Ventilation is simply the process of supplying a continuous supply of air through the attic space. “Proper ventilation” consists of 50% intake, under the eaves, and 50% exhaust near or at the roof peak, at least 3” higher than the intake system.
To fight the enemies, heat and moisture, you must ventilate year round.
Heat in unventilated attics may cause temperatures to exceed 150 degrees causing damage to shingles, rotten sheathing and possibly radiate to the living area. Moisture being the #1 enemy, causes rot, mildew, mold, blisters, and renders insulation to be ineffective.
Proper ventilation reduces energy bills, winter ice build up and eliminates mold/mildew, which can lead to health problems. Be ventilating, you are extending the life of other building components, shingles, insulation etc…
Heat rises, and in your house, it has no place to go. Therefore, it sits there and increases your air conditioning bill and makes your attic useless and deteriorates the roof. Adequate ridge, roof, or gable vents are important. In winter, the vents sweep moisture out of the roof preventing mold and condensation problems. The ceiling insulation is you building envelope for heat and ventilation should have a negligible effect.
Actually, your total venting should be at least a square foot for every 300 square feet of attic with a vapor barrier and 1 to 150 without. Half of the required vent area should be roof, gable or ridge vent, and the other half soffit vent. Having excess soffit vents does not hurt, but inadequate roof vent can. I have replace roof decks that rotted on a less than 15-year-old unvented section of roof while in the same house; the properly vented attic space roof deck looked like the day it was built.