Moisture and Buildings Building Science...– e.g. steel stud, vs wood stud vs concrete block – 1:...

Building Science

Moisture

Joseph Lstiburek, Ph.D., P.Eng John Straube, Ph.D., P.Eng

presented by www.buildingscience.com Building Science 2008

Moisture and Buildings

•  Moisture is involved in almost all building envelope performance problems –  In-service .... Durability

•  Examples: –  rot, – corrosion, – mould (IAQ) –  termites, (!), – staining – etc.

Heat Air and Moisture No.2/78

Building Science 2008

Moisture Damage

•  Damage caused by – Very high humidity for a long time – Wet (100%RH) for a shorter time

•  Time required depends – on material – Temperature

•  Higher temperature accelerates process

Heat Air and Moisture No.3/78 Building Science 2008


RH>80%, surface wet warmer= better

Building Science 2008 Heat Air and Moisture No.5/78

Near saturation combined with hard freeze Building Science 2008



RH>80%, surface wet warmer= better Building Science 2008

Moisture Control

•  Moisture-related Problems 1. Moisture must be available 2. There must be a route or path 3. There must be a force to cause movement 4. The material must be susceptible to damage

•  Theory: – eliminate any one for complete control

•  Practice: – control as many as possible


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Drying

Wetting

kg

Wetting

Moisture Balance

Safe Storage Capacity



Drying

Wetting

kg

Wetting

!

Moisture Balance: Accumulation



Drying

Wetting

kg

Wetting

Moisture Balance


• Drainage • Air convection • Evaporation-Diffusion Heat Air and Moisture No.11/78


Wall + Roof Wetting Sources/ Mechanisms

1.  Rain –  absorption –  penetration –  splash and drips

2.  Water Vapor Movement –  Diffusion –  Convection (air leaks)

3.  Built in 4.  Ground

–  Capillary (wicking) –  Gravity –  Diffusion



Wall + Roof Drying Sinks and Mechanisms

1.  Surface Evaporation •  Wicking to surface

2. Vapor Movement i) Diffusion

ii) Convection 3. Drainage 4. Intentional Convection =

Ventilation Drying


Note above and below grade Building Science 2008

Ventilation Drying

Clear Air Spaces

Vent Holes Above& Below Window

Vent Holes at Top& Bottom of Wall

• Ventilation provides drying to the exterior • Can be important for: 1. vapor impermeable cladding

•  metal panels •  most roofing

2. systems which retain rainwater

• Improves survivability of small rain leaks and condensation


Vented roofs soffit to ridge/gable


Storage •  Bridges gap in time between wetting and

drying •  How much moisture for how long before

damage •  Safe storage: safe against what?

–  mold, rot, freeze-thaw, corrosion •  Basic mechanisms

–  Absorbed into materials= capillary pores (bound liquid)

–  Adsorbed to materials = sorption (vapor) –  pools and puddles (free liquid)


Moisture Storage in Assemblies

1.  Trapped / undrained 2.  Surface tension

•  Liquid or solid 3.  Adsorbed 4.  Absorbed 5.  Vapor

•  small

3,4



Design Choices

•  Either avoid wetting •  Or, provide enough drying to accommodate wetting •  Depending on the storage provided

The balance has shifted over time •  Amount of storage has changed over last 100 yrs

–  e.g. steel stud, vs wood stud vs concrete block –  1: 10 : 100+

•  Wetting is usually less •  Drying is often much less


Design Solutions

•  Balance wetting, drying, and storage •  Practical Rules

– Provide a continuous plane of rain control including each enclosure detail

– Provide continuous air barriers and insulation to control condensation problems

– Allow drying of built-in and accidental moisture – beware drying retarders



The Water Molecule

•  Asymmetrical = polar •  Small: one billion = one foot

0.28 nm ≅ 3 Å

+

- Heat Air and Moisture No.19/78


The Polar Molecule

•  Hydrogen end is “more” positive •  Oxygen end is “more” negative

O 2-

H +

H +

O 2-

H +

H + +

_

+

_



Water: Liquid vs Vapor

•  Vapor is a single molecule •  Liquid is molecular clumps, 60 or more •  Tyvek vs asphalt

Vapor

Liquid


Moisture as a Gas (water vapor)



Building Science 2008 -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

100% RH

75% RH

50% RH

25% RH

Psych Chart: Air Vapour Content vs Temperature

Saturation

50%

RH

25%RH

75%

RH

100%

RH

100%RH

Temperature

Air

Moi

stur

e C

onte

nt

= va

pour

pre

ssur

e (P

a, in

Hg)

, h

umid

ity r

atio

(g/k

g, g

rain

s/pd

)


