ATTIC VENTILATION
Attic ventilation serves two primary purposes: reducing attic temperature during hot weather and reducing attic moisture during cold weather. This moisture is caused by regular household activities such as cooking, showering or laundering which produce water vapors that can become condensed and trapped in your attic, producing damaging mold and mildew. Vents are classified as either intake vents (where air comes in) or exhaust vents (where air flows out). These two must work in conjunction in order for attic ventilation to work properly. Your attic should have enough unobstructed intake vents (soffits) and enough exhaust vents to allow air to flow through freely. Read what energy star says about attic ventilation.
Following are some general attic ventilation sizing guidelines:
Intake Vents:
• one soffit vent for every 100 square feet of attic
Exhaust vents:
• one solar vent (1550 CFM) for every 2,000 square feet of attic (may require more depending on roof pitch)
OR
• one 14 inch wind turbine for every 500 square feet of attic
Ideally, you would like your attic temperature to be close to the temperature outside, but unfortunately most attics are under-ventilated. During the summer, it is not unusual for attic temperatures to reach 30 to 50 degrees above the outside temperature! This heat trapped inside your attic will eventually work its way through your insulation and into your living space making your home warmer and your AC work harder. To solve this, we offer both state of the art solar vents and 14 inch wind turbines for those customers on tighter budgets. FactsFacts.com ventilation diagrams.
Your attic needs 100% ventilation in the summer and only 20% in the winter. 
Moisture vapor can rise into the attic from a bathroom, shower, or your dish washer. You will able to eliminate this with a little ventilation.
Stop turbine heat loss in the winter! Facts about condensation.