Sealing cracks on its own rarely fixes a radon problem. EPA does not recommend sealing alone, because it has not been shown to lower radon levels significantly or consistently. Radon entry points are hard to find and seal for good, and normal settling of a house opens new cracks and reopens old ones (EPA, Consumer’s Guide to Radon Reduction). Kansas State University’s EPA-funded radon program puts it the same way: caulking and sealing alone has proven not to be a reliable or durable technique (sosradon.org).
Sealing still matters. It is a basic part of almost every radon mitigation system.
Why sealing helps a mitigation system
A radon mitigation system uses a fan to create suction under the floor. Cracks and gaps let house air leak into that suction instead of soil gas, so the system works harder and pulls heated or cooled air out of the home. Sealing them (EPA, Consumer’s Guide):
- limits the flow of radon into the home;
- makes a soil suction system more effective and cheaper to run;
- reduces the loss of conditioned air.
That is why installers seal cracks as part of the job (MDH’s 10-step process lists it as a separate step).
Where radon gets in
The usual entry points are cracks in solid floors and walls, construction joints, gaps around service pipes, gaps in suspended floors, cavities inside walls, and the water supply (EPA, A Citizen’s Guide to Radon).
What to seal, and with what
EPA’s Radon Mitigation Standards spell out how sealing should be done during mitigation (EPA, Radon Mitigation Standards):
| Opening | What the standard calls for |
|---|---|
| Cracks in slabs, small gaps around pipes | Caulks and sealants designed for the job. Urethane sealants are recommended for durability |
| Gaps wider than ½ inch | Push in a foam backer rod first, then apply sealant over it |
| Large holes below ground (plumbing rough-ins) | Non-shrink mortar, grout or expanding foam |
| Perimeter channel (“French”) drains | Backer rod and urethane sealant, keeping the channel working as a water drain |
| Sump pits | A rigid, airtight cover. See radon sump pump covers |
| Tops of hollow block walls | Closed and sealed |
| Openings to a crawlspace | Closed and sealed if only one area is being mitigated |
The standards also say that openings an installer can’t reach or seal must be disclosed to the homeowner and included in the job documentation.
If a sealant contains volatile solvents, the work area should be ventilated during and after use, as the product’s manufacturer recommends (EPA standards).
About “radon sealer” products
You’ll see concrete sealers and coatings sold as radon sealers. EPA’s guidance does not support sealing or coating as a stand-alone radon fix, for the reasons above. If you use one, treat it as an extra, test before and after, and don’t let it replace a soil suction system when your level is 4 pCi/L or more.
If you seal it yourself
CDC lists sealing cracks in floors and walls with plaster, caulk or other materials designed for the purpose as one step that can help lower radon (CDC). If you try it:
- Test first so you know your starting level.
- Seal the openings above with the right materials.
- Test again afterward. CDC and EPA both say to retest after any change meant to lower radon.
- If the level is still 4 pCi/L or more, plan for a radon mitigation system.