Moisture Transport

• Quantitatively and qualitatively describe transport by

1. Liquid flow

2. Capillary suction

3. Air movement

4. Vapor diffusion

Capillary Suction

• Paper towel example• What makes a good capillary medium?

• Small pores (but not sealed)• Small contact angle (hydrophilic)

• What is the driving force?• Surface tension

• Units on surface tension? • Is surface tension a function of temperature?

• Is it only a liquid phenomena?Ref: Carey (1992) Liquid-Vapor Phase-Change Phenomena

Capillary Action (quantitative)

• Liquid water• Water moves from big capillary pores to small

capillary pores

• Water vapor (at equilibrium)• s = ρRTln

• Temperature does influence vapor motion through capillary pores

• Capillary vapor transport is from high T to low T

How do we stop capillary action?

• Get rid of the moisture source

• Make the pores bigger• Capillary break

• Seal the pores

• Give the water someplace else to go

Stopping Capillary Suction Below Grade

• Bituminous liquid (tar-like material) to seal pores on exterior of foundation• Does not span big cracks

• Gravel around foundation (with below grade drain)

• Install capillary breaks• Air gaps, insulation gaps

Stopping Capillary Suction Above Grade

• Paint

• Caulk small air gaps• Disadvantages?

• Make large air gaps (vented) between siding and wall and between shingles and roof decking

• Use building paper or bricks or other material to absorb moisture

Air Movement

• Simplest form of vapor transport

• Driving force?• Air moves from high pressure to low pressure• Pressure increases with temperature (IGL)

• Flow is from high temperature to low temperature

OH

airWQV

2

Source Control

• Exhaust ventilation• Bathrooms, kitchens, dryers, unvented combustion,

wood storage, construction materials

• Condensate drainage• Vapor-diffusion barrier

• Dilution

• Dehumidification

How to Stop Air Movement

• Air retarders

• Air sealing• Caulk and foam• Dense-pack cellulose insulation

• DO NOT FORGET ABOUT VENTILATION

Vapor Diffusion

• Movement of water vapor from high concentration to low concentration• Mechanism is random molecular motion

• Some materials are impermeable to vapor diffusion

• Other materials retard vapor transmission

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

Permeability and Resistance

• ASHRAE ch. 25 Table 9• What has greater average permeability?

• Brick• Concrete• Aluminum foil• Air• Polyethylene• Latex enamel paint• Latex primer/sealer paint

Average Permeability

Material Average Permeability [ng/(s·m·Pa)]

Aluminum foil 2.6 × 10-5

Polyethylene 4.6 × 10-5

Latex enamel paint 1.1 × 10-2

Latex primer/sealer paint 2.2 × 10-2

Brick 4.6Concrete 4.7

Air 174

More questions

• Does permeability or permeance matter?

• How do you measure permeability/permeance?• Wet-cup/dry-cup tests

• What is a vapor-barrier/ vapor-retarder?

• How do tears, voids, gaps affect vapor-retarder performance?• Is this the same as for air barriers?

Protecting against Vapor Diffusion

• Above grade• Use a vapor retarder

• Interior in heating climates– Caveat about cladding moisture

• Exterior in cooling climates

• But, what happens in the “other” season?

• And, what happens when moisture does get into the building assemblies?• “Smart” retarders

1. Impermeable to vapor, but “permeable” to liquid

2. Low permeability at low RH, high permeability at high RH

Protecting against Vapor Diffusion

• Below grade• Damp proofing• Vapor diffusion retarders on different surfaces• Insulation on exterior of foundation

Moisture Modeling

• Thus far, largely qualitative analysis• In order to make informed decisions need to do

quantitative analysis• Challenges/barriers

• Building assemblies are not well characterized• Discontinuity between design and construction phase• Modeling liquid water flow is practically impossible

and not particularly desirable• Very expensive to do completely

Strategies for Modeling

Strategy 1• Assume that only one water transport method is

active• Back of the envelope calculation to figure out which

method is the most important

• Combined thermal and moisture transport calculations

• Usually assume equilibrium, 1-D transport

Detailed Modeling

• Strategy 21. Divide building materials into small volumes

2. Consider all transport mechanisms and calculate liquid and vapor transport to and from each volume

3. Simultaneous energy, mass balances for each volume including phase change

• Computationally intensive• Requires

• Material properties

• Excellent geometric description

Example of Strategy 1

• New building• Verified construction to limit liquid water entry• Foundation/cladding designed to eliminate

capillary suction• ADA verified with blower door testing• Interested in steady-state moisture transport

Review of Heat Transfer

• For series heat flow

W

Cm

BTU

FfthrR

m

W

fthr

BTUq

R

Tq

22

22,,,,

total

i

total

i

R

R

T

T

q = heat flux (heat flow)

ΔT = Temperature difference

R = thermal resistivity

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

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