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The function of a vapor barrier is to retard the migration of water vapor. Where it is located in an assembly and its permeability is a function of climate, the characteristics of the materials that comprise the assembly and the interior conditions. Vapor barriers are not typically intended to retard the migration of air.
Vapor intrusion is the movement of chemical vapors from contaminated soil and groundwater into nearby buildings. Vapors primarily enter through openings in the building foundation or basement walls ─ such as cracks in the concrete slab, gaps around utility lines, and sumps.
Vapor barriers or retarders control water vapor transport driven by diffusion. Diffusion is simply the movement of water vapor from regions of more to less vapor (without the movement of air). Air barriers control the moisture that is transported along with airflow.
The vapor permeance characteristic of the sheathing/cladding assembly is defined by the effective wet cup permeance of both the cladding and sheathing combined. Three categories are established: • Vapor impermeable Less than or equal to 0.1 perm • Vapor semi-impermeable Less than or equal to 1 perm and greater than 0.1 perm
Vapor intrusion is the movement of chemical vapors from contaminated soil, groundwater or sewer lines into nearby buildings. Vapors enter buildings primarily through openings, such as cracks and seams in the foundation or basement walls, gaps around utility lines, and sump pits.
Figure 6-1 Overview of Planning, Scoping, and Conducting Vapor Intrusion Investigations Figure 7-1 Sample Depiction of Subsurface Vapor Source and Data to Support Pre -emptive Mitigation/Early Action for Multiple Buildings , Each with Limited Data
Builders are constructing tight houses for two rea-sons: (1) to keep heated air or air-conditioned air inside the house and hence reduce utility bills; (2) to keep moisture out of wall cavities. Moisture can move into walls in one of two ways: by vapor diffusion or by air leaks. The movement of moisture through a material is called vapor diffusion.
