There are two main areas that require ventilation. The roof needs to breathe and maintain a cool temperature. Preferably you want to have the attic temperature equal to the exterior temperature. This prevents ice dams and icicles building up.
If the attic is too hot it warms the roof deck, which in turn does two things. In the summer, it ‘tricks’ the roof, as we call it in our terminology. The inside attic temperature gets so hot that it breaks down the molecular structure of the roof by drying out all of the oils in the shingles. This creates cracking and curling of the shingles and dramatically decreases the life of the shingle by as much as 50%. To avoid this, we want to cool down the roof in the summer. This is done by installing a ridge vent at the top of the roof and soffit units at the base.
This allows the warm air to rise to the center of the attic and flow out the ridge vent. In turn it creates a pulling or movement of the air, by drawing new air thought the soffit. This creates a wind tunnel like phenomena.
If the roof has a finished ceiling then the roof would require a baffel system. Baffels are made of styrofoam and are installed over the insulation, between the roof rafters and under the plywood. They run from the soffit to the ridge and move the air under the deck surface to keep the deck cool.
Winter Condition
The same type of ventilation works for the winter. In this case the changing of the air in the attic keeps the deck cool. This prevents the snow from melting on cold days and prevents ice-dam. An ice dam is when there is snow on a roof and the basic layer melts while the top layer is still frozen. Since the roof is only designed to shed water and is actual not waterproof. The melted snow works its way under the shingle and creates leaks. The same melted snow drips over the face boards of a house, thus creating icicles.
