Objectives

• Analyze potential for mold and condensation in different climates and different building types

• Describe moisture transport in buildings• Mechanisms• Driving forces

Cooling climate

• Summer – AC cools surfaces• RH near surfaces increases

• Outdoor air is “poison” from a moisture control perspective• Does not let surfaces dry out

How do you limit potential for condensation?

• Duct and diffuser design

• As low as possible ventilation rates

• Treat (dehumidify) incoming air

• Maintain building at positive pressure• Vapor diffusion retarder

Heating or Mixed Climate

• What influences RH near surfaces of exterior walls?

• Insulation level• Thermal bridging

• Temperature difference (inside temperature)• Indoor moisture levels

• More likely to get mold at summer or winter design conditions in Austin?

Mold in Austin

• ASHRAE design weather conditions:• Winter -1.3 °C DB

• Inside 21 °C, 50 % RH

• Summer 35.8 °C DB, 23.4 °C WB • Inside 26 °C, 50 % RH

• Winter inside air dew point: 10.2 °C• Winter DB corresponding to 70% RH: 16 °C• Summer outside air dew point: 18.1 °C• Summer DB corresponding to 70% RH: 20.3 °C

• What temperature is air blowing out of a register?

Other Causes of Mold

• Plumbing leak, condensation, envelope leak

• Building materials get wet• Don’t dry out because of

• Insufficient air flow

• Air that is too humid

• Oversized air conditioners

• Mold growth hard to arrest• http://www.epa.gov/iaq/molds/moldresources.html

Exterior Closet Example

• Heating climate (dry outside air)

• Closet in exterior corner of bedroom• Mold growth

• What can you do to control mold?

• In what order would you implement suggestions?

Stopping Condensation

• Raise surface temperature• Reduce heat flow to exterior• Increase heat flow to surface

• Reduce interior moisture (vapor pressure)• Source control• Dilution• Dehumidification

Causes of moisture problems

• Exterior corners• Low air circulation• Wind-washing• Higher heat loss (wood instead of insulation)• Greater surface area to volume ratio

• Exterior wall/roof/floor intersections• Higher heat loss• Wind-washing

More causes of moisture problems

• Impermeable wall covering in cooling climates

• Thermal bridges

• Insulated walls in heating/mixed climates

Condensing Surfaces

Moisture Movement

• Thus far we have studied moisture problems

• How does moisture move through buildings?

• MCH does this anecdotally and qualitatively• We will explore this quantitatively

• Everything can be explained with the 2nd Law of Thermodynamics

Moisture Transport

• Quantitatively and qualitatively describe transport by

1. Liquid flow

2. Capillary suction

3. Air movement

4. Vapor diffusion

Liquid Flow

• Requirements• Liquid water• Driving force• Hole

• Driving forces?

How do you stop liquid flow?

• Minimize moisture source or minimize driving force• Site selection• Drainage• Overhangs/architectural detailing• Irrigation

• Minimize holes• Waterproofing

Site Selection

• Grading• Building should be at high point• Ground should slope downwards away from

building• Sounds simple, not always possible

• Water flows downhill, account for it

Drainage

• Gutters and downspouts• Direct away from

building

• Impermeable caps• Free-draining backfill

material• Also building materials

• Drain away moisture that enters building• Flashing

Driving Force Minimization

• Leaky siding, air gap, tight sheathing• Drainage/flashing at bottom of air gap

• No straight-through holes

• Kerfs to address surface tension

• Minimizing wind exposure• WDR (momentum)• Pressure difference

Waterproofing

• Idea is to eliminate holes

• Membranes/barriers• Downside?

• Control joints• Organized concrete failure

• Treat failed cracks

• Disadvantages?

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

• What direction is vapor flow?

• 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

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

Objectives

• Quantitatively and qualitatively describe transport by

1. Liquid flow2. Capillary suction3. Air movement4. Vapor diffusion

• References: MCH ch. 2, ASHRAE ch. 23 & ch. 24