See this recent thread on this very topic. In brief, to avoid moisture accumulation under the mats and the mould that feeds, you need to install a subfloor or remove the mats frequently for cleaning.

Do not impede the air to your mechanicals.

use pressure treated 2x for base plate, also helps to put a membrane/ thin sheetmetal or termite gasket under the plate to prevent termites from migrating up through a crack in the cement and into the house framing, this is especially common in coastal area.