Most of the radon information online assumes your house sits on a basement slab. A large share of homes in Knox County do not. From the ranches in Fountain City to the newer builds in Farragut and Hardin Valley, crawl spaces are the norm across East Tennessee, and they change both how radon gets into the house and how it is kept out. Here is what is different, what the fix looks like, and what it costs.
Radon rises out of the soil across the entire footprint of the crawl space. With a slab, the concrete slows it down and the gap under the slab can be depressurized with a single suction point. With a crawl space, the gas enters the crawl freely, then moves up into the living space through the subfloor, plumbing and duct penetrations, and every gap around the rim joist. A vented crawl space dilutes it somewhat, but not reliably, and in winter when the house is warm and the crawl is cold, the stack effect pulls air upward out of the crawl and into the rooms above. That is when readings peak.
The standard mitigation for a crawl space home is called sub-membrane depressurization. A heavy plastic membrane, usually 6 to 10 mil, is laid across the entire crawl space floor and sealed to the foundation walls and around every pier. A perforated pipe or a suction point goes under the membrane, connects to a fan mounted outside the living space, and vents above the roofline. The fan creates a slight vacuum under the plastic, so the radon is pulled out and exhausted before it can rise into the house. Properly installed, it routinely brings a home from 8 or 10 pCi/L to under 2. Our mitigation systems page covers the equipment.
Some homes are a mix, with a partial basement and a crawl space next to it. Those need a system that handles both, sometimes with one fan and two suction points. It is a common configuration in Knoxville and nothing unusual for an experienced installer.
A crawl space system costs a bit more than a slab system because of the labor of installing and sealing the membrane. In Knoxville a typical sub-membrane system runs $1,500 to $2,200, with the higher end for large or hard to access crawl spaces and the combination basement and crawl layouts. If your crawl space is already encapsulated, the existing liner can sometimes be used, which brings the price down. See the mitigation cost guide for what drives the number.
A sealed membrane with a fan drawing air from under it also pulls moisture out of the crawl space. Many homeowners find the musty smell fades, the crawl stays drier, and the hardwood floors above stop cupping in summer. It is not a substitute for dedicated moisture control on a truly wet crawl, but it helps, and it is one reason encapsulation contractors and radon installers often end up on the same job.
The test goes on the lowest level of the house that people live in, not in the crawl space itself. A monitor placed in the crawl will read very high and tell you nothing about what you breathe. Put it in a first floor room, away from exterior doors, for a 48 hour test with the house closed up. If you are under contract, our real estate testing is built around inspection deadlines, and Tennessee's disclosure rules are explained on our disclosure guide.
Call (865) 563-8112 and tell us the square footage, whether the crawl is vented or encapsulated, and what your test showed. We will quote a system with a written guarantee over the phone.
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