RR-0404: Roof Design

Roofs can be designed and constructed to be either vented or unvented in any hygro-thermal zone. Air barrier systems are typically the most common approach, however, air pressure control approaches are be-coming more common especially in cases involving remedial work on existing structures. Vapor diffusion should be considered as a secondary moisture transport mechanism when designing and building roofs. Specific vapor retarders are often unnecessary if appropriate air movement control is provided or if control of condensing surface temperatures is provided.