-10 ºC 14 º F

0 ºC 32 º F

10 ºC 50 º F

20 ºC 68 º F

30 ºC 86 º F

40 ºC 104º F

Water vapor in Air


95 F

72 F 50 F

32 F

1 2 4 8

Water vapor in Air

Building Science Heat Air and Moisture No.26/78

95 F

72 F

50 F

32 F

1 2 4 8

0 C

10 C

22 C

35 C

Water vapor in Air


95 F 72 F

50 F

32 F 50%RH 0 C

10 C

22 C

35 C RH is relative


95 F 72 F

50 F

32 F

= =

50%RH 25%RH

Condensation


95 F 72 F 50 F

32 F

1/2 = = =

50%RH 25%RH 100%RH


Methods to get Condensation


Definition of “dewpoint”


95 F 72 F

50 F

32 F 50%RH 0 C

10 C

22 C

35 C Definition of “dewpoint”


95 F 72 F

50 F

32 F 25%RH 0 C

10 C

22 C

35 C


-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

100% RH

75% RH

50% RH

25% RH

Indoor Conditions

•  68 – 78 F (20-26 C) •  25 to 60%RH • (Could be 20 to 70%) • Dewpoint: 30 - 60F

Summer

Winter

-10 ºC 14 º F

10 ºC 50 º F

20 ºC 68 º F

30 ºC 86 º F

40 ºC 104º F

0 ºC 32 º F


Temperature (°C)

0

500

1000

1500

2000

2500

3000

3500

4000

100% RH

75% RH

50% RH

25% RH

Outdoor Conditions Summer / Winter • January 35-50F / 0-10 C &80-100% • July 75-95F /24-35°C & 50-90% • Dewpoint 60-85 F

-10 ºC 14 º F

0 ºC 32 º F

10 ºC 50 º F

20 ºC 68 º F

30 ºC 86 º F

40 ºC 104º F

Warm weather condensation


95 F 72 F

50 F

32 F

100% 75 %

0 C

10 C 22 C 35 C

Dehumidification


72 F

50 F

32 F

=

50%RH 95 F

75 % =

0 C 10 C

22 C 35 C


Cooling to get Condensation

Heat Air and Moisture No.37/78 Building Science


95 F 72 F 50 F

32 F

1/2 = = =

50%RH 25%RH 100%RH

Cold weather condensation

0 C 10 C

22 C


95 F 70 F 50 F

32 F

= = =

25%RH 12%RH

100%RH

Control interior RH

50%RH

0 C

10 C 21 C

Powerful means of controlling condensation in cold climate buildings


Temperature (°C)

0

500

1000

1500

2000

2500

3000

3500

4000

100% RH

75% RH

50% RH

25% RH

Indoor vs Outdoor Conditions

-10 ºC 14 º F

0 ºC 32 º F

10 ºC 50 º F

20 ºC 68 º F

30 ºC 86 º F

40 ºC 104º F

Vapour Pressure Difference

Inward drive bigger than outward

Tighter control of T/RH increases drive

Summer

Winter





Water Vapour Transport

•  Vapour Diffusion (like heat conduction)

– more to less vapour •  Air Convection (like heat convection)

– more to less air pressure –  flow through cracks and holes – vapour is along for the ride


Vapour Diffusion

•  Slow process – through open pores •  Some materials allow easy diffusion

–  Many, very open pores –  e.g. batt, gypsum, cellulose, etc.

•  Many materials slow/retard diffusion –  small pored materials –  e.g., concrete, brick, stone

•  Some stop, or practically stop it –  crystals, or micropore –  e.g., many plastics (poly), metals, glass





Temperature (°C)

0

500

1000

1500

2000

2500

3000

3500

4000

100% RH

75% RH

50% RH

25% RH

Outdoor Conditions Summer / Winter • January 35-50F / 0-10 C &80-100% • July 75-95F /24-35°C & 50-90% • Dewpoint 60-85 F

-10 ºC 14 º F

0 ºC 32 º F

10 ºC 50 º F

20 ºC 68 º F

30 ºC 86 º F

40 ºC 104º F


Temperature (°C)

0

500

1000

1500

2000

2500

3000

3500

4000

100% RH

75% RH

50% RH

25% RH

Indoor vs Outdoor Conditions

-10 ºC 14 º F

0 ºC 32 º F

10 ºC 50 º F

20 ºC 68 º F

30 ºC 86 º F

40 ºC 104º F

Vapour Pressure Difference

Inward drive bigger than outward

Tighter control of T/RH increases drive

Summer

Winter





Cold-weather air leakage condensation

Air leakage pathways


Building Science

Water Vapor in Walls Enclosures No. 53 /

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Moisture and Buildings Building Science...– e.g. steel stud, vs wood stud vs concrete block – 1:...

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