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How does radon get into a house?

By Radon411 Editorial TeamUpdated How we research

Short answer

Radon gets into a house from the soil beneath it, through openings in contact with the ground: cracks in floors and walls, construction joints, gaps around pipes, sump pits and crawlspaces. The house pulls it in, because the air pressure inside is usually lower than in the soil around the foundation.

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

Because radon enters through the ground rather than through anything specific to a home's age or type, it can be high in a new home or an old one, a well-sealed or drafty one, and a home with a basement or without.

Radon in well water is a second, usually much smaller route: it is released into the air when the water is used for showering and other household uses.

What to do next: test the lowest lived-in level; if the result is high, a soil suction system blocks the main route by drawing radon from under the house before it can enter.

The ways radon gets in

Radon moves up through the ground into the air above it, and a house sitting on that ground lets it in through openings between the soil and the home. Radon finds the cracks, joints and gaps, and once inside, the house traps it.

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 main ways soil gas enters a house, from cracks in solid floors to the water supply.
Entry routes, and where they are found
RouteWhere it is
Cracks in solid floors and wallsBasement floors and walls and slab-on-grade floors
Construction joints and floor-wall jointsWhere the slab meets the foundation wall, and other joints in the concrete
Gaps around service pipes and cablesWhere pipes and cables pass through the floor or walls
Cavities inside walls and pores in hollow-block wallsBlock foundations and cavity walls
Gaps in suspended floorsRaised floors over open ground or a crawl space
Sump holes and drainsAn open sump pit, sumps and drains
The water supplyPrivate wells and other ground-water supplies

What pulls radon in

Openings alone are not the whole story; something has to draw the gas through them. Air pressure inside a home is usually lower than in the soil around its foundation, so the house acts like a vacuum, pulling soil gas in through foundation cracks and other openings. Homes commonly run at slightly negative pressure compared with outdoor air, and some of the replacement air comes from the soil.

What lowers the pressure inside a house
EffectHow it works
Stack effectWarm air rises and escapes from the upper part of the house; replacement air enters low down, some of it from the soil
WindStrong winds blowing over the house push and pull air into and out of it
Appliances and exhaust fansWater heaters, fireplaces, clothes dryers, older furnaces and exhaust fans remove air from the home, which can drive soil gas in

How much of this reaches the rooms depends on the routes available, the ground under the house and how fast indoor air is exchanged with outdoor air, which in turn depends on construction, ventilation habits and air-tightness. That is why levels vary from day to day and from hour to hour. The wider causes are covered in what causes radon in a house.

How the foundation shapes the way in

Homes are generally grouped by foundation: a basement; a slab-on-grade, concrete poured at ground level; or a crawlspace, a shallow unfinished space under the first floor. Many homes combine them, with a basement under part of the house and a slab or crawlspace under the rest. Every type can let radon in.

Radon entry by foundation type
FoundationHow radon gets inUsual way to stop it
BasementCracks, floor-wall joints, block walls, pipe gaps and an open sump; levels are usually highest down hereSubslab, sump-hole, drain-tile or block-wall suction
Slab-on-gradeCracks and joints in the slab and gaps around pipes through itSubslab suction
CrawlspaceThe crawl space is directly connected to the soil, creating a pathway into the homeA plastic sheet over the soil with a pipe and fan drawing from under it

A house without a basement is not protected: radon problems are found in homes with and without basements. See can a house have radon without a basement and can radon come through concrete.

Water and building materials: the minor routes

Soil gas is the main way in. Radon can also arrive in well water and be released into the air when water is used for showering and other household uses, but radon entering through the soil is usually a much larger risk than radon entering through water. Building materials such as granite and some concrete products can give off radon too, though they rarely cause radon problems by themselves.

How large each route usually is
RouteRelative importance
Soil gas through the foundationThe principal source of elevated radon in U.S. homes
Well water or a ground-water supplyUsually a much smaller risk than soil gas
Building materialsRarely a problem by themselves

How entry is found and stopped

A contractor finds the routes by watching air move. A small amount of chemical smoke puffed into holes, drains, sumps or along cracks shows where air is flowing, and those flows mark possible radon routes. A soil communication test, a vacuum in one small hole and smoke in another, shows how easily air moves under the slab.

Cross-section of a house with a sub-slab depressurization system: soil gas drawn from beneath the slab into a suction point, up a vent pipe, through a fan outside the living space and out above the roofline.
Read this figure as text
Sub-slab depressurization system in a house cross-sectionGround levelLiving spaceBasement12345
  1. Soil gas drawn from beneath the slab
  2. Suction point through the slab
  3. Vent pipe
  4. Fan, outside the living space
  5. Discharge above the roofline
A sub-slab depressurization system: the fan creates a vacuum under the slab and vents the soil gas above the roof instead of letting it into the house.

Sealing alone cannot close every route: the places radon enters are hard to find and seal permanently, and normal settling opens new ones. Soil suction works instead by drawing radon from below the home and venting it above the roof, with sealing making it more effective. New homes can be built to resist entry from the start, with a gas-permeable layer under the slab, plastic sheeting, sealed below-grade openings and a vent pipe to the roof. See how to reduce radon.

Questions about how radon gets in

How does radon enter a home?

As soil gas, through cracks, joints and gaps where the house meets the ground, drawn in because the air pressure inside is usually lower than in the soil.

Can radon come in through a sump pump?

Yes. An open sump hole is a direct opening to the soil. A capped sump can still drain water and serve as a suction point for a radon system.

Does radon come through walls?

Through cracks in walls, small pores in hollow-block walls and cavities inside walls, especially foundation walls below ground.

Can radon come up through floor drains?

Drains connected to the soil are one of the routes radon uses to enter buildings.

Can radon reach the upper floors?

Yes. Levels are usually highest on floors in contact with the ground, but considerable radon can also be found above the ground floor.

Can a well bring radon into a house?

Yes. Radon in well water is released into the air when water is used. See does radon come from well water.

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