# vapor barriers & vapor retarders - barriers vapor  · vapor barriers & vapor retarders

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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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