moisture physics 708 - university of waterloo

Dr John Straube Presentation Moisture Physics 1

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Understanding Water:Understanding Water:The Physics of Moisture DynamicsThe Physics of Moisture Dynamics

www.civil.uwaterloo.ca/beg

BEGBuilding Engineering Group

Dr John F StraubeDepartment of Civil Engineering

+School of ArchitectureUniversity of Waterloo

Waterloo, Ontario, Canada

2/24/2005 2 /170

OverviewOverview

Moisture and Building ScienceMoisture storage in porous materialsMoisture transport through porous materials Material performance thresholds

Modeling

2/24/2005 3 /170

Moisture PhysicsMoisture Physics

The Basics: The Moisture BalanceWetting, Drying, Storage

The Water MoleculePhases, vapour pressure

Porous Media – wood, concrete drywallMoisture StorageMoisture Transport – diffusion, capillary

2/24/2005 4 /170


2/24/2005 5 /170

The Water MoleculeThe Water Molecule

Asymmetrical = polar Small: one billion = one foot

0.28 nm≅ 3 Å

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The Polar MoleculeThe Polar Molecule

Hydrogen end is “more” positiveOxygen end is “more” negative

O2-

H+

H+

O2-

H+

H+

+

_ +

_

2/24/2005 7 /170

Polar ImplicationsPolar Implications

Boiling point higher than predicted

Adsorptionwater vapour sticks to surfaces

Surface tensionliquid water sticks to itself and surfaces

Surfactantsinteract with water at surfaces

2/24/2005 8 /170

Moisture PhasesMoisture PhasesAs for most materialsAll four phasesoccur commonly in buildings


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Moisture as a Gas (water vapor)Moisture as a Gas (water vapor)

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Vapour PressureVapour PressureFor water vapour in a containerHigher temperature

= more energy= higher velocity= harder collisions with wall (higher pressure)

Greater number of molecules = more collisions with walls (higher pressure)= pressure simply another measure for moisture content

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Water Vapour in AirWater Vapour in Air

Water vapour exists in all airAir has a maximum vapour holding capacity

Capacity changes dramatically with temperatureWhen the maximum holding capacity is exceeded, condensation occurs

These facts are summarized by thepsychrometric chart

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2/24/2005 13 /170-10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0

Temperature ( °C)

0

500

1000

1500

2000

2500

3000

3500

4000

Vap

our

Pre

ssure

(Pa

)

100% RH75% RH50% RH25% RH

Psych Chart: Air Vapour Content vs Temperature

Saturation

50%

RH

25%RH

75%

RH

100%

RH

100%RH

TemperatureA

ir M

oist

ure

Con

tent

= va

pour

pres

sure

(Pa,

in H

g),

hum

idity

rat

io (g

/kg,

gra

ins/

pd)

-10 ºC14 º F

0 ºC32 º F

10 ºC50 º F

20 ºC68 º F

30 ºC86 º F

40 ºC104º F

2/24/2005 14 /170

Moisture as a LiquidMoisture as a Liquid

Polar molecules stick to each otherLiquid water exists in clusters

e.g., H120O60 at room temperatureClusters get smallerwith higher temp

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Vibrating Liquid WaterVibrating Liquid Water

Red= Oxygen (big, M=16)Blue = Hydrogen (small, M=1)

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Moisture as LiquidMoisture as Liquid

Single vapor molecule is smallLiquid cluster is largeHence, Gore-Tex & Tyvek

Vapour molecules pass through small openingsLiquid molecules repelled by hydrophobicE.g., try a pin hole in housewrap


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Moisture as a SolidMoisture as a SolidCanadian moisture jokes

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Ice : Moisture as SolidIce : Moisture as Solid

Ice forms a crystal and expands as it freezese.g., lower density by 9%

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Surface TensionSurface Tension

Water is attracted to selfCreates a “membrane” or “surface film”

+ _ + _ + _

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Surface Tension: Surface Tension: WettableWettableWater attracted to self

more than surfaceWater attracted to

surface more than self


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Capillary Suction PressureCapillary Suction Pressure

Silicon dioxide (glass) – attracts waterResult - meniscus, capillary suction

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Capillary PressuresCapillary Pressures

