Snow melt of an attic without excessive heat build-up and with functional attic ventilation should be totally and uniformly white. If the snow melt were the result of heat from the sun, all of the roof would be clear, without any white spikes on the roof being present. Attic heat is the enemy of all roofs, and poses a problem for the insulation, h.v.a.c. systems and health factors, too. Twelve inches of insulation, creating an R-38, is optimal. Remember, only have one vapor barrier and install it facing toward the heated surface. If additional layers of insulation are needed to attain 12", use either unfaced insulation or blown-in insulation. Air flow is easily explained using a ketchup bottle as example. When you open a new bottle of ketchup and turn it over, the ketchup does not immediately come out. In order for the ketchup to exit the bottle, air must enter the bottle and occupy the space just left by the ketchup. The same scenario holds true for your attic. In order for heated attic air to exit the attic, make-up air must be able to enter the attic. Having only inlet vents, or having only outlet vents in a balanced proportion.