The reason that your floors are varying in temperature like this is due to the stack effect. The outside air is drawn into the lowest area of your home--the crawlspace--and then drawn upward into your home. In the winter, cold air is pulled into the crawlspace through the vents, etc and forces your heater to work overtime to heat your home. In the summer, the hot, humid air does the same thing--bringing with it all the allergens and spores from outside.