Result of surface tension = attraction to surfaces

pressure varies with pore sizee.g., height rise in a glass tube

Gravity Down

Surface tension up

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Surface TensionSurface Tension

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Capillary rise between platesCapillary rise between plates

Paper clip

Elasticband


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Calculating capillary riseCalculating capillary rise

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Capillary rise versus diameterCapillary rise versus diameter

0

10

20

30

40

50

60

70

80

90

100

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

diameter [inch]

capi

llary

rise

[inc

h]

0

10

20

30

40

50

60

70

80

90

100

0.000 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 0.010

diameter [inch]

capi

llary

rise

[inc

h]

2/24/2005 27 /170

Particle Size Particle Size vsvs Capillary SuctionCapillary Suction

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

0.00001 0.0001 0.001 0.01 0.1 1 10Radius [mm]

Cap

illar

y Pr

essu

re (P

a)

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SurfactantsSurfactants

Changes the way water molecules interact with surfaces

dramatically reduces surface tensionSoap is a surfactant

connects water to greaseone end is polarone end is non-polar (sticks to oil and grease)


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Soap and WaterSoap and Water

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Nature of Porous MaterialsNature of Porous Materials

Many materials interact with moisture!Many building materials are porous

woodconcrete, brick, gypsum

Nature of material is as important as nature of water

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Slice through a porous materialSlice through a porous material

e.g., wood, brick, gypsum, concrete

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Real MaterialsReal Materials


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Clay BrickClay Brick

Mercuryporosimetry

Pressureplate

BrickSucks!0

5

10

15

20

25

30

1E-91E-81E-71E-61E-51E-4

Pore Radius (m)

Cum

ulat

ive %

of T

otal

Volu

me

Vacuum saturation

24 hr cold absorption

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Simple ModelsSimple Models

Cubic packing of spherical particles

Random particles size and packing

Simple Reality

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Isotropic Isotropic matlmatl’’ss: Wood Structure: Wood Structure

Equivalent to a bundle of tubesflow along grain much faster

Surface tension in Surface tension in small tubes can small tubes can feed water up feed water up 100m (300 ft) tall 100m (300 ft) tall treestrees

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Storage MechanismsStorage Mechanisms

Basic mechanisms in materials

capillary pores (bound liquid, vapour, ice)sorption (adsorbed)

in enclosurespools and puddles (free liquid)


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Adsorbed State of MoistureAdsorbed State of Moisture

Poorly understood by mostWater vapour molecules stick to surfaces

like dust on glass tabledynamic balancemolecules stick and leavedepends on energy of water vapour

Very important for porous materials with large surface areas

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Adsorbed MoistureAdsorbed Moisture

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Adsorption Lower RHAdsorption Lower RH

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Adsorption higher RHAdsorption higher RH

More H20 sticks


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Adsorption Adsorption -- higher temperaturehigher temperature

HigherEnergyLess H20 sticks

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Materials Materials -- Lots of surface areaLots of surface area

0.0001

0.001

0.01

0.1

1

10

100

1000

0.00001 0.0001 0.001 0.01 0.1 1 10Radius [mm]

Surf

ace

Are

a pe

r cc

[m2]

Assuming spherical particles and cubic packing

Surf

ace

Are

a (m

2pe

r cc)

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One layer of molecules

0 20 60Relative Humidity

Moi

stur

e C

onte

nt

40

Low RH

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Moi

stur

e C

onte

nt

40

Multi- layer of molecules


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Moi

stur

e C

onte

nt40


Mid RH

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Moi

stur

e C

onte

nt

40


80

High RH

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CondensationCondensation

Thick layers formno longer attracted strongly to surface

Strongly attached

Loosely attached

No longer attached

C: Interconnected layers, (internal capillary condensation)

A: Single-layer of adsorbed molecules

B: Multiple layers of adsorbed molecules

D: water in pores, capillary suction

E: Supersaturated Regime

A

wcrit

wcap

wsat

Adsorbed Water: Hygroscopic Regime

B C

E

Supersaturation - all pores filled

Relative Humidity (%)

Capillary Saturation

00 2020 100100808060604040

DAbsorbed Water: Capillary Regime

Free Water: Supersaturated Regime

Moi

stur

e C

onte

nt (

w%

)


