How does radon enter a home?

How Pressure Draws Radon Into a Home

The intuition most people start with is that gas seeps in the way water finds a low spot. What actually happens is closer to the opposite: the house draws it. Warm indoor air rises and leaks out through recessed lights, attic hatches, chimney chases and bath fans, and the air that replaces it has to come from somewhere. A meaningful share arrives through the foundation, carrying whatever is in the soil with it.

Anything that exhausts air makes this stronger. Clothes dryers, range hoods, bathroom fans and combustion appliances all push the lower level a little further below the pressure of the ground outside. This is why the same house can read differently in January and July, and why a test run with windows open is not a test.

The Most Common Radon Entry Points

An open or loosely covered sump pit is usually the largest single path, because it is a direct connection to the drainage material under the slab. After that: the joint where the slab meets the foundation wall, which exists in nearly every basement and is rarely sealed; shrinkage cracks in the slab; and the annular gaps around plumbing and utility penetrations, which were sized for the pipe rather than sealed to it.

Block walls deserve their own mention. The hollow cores form a continuous cavity connected to the footing, and gas entering at the bottom can emerge anywhere along the wall, including through the block face itself. A block wall does not need a visible crack to be a path.

Why sealing on its own usually disappoints

Sealing is worth doing and rarely sufficient. The reason is that the pressure difference driving the entry is unchanged, so closing some openings simply redirects the flow to whatever remains, including paths through porous concrete that no caulk addresses. Homeowners who seal thoroughly and retest often find the number barely moved.

An active system works on the cause rather than the symptoms. A fan holds the space beneath the slab at lower pressure than the basement, which reverses the direction of flow at every opening at once. Sealing then becomes useful for a different reason: it stops the fan from wasting its capacity pulling conditioned indoor air down through the cracks instead of soil gas from further out.

Still deciding what to do about a reading? The testing page covers how a measurement is taken, and what a system costs breaks the ranges down by method. If you got here from a map or a ZIP-code lookup, what Ohio's radon maps show explains what those averages are actually measuring.