The purpose of ventilating the attic in a residential roof situation is to reduce the temperature gradient between the outside temperature and the attic temperature. This temperature gradient, coupled with other factors such as dew point and relative humidity, may result in condensation in the attic and the accumulation of an unhealthy attic environment.
Another consequence of a significant temperature difference between the attic and outside temperature during below-freezing temperatures outside is the ice dam.
When the outside temperatures are below freezing, and the attic’s temperature at the roof’s peak is above freezing, an ice dam will form at the roof’s perimeter. Higher temperatures at the apex of the roof are caused when the heat is lost through the ceiling.
This happens as ice on the roof’s peak melts, and the water runs down from the top to the roof’s edge beyond the outside wall. Once the water gets to the eave area and the deck’s underside is back to freezing temps, the water re-freezes turning to ice due to the lack of heat loss through the ceiling. This forms a solid dam trapping water behind it. The water will leak into the house through the nail holes in the shingles. Even correctly installed shingles will leak. Shingles are made to shed water. They do not resist poinding water, which is undesirable since they are not waterproof.
Ventilating the attic will allow the trapped heat at the roof’s peak to be discharged into the outside environment. This will lower the attic’s temperature reducing the chance of water melting and flowing down the roof to the roof’s edge.
Roof ventilation is meant to lessen the temperature gradient between the outside and the attic’s atmosphere. Never cover your vents to trap heat (this could guarantee an ice dam).