Moisture Transport

1. Moisture Transport • Quantitatively and qualitatively describe transport by • Liquid flow • Capillary suction • Air movement • Vapor diffusion

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. Source Control • Exhaust ventilation • Bathrooms, kitchens, dryers, unvented combustion, wood storage, construction materials • Condensate drainage • Vapor-diffusion barrier • Dilution • Dehumidification

9. How to Stop Air Movement • Air retarders • Air sealing • Caulk and foam • Dense-pack cellulose insulation • DO NOT FORGET ABOUT VENTILATION

10. 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

11. 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

12. 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

13. Average Permeability

14. 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?

15. 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 • Impermeable to vapor, but “permeable” to liquid • Low permeability at low RH, high permeability at high RH

16. Protecting against Vapor Diffusion • Below grade • Damp proofing • Vapor diffusion retarders on different surfaces • Insulation on exterior of foundation

17. 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

18. 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

19. Detailed Modeling • Strategy 2 • Divide building materials into small volumes • Consider all transport mechanisms and calculate liquid and vapor transport to and from each volume • Simultaneous energy, mass balances for each volume including phase change • Computationally intensive • Requires • Material properties • Excellent geometric description

20. 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

21. Review of Heat Transfer • For series heat flow q = heat flux (heat flow) ΔT = Temperature difference R = thermal resistivity

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

26. 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