Basics · 8 min read
How Radon Enters a Home
Radon enters through cracks, joints, and penetrations because homes are slightly negative in pressure relative to the soil. Here are the pathways and the physics.
Radon does not force its way into a house. It is drawn in, because the air pressure inside a building is usually slightly lower than the pressure in the soil beneath it.
That pressure difference is small, often less than the weight of a sheet of paper spread across a square foot, but it operates continuously across the entire foundation. Over a year it moves a great deal of soil gas.
The stack effect
Warm indoor air rises and escapes through upper-level leaks, attic hatches, recessed lights, and gaps around plumbing vents. Air that leaves must be replaced, and the replacement enters low in the building.
In a home over soil, some of that makeup air comes through the foundation. The taller the building and the greater the indoor-outdoor temperature difference, the stronger the effect.
Mechanical depressurization
Bathroom fans, range hoods, clothes dryers, and central vacuums all push air out of the house. Unless an equal volume is deliberately supplied, the interior goes slightly negative.
In tightly built homes this effect can dominate. A powerful kitchen exhaust hood running during dinner can measurably increase soil gas entry for as long as it operates.
Common entry points
Soil gas enters wherever the foundation is not airtight, which is everywhere to some degree.
- Shrinkage and settlement cracks in the slab
- Control joints and perimeter expansion joints
- Plumbing penetrations at toilets, tubs, and utility lines
- Open sump pits and floor drains
- The joint where the slab meets the foundation wall
- Exposed soil in crawl spaces and under torn vapor barriers
- Hollow block walls and utility chases that act as vertical conduits
- Porous concrete itself, through diffusion
Why sealing alone is not enough
Sealing visible openings helps, and it is part of every good mitigation job. It is not a solution on its own because concrete is porous and develops hairline cracks across its full area as it cures and settles.
The durable fix reverses the pressure relationship instead of trying to plug every hole. Once the space beneath the slab is held at lower pressure than the room above, air flows downward through those cracks rather than gas flowing upward.
What changes entry rates
Weather matters. Rain saturates surface soil and caps the ground, directing gas laterally toward the relatively dry soil beneath a building. Wind creates pressure differences across the building envelope. Barometric pressure drops draw gas out of the soil.
Building changes matter too. New windows, added insulation, and duct sealing all reduce air exchange, which raises indoor concentration from an unchanged soil source.
Key takeaways
- Homes pull soil gas in because they run slightly negative relative to the ground.
- Every foundation has entry points, including porous concrete itself.
- Sealing supports mitigation but never replaces depressurization.
- Weather and building tightness both change measured levels.