Looking for Moisture in All the Right Places
(locating moisture)

Vapor Diffusion

Vapor diffusion is when water or moisture in a gaseous state moves through building materials. Diffusion is when vapor movement occurs independent of air flow. Moisture will flow by diffusion because of a concentration gradient as well as a temperature gradient (from “more to less” as well as from “warm to cold”).

  1. Water vapor moves from warm to cold.
  2. Water vapor moves from more to less concentration.
  3. Negative house pressure can pull water vapor into the inside of walls, floors, etc.
  4. If a vapor barrier is installed incorrectly, moisture will collect in the wall insulation. The vapor barrier should be placed on the outside of the wall’s insulation.

Water vapor barriers or retarders should be placed in the vented attic, crawl space, and outside wall to reduce the diffusion of water vapor into the home. The vapor retarder should be placed on the warm side of the building envelope to prevent the transport of water vapor to a place in the wall or ceiling where it could condense and cause mold growth. Where you live and what your climate is will determine where you should place the vapor barrier.

For more detailed information, go to http://buildingscience.com.

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Controlling Moisture to Control Mold Mini-Course developed by Janie L. Harris, M.Ed., CRS, Extension Housing and Environment Specialist, Texas AgriLife Extension Service, Texas A&M System, College Station, Texas.

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Last updated: 26 July, 2010

Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.