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Moisture RegionsMoisture Regions

Hygroscopicdry to the touch but still moisturee.g., wood can store over 25% by weight

Capillary rangewet to touch, but no draining (sponge)e.g., brick 5 to 20% by weight

Over-saturated rangewater drains from materiale.g., crushed stone

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Predicting SorptionPredicting Sorption

BET with Rounsley enhancement

XX

CC

n

1 1 111

=+ −

−−

⎡

⎣⎢

⎤

⎦⎥

φφ

φφ( )

where X is the equilibrium moisture content (M%), X1 is the moisture content in a mono-layer of adsorbate (M%),

φ is the relative humidity, C is a constant, comprising the heat of adsorption, heat of condensationand the universal gas constant (typically 20 to 50), n is the maximum number of layers that can be adsorbed (typically 4-6).

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Vapor pressuresVapor pressures

Curved SurfacesKelvin’s Equation

TRr cos2

wPcapP

ln wv ⋅⋅⋅⋅⋅−

=⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

ρθσ

where, ρ is the density of water,

Pcap is vapor pressure over the curved surface of water in a capillarypore, Pw is the water vapor pressure over a flat surface at the sametemperature, Rwv is the water vapor gas constant, and other terms are as before.

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Kelvin: Capillary condensationKelvin: Capillary condensation


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Sorption + CondensationSorption + Condensation

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SorptionSorption

0%

5%

10%

15%

20%

25%

30%

0% 20% 40% 60% 80% 100%Relative Humidity

Moi

stur

e C

onte

nt (M

%) concrete

brickcement stuccosoftwood

to 140%

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Sorption IsothermSorption Isotherm

0

5

10

15

20

0 20 40 60 80 100RH (%)

MC (M

%)

SprucePlywoodFibreboardRed BrickMortarConcreteStucco

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HysteresisHysteresis of Sorptionof Sorption

Ink Bottle effect


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Sorption Sorption vsvs TemperatureTemperature

Sorption vapor vs temperature and MC

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Temperature EffectsTemperature Effects

0

5

10

15

20

25

30

0 20 40 60 80 100 120Relative Humidity (%)

Mos

iture

Con

tent

(w%

)

02749

Softwood

Temperature

Its RH not vapour pressure

ºC/ 80 º FºC/ 120 º F

ºC/ 32 º F

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Sorption testSorption test

Tupperware Standard- Balanced ASTM

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Significance Of RegionsSignificance Of Regions

mould, corrosiontend to begin around 80%layer is thick enough that “free” H20 is avail.

Swellingonly occurs in hygroscopic range

Freeze-thawcapillary range


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SwellingSwelling

1

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SwellingSwelling

2

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SwellingSwelling

3

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SwellingSwelling

4


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Swelling Causing CurvingSwelling Causing Curving

Swelling on Bottom Side

• E.g., concrete slab curl• Deck Board cupping

Shrinkage on Top Side

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Moisture SwellingMoisture Swelling

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Wood Swelling and ShrinkageWood Swelling and Shrinkage

∆=3.5%

Wood acts as a bundle of tubesShrinks across grain, not alongOnly during adsorptionE.g. Drywall cracks

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Colonial DoorsColonial Doors

DesignedDesigned so that frame so that frame has nearly no cross grainhas nearly no cross grainCrossCross--grain panels slidegrain panels slide

Detail Section


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Wood expansionWood expansionConcrete ShrinkageConcrete Shrinkage

Stucco ShrinkageStucco Shrinkage

Frame ShrinkageFrame Shrinkage

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Brick expansionBrick expansionConcrete ShrinkageConcrete Shrinkage

Adobe shrinkageAdobe shrinkageStucco Stucco spallsspalls

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SolutionsSolutions

Allow movementCupping

Reduce gradientReduce thicknessReduce stiffness & restrain

Shrinkage swellingReduce range of RHReduce rate of changeReduce cross grain!

