Crawl spaces can be a major source of radon entry.
If your home has a crawl space and the floor is concrete, the crawl space is treated much like the basement floor would be. Large cracks and opening would be sealed.
If the floor of your crawl is dirt or gravel and is accessible, then the floor is covered and sealed to block radon entry.
In Illinois when covering a crawl space floor, a minimum of 6 mil or 3 mil cross laminate polyethylene must be used. At VSI we use 3.5 mil cross laminate because of the added strength over single polyethylene as shown below. You can also use your crawl space for storage after we have sealed it, just be careful when storing objects with sharp edges.
| 3.5 mil cross
|6 mil Poly||2705||6.4|
|10 mil Poly||2215||8.1|