I'm in Springfield, MO. This single story house was built in 1906, still has the stone foundation and below ground is about 25% basement and 75% crawl space. It was flipped last year before I bought it. A radon test in the basement = 4.4. I covered the sump hole with lexan (nice fit but not air tight) and put in a 6" PVC exhaust pipe with an inline exhaust fan to the outside. I'm waiting for the results of a second radon test.
I used canned spray foam and steel wool in gaps in the foundation. I began using Reflectix on the inside of the foundation (light weight and easy for me to transport and handle). I bought but haven't yet installed vapor barrier. The crawl space has a lot of debris the house flipper never removed. I bought a crawl space vent power fan attached to a humidistat but shut it off and covered the vent when the weather got cold.
I don't know if I should continue with trying to encapsulate the crawl space (will it help reduce radon?) and get a good dehumidifier or just ventilate. What's the best/right path? I'm a 66 yr old female DIY'er with no experience in these matters.