Proper roof venting is critical to good construction practices and maintenance. The purpose of roof venting is to reduce temperature build-up inside the roof cavity during the summer months and to reduce fungal wood rot and other moisture related problems associated with condensation in the roof cavity during the cooler months of the year.
A good venting system allows for a continuous flow of air which enters the roof cavity at a low spot in the roof (for example, under the eave) and exits the roof at a high spot (for example, at or near the ridge of the roof). As a general rule, there is no need to provide for any sort of mechanical assistance, fans, or other devices to the roof venting system. 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. It is not uncommon to find an improperly vented fifteen-year-old roof with badly curled (and thus non-functional) roofing material and advanced fungal wood rot on the lower side of the roof 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.
During the winter months, when the temperature outside is relatively cool and the temperature on the inside of the house is 20+ degrees warmer, the air on the inside of the house is capable of holding significantly more moisture than the air on the exterior of the home. 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. If the attic space is not properly vented, this warm, moisture-laden air will come in contact with cold surfaces such as the roof sheathing; condensation will occur at this point. 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. However, the building code and most roof manufacturers recommend venting at a rate of one square foot per 150 square feet of ceiling space.
The code allows for venting at a rate of one square foot per 300 square feet of ceiling space where a complete vapor barrier exists at the bottom of the insulation. The code requirement is a minimum requirement. Smaller attics and low sloped roofs typically require more venting. The code also appears to allow for roofs without any venting in roofs without any cavities. For example, some cathedral ceilings are insulated with rigid insulation, with the sheathing and roofing material installed directly on top of the rigid insulation. In order for such a roof system to function without subsequent moisture problems, a high quality complete vapor barrier needs to be installed on the under side of the insulation. Our experience suggests that such roof systems often fail, resulting in condensation and associated wood-destroying problems. This experience appears to be borne out by insulation experts at the Bonneville Power Administration. We recommend roof venting systems in almost all roof types.
At least one-half of the venting must be in the form of high vents (for example ridge vents), with the balance in the form of low vents (for example, soffit vents). The vented roof structure and insulation must be configured in such a way as to allow for a 1-1/2 inch air space between the insulation and the roof sheathing and a continuous air slow between the low and the high vents.
See also Topics: The Sound Roof, Q&A #20: Roofs