Many years ago, roofs were never built airtight. The building itself often acted as a chimney. Roofing materials were porous to air movement, thus providing natural ventilation of attic spaces. Modern home construction methods result in better insulated homes, which saves money on energy bills, but makes the need for a well ventilated attic important.

 An effective ventilation system will help:

• reduce heating and cooling cost
• reduce attic heat build-up
• reduce attic moisture and condensation
• prevent weather infiltration (drifting snow, wind-driven rain, etc.)
• reduce the chance of ice dam buildup

The benefits of proper ventilation for the homeowner are a cooler attic in the summer, a dryer attic in the winter and protection of the structural integrity of the home from moisture damage.

Even if you feel you’ve had satisfactory ventilation performance with your old roof for as long as 20 years, it might be necessary to add ventilation with your new roof to meet new standards. Most shingle warranties require a lot of ventilation–as much as “one square foot of net free ventilation area for each 150 square feet of floor space to be vented; or one square foot per 300 square feet when both ridge and soffit ventilators are used.”

Summer Heat
In the summer, heat builds up in the attic space which will cool at night if the attic is properly ventilated. However, if the attic is unventilated or ventilated incorrectly, the heat will build up over a period of days and continue to increase the temperature inside the attic. According the Air Vent Inc.'s "Principles of Attic Ventilation," a 90 degree day with no attic ventilation can heat the roof sheath to 170 degrees or more. 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. Overheated ceiling insulation conducts the heat through the ceiling down to the residents of the home. This causes the air conditioning system to operate for longer periods of time to reduce the heat.

Winter Moisture
The well insulated attic also will hold moisture in the winter and moisture causes considerably more problems than excessive heat. During the winter, air inside the home is warmer and carries more water vapor than the colder, dryer air into the attic. Cooking, laundry, showers, and humidifiers add moisture to the air inside the home. Atmospheric pressure causes the vapor moisture to naturally migrate from high humidity air to low humidity air and as it moves into the attic, the moist air will condense into liquid on the cooler rafters, trusses, and roof sheathing. As the air gets colder, its ability to absorb moisture is reduced. And, as the insulation levels are increased in the ceiling, the attic becomes colder and more likely to retain the moisture, which can damage the structure.

Ice Dams

Ice dams are barriers to water runoff on the roof. They usually form at the roof edge, just above the gutter, in cold, snowy climates. They form when snow melting on warmer areas of the roof, usually near the ridge, runs down and refreezes at colder roof overhangs. Warm spots on the roof are caused by the heat that escapes from the living space into the attic. Once this cycle of melting and refreezing begins, a barrier is formed, trapping the snowmelt and allowing it to flow up and under shingles. As water begins to penetrate sheathing, insulation, wall cavities, and sheetrock or plaster, major damage to your home may occur including:

• paint can begin peeling on both inside and outside walls
• roof coverings, fascia and gutters can be damaged
• structural damage can result from the weight of the ice dam

Shingle roof systems are designed to channel water from the high points of the roof to the lower sections where it should exit the roof either into the gutter system or simply onto the ground. Shingle roof systems are not designed to hold water or prevent water from entering that runs or is blown uphill.
 

Ice dams are the result of winter storm conditions. Because they are storm related, and not a roof flaw, damage caused by water is not covered by the shingle manufacturer warranty.The following steps can minimize the chance your home will be affected:

• Make sure your attic area has adequate insulation.
• Make sure your attic insulation does not cover your soffit air in-take vents. Add soffit vents if your home currently does not have any.
• Make sure your attic area has sufficient air exhaust outlets (gable vents, ridge vents and free air vents) and that the airflow is distributed throughout the attic space.
• Products such as an ice & water shields can be installed at the eaves of the home, around dormers and in the valley areas under the shingles, which will prevent water from entering the home unless it dams higher the protection, provides.
• Homes and buildings with cathedral or open ceilings may want to consider installing ceiling fans near the peak of the ceiling to help push the warmer air away from these areas.