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Home , Air barrier, Vapor barrier

3/27/19

AIR BARRIER ASSOCIATION OF AMERICA MARCH 2019 Why specialists need to understand water.

Dr John Straube, P.Eng. Principal & Senior Specialist, RDH Building Science Associate Professor, University of Waterloo

ABAA

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1 3/27/19

Learning Objectives:

• Recognize the differences and relationship between vapor, air, and thermal control. • Understand key building science principles that determine how and liquid water move through materials and assemblies. • Identify critical details and transitions that impact rain and air control. • List common moisture-related problems and their impacts on buildings and building occupants.

Today

• Vince– control pressure to control flow • Chris- performance & energy • Sarah/Andrew- water leaks

2 3/27/19

Why care about moisture?

• Moisture is involved in most building enclosure performance problems – Construction cycle and in-service durability • Examples: – rot, – corrosion, – mould (IAQ) – buckling – termites, insects – staining – etc.

Why Air Barriers and h20?

1. Air barriers limit airflow and hence limit condensation 2. Many air barrier products also used as Water Resistant Barriers 3. Air barrier products may also be vapor control layers (& hence control condensation)

3 3/27/19

Some definitions

• Water … usually liquid H2O • Moisture … liquid and vapor? • Ice .. Unambiguous solid water • So, – Air Barrier? – ? – Moisture barrier? – Water Barrier?

The Moisture Balance

Wetting Wetting

Safe Storage Capacity

Drying

4 3/27/19

The Moisture Balance

Wetting

Wetting Safe Storage Capacity

Drying

Rain •absorption The Moisture Balance•penetration

Wetting Wetting

Condensation • air • vapor diffusion Plumbing Safe Storage Capacity Built-in

Drying

•Drainage •Air convection •Evaporation-Diffusion

5 3/27/19

Rain •absorption The Moisture Balance•penetration

Wetting Wetting

Condensation • air convection • vapor diffusion Plumbing Safe Storage Capacity Built-in

Drying

•Drainage •Air convection •Evaporation-Diffusion

Review of enclosure design basics

6 3/27/19

The Perfect Enclosure?

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Water Barrier and Air and Vapor Barrier,

Building Science Airflow Control No. 14/79

7 3/27/19

Walls we commonly build

• Hybrid/split walls • Water-Air control in middle • Don’t always need vapor layer

Water Barrier & Air Barrier outside insulated frames

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8 3/27/19

Vapor permeable on exterior

Air Barrier, WRB, VB Placement Considerations permeable permeable Air-Water-Vapor Air-Water Vapor Water Air-vapor

All Exterior Split (exterior & Interior (stud) insulated stud) insulated insulated)

9 3/27/19

Air Barrier, WRB, VB Placement Considerations permeable Air-Water-Vapor Air-Water-Vapor Air-Water Vapor

All Exterior Split (exterior & Interior (stud) insulated stud) insulated insulated)

There are some variations

10 3/27/19

Back to Moisture:

Psychrometrics

• The study of air and its water and energy content • Explains when, where, how much condensation

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Water Vapor Storage in Air 35 C 95 F

21 C 70 F 10 C 50 F 0 C Water Vapor in Air Air in Vapor Water (Moisture Content) (Moisture 32 F

1 2 4 8

Temperature

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Relative 35 C

21 C 95 F

10 C 50 F 70 F 0 C

32 F 50%RH

Water vapor in Air

22 C 35 C 72 F 95 F 50%RH 25%RH 10 C 50 F 0 C 32 F

= =

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Dewpoint Temperature

7.6 g/kg 50 F 70 F 95 F 100%RH 50%RH 25%RH

21 C Dewpoint of 50F 10 C

1/2 = = =

Dewpoint of 50F Dewpoint of 50F Dewpoint of 50F

Cold weather condensation

50 F 70 F 95 F 100%RH 50%RH 25%RH

21 C 0 C 32 F 10 C

1/2 = = =

1/2

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Surface Condensation

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Warm weather condensation

Beware in humid summer climates and 95 F cold occupancies 70 F 35 C 10 C 21 C 50 F 0 C 32 F

100% 75 %

2

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Dehumidification

70 F 95 F 50%RH 35 C 21 C 0 C 50 F 32 F 10 C = = 75 %

4

Interstitial Condensation

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Interstitial Condensation

• Condensation within enclosures • Hidden and hence can be problem

• Water vapor delivered by air leakage and/or diffusion

Condensation • Plane • Condensation occurs on solid surfaces • Not within batt

• “Next solid surface colder than dewpoint”

17 3/27/19

Where does condensation occur? Diffusion or Air Leakage (Convection)

Heat Air and Moisture No.35/78

Which wall is at risk?

