Proper attic ventilation is necessary to help prevent moisture problems and heat build-up, as well as helping to reduce heating and cooling costs. The benefits of proper ventilation can include a cooler attic in the summer, a dryer attic in the winter and assistance with air movement year round.
In the summer it is inevitable that heat will build up in the attic during the day, and cool off at night if the attic is properly ventilated. If the attic is unventilated or ventilated incorrectly, the heat will continue to build-up over a period of days and increase the attic temperature. On a 90-degree day, an unventilated attic can heat up to more than 170 degrees. Heat radiating from the sheathing down to the attic floor can raise its temperature to as much as 140 degrees. Without adequate ventilation, today’s heavier insulation will absorb and hold more heat built-up during the day and make the attic less likely to cool at night. This overheated ceiling insulation conducts heat through the ceiling down to the living space of the home, which may cause the air conditioning system to operate for longer periods of time to reduce the heat.
Contrary to popular belief, a poorly ventilated attic can cause more problems in the winter than in the heat of summer. During the winter, air inside the home is warmer and carries more water vapor than the colder, dryer outside air. Cooking, laundry, showers and humidifiers all add moisture to the air inside the home. Physics causes this warmer, wetter air to move to the ceiling and possibly into the attic if the insulation is inadequate and the attic space itself is improperly ventilated. Once in the attic, the moisture will rise to the bottom of the actual roof sheathing, trusses and rafters and then freeze when it hits that cold surface. At this point, if you were to peek into the attic, you would likely see ice stalactites hanging from the ceiling. When the weather improves slightly, and the sun beats down on the roof material causing the temperature to rise, the ice melts and drips back down onto the floor of the attic causing water spots on the ceiling of the living space. If you see water spots on the ceiling in in winter, and have no leaking with rains, more than likely you have a condensation issue, not a roof leak.
In some parts of the United States such as the Greater Chicago area, snow melting on a roof surface can cause an ice dam. The way this usually occurs is that the temperature warms up slightly causing the snow pack on the roof to melt. Gravity pulls the melting snow from a warmer area of the roof near the ridge down to the overhang which is cooler refreeze it into ice. If this happens once, it is usually not a big problem, but the freeze/thaw cycle over a couple of weeks can cause this melting snow to eventually back up under the shingles to cause major damage. The results can be soaked insulation, stained sheet rock or peeling paint. In some cases, the build up of the ice weight can cause structural damage.
For more information on ventilation visit the Air Vent, Inc. website.