Where to Place a Radon Test

You ordered a kit. You opened the canister. Now where do you put it?

This isn't a trick question — but it does have a right answer, and a wrong placement can make a 4.5 pCi/L home look like a 2.8 pCi/L home (or vice versa). The EPA and AARST have specific placement requirements; here's the plain-language version.

The rule is: test in the lowest part of your home that you actually use. EPA defines this as the lowest livable level.^[1]

• Finished basement that you use (TV room, office, gym, bedroom) — test here.
• Unfinished basement that you walk through to do laundry — test here.
• Unfinished crawlspace you don't enter — don't test here. Test the floor above it.
• Slab-on-grade home (no basement) — test the ground floor.
• Walk-out basement — test the basement, even if it has a daylight door.

The idea is to measure radon where you actually breathe. A test in a sealed-off attic doesn't tell you much.

Within the lowest livable level, EPA placement guidance is specific:^[1]

• 2 to 6 feet above the floor. A shelf or table works. Don't put it on the floor; don't tape it to the ceiling.
• At least 1 foot from exterior walls. Walls have small drafts.
• At least 4 inches from any other surface (table edge, book, lamp).
• Away from drafts — supply or return vents, ceiling fans, exterior doors, fireplaces.
• Away from direct sunlight — heating affects readings.
• Away from high humidity — bathrooms, kitchens, laundry rooms.
• Not near electronic equipment that gives off heat (TV, computer, modem).

A reading taped to the wall right above a heating vent will be different from a reading on a shelf in the middle of the room. That's not measurement error — it's measurement bias.

For short-term tests (2–7 days), the home should be under closed-house conditions for 12 hours before the test starts and the entire duration of the test.^[2] Closed-house means:

• Windows and exterior doors closed except for normal in-and-out traffic.
• Whole-house fans not running.
• Window-mounted HVAC units running normally; window fans not running.
• Internal doors can be open or closed as usual — don't change the rest of your routine.

The closed-house requirement doesn't apply to long-term tests (90+ days). Those average across normal living conditions over enough time that day-to-day variations cancel out.

Most homeowners get the basic placement right but trip on one of these:

• Placing the test on the floor. The reading will be slightly higher than at breathing height. Use a shelf, table, or chair (2–6 feet up).
• Placing in the laundry room. The humidity from the dryer and washer affects the reading. Use a different room.
• Placing on an exterior windowsill. Drafts and proximity to outside air bias the reading.
• Putting a short-term test in the master bedroom on the second floor. Test the lowest livable level, not the most-occupied room.
• Running the test while windows are open. Closed-house conditions matter for short-term tests.
• Moving the test mid-test. Don't.
• Testing in a crawlspace. Test the floor above it — that's where you breathe.

Slab-on-grade homes

No basement, no crawlspace. Test the ground floor — but place the test in a room that gets daily use (living room, home office, bedroom) rather than a transient space like a foyer or laundry room.

Walk-out basements

The basement is still the lowest livable level even if it has a daylight door. Test the basement, with the daylight door closed for the closed-house duration.

Split-level homes

The lowest level used daily — typically a family room or office on the half-basement level — gets the test. If both half-basements are used (rare), test the lower one.

Tri-level homes

The lowest occupied level. Tri-levels often have a basement and a half-basement; test the basement.

Apartments / multi-family

The unit you actually live in. Ground-floor and basement apartments are higher priority than upper-floor units. EPA guidance for multi-family testing is largely the same as single-family within your own unit.

Multiple test locations

If your home has multiple foundation zones (basement + crawlspace under different parts), professional measurements may test each zone separately. For a homeowner kit, test the lowest livable level where you spend the most time.