Vapor permeability is a critical property of wall design. Wall systems must be protected from water and air leakage, but at the same time must be allowed to dry. In this lesson, you will explore the mechanics of water vapor permeance and learn how the unique structure of Tyvek resists the penetration of water and air into walls, while allowing individual water vapor molecules to escape. In addition, you will be introduced to the performance standards used to evaluate vapor permeable membranes.

Learning Objectives
At the conclusion of this lesson, you will be able to:

• Describe how the physical structure of a housewrap material affects permeability
• Summarize the effects of vapor permeability on a wall system
• Distinguish between water vapor transmission rates (MTVR) and water vapor permeance (WVP)
• Understand the effect of air pressure on water permeance
• Explain the ASTM test procedures for establishing moisture vapor transmission rates of weather resistant barriers
• Compare ASTM E96 "dry cup" and "wet cup" methods

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