ProAir uses only the highest quality counter-flow plate exchangers, made from pure polystyrene sheets, one fifth of a millimetre thick. Pure polystyrene has efficiency advantages for this application over the more traditional aluminium exchangers.
Aluminium is a metal and is a better conductor of heat than polystyrene. It is most important to have heat travel across the thickness of the plate. It is actually disadvantageous to have heat travel along the plate. In this setup the polystyrene performs better. The reason it is a disadvantage to conduct along the plate is that a totally efficient exchanger will have the end with the cold air entering at the temperature of the cold air and the end with the warm air entering at the temperature of that warm air. This cannot happen with a metal exchanger, as there will always be lateral conduction, especially when there might be a temperature difference of 40o C across it.