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How does humidity affect radon testing?

By Radon411 Editorial TeamUpdated How we research

Short answer

Humidity is a placement rule for radon tests: keep the device away from drafts, high heat, high humidity and exterior walls, and do not test in a bathroom, kitchen, laundry room or other enclosed humid area. Use a regularly used room on the lowest lived-in level instead.

Next step: pick the right test for your home. Which test to use

The test belongs on the lowest lived-in level, the basement if it is frequently used, otherwise the first floor, in a room used regularly, like a living room, playroom, den or bedroom, at least 20 inches above the floor.

What changes a result is weather and ventilation. Do not run a of just 2 or 3 days during unusually severe storms or high winds, and keep from at least 12 hours before a test of 2 to 4 days and through the whole test. Every radon test should run at least 48 hours.

If a spot is damp, choose another regularly used room on the same level, away from kitchens, bathrooms and laundry rooms. Improper testing may yield inaccurate results and require another test.

What to do next: read your device's placement instructions before you start, and if a test ran somewhere damp, repeat it in a better spot.

Humidity is a placement rule

Humidity enters radon testing as a rule about where the device sits. Place the kit away from drafts, high heat, high humidity and exterior walls, and do not test in an enclosed area of high humidity or high air velocity, such as a kitchen, bathroom, laundry room or furnace room.

Damp and drafty spots to avoid, and better choices
AvoidWhyChoose instead
BathroomEnclosed area of high humidityA bedroom or den on the same level
KitchenEnclosed area of high humidity or air velocityA living room or playroom
Laundry or furnace roomEnclosed area of high humidity or high air velocityA regularly used room nearby
Crawl spaceOn the list of places not to testThe living space above it
Next to an exterior wall or a draftPlacement rule for every testThe interior of the room, at least 20 inches up

Where to put the test instead

Put the device on the lowest lived-in level, the basement if it is frequently used, otherwise the first floor, in a room used regularly, like a living room, playroom, den or bedroom, but not a kitchen or bathroom. Set it at least 20 inches above the floor where it won't be disturbed. In a multi-unit building, place the device in the lowest level within your unit.

The conditions that do change a result

The conditions tied to the result itself are weather and ventilation. Do not run a of just 2 or 3 days during unusually severe storms or periods of unusually high winds. Keep windows and outside doors closed as much as possible, and for a test of 2 to 4 days, keep from at least 12 hours before the test and through the whole test.

During a short-term test
AllowedNot allowed
Heating and air-conditioning that recirculate airFans or machines that bring in outside air
Normal entry and exit through doorsWindows or outside doors left open
Running the home's heating and cooling normallyMoving or disturbing the device

Every radon test should run for a minimum of 48 hours. See what can affect a radon test.

What your kit's instructions add

Follow the instructions that come with your test kit carefully, and leave the test in place for the instructed amount of time; the instructions may add their own placement limits. Passive devices also differ: some have features that offer more resistance to test interference or disturbance than others.

Long-term tests and seasons

Because radon levels tend to vary from day to day and season to season, a short-term test is less likely than a to show your year-round average. A long-term test stays in the home for more than 90 days, so it spans whatever damp or dry spells come along.

TestHow long it runsWhat it is for
Short-term testStays in the home from 2 to 90 days, depending on the device, and for a minimum of 48 hours.A faster screening result. Because radon levels vary from day to day and season to season, it is less likely than a long-term test to tell you the year-round average.
Long-term testStays in the home for more than 90 days.It is more likely than a short-term test to tell you the home's year-round average radon level.
Short-term and long-term radon tests: how long each stays in the home.

Questions about humidity and radon tests

Does humidity affect radon test results?

The rule is about placement: keep the device away from high humidity, and do not test in enclosed humid rooms such as bathrooms, kitchens and laundry rooms.

Can I put a radon test in the bathroom?

No. A bathroom is an enclosed area of high humidity. Use a regularly used room on the same level.

Should I run a dehumidifier during a radon test?

The rules allow heating and cooling to run normally and recirculating fans; they bar fans or machines that bring in outside air. Follow your kit's instructions too.

Can I test in a damp basement?

If the basement is used regularly, test there, in a drier room away from moisture sources, at least 20 inches above the floor.

Where should a radon test go in an apartment?

In the lowest level within your unit, in a regularly used room away from humid areas such as the bathroom and kitchen.

Does rain affect a radon test?

Do not run a short-term test of just 2 or 3 days during unusually severe storms or periods of unusually high winds.

What if my test sat somewhere humid?

Improper testing may yield inaccurate results and require another test, so repeat it in a better spot.

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