Radon can enter your home via a number of entry points or pathways:
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- Cracks in concrete slabs.
- Spaces behind brick veneer walls that rest on uncapped hollow-block foundations.
- Pores and cracks in concrete blocks.
- Floor-wall joints.
- Exposed soil, as in a sump or crawl space.
- Weeping (drain) tile, if drained to an open sump.
- Mortar joints.
- Loose fitting pipe penetrations.
- Open tops of block walls.
- Building materials: brick, concrete, rock.
- Well water.
In general, whenever air enters a home from the underlying soil, some radon will likely come with it. Any home can have a radon problem, no matter what type of foundation it has.
If you have a basement: A basement provides a large surface area in contact with soil material. Radon can enter through cracks in the concrete, or through floor-to-wall joints or control joints. But radon can enter a home regardless of whether or not there is a basement.
Slab-On-Grade: Slabs built on grade can have many openings that allow radon to enter, just as in a basement.
Crawl Space: Homes with crawl spaces can also have elevated radon levels. The vacuum effect can draw radon gas from a crawl space into the home.
Manufactured Homes: Unless these buildings are placed on supports without skirting around them, interior air pressure vacuums can cause radon to enter manufactured homes, as well.
Radon and Your Home’s Air Pressure
Other factors also contribute to air pressure changes in a home, including:
- Stack Effect: As warm air rises to the upper portions of a home, it is displaced by cooler, denser outside air. Some of that displaced air comes from the soil.
- Down Wind Draft Effect: Strong winds can create a vacuum as they flow over the top and around the home.
- Vacuum Effect: Combustion appliances like furnaces, hot water heaters and fireplaces, as well as exhaust fans and vents, can remove a considerable amount of air from a home. When air is exhausted, outside air enters the home to replace it. Some of this replacement air comes from the underlying soil.
What Happens After Radon Enters a Home?
Radon levels are often highest at the entry point – typically in the lower part of a building. As radon gas moves upward, diffusion, natural air movements and mechanical equipment (such as a forced-air ventilation system) distribute the radon through the home. Radon gas becomes more diluted in the upper levels of the home because there is more fresh air for it to mix with.
Greater dilution and less house vacuum effect occur when the house is more open to the outdoors, as during the non-heating season. This generally results in lower indoor radon levels in the summer compared to the winter.
Frequently Asked Questions
Radon enters wherever the home meets the soil. Common paths include cracks in the foundation floor and walls, gaps around pipes and utility lines, construction joints, sump pits, crawl space floors, and openings in block walls. Pressure differences pull the gas through these points, and it then collects in the living space above.
The source is the ground beneath the home, where uranium in soil and rock naturally breaks down and releases the gas. It travels up through the soil and finds entry points in the foundation. In some homes, well water can also carry radon that is released into the air during showering and other use.
Yes. A slab is not a sealed barrier. Hairline cracks, the perimeter joint where the slab meets the wall, and openings around plumbing all allow gas to pass. Even a sound looking slab can let radon through, which is why slab on grade homes test just as high as homes with basements.
Radon enters from below, through the foundation, not through windows and doors at normal levels. Opening windows can temporarily dilute indoor concentrations, but it does not stop the gas from entering and is not a lasting solution. A mitigation system addresses the entry points at the source.
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