Radon in Basements vs. Upper Floors

If you have looked into radon at all, you have probably heard that basements are where the problem lives. That is mostly true, but not completely. This guide explains why radon concentrates where it does, what that means for different floors of your home, and where to actually place your test kit.

🏠Why basements tend to have higher radon

Radon enters your home through the foundation: cracks in concrete floors and walls, construction joints, gaps around pipes, sump pits, and bare soil in crawl spaces. The driving force is simple physics: your home is typically at slightly lower pressure than the soil beneath it, which draws soil gases, including radon, upward through any available opening.

Because basements are in direct contact with the soil on all sides (floor and often walls), they have the most entry points and the highest concentrations. This is consistently confirmed in testing data. The EPA notes that basement levels are typically 2 to 3 times higher than ground-floor levels in the same home.

📐What about upper floors?

Radon concentrations generally decrease as you go up. By the second floor, levels are typically much lower than the basement, often below 1 pCi/L even when the basement reads 6 or 8. The reason is dilution: upper floors have more ventilation, more distance from the source, and fewer foundation contact points.

That said, upper floors are not guaranteed safe:

• •Tight construction. A well-sealed upper floor with minimal ventilation can trap radon that migrates up through interior stairwells, HVAC ducts, or plumbing chases.
• •Stack effect. In winter, warm air rises and exits through upper levels, pulling replacement air, including radon, upward from the basement through the interior of the house.
• •Forced air HVAC. Ductwork that connects basement returns to upper-floor supply registers can distribute radon-laden air throughout the house.

🧪Where to place your test

The EPA recommends testing in the lowest livable area of your home: the lowest floor where someone regularly spends time. The goal is to measure what your family actually breathes.

• •Finished basement where anyone watches TV, sleeps, exercises, or works. Test there.
• •Unfinished basement that nobody uses. Test on the ground floor above it instead, unless you plan to finish the basement. If you are finishing it, test the basement before and after.
• •Crawl space: do not test in the crawl space itself. Test on the floor above it.
• •Multi-story with no basement: test on the ground floor.
• •Apartment or condo: test in your unit. Ground-floor and below-grade units are higher priority, but upper-floor units can still have elevated levels in buildings with shared HVAC or vertical air pathways.

⚡What if my basement is high but upstairs is fine?

This is common. A basement reading of 6 pCi/L with a ground floor reading of 1.5 pCi/L is a realistic scenario. What matters is where people spend time:

• •If the basement is a finished living space, mitigate based on the basement reading.
• •If nobody uses the basement and you tested the ground floor below 2 pCi/L, you are likely fine, but consider mitigation if you ever plan to finish the basement.
• •If you have bedrooms on different levels, the room where someone sleeps matters most. Long-term exposure during 8 hours of sleep every night adds up faster than occasional time in a living room.

A mitigation system pulls radon from beneath the foundation, which lowers levels on every floor, not just the basement. It is a whole-house solution, even though the biggest improvement is at the lowest level.