A good venting system works adequately by means of a natural convection current where warm air rises and pulls the cooler, drier air behind it. A good roof venting system must also be designed in such a way as to provide for proper air flow at all portions of the roof. Improperly vented roofs often result in premature deterioration of the roofing material and fungal wood rot in the roof framing and sheathing. With the introduction of more insulation and weather sealing, as well as other building techniques which reduce venting, proper roof venting techniques are even more critical than in the past. When the home is occupied, one can expect that the warm interior air will be saturated with moisture, the result of normal living activities.
Human beings give off approximately one quart of moisture a day.
Other moisture sources are pets, household plants, and normal activities such as washing or cooking. A problem arises when warm, moisture-laden air travels through walls and ceiling surfaces and into the attic space. With sufficient and continuous condensation, a perfect medium develops for fungal wood rot. If, however, the roof cavity is properly vented and a continuous flow of cool, dry air moves through the attic space, the possibility of condensation is reduced and the development of fungal wood rot is almost completely eliminated. The exact amount of venting needed in any particular roof structure is not always easy to calculate.