After this last winter the nails in the drywall on my exterior walls are all discolored. I can see pretty much all of them, especially near the ceiling and on the NW side of my house. My house is an old school house that was remodeled about 20 years ago. There is about 5 feet of dead air space above my ceilings that definitely needs better insulation. The walls are all insulated (I believe), but there really isn't anything to stop the air from the dead air space above my ceiling from moving down the wall cavities. Is the discoloration likely due to condensation from that cold air coming down the wall cavity from above the ceiling? What can I do to fix that - spray foam I am guessing would be one possibility as this would insulate the walls better, and also serve as a better vapor barrier. Any other method?