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Moisture Transport in Building Materials: A Comprehensive Review

Explore the complexities of moisture transport in building materials, from vapor diffusion to liquid flow, capillary action, and more. Learn about important terms and governing equations for understanding moisture transfer dynamics.

patricia-fitzpatrick
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Moisture Transport in Building Materials: A Comprehensive Review

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  1. Example – 8” Concrete Wall perm = grain/(hr∙ft2∙in Hg)

  2. Review of Terms • -ity terms – material properties (k, µ) • Inverse of 1. (resistivity, diffusion resistivity) • -ance terms – 2. × thickness (R, Z) • Inverse of 3. (U, M)

  3. Governing Equation For Diffusion • w water vapor flux [M/t/A, kg/s/m2] • µ permeability [perms∙in, perm = grain/(hr∙ft2∙in Hg)] • Permeance [ng/(s·m2·Pa)] • p is water vapor pressure • x is distance along flow path • Water diffuses from high vapor pressure to low vapor pressure • Permeability is a function of temperature in materials • Very ugly non-linear relationship

  4. Series Moisture Transfer ΔP = water vapor pressure difference Z = Diffusion resistance

  5. Saturation Vapor Pressure • Function of temperature Ref: ASHRAE Handbook of Fundamentals

  6. If Condensation Occurs • Set vapor pressure to saturation pressure at most likely point • Divide wall into two sections • Use relationship on each side of condensation • Recalculate vapor pressures

  7. Cautionary Notes • 1-D moisture and thermal transport • Assumed steady-state and equilibrium conditions

  8. Equilibrium Moisture Content • Mass (or volume) ratio between moisture in material and wet (or dry) material • http://www.forestprod.org/cdromdemo/wd/wd4.html

  9. Review moisture transport • Moisture transport • Liquid flow • Vapor diffusion • Air movement • Capillary action • What driving force is responsible for each one? • What do you need for each transport method?

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