Well-ventilated attics are like a good pair of walking shores, you don't often think about them, but they affect your life everyday.
Poorly ventilated attics do several bad things.
Thing #1: It allow heat to build up over the course of the day. That heat radiates downwards through the insulation and is transferred to the top-side of the ceiling. As the ceiling heats up, the AC must work harder to cool. Utility bills go up. AC life expectancy goes down. No good!
Thing #2: If your heating & cooling system is actually located in the attic, then it's fighting the heat to produce cool air. See Thing #1
Thing #3: The built-up heat in the attic can exceed 150° F. If this were water, it would be scalding hot! This heat can actually cook the shingles in the roof and significantly reduce their life expectancy.
How do you know if you have a well-ventilated attic or a poorly ventilated one? Easy. Take its temperature. The interior temperature of a well-ventilated attic should not be more than 20° hotter than the outside air. If it's hotter, then it's too hot.
How do you fix a hot attic? Add ventilation, of course. That can mean adding a ridge vent at the peak of the roof (a job for a qualified roofing contractor). Or, you can add a electric fan controlled by a thermostat (again, installation is the job for a professional).
Sometimes, the ventilation under the eaves of the house (soffits) has become blocked with insulation. That will restrict the inflow of cooler air into the attic, as the hotter air exists the ridge or the fan. Try pulling the insulation off of the tops of the eaves from the attic by using a plastic toy rake and duct-taping it to a long broom handle.
Sometimes there is not enough soffit ventilation and more must be added. At least 50% of the total ventilation should be in the soffits.
Good luck!
Tom Sinclair
About the House - Home Inspections'
Fairhope, Alabama 36532
(251) 990-3169
For more information and a longer discussion of this subject try these links:
http://www.askthebuilder.com/Ventilation.shtml
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