Attic Ventilation, The Basics
It is estimated that 90% of the homes in the U.S.A. do not have proper attic ventilation. It is required in many cases to limit the temperature in the attic as well as to prevent the buildup of moisture. To understand why it is important, we must look at the basics of heat transfer. Basically, a larger temperature difference between two areas results in a larger amount of heat being transferred from the warmer area to the colder area. The temperature difference between your attic and the living areas affects the rate of heat transfer between them. In dominantly cold areas, attic ventilation is neither required nor desirable. In these areas, moisture removal is not required under normal conditions. (That is if bath fans are used and properly installed.) It typically does not get hot enough in these areas to require it in the summer time.
The disadvantage of having a vented attic in these areas is that the attic temperature is kept colder in the winter time which allows more heat to be transferred from the living space to the attic.
This in turn requires more operation of the heating system to replace the heat that was transferred to the attic.
In moderate climates, ventilation is required but the proper design is critical. If there is too little air flow to cool the attic in the summer, the life of roofing materials can be shortened and your cooling bills can be higher. This is because the attic temperature will be higher and more heat will be transferred from the attic to the living areas. If there is too much ventilation, your winter heating bills will be higher than they should be because the attic temperature will be lower at this time of year. Proper ventilation is a balance between the summer and winter requirements.
In hot climates where there is little or no need for heating systems, maximum attic ventilation is recommended. This will minimize the air temperature in the attic and lower the load on the cooling system. (In this case, more is better.)