18 3/27/19

See: BSD-163 at buildingscience.com

Tback of sheathing = Tinterior – (Tinterior-Texterior) Rbatt/Rtotal Interior 70 F 21 C air 60 F

50 F 10 C

40 F

Back of 30 F 0 C sheathing Safe range of 20 F indoor air dewpoint 10 F -12 C Exterior air 0 F R10 R19

Condensation on cold sheathing

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Rim joist

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“Condensation occurs on the first solid upstream facing surface downstream of the dewpoint being exceeded” Condensation occurs at the first solid upstream-facing surface after “dewpoint”

Heat Air and Moisture No.41

Inside of roof sheathing from attic hatch

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Condensation on cold surfaces

Vapor Movement

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Vapor Transport

• How does vapor get to cold surface? 1. Vapor Diffusion 2. Air leakage (convection)

• Air leakage moves much more vapor in most cases! • Diffusion still important for drying wet materials, inward solar drives

Vapor convection (w/air flow)

• Vapor molecules flow along with air (O2 and N2 molecules are much more numerous and not shown)

∆Pair

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Diffusion through layers

• Vapor molecules moves through microscopic pores in materials

∆Pvapor

Forces Driving Vapor Flow

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How much does diffusion matter?

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(-4 F) (70 F)

When is a layer permeable?

• Test the vapor permeance of a layer • Most materials provide date • Beware: – units (WVTR vs perm) – Test conditions – thickness

25 3/27/19

Vapor permeance ASTM Testing

• ASTM E96: Wet cup, dry cup

Vapour Permeance

Dry Cup Wet Cup

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Room filled with permeance samples @50%RH

Why test wet and dry?

• The permeance of many products varies between wet and dry cup! • Interiors during cold weather have a low RH – Thus vapor barriers best rated by dry cup • Exteriors during cold weather, or when wall is wet, have a high RH – Permeable WRBs best rated by wet cup

27 3/27/19

Smart Membranes

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Inward Drives

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Building Science Heat Air and Moisture No.57/78

Inward Drive Summer

Trump Taj Mahal, Atlantic City, NJ

Mark Makela for The New York Times

29 3/27/19

Design for Diffusion

• Should balance between cold weather and warm weather performance • Location of low permeance layers matters – “vapour barrier” label is not important: materials that have low permeance act as vapor barriers • Ventilation (airflow) of inner or outer layers bypasses vapor diffusion resistance – E.g. vinyl siding, metal panels, roofing

Building Science Heat Air and Moisture No.59/78

Details

• Sometimes small areas of low permeance matter

30 3/27/19

Self-adhered membrane Formable self-adhered Liquid applied membrane membrane Pre-formed • flashing system

Swimming pool in cold climate

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Vapor barrier flashing limits outward drying

Swimming pool in cold climate

31 3/27/19

Water pooling • Small leaks through window & opening • Sits on sub sill flashing • Can diffuse into framing

• Need to balance

Impermeable sub-sill over permeable air-water barrier

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When don’t air barriers stop condensation?

Answer: when they are misplaced

• Cool air is denser • Cool air falls • =convection loop

• Perfect air barrier ….. But energy loss and condensation

Airtight plane

33 3/27/19

Case Study: Architectural Precast • Air gap intentionally provided in design! • Imperfect air seal at upper and lower levels allowed

Precast is so impermeable, No wicking, almost zero drying

Interior/back of precast panel when insulation removed during inspection

Building Science Heat Air and Moisture No.68

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Bottom Exit

Top entry

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Solid wall

Poly

Interior Retrofits

• Loops can occur • Require continuous air space between masonry & insulation • Masonry can wick small amounts away to outside

• Any outward air leaks just make problem worse

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Convection Loop Solutions

• Locate air barrier closer to inside • Make it very airtight (hard) • Avoid airgaps!! Press insulation tight to surface – Eg spray foam, attached boards, compressed fibrous batt

Wind Washing

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Avoid large gaps behind insulation

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Choose mid-density fibrous

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38 3/27/19

Wind washing Solutions

• Pretty easy: • Keep AB in contact with insulation • Use foam or mid-density fiber

Wind washing & convection solved?

• Split wall

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Air Barriers and Rain Penetration

Water penetration & air barriers

• Air leakage through air barrier may increase rain leakage • Is the air and water barrier at the same location?

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www.BuildingScience.com

Pressure Equalization

www.BuildingScience.com

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Pressure Equalization

Pressure Equalization: Reality Air-Water Reality: control layer Cladding leaks because of gravity, and penetrations dump water on drainage plane

• If you get perfect pressure equalization … • all air pressures are across the water control layer, so • if water is on drainage plane it will be driven through any holes by air pressure!

42 3/27/19

Conclusions

• Stop air leakage through enclosure! But . . . Also consider • vapor permeance • Location, location • Climate • occupancy

AIR BARRIER ASSOCIATION OF AMERICA Discussion + Questions [email protected]

Learn more at rdh.com

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