I learned efficiency in heat transfer by opposite current flows. For example, a heat trade between outside air coming in, and inside air blowing out is most efficient if the highest temperature has the exchanger with the coldest temperature, and second and third stages of exchange always move so that the last exchanger have both sides closest in temperature.

If you want to squeeze the most that you can in a passive system, this principle will help.

Don't forget, in insulating, you want the least transfer, so you go in the opposite direction.