Having proper ventilation installed on your roof is very important.  Proper ventilation can decrease your energy bills by 40%.  Air enters your home and flows into the house by opening, joints, and cracks in the walls, floors, and ceiling, and windows/doors.

A vented attic works in all conditions whether it is hot, cold, or mixed.  Without proper ventilation it can lead to excessive energy loss, ice dams, mod, rot and lots of headache for you.

In cold temperatures the primary purpose is to main a warmer temperature as to not cause ice dams created by melting snow and to vent any moisture that moves from the conditioned living space to the attic.  In hot climate the primary purpose is to eliminate the hot air from the attic and roof to not cause blistering or cooked shingles.