The rim joist area counts for almost 30% or the air that leaks into the crawlspace. The evidence of this leakage is the discoloration of the fiberglass batt insulation stuff in the rim joist cavity. With proper spray foam and crawlspace encapsulation the floors stay a nice even temperature year round.
It also helps to make sure your duct work is actually connected. But that more of an issue of why some rooms don't heat or cool evenly while others are hot.