We were advised during our initial blower door test to condition the crawlspace under the rear of the house. That means closing it off to outside air and introducing “conditioned” interior air i.e. heated and cooled air from the interior of the house. We also needed to remedy high Radon levels so the crawlspace treatment is meant to do that as well. These are resources that lay out the case for closing off outside crawlspace ventilation and having the space remain warmer and dryer.