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Measuring Moisture StorageMeasuring Moisture Storage

Sorption, capillary saturation, pore volumeMaterial Density (Dry)

kg/m3

Open Porosity

(%)

MC @ ≅95%RH

(M%)

wcap

(M%)

Concrete 2200 15-18 4-5 6-8 Brick 1600-2100 11-40 3-8 6-20 Cement Mortar 1800-1900 20-30 5-7 14-20 Softwood 400-600 50-80 20-30 100-200 Fibreboard 240-380 60-80 20-25 100-200 Wood chipboard 700 50-70 15-20 100-150 Expanded polystyrene

32 95 5 > 300

Gypsum (exterior) 1000 70 10 50-100


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Moisture TransportMoisture Transport

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Moisture TransportMoisture Transport

Moves from high concentration to lessThis does not always mean more moisture content or more vapour pressure!Each phase should be considered separately

liquidvapor (diffusion and air leakage)adsorbedice

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Liquid flowLiquid flowMay be driven by:

gravitypressure differences (air or water)capillary pressures

Capillary – 0 to 10 MPa (M!)Gravity always acts, downward

About 10 000 Pa/m (1.3 psi/yd =15 psi/33 ft)Air pressure varies

wind typically < 100 Pa (0.015 psi= 2 psf) short 1000 Pa bursts (0.15 psi = 20 psf)

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Capillary FlowCapillary Flow

Eg. : Crushed stone, air gapslarge pores - no suction (“wicking”)


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Capillary FlowCapillary FlowExample: Sand, siding lapsSmaller pores - some wicking (inches to feet)

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Head Joints Head Joints –– capillary crackcapillary crack

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Water Penetration MechanismWater Penetration Mechanism

Hypothetical area of bonded mortar in head joint

Potential interfacial crack plane locations

Water drawn into crack by capillarity

Hydrostatic head: highest entrance to deepest exit

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Pressure and Masonry PermeancePressure and Masonry Permeance

Application Rate:200 l/m /hr2

Suction Pressure0

123456

-125 -100 -75 -50 -25 0 25 50 75 100 125

Pressure Difference (Pa)

Pene

trat

ion

Rat

e (l/

hr/m

2 )


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ImplicationsImplications

Air pressure does not substantially increase water permeance of masonryHence, pressure equalization is not that importantDrainage is the key

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Capillary FlowCapillary Flow-- concrete sucksconcrete sucksExample: Clay or siltWicking (dozens - hundreds of ft)

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Particle Size Particle Size vsvs Capillary SuctionCapillary Suction

1.00E+01

1.00E+02

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

0.00001 0.0001 0.001 0.01 0.1 1 10Radius [mm]

Cap

illar

y Pr

essu

re (P

a)

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Measuring Liquid TransportMeasuring Liquid Transport

Water uptake


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Typical resultsTypical results

0

5

10

15

20

25

30

0 2 4 6 8

Square Root of Time

Weig

ht g

ain (

g sin

ce st

art)

Slope = waterabsorptioncoefficient

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Liquid transport: wettingLiquid transport: wetting

Role of pore structureRole of pore structure

Place porous material in contact with water

Small capillaries have large suction and large flow resistance

Large capillaries have less suction and little flow resistace

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Role of pore structure




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Liquid transport: redistributionLiquid transport: redistribution




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HydrophobicHydrophobicExample: silicone impregnation, many (clean) plastics and polymersRepels water – no wicking!

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Water RepellentsWater Repellents

Normal, capillaryactive material

water

material material

Hydrophobicallytreated material

Waterflowing in:system not inequilibrium

No water flow

E.g., silicone


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Water Vapour TransportWater Vapour Transport

Vapour Diffusion (like heat conduction)

more to less vapourAir Convection (like heat convection)

more to less air pressureflow through cracks and holesvapour is along for the ride

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Vapour DiffusionVapour DiffusionSlow process – through open poresSome materials allow easy diffusion

Very open porese.g. batt, gypsum, cellulose, etc

Many materials resist diffusionsmall pored materialse.g., concrete, brick, stone

Some stop, or practically stop itcrystals, or microporee.g., many plastics (poly), metals, glass

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DiffusionDiffusion

Each layer reduces vapor flow

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AirflowAirflow

Vapor flows with airConstant flow across layers


2/24/2005 105 /170-10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0

Temperature ( °C)

0

500

1000

1500

2000

2500

3000

3500

4000

Vap

our

Pre

ssure

(Pa

)

