Effect of Cavity Ventilation on Moisture in Walls and Roofs

Moisture damage in walls and roofs is often associated with air leakage. Many believe that installing vents prevents moisture problems; venting roofs is common building practice, and providing ventilation openings at top and bottom wallplates is accepted practice in U.S. mobile home manufacturing. This paper presents a discussion of the effect of airtightness and vents on moisture and airflow in walls typical of U.S. mobile home construction. We used mea sured airtightness data for wall components to calculate airflows, moisture flows, and wetting or drying potentials for U.S. cold winter conditions. Vents can significantly increase air leakage through the wall. If the airflow is exfiltrative, more wetting may occur during winter in vented walls than in unvented walls. There is no guarantee that vents will provide enough ventilation with outdoor air to offset this increase in wetting. Thus, we conclude that installing vents in walls is not a reliable moisture control strategy. In addition, vents degrade thermal performance and increase construction cost. An effective air barrier and vapor retarder should provide more reliable moisture control and superior thermal peformance. Similar conclusions can be drawn for cathedral ceil ings, although we believe field studies of unvented cathedral ceilings are warranted. We do not recommend change in the current practice of providing attic ventilation at this time