Radon and the Sealed Crawl Space: A Marion County Guide
Radon is a gas from the ground that you cannot see or smell. In a house over a crawl space, the dirt floor is where it starts, which makes sealing that floor a good moment to deal with it.
Marion County is in EPA Radon Zone 1, where the predicted average indoor screening level is above 4 pCi/L. The EPA calls a sealed plastic sheet over the earth floor, with a fan drawing from under it, the most effective fix for crawl space homes. Indiana requires radon testers and mitigators to be certified by the Indiana Department of Health.
What Zone 1 tells you, and what it does not
The EPA drew its zone map in 1993 from indoor measurements, geology and soil data, county by county. It says the map should not be used to decide whether a single home needs testing: testing is advised for all homes, in every zone. Fix at 4 pCi/L or above, and consider fixing between 2 and 4. A neighbor's low result says little about your house.
Does sealing a crawl space lower radon?
Not reliably on its own. The EPA's Consumer's Guide to Radon Reduction says it does not recommend sealing alone, because sealing has not been shown to lower radon significantly or consistently. What works is turning the liner into a membrane: a high-density plastic sheet over the earth floor, a vent pipe, and a fan that pulls radon from under the sheet and sends it outdoors. The EPA calls this submembrane suction and rates it the most effective method for crawl space homes. Pulling air from the whole crawl space with a fan is listed as less favorable, partly because it can draw conditioned air out of the house.
Who can do the radon part in Indiana?
Under the Indiana Radon Rule (410 IAC 5.1), anyone testing for, analyzing or mitigating radon in Indiana homes must be certified by the Indiana Department of Health. The department points homeowners to a state license search to check a company. A crawl space installer may or may not hold that certification, so ask. If they do not, the liner can still be laid so a certified mitigator can add the pipe and fan later.
A sensible order of work
- Test the lowest lived-in floor before any work.
- Fix water and rotted wood (see crawl space repair).
- Lay the liner with sealed seams and edges (see vapor barriers and liners).
- If the result was high, add submembrane suction.
- Retest no sooner than 24 hours after the fan starts and within 30 days, then every two years, as the EPA advises.
The EPA suggests that follow-up test be independent of the installer. For how radon work sits next to the rest of the bill, see the encapsulation cost page.
Describe the crawl space in a sentence or two
Pick the closest match, then add what you noticed: a smell upstairs, water on the plastic after a storm, a floor that gives underfoot. A crawl space specialist who works in Indianapolis phones you to talk it through.