100% RH75% RH50% RH25% RH

Air Leakage vs Diffusion

Summer

Winter

Air LeakageCondensation

Diffusion

50%

RH

25%RH

75%

RH100%

RH

•Air barrier systems essential•Low perm vapor barrier rarely needed

-10 ºC14 º F

0 ºC32 º F

10 ºC50 º F

20 ºC68 º F

30 ºC86 º F

40 ºC104º F

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VapourVapour Permeability MeasurementPermeability Measurement

e.g. ASTM E96

Procedure A Procedure B

Water= 100%RH

Specimen

23 °C /50%RH

Dessicant= 0%RH

Specimen

23 °C /50%RH

Water= 100%RH

Specimen

23 °C /50%RH

Procedure BW

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The Tupperware standardThe Tupperware standard

Can conduct tests in your own kitchen with special salts and good scale

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Typical saltsTypical salts

Italics are temperature insensitive

Salt RH% Salt RH

Lithium Chloride 11.3 Sodium Chloride 75.3

Potassium Acetate 22.5 Potassium Chloride 84.3

Magnesium Chloride 32.8 Barium Chloride 90

Potassium Carbonate 43.2 Potassium Nitrate 93.6

Magnesium Nitrate 52.9 Potassium Sulfate 97.3


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VapourVapour Permeance Permeance vsvs RHRH

Relative Humidity (%)1000 50

δp

Test Results

““PermeancePermeance””changes with RHchanges with RHother transport other transport mechanisms as workmechanisms as work

Example of Building Paper

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Adsorbed FlowAdsorbed Flow

Also called surface diffusion

Driven by RH differences!

Affects highly porous, especially fibrous natural materials

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Adsorbed moisture flows from more MC to less MC (or from high RH to low RH)

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Competing MechanismsCompeting Mechanisms

Tightly bound adsorbed molecules --no surface diffusion

Surface diffusion flow direction opposes vapour diffusion !

E.g. Cold and humid outsidewarm and dry inside


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VapourVapour Permeance: StuccoPermeance: Stucco

0

100

200

300

400

500

600

700

0% 20% 40% 60% 80% 100%Relative Humidity (%)

Perm

eanc

e (fo

r 19

mm

thic

k)

Mean

Lower

Upper

Result of Adsorbed Flow

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VapourVapour Permeance: SheathingPermeance: Sheathing

0

500

1000

1500

2000

2500

0 0.2 0.4 0.6 0.8 1

Relative Humidity

Per

mea

nce

(ng/

Pa

s m

2) 11.1 mm OSB

12.7 mm Plywood

Dry

Cup

Wet

Cup

Result of Adsorbed Flow

Kraft Paper, Smart Retarder are similar

101

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Vapour BarriersVapour Barriers

Control vapour diffusion onlyVapour barriers in Code: <1 US perm

based on Rowley 1937 no good science

Vapour retarder approx 2-5 US permMeasurement Units

Metric perms ng /(s·m2·Pa)US perm grain/(hr·in Hg· ft2)WVT grams/(sq ft/24 hours)

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LowLow--permeance Materialspermeance Materials

6 mil Poly 3.5 metric (0.1 US perm)Vinyl wall paper 15 metric perm (0.3 US)Concrete 1/2 perm for 8” foundation wallDrywall with a VB paint 0.3 to 1 permBrick veneer (1/2 - 2 US perms)Extruded foam 1/2 - 1 US perms for 1.5”Plywood 0.5 to 20 US perms (dry to wet)Kraft paper 0.3 to 2 US perms (dry and wet)


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SemiSemi--permeable Materialspermeable Materials

Plywood 0.5 to 20 US perms (dry to wet)Expanded foam 2.5 - 5 US perms for 1 inchSpray PUR about 2 US permsDrywall with latex paint (2-5 US perms)

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HighHigh--perm Materialsperm Materials

Fibreboard over 20Plywood 0.5 to 20 (dry to wet)Icynene open cell spray foam 10 - 13Tyvek other housewraps 20 to 50 permsBuilding paper over 10 to 30

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Summary Moisture TransportSummary Moisture Transport

Moisture in 3 phases moves with three mechanisms

Liquid capillary (suction)Adsorbed flow (RH)Vapour diffusion (vapor pressure)

Moisture storage in all three phases, only two important for hygroscopic

Adsorbed (RH)Liquid (capillary suction)

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Moisture ThresholdsMoisture Thresholds


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Material Performance ThresholdsMaterial Performance Thresholds

Depends on What PerformanceCorrosionMouldDecayFreeze-thawDissolution/Dissassociation

Also, mechanical properties, insulating, etc.

