vapor permeance

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A measure of a material’s resistance to water-vapor transmission, expressed in perms. Equal to the ratio of (a) the rate of water vapor transmission through a material or assembly between its two parallel surfaces to (b) the vapor pressure differential between the surfaces.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.
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In this article, the term "vapor permeance" refers to the coefficient for moisture transfer when water vapor pressure is the driving potential, even though moisture transfer involves combined vapor and bound water diffusion (Stamm 1964).
where M is water vapor permeance (kg [Pa.sup.-1] [s.sup.-1] [m.sup.-2]) and [[DELTA]p.sub.wv] is the difference in water vapor pressure (Pa) across the specimen.
* Walter Payton, arguably one of the greatest football players of all time, and, according to Mike Ditka, one of the greatest human beings of all time, is known to his fans by his nickname "Sweetness." In "Joe World" there is a "sweet spot" for vapor permeance--an optimum range for some types of wall assemblies--so it is inevitable that Walter "Sweetness" Payton would find the "sweet spot" for vapor permeance had he been an engineer rather than a Chicago Bear.
Understanding vapor permeance and condensation in wall assemblies, 11th Canadian Conference on Building Science and Technology Banff, Alberta.
This overlap zone is a possible route for two dimensional vapor transfer, leading to an overestimate of the vapor permeance, since the total flow through the specimen in the test assembly is greater than that through the exposed area of the specimen.
Intercomparison on measurement of water vapor permeance. Nordtest-project agreement 1529-01.ACGIH.
"Understanding Vapor Permeance and Condensation in Wall Assemblies." 11th Canadian Conference on Building Science and Technology.
"Understanding Vapor Permeance and Condensation in Wall Assemblies." According to Professor Eric Burrnett, who founded the group, the acronym "BEG" also signifies what they have to do to get funded.
Often, especially for membranes and composite materials, one calculates the water vapor permeance [[delta].sub.l] of a product at a given thickness from the above measurements as
[[delta].sub.l] = water vapor permeance for thickness, l, kg x [m.sup.-2][s.sup.-1][Pa.sup.-1]
The floor joist sees the dew point of the air in the crawlspace (we can ignore the vapor permeance characteristics of the fiberglass batt insulation since it is so vapor open.
First, we make the simple engineering assumption that we can ignore the vapor permeance (or vapor resistance) of the fiberglass batt insulation.