Example – 8” Concrete Wall

Conductivity, k

BTU∙in/(h ft2 °F)

Resistivity

h ft2 °F/(BTU∙in)

Resistance, R

h ft2 °F/(BTU)

Conductance, U

BTU/(h ft2 °F)

~15 0.067 0.53 1.9

Permeability, µ

perm∙in

Diffusion Resistivity

rep/in

Diffusion Resistance, Z

rep

Permeance, M

perm

~3.2 0.31 2.48 0.40

perm = grain/(hr∙ft2∙in Hg)

Review of Terms

1. -ity terms – material properties (k, µ)

2. Inverse of 1. (resistivity, diffusion resistivity)

3. -ance terms – 2. × thickness (R, Z)

4. Inverse of 3. (U, M)

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

x

pw

d

d

Series Moisture Transfer

rep

permZ

Z

P

x

pw

1,

d

dtotal

i

total

i

Z

Z

P

P

ΔP = water vapor pressure difference

Z = Diffusion resistance

Saturation Vapor Pressure

• Function of temperature

Ref: ASHRAE Handbook of Fundamentals

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

total

i

total

i

Z

Z

P

P

Cautionary Notes

1. 1-D moisture and thermal transport

2. Assumed steady-state and equilibrium conditions

Equilibrium Moisture Content

• Mass (or volume) ratio between moisture in material and wet (or dry) material

• http://www.forestprod.org/cdromdemo/wd/wd4.html

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?