Weatherization and Radon It's important to note also that the study referred to here is from old weatherization practices. Since this study was published, new requirements were implemented. Now, weatherized homes must meet ASHRAE 62.2.
DOE has the BARRIER study, which I believe is nearly complete, and it looks at effects of enhanced and standard practices on radon (and I believe moisture is included). But also, DOE is now into year two of the BEX study to look at if and how the new standard practices affect radon. This large study includes 200+ homes.
Essentially, the ventilation requirements are being met by exhaust-only ventilation, which as you can imagine, has it's own set of possibilities for what can happen. A pilot study was completed before the larger study that is occurring now (but it only looked at a handful of homes). The pilot study concluded that meeting ASHRAE 62.2 didn't raise radon.
In the meantime, any homes in this study that go from below the action level to above, will be mitigated (not at owner's expense).