What many don't realize is that the big attic problem is moisture, which is more noticeable in the winter. It can cause more damage and cost more money than excess heat can. On a cold night, when the roof deck cools, the moist air trapped just under the roof will condense and often form frost. Once it warms, the frost melts and drips onto the insulation, which can ruin it. Sometimes there is enough moisture to stain ceilings.
Ventilation is important all year long.
In the summer, the more heat you vent out of the attic, the less that finds its way into the house. Much of the moisture that leaves the house heads into the attic, and it, too, must be vented. Many people worry about losing heat through open attic vents, but the only heat lost is what has already escaped to the attic. The job of the insulation on the attic floor is to keep the heat in the living space. They might freeze if the attic is closed tight.