Nothing short of a Tornado or Hurricane can destroy a home like a poorly ventilated attic. The temperature of an attic space should be the same as the outdoor air temperature. This can only be achieved with circulation of air from outdoors at the underside of the eaves, called the soffit and the peak area of the roof at vent openings. The sun shining on a roof heats the air in the space and creates a natural convection current. Hot air rises and exits the high area of the roof at the vents and draws in ambient outdoor air at the soffit. Air leakage from the house through unsealed openings such as pot lights, sewer vents and exhaust stacks can result in warm moist air condensing in the attic. This condensation can cause mold and also water drips that soak both insulation and the ceiling drywall. The repair costs can be mind boggling. It is important to have a minimum of R-40 insulation in the attic and make sure that the insulation is not blocking the soffit inlets.
Some attics will use a power exhaust fan controlled by a thermostat to purge the attic of hot air. The only time this is practical is in the summer and sometimes the noise of the fan will resonate throughout the home. The trouble with this method is that the attic needs ventilating all year round. Check the temperature of your attic on the coldest day of winter and the hottest day of summer. If there is more than 5 to 10 degrees F. difference there is a problem.
A good roofing contractor will replace the vents when redoing your shingles, but extra vents can be added at any time. It is simple enough for the average home owner to inspect for mold or condensation on the underside of the roof boards and to ensure that ducts from exhaust fans are properly terminated and insulated. When looking towards the eave of the house from the attic there should be light visible where there are soffit vent openings. This is also a good time to check for any openings that would permit birds or rodents access to your attic.