QORVAN

Radon in the Basement

Basements usually have the highest radon levels in a house. Learn why, what a basement reading means, and how to test it properly.

Updated · 2 min read · By QORVAN Editorial Team · Reviewed by QORVAN Product Team

A tidy unfinished home basement with a concrete floor and small window

Basements usually have the highest radon levels in a house. Radon rises from uranium in soil and rock, and the basement is closest to the soil — with foundation cracks, floor joints, sumps and utility penetrations giving gas an easy route in.

Why the basement reads higher

  • Proximity to the source. Soil gas enters at or below the foundation.
  • Air pressure. A house is usually slightly depressurised relative to the soil, so gas is drawn in at the lowest point.
  • Less air movement. Unfinished basements often have less ventilation than living areas.

Stack effect can still move radon upstairs, especially in winter when warm air rises through the house. A low basement reading does not guarantee low levels everywhere.

How to test a basement

Place the detector on the lowest lived-in level, about 20 inches or more above the floor, away from drafts, sumps and appliances. If you use the basement as a bedroom, office or family room, that is where the reading matters most.

If the basement is only storage and you never spend time there, test the lowest floor you do use — where to put a radon detector covers this in detail.

What to do with the result

The EPA recommends fixing a home at 4 pCi/L or higher, and considering it between 2 and 4 pCi/L. Sealing cracks helps but is not a fix on its own; see how to reduce radon.

Basement levels change with weather, season and how you heat and ventilate. A continuous monitor shows the trend; a single test shows one moment. Read why radon levels change for the drivers.

Frequently asked questions

Should I test the basement or the living room?
Test the lowest level you actually use. If that is the basement, test there; if the basement is only storage, test the ground floor instead.
My basement reading is high but upstairs is low. Is that normal?
Yes. Radon usually falls off with height and with more air movement, though stack effect can push it upward in some homes.
Do I need to fix my basement if nobody sleeps there?
The EPA’s action level applies to the home, not one room, because soil gas moves and household air mixes. If levels are high, mitigation is recommended regardless of how you use the basement.

Sources

  1. A Citizen's Guide to Radon — U.S. EPA
  2. Consumer's Guide to Radon Reduction — U.S. EPA