Why a house pulls soil gas in
Warm air rises. In a heated house it escapes through the upper floors, the attic hatch, recessed lights and every small gap near the roof. Air leaving at the top has to be replaced, and the replacement is drawn in lower down. The result is that the bottom of your house sits at very slightly lower pressure than the ground underneath it.
That pressure difference is tiny, a few pascals, and it is enough. Soil gas moves toward low pressure, so the house becomes the path of least resistance for radon that would otherwise have drifted into open air. Engineers call this the stack effect, and it is the same principle that makes a chimney draw.
The York County record shows the effect directly. February averages 13.19 pCi/L and July 6.87, a difference of 92 percent between the same houses in the same county. Nothing about the ground changed between those months. What changed is how hard the buildings were pulling.
Several ordinary things make the pull stronger: a fire in the fireplace, an extractor fan or dryer venting outdoors, a forced air system with leaky return ducts in the basement, and cold weather generally. Each of them removes air from the house and something has to replace it.
The routes radon uses
Given a pressure difference, radon needs openings, and every slab has them. These are the ways in, roughly in order of how much they usually contribute.
| Route | Why it matters |
|---|---|
| Slab to wall joint | The continuous gap where the floor slab meets the foundation wall, usually the single biggest opening in the house |
| Sump pit | An open hole straight into the gravel and soil beneath the slab, often uncovered |
| Cracks in the slab | Shrinkage and settlement cracks, which need be no wider than a hair to pass gas |
| Pipe and service penetrations | Gaps around drains, water lines and conduit where they pass through the floor |
| Exposed soil in a crawl space | No barrier at all, so the whole floor area is an opening |
| Hollow block walls | The cavities connect to the footing and act as a chimney inside the wall itself |
| Floor drains and untrapped openings | Direct connections below the slab, particularly when a trap has dried out |
None of these is a defect. A slab to wall joint exists because concrete has to be free to move, and a sump pit exists because the house needs drainage. The openings are doing their jobs; radon just uses them.
Why sealing alone rarely works
The obvious response is to close the openings, and it is the one most people try first. Caulking cracks, sealing the sump lid and laying plastic over a crawl space floor all help, and on their own they usually disappoint.
The reason is that sealing does not remove the pressure difference that is driving the gas. It only narrows the routes. A house still pulling on the soil will find whatever opening remains, and the openings you cannot see are generally more numerous than the ones you can. In the county mitigation record, sealing on its own accounts for 20 of 11,470 jobs, which tells you what the trade thinks of it as a standalone fix.
What works is reversing the pressure rather than fighting it. A mitigation system draws continuously on the gravel beneath the slab so that the space under your floor is at lower pressure than the room above. Soil gas then leaves through the pipe because that is now the easy route. Sealing still matters in that arrangement, but as a supporting measure that lets the fan work efficiently rather than as the fix itself. The page on how a radon mitigation system works covers the parts in detail.
What actually helps before you mitigate
- Cover the sump pit. A sealed lid with a gasket is cheap and closes the single largest deliberate hole in the floor. It also makes a future mitigation system work better.
- Lay a barrier over crawl space soil. Sheet polyethylene over exposed earth, lapped and sealed at the edges, reduces entry across the whole area.
- Stop making the pull worse. Combustion appliances drawing their air from inside the basement, and dryers or extractors running without makeup air, all deepen the vacuum.
- Do not rely on ventilation. Opening basement windows does lower the reading while they are open, and it is not a fix. It is also impractical for most of the York County year, when the stack effect is strongest.
All of these reduce entry. None of them substitutes for a measurement, and the state record shows that once a system goes in, levels fall by around 92 percent. If your test result sits at or above 4 pCi/L, the useful next question is what mitigation costs in York County.
Common questions about radon entry
How does radon get into a house?
A heated house loses warm air at the top, which leaves slightly lower pressure at the bottom and draws soil gas in through openings in the floor. The main routes are the slab to wall joint, the sump pit and cracks in the slab.
Why is radon worse in winter?
Cold weather deepens the stack effect and windows stay shut. York County results average 13.19 pCi/L in February against 6.87 in July, a 92 percent difference in the same houses.
Can I fix radon by sealing cracks?
Sealing helps but rarely solves it, because the pressure difference driving the gas remains and finds the openings you cannot see. In the York County record, sealing alone accounts for 20 of 11,470 mitigation jobs.
Does opening windows reduce radon?
While they are open, yes. It is not a fix, and it is impractical through a Pennsylvania winter, which is exactly when the stack effect pulls hardest and levels run highest.
Does a crawl space make radon worse?
Exposed soil in a crawl space is an opening across the whole floor area rather than a crack, so it can contribute heavily. A sealed membrane over the earth is the usual first step.
Do gas appliances affect radon levels?
Anything that removes air from the house deepens the vacuum that pulls soil gas in. Combustion appliances taking their air from the basement, extractor fans and clothes dryers all make the pull stronger.