Reviewed and updated September 10, 2026. This article provides general public-health and referral guidance, not testing, medical, legal or engineering advice.
Use the lowest level that could be occupied regularly
EPA's current home-buying guidance says to place the device on the lowest level that could be used regularly, whether finished or unfinished. The question is not whether a basement currently has a sofa. It is whether a buyer or household could reasonably use it as a bedroom, play area, family room, den, exercise room or workshop.
If the basement is inaccessible, wet, extremely drafty or plainly unsuitable for occupancy, the first floor may be the meaningful location. A crawlspace is not a test room. When the property or intended use is unclear, a certified tester can document the decision.
Finished basements deserve their own attention
A finished lower level increases the time people may spend close to the soil-gas entry points. Bedrooms, home offices and playrooms make placement consequential. Put the device in a representative occupiable room, away from exterior walls, direct drafts, heat and humidity, following the device instructions.
Do not hide the device in a closet or place it in a bathroom, kitchen, laundry room or furnace room. Those spaces can produce conditions that are not representative or outside the testing protocol.
Foundation openings explain why basements matter
Radon can enter through slab cracks, wall-floor joints, block walls, utility openings, drains and exposed soil around a sump or crawlspace. Indoor pressure can draw soil gas through those pathways. The existence of a crack does not quantify the radon level, and a visually perfect slab can still test high.
A sump opening is both a potential soil-gas pathway and part of the drainage system. Read how sumps interact with radon mitigation before applying permanent sealants or covers that could compromise pump access.
Season and weather affect protocol, not the need to test
Radon can vary across days and seasons. Heating-season testing is useful because closed-house conditions can be easier to maintain, but winter does not guarantee the highest result in every home. A short-term test is a snapshot under its documented conditions.
Do not run a short test lasting less than four days during severe storms or high winds, according to EPA's buyer and seller checklist. If the decision is not urgent, a long-term test across more than 90 days better represents the annual average. See the winter testing guide for a more careful seasonality discussion.
Test again when use or structure changes
EPA recommends retesting when living patterns change and a lower level becomes occupied. Renovations, foundation work, new HVAC equipment or mitigation-system changes can also make an older report less representative.
If a valid basement result is 4 pCi/L or higher, follow the confirmation guidance for that test type and discuss mitigation. A system should be designed for the foundation and verified with a post-mitigation test, not judged only by whether the fan is running.
Sources
Primary guidance used for this article
We use government and public-health sources, distinguish historical datasets from current household measurements, and do not invent local results. Read our editorial policy.