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Where is radon found in homes?

By Radon411 Editorial TeamUpdated How we research

Short answer

Radon is usually highest in basements, cellars and other living spaces in contact with the ground, where it enters from the soil through cracks and openings in the foundation. Upper floors can have considerable radon too, so test the lowest lived-in level of any home below the third floor.

Next step: a test is the only way to know your home's level. How to test your home

Radon moves up through the ground and into a home through cracks and other holes in the foundation: cracks in floors, floor-wall junctions, gaps around pipes or cables, hollow-block walls, sumps and drains. Lower indoor air pressure draws it in, and the home traps it, where it can build up.

Smaller sources add to it. In homes on wells, radon in water is released into the air when the water is used, though radon from the soil is usually a much larger risk, and building materials rarely cause radon problems by themselves.

Levels vary between neighboring homes and within one home from day to day and hour to hour, so a test of your own lowest lived-in level is the only way to know: the basement if it is frequently used, otherwise the first floor.

What to do next: test the lowest lived-in level with a of at least 48 contiguous hours under , or a of more than 90 days, and read the result with the interpreter.

Lowest floors first

Radon levels are usually higher in basements, cellars and living spaces in contact with the ground, and are often higher in basements and lower levels. That is where radon arrives: it moves up through the ground to the air above and into a home through cracks and other holes in the foundation.

Cross-section drawing of a house with a basement and a crawl space, numbered at the seven ways EPA lists for soil gas to get in: cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, the water supply.
Read this figure as text
House in cross-section showing where soil gas entersGround levelSoil and rockBasementSolid floorCrawl spaceSuspended floor1234567
  1. Cracks in solid floors
  2. Construction joints
  3. Cracks in walls
  4. Gaps in suspended floors
  5. Gaps around service pipes
  6. Cavities inside walls
  7. The water supply
The routes soil gas takes into a house, all at or near the lowest level.

But considerable radon concentration can also be found above the ground floor, so an upper floor is not automatically low.

Where radon gets in

Air pressure inside a home is usually lower than in the soil around its foundation, so the house acts like a vacuum, drawing radon in through foundation cracks and other openings. That is why radon can build up in any home, with or without a basement, sealed or drafty, new or old.

Common radon entry points
Entry pointWhere you find it
Cracks in floorsBasement and slab floors
Floor-wall junctionsWhere the floor meets the foundation wall
Gaps around pipes or cablesService entries through the floor or wall
Pores in hollow-block walls and cavity wallsBlock foundations
Sumps and drainsBasements

Water and building materials

Two smaller sources add to soil gas. Radon dissolved in water is released into indoor air when the water is used, mostly during showering and other household use; it matters in homes on wells, but radon from the soil is usually a much larger risk. Some radon also comes from building materials, though materials rarely cause radon problems by themselves.

Where a home's radon comes from
SourceHow much it usually matters
Soil under and around the houseUsually the much larger risk
Well waterReleased into the air when water is used; smaller than soil
Building materialsRarely cause radon problems by themselves

See does radon come from well water.

Why levels differ from room to room and house to house

Radon concentrations vary considerably between neighboring buildings, and within one building from day to day and hour to hour. How much indoor air trades with outdoor air depends on the construction, the occupants' ventilation habits and how airtight the building is.

Where to test

Test the lowest lived-in level: the basement if it is frequently used, otherwise the first floor. If you later start using a lower level, such as a basement, retest on that level. Testing is recommended for all homes below the third floor. Put the device in a room that is used regularly, like a living room, playroom, den or bedroom, but not the kitchen or bathroom.

Where to test, by how you use the home
Your homeTest here
Basement used regularlyThe basement
Basement not used, or no basementThe first floor
Apartment below the third floorYour unit
A lower floor you have just started usingRetest on that floor

See where a radon test should be placed.

Questions about where radon is found in homes

Where is radon most likely found in a house?

In basements, cellars and living spaces in contact with the ground, where it enters from the soil.

Can radon be on the second floor?

Yes. Considerable radon concentration can be found above the ground floor.

Is radon higher in the basement or upstairs?

Usually in the basement and lower levels, the parts of the home in contact with the ground, though considerable radon can be found above the ground floor.

Does a house without a basement have radon?

It can. Radon can build up in any home, with or without a basement. See do I need a radon test without a basement.

Can radon come from my water?

In homes on wells, radon in water is released into the air when the water is used, but radon from the soil is usually a much larger risk.

Do granite or concrete give off radon?

Building materials give off some radon, but they rarely cause radon problems by themselves. See do granite countertops give off radon.

Does every room need a radon test?

No. One test in the lowest lived-in level is the starting point: the basement if it is frequently used, otherwise the first floor.

Why is my neighbor's radon different from mine?

Radon concentrations vary considerably between neighboring buildings.

Learn

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