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  • Vapor Barriers & Vapor

    Retarders

    Presented by

    Roger V. Morrison, PE, RRC

    Deer Ridge Consulting, Inc.

  • What were gonna talk about

    1. Water vapor drive and transmission

    2. Psychrometrics

    3. Vapor barriers and vapor retarders

    4. SPF vapor transmission properties

    5. Best practices

    6. Analytical techniques

  • Water Vapor Transmission

    Fundamentals

    Basic flow equation

    Flow = drive / resistance= drive x permeance

    D = P x M

    D = water vapor diffusion

    P = water vapor pressure (absolute humidity)

    M = permeance

  • Fundamentals (cont)

    Diffusion

    Water vapor molecules wending their

    way through solid or porous materials

    Drive

    Water vapor moves from regions of high

    humidity to regions of lower humidity

    Absolute humidity is the driving force

  • Fundamentals (cont)

    Permeance

    Tendency of a material to allow water

    vapor to diffuse through it

    Dependent on:

    Physical properties of the material

    (permeability)

    Thickness

  • Fundamentals (cont)

    Permeance

    The tendency of a material to allow the

    passage of water vapor at a specified

    thickness

    Permeance values MUST report the thickness

    Grains H O

    ft hr in HgPerm2

    2

    Note: 1 pound = 7000 grains

  • Fundamentals (cont)

    Permeability

    The tendency of a material to allow the

    passage of water vapor

    NOT thickness dependent; thicknesses are

    NOT reported

    Divide by the actual thickness to determine the

    Permeance

    Grains H O in

    ft hr in HgPerm inch2

    2

  • Fundamentals (cont)

    Permeance (perm)

    Specify the

    thickness when

    reporting this

    value

    Grains H O

    ft hr in Hg

    2

    2

    Permeability

    (perm-inch)

    Divide by the

    thickness to get

    permeance

    Grains H O in

    ft hr in Hg

    2

    2

  • Psychrometrics

    Warm air can hold more water vapor

    than cold air

    Air can become saturated with water

    vapor (Condensation)

    The saturation threshold increases

    with temperature

  • Psychrometric Chart

    TEMPERATURE

    ABSOLUTE HUMIDITY

    SATURATION

    100% REL HUMIDITY

    50% REL HUMIDITY

  • Psychrometric Chart

  • SPF is a very useful material

    to control temperature and

    humidity profiles in building

    assemblies

  • More Fundamentals

    Water vapor concentration (absolute

    humidity) can build up within

    building assemblies due to water

    vapor drive

    Condensation occurs when the

    temperature within the assembly

    drops below the dew point

  • More Fundamentals (cont)

    AVOID condensation by:

    Preventing building components from

    dropping below the dew point

    Reducing water vapor entering the

    building component

    Increasing water vapor leaving the

    building component

  • More Fundamentals (cont)

    Preventing building components from dropping below the dew point

    Reducing water vapor entering the building component

    Increasing water vapor leaving the building component

    Placing INSULATION on the cold side of vapor retardant materials

    Installing vapor RETARDANT materials on the warm side of the assembly

    Installing BREATHABLE materials on the cold side of the assembly

    Avoid condensation by:

  • Barriers & Retarders

    The definition keeps changing

    Vapor Retarder is the preferred

    term but youll commonly hear the

    term Vapor Barrier

    Common definition:

    Vapor retarder 1 perm

    Not a very good definition

  • Vapor Barriers & Retarders

    Impermeable Class I VR < 0.1 perm

    Semi-

    Impermeable

    Class II VR 0.1 1.0 perm

    Semi-Permeable Class III VR 1.0 10.0 perm

    Permeable > 10.0 perm

  • Vapor Barriers & Retarders

    0.1

    1.0

    10

    Polyethylene

    Metal Foils

    Butyl Coatings

    Roof Membranes

    SPF (closed-cell) > 2

    Kraft Paper Laminate

    Plywood

    Vinyl Coverings

    SPF (closed-cell) < 2

    SPF (open-cell) > 2

    Polystyrene Foam

    Acrylic/Silicone Coatings

    SPF (open-cell) < 2

    Sheetrock

    Fiberglass

  • Spray Foam Properties

    Closed-cell, 2 pcf SPF

    Permeability = 1.8 perm-in

    Permeance = 0.9 perm @ 2

    Open-cell, pcf SPF

    Permeability = 15 perm-in

    Permeance = 4.3 perm @ 3.5

  • SPF: Retarder or Not?

    0.1

    1.0

    10

    1 ocSPF (15)

    2 ocSPF (7.5)

    3.5 ocSPF (4.3)

    6 ocSPF (2.5)

    1 ccSPF (1.8)

    2 ccSPF (0.90)

    3.5 ccSPF (0.51)

    6 ccSPF (0.30)

  • Theory of Relativity

    Water vapor retardance is relative:

    When determining the need for and

    type of vapor retarder, consideration

    must be given to all the other

    materials with which it is assembled.

    What may be a retarder in one case

    may be considered a breather in

    another.

  • Best Practices

    When this cannot be done, install a vapor

    retarder such that the vapor retarder is

    positioned as close to the side with the

    highest absolute humidity as possible and

    the vapor retarder has an installed perm

    rating substantially less than that of the

    next lowest component.

    Install the building materials such that relative

    vapor retardance increases toward the side

    with the higher absolute humidity (usually the

    warm side).

  • Analytical Techniques

    Dew Point Analysis

    WUFI Simulation

  • Dew Point AnalysisRoof ProfileTemperature ProfileHumidity ProfileDesign ConditionsExterior

    10o F

    50 % RH

    Interior

    70o F

    40 % RH

    Dew Point

    Condensation

    Saturation

    Line

    Calculated

    Humidity

    Line

    T TR

    RT Tx i

    x

    i elog logP AB

    TC TP P

    M

    MP Px i

    x

    i e

    ( / )

    ( / )

    1

    1

  • WUFI Simulation

  • Review

    Water vapor moves from regions of

    high absolute humidity to regions of

    lower absolute humidity

    Rate of vapor movement is

    determined by the permeance of the

    building assembly components

  • Review (cont)

    Problems are likely to occur if

    condensation occurs

    Condensation can be avoided by the

    judicious use and placement of

    insulation and breathable and

    retardant materials

  • Review (cont)

    SPF can be used to control both

    thermal and humidity profiles in

    building assemblies to avoid

    condensation

    Analytical techniques may be used to

    predict condensation potential in

    building assemblies

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