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Performance of What?Performance of What?

Materials - asphalt, paperLayers - building paperSub-assembly - lapped, between airspace/sheathingAssembly - drained stucco over steel studEnclosure - wall, joints, windowBuilding - 12 storey apartment bldgSite - seashore or shelteredClimate -Miami or Minneapolis

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Material PerformanceMaterial Performance

How to predict performance?1. We test materials or layers. 2. Model assemblies in specific climates.

Must know loads, microclimate

““No Wrong Material, No Wrong Material, Just Materials Used the Wrong WayJust Materials Used the Wrong Way””

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Florida Pier

Structural Steel Waterloo

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Steel CorrosionSteel Corrosion

Electrochemical Process - oxygen + electrolyteCan begin if RH>80%Coatings protectZinc galvanizing is sacrificialFactors

Temperature (Arhenius Law)Time of Wetness (TOW)pH of environmentSalinity

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RH, T and RH, T and CorrosionCorrosion


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CorrosionRate

(g/m2/day)

0 7 14pH

pH and Steel CorrosionpH and Steel Corrosion

(Base)(Acid)

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CorrosionRate

(g/m2/day)

0 7 14pH

pH and Zinc CorrosionpH and Zinc Corrosion

steel

zinc

acidic basic

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• Spores and toxins Spores and toxins cause cause immunoimmuno--depressiondepression

•• LBL estimates US LBL estimates US health and health and productivity losses at productivity losses at $30 to 100 billion/yr$30 to 100 billion/yr

•• ““Sick Building Sick Building SyndromeSyndrome””

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Mould Growth On SurfacesMould Growth On Surfaces

SurfaceSurface Humidity > about 80%RHTemperature 5C/40F - 50C/120F

Food Source (cellulose, soap, wood, oil)pH - usually less than 8 - 10

GrowthRate

0C/32F Temperature 50C/120F


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From: Klaus Sedelbauer PhD Thesis

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Germination & GrowthGermination & GrowthTime for spore germination

Rate of GrowthPlots for perfect food source

From: Klaus Sedelbauer PhD ThesisBrampton stripmall

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Minimum LevelsMinimum Levels

Different species = different conditionsDynamically varying mix as substrate and conditions change

From: Clarke, Bldg & Environ

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Wood DecayWood Decay

Surface Humidity > 95%RH, MC>30%Temperature 5C/40F - 50C/120F

Highly species, sample, exposure dependent

GrowthRate

0C/32F Temperature 50C/120F

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Waterloo, PatioWaterloo, Patio

Buffalo, NYBuffalo, NY

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FreezeFreeze--thawthaw

Must be nearly saturated while freezingFactors

degree of saturationhow coldrate of freezingpores size distributionliquid diffusivity

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SubfluorescenseSubfluorescense

Salts absorbed in porous mineral material while dissolved in liquid waterWater evaporates salts recrystalize


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DissolutionDissolution

Water is the universal solventAvoid capillary saturatione.g.:

EIFS finish re-emulsificationGypsum becomes goopaper unglues

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MicroclimateMicroclimate

Surface HumidityT= 20 CRH = 60%

0

Temperature

VapourVapourPressurePressure

40-20 20

T= 14.5 C, RH=85%

DamagePossible

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MicroclimateMicroclimate

pH Acid or Alkaline

pH=10

pH=4?

Steel in Concrete

Screw in Gypsum


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ConclusionsConclusions

Moisture control is a balance of wetting and dryingWater is a unique molecule

Adsorption – swelling/shrinkingCapillary / surface tension

Separate the phases ….Vapor, liquid, adsorbed, solid

… and transport modes …Diffusion, convection, capillary, adsorbed

… when thinking about moisture

moisture physics 708 - university of waterloo

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