There is some question about the benefit of foil facing. In many climates, there will be times during the Summer when the foil will create too much heat behind the finish siding, which will shorten the life of the siding, or even damage it on occasion. However, there may be an advantage to facing the foil to the interior to reflect heat from the inside back into the room. Still, the vapor barrier provided by the foil is useful pointed to the exterior. The alternative is to apply Typar or Tyvek building wrap.