Sanity Check

Half of our 1st floor is above a crawlspace and the other half is above a basement. We got a radon fan installed and connected to the sump pit in the basement.

Before doing any work, the contractor did say that the crawlspace will need to be dealt with as well if the levels don't drop enough. Since we have some additions on our house, there is no easy way to vent the crawlspace to the outside, and I've also heard that is not recommended in climates where the pipes can freeze.

Our radon levels dropped from 8 to around 3.5 - 4 and in order to get it lower, they are recommending installing a vapor barrier in the crawlspace and connecting it to the system somehow.

Is this common practice? Does it all make sense before I spend more money?