Vapor diffusion

Insulation · The Basics · printed page 234 (PDF page 235)

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Vapor Diffusion Insulation – the Basics

Text in the illustration

membrane (air barrier) stops air, but allows migration of moisture (vapor diffusion) towards the area of lower vapor pressure
initial state
70°F
80% relative humidity
20% relative humidity
migration
over time
the relative humidity on either side of the air barrier eventually equalizes
air barrier
grains of moisture
50% relative humidity

Generated explanation

Two schematic house sections illustrate water vapor diffusion through an air-barrier membrane. In the initial state, both sides are at 70°F, but relative humidity is 80% on the left and 20% on the right; an arrow shows moisture migrating toward the lower-vapor-pressure side. Dots represent moisture, with more dots initially on the high-humidity side. The lower panel shows relative humidity eventually equalizing at 50% on both sides while temperatures remain at 70°F. The illustration distinguishes stopping air movement from stopping water vapor diffusion.

Components

air-barrier membrane wall assembly roof outline

Materials

air-barrier membrane