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TRANSCRIPT
Vapor Barriers & Vapor
Retarders
Presented by
Roger V. Morrison, PE, RRC
Deer Ridge Consulting, Inc.
What we’re 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 you’ll 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, ½ 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