Why Condensation Occurs

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