes
In multi-story homes with a basement, the default for many families is still the basement if it is used as a living space. If the basement is not used regularly, the first floor is often the primary placement.
The big misunderstanding is assuming upper floors are “safe” without testing. Radon levels typically decrease as you go up, but a home can have elevated levels on the first floor, especially in tight homes with strong stack effect and consistent soil gas entry. Test the lowest lived-in level and do follow-up testing if needed.
If your children sleep on the second floor but spend afternoons and evenings in a basement rec room, the basement is still the important placement because of time spent.
Walkout basements and daylight basements
Walkout basements can feel “less basement-like” because they have doors and windows. Some homeowners assume that means radon is unlikely. In reality, a walkout basement is still often partly below grade and still connected to soil. It can still test high.
If your walkout basement is used as living space, test it like a normal finished basement. Place the test in a regularly used room, not right next to the walkout door or a drafty window.
If the walkout basement is unfinished and unused, test the first floor as the lowest lived-in level, but consider also testing the walkout basement if you plan to finish it later.
Additions, sunrooms, and partial slabs
Additions can create mixed foundation conditions: part basement, part slab, part crawlspace. This is where “one placement” can feel insufficient.
If the addition is a heavily used space (a new family room or a home office) and it sits on a different foundation type, you may want to test the addition area as well, especially if your first test result is borderline.
The goal is not to test everything forever. The goal is to test the areas where your family actually spends time, especially the areas closest to the soil and the areas with different foundation conditions.
Apartments and condos (ground floor vs upper floor)
Radon matters most in ground-contact units. If you live in a basement apartment, garden-level unit, or first-floor unit over a crawlspace or slab, testing is relevant. Upper-floor units generally have lower risk, but radon can still be present depending on building design and airflow patterns.
If you are on the first floor and the building has a basement or underground garage below you, testing can still make sense because those spaces are ground contact and can influence the building’s airflow.
EPA provides radon resources for the real estate community and includes materials for tenants, which can help renters navigate testing questions. EPA radon resources for real estate community
Real estate testing placement notes
During a home sale, placement disputes can derail deals. The simplest way to keep a transaction clean is to use a qualified radon measurement professional and follow transaction-focused guidance. EPA’s Home Buyer’s and Seller’s Guide to Radon includes a radon testing checklist and emphasizes proper conditions to avoid invalid tests and retesting. EPA Home Buyer’s and Seller’s Guide (PDF)
A key real estate point is that the placement should reflect how the buyer might use the home, not only how the seller uses it today. A seller may not use the basement, but a buyer may plan to turn it into a playroom. That is why many transaction tests focus on the lowest level that could be lived in.
Also note: consumer digital radon monitors are not the right tool for transaction testing. CRCPD issued an advisory stating consumer digital radon monitors are not for testing during real estate transactions and recommends verification using approved methods. CRCPD advisory (PDF)
Common placement mistakes
Placing the test in the kitchen or bathroom. EPA recommends avoiding these rooms. EPA Citizen’s Guide (PDF)
Placing it near a window, exterior door, or supply vent. Drafts distort results and can give false confidence.
Testing in a utility room instead of a living area. Utility rooms have different airflow, equipment heat, and sometimes more openings.
Testing a basement storage room when nobody lives there. If the family does not use the basement, the result might not represent exposure. If the basement will be used later, then it becomes relevant again.
Testing “too high” or “too low.” A test placed on the floor, on top of a refrigerator, or near the ceiling is less representative of breathing-zone air.
Only testing the second floor because it feels safer. Radon risk usually starts at ground contact. Test lower levels first.
Do you need more than one test location?
Most homes start with one test on the lowest lived-in level. That is usually enough to decide whether follow-up is needed.
You might consider a second test location if:
- Your home has mixed foundation types and a heavily used addition on a different foundation.
- You have a split-level and you spend major time on two different lower levels.
- Your first result is borderline and you want to understand exposure in a different living area.
- You plan to change how the home is used (finishing a basement, moving a bedroom downstairs).
EPA’s Citizen’s Guide explains follow-up testing logic and emphasizes long-term tests for better year-round averages when needed. EPA Citizen’s Guide (PDF)
After placement: closed-house conditions and timing
Placement is only half the equation. The other half is running the test under the right conditions. For short-term tests, EPA recommends closed-house conditions, especially for very short tests. It also advises avoiding short-term testing during unusually severe storms or periods of unusually high winds. EPA Citizen’s Guide (PDF)
If you are doing a real estate test, follow the transaction checklist guidance and do not let showings or move-outs accidentally invalidate the conditions. EPA Home Buyer’s and Seller’s Guide (PDF)
FAQs
Should I test the basement even if it is unfinished?
If you do not use it regularly, EPA’s “lowest lived-in level” concept often points to testing the first floor. But if you plan to finish the basement or your family spends time there, test the basement now. The most important idea is testing where people will live. EPA Citizen’s Guide (PDF)
Where do I place a test in a slab-on-grade home?
Usually in a regularly used room on the main floor, away from drafts and not in the kitchen or bathroom.
Should I test a crawlspace?
In most cases, no. Test the indoor air on the lowest lived-in level. Crawlspace conditions are not the same as living-space conditions.
Can I put the test in the laundry room because it is in the basement?
It is better to use a regularly used room because laundry rooms often have airflow and humidity swings that are not representative.
What if my result is high?
If your result is at or above 4.0 pCi/L, EPA recommends fixing the home. If it is between 2.0 and 4.0 pCi/L, EPA recommends considering action. EPA action level guidance