While sheathing provides various benefits to the building envelope, it is essential to differentiate between its primary functions and its capability as a vapor barrier. Sheathing, especially when made from plywood or OSB, can possess some vapor resistance due to the nature of the materials. These types of sheathing can retard the passage of water vapor to a certain extent. However, it is crucial to note that sheathing alone cannot offer the same level of vapor barrier performance as dedicated vapor barrier materials.
Factors Influencing Vapor Barrier Capabilities of Sheathing:
Several factors affect the vapor barrier capabilities of sheathing material:
- Permeability: Different types of sheathing materials have varying permeability levels. While some types of sheathing, such as foam boards, have low permeability, plywood and OSB have a moderate level of permeability. This permeability allows some passage of water vapor, limiting their effectiveness as standalone vapor barriers.
- Thickness: The thickness of sheathing also influences its vapor barrier capabilities. Thicker sheathing materials tend to provide more resistance to vapor movement. However, when relying solely on sheathing as a vapor barrier, the thickness required to achieve an effective barrier would be impractical and not cost-efficient.
- Installation and Sealing: Proper installation and sealing of sheathing play a vital role in determining its effectiveness as a vapor barrier. Gaps or improper sealing can compromise its performance, allowing moisture to penetrate the building envelope.
While sheathing does possess some vapor resistance properties, it cannot replace the dedicated function of a vapor barrier. Sheathing has its primary role of providing structural support and protection to the building envelope. Therefore, it is advisable to use sheathing in combination with proper vapor barrier materials to ensure adequate control of moisture movement within a building. By understanding the characteristics, limitations, and best practices associated with sheathing and vapor barriers, builders and homeowners can create more durable and energy-efficient structures.