Holy smokes Harry you asked THE $64 question. I will try to be brief.

The best solution in the St. Louis area part of the country is that you totally insulate and seal up the crawl space so it stays relatively warm and dry to avoid condensation. This is done with 2” rigid insulation, no ventilation, a complete moisture barrier enclosure that is similar to swimming pool liner, and a dehumidifier running to mechanically dry the air.

This is achieved by insulating the concrete perimeter foundation with 2” rigid foam insulation and fibrous batts between the joists at the band joist; closing and insulating the air vents; totally sealing the crawl space with a “plastic bag” enclosure to include sealing the crawl space soil floor (or concrete) and crawl space walls; and leaving the floor system above the crawl space either un-insulated or insulated. Then set a dehumidifier to a maximum of 50% relative humidity inside the crawl space to keep the air “dry”. This 50% humidity keeps the insects down due to the dryness, the crawl space air warm enough to control condensation, and therefore limits any mold condition inside the crawl space. This is a costly solution but the best that I know. The method treats the crawl space as a conditioned space just like a basement.

Our latitude on the globe causes our ambient humidity (general air all around us) to often rise above the 50% relative humidity, and that means that the crawl space (and attic spaces) cannot be adequately kept dry if it is ventilated. The building codes suggest either the full and continuous vented method OR this enclosed method with a dehumidifier. Most folks are not aware of the dehumidifier option, and the vented method is equally ineffective in the St. Louis area because all our humidity is generally above 50% during the warmer half of the calendar year, and below 50% during the cooler months. That is why the mechanical dehumidifier method is preferred, even though it is not yet very well known.

Attics are to be fully insulated and fully ventilated year round.