This is why you see the frost.
Frozen condensation in attic.
Signs of condensation in the attic may include the following.
Moisture forming on the glass window panes or other flat surfaces.
Noticeable musky damp odor.
The air in the attic has water in it and it condenses on the cold metal.
Condensation forms on the outside of the glass where the cold glass is coming into contact with the warm humid air.
Sagging ridgeline drooping ceilings water leaks on interior walls and ceilings jammed doors cracked interior walls near the center of your home creaking sounds.
A point often overlooked is that since it s winter the water freezes on the cold nails.
Well just like any drain line the water should exit the drain and there should be no concern about freezing.
The warm moist air trapped inside of your home is coming into contact with the cold wooden surfaces in your attic forming large damaging water droplets.
If there is a condensate trap insulate it or just remove it since it probably just runs outside from an attic space.
The same thing may be happening on the inside of your attic.
Recognize the signs of stress when too much snow and ice has accumulated on your roof or when you have too much condensation in your attic.