Vapor Drive

[JackCrump]

Can someone give me a definition of "vapor drive" as it relates to moisture migration through building walls. I need a reasonably technical definition that is still understandable to a non-technical person such as an insurance company claims rep or attorney. Thanks

 

[boo1]

Moisture flow by vapor diffusion is governed by the second law of thermodynamics (basic physics to some, not so basic to others). 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&quot. This means that it tends to go from the inside out up north and from the outside in down south. In the middle of the country part of the year it goes from inside out and part of the year it goes from outside in.

 

[Ron]

In short, as boo1 said, moisture moves TOWARD dry and/or cool conditions.

The mechanics are a bit more complex than that for a building. For example, moisture generally first moves into the exterior finish materials by capillarity, or directly through a leak. It moves laterally and downward.

When a void or cavity is encountered, the relative humidity in the void or cavity is increased by the moisture intrusion. Now we get condensation. There is nothing to drive the moisture backward, so it is cumulative and progressively worsens unless the source is stopped.

The accumulated moisture then "attacks" the next dry area or cooler area. The same mechanics prevail and it will move to some other location, inward and downward. In the process, given reasonable conditions, mold will grow!