Traditionally, 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. With the advent of bituminous roofing materials, and later composition shingles, the roof became better insulated. It quickly became apparent that ventilation was necessary to prevent moisture problems.

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 to 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 can 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.

The well insulated attic also will hold moisture in the winter and moisture causes considerably more problems that 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.

For more information on ventilation, please see our product page.