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Air Barriers vs.
Vapor Barriers Roger G. Morse AIA, Stephen M. Lattanzio PE
Paul Haas CIH, Gordon A. Brandon Brian Crowder
Morse Zehnter Associates Rensselaer Technology Park
165 Jordan Road Troy, NY 12180
Moisture Intrusion into Exterior Wall Cavity
• Liquid Moisture • Air Infiltration • Vapor Diffusion
Air Infiltration vs. Vapor Diffusion
Methods Used to Determine Location of Vapor Retarder
• Rule of Thumb – Mapping and prescriptive requirements
• Dew Point Calculation • Hour-by-hour simulation
Location of Vapor RetarderRule of Thumb
• Rule of Thumb: Locate on warm side of insulation – North – toward inside – South – toward outside – Mixed Climate - ????
• Problem – Most places have both a heating and a cooling season
Unforgiving Northern Construction
Forgiving Northern Construction
Unforgiving Southern Construction
Forgiving Southern Construction
Mapping Climate/Hygrothermal Zones
Specific Requirements for Zone
Calculation Methods
• ASHRAE Dew-Point Method – Steady state for given outside temperature
and humidity
ASHRAE Dew-Point Method
Location of Condensation
Dew Point Calculation
Calculation Methods
• Hourly Simulations – Hour by hour weather data – Typical
Meteorological Year (TMY)– WUFI
– MOIST (no longer supported)
Hygrothermal Modeling
• Hygrothermal Modeling Software • WUFI-ORNL/IBP Hygrothermal Model for
Architects and Engineers. Dr. Achilles N. Karagiozis, A. Oak Ridge National Laboratory, Kuenzel, H. Fraunhofer Institute fur Bauphysik, Holzkirchen, Germany
• http://www.ornl.gov/sci/btc/apps/moisture/
WUFI Input Screen
WUFI Output Screen
WUFI Output Screen
WUFI Water Content Profiles
Construction with vinyl wall covering Construction without vinyl wall covering
WUFI Temperature and Humidity ProfilesConstruction with vinyl wall covering Construction without vinyl wall covering Gypsum board wall side face (3) Gypsum Gypsum board wall side face (5) Gypsum board room side face (4) board room side face (6)
Test House # 1
Bonus Room in Attic
Mold Spots Under Plastic Wall Covering
Mold Through Gypsum Board
2×4 STUD (TYP.)
KRAFT PAPER FACED FIBERGLASS BATT
INSULATION
½" GYP. BOARD
CHAIR RAIL
PLASTIC COVERED WALL PAPER
WAINSCOT
VENTILATED ATTIC
AIR CONDITIONED ROOM
Batt Insulation on Attic Side
Attic Ventilated
Attic Side of Walls
Kraft Paper Toward Room
Kraft Paper Laminated Material
Engineered for Perm of 1.0
Mold on Attic Side of Gypsum Bd
Mold on Attic Side of Gypsum Bd
Locations with Painted Surface ATTIC
½" GYP. BOARD 2×4 STUD KRAFT PAPER FACED (TYP.) FIBERGLASS BATT
ROOM INSULATION
AIR FLOW
BATT INSULATION
KRAFT PAPER FACING
SPOTS OF MOLD GROWTH ON PAPER FACE OF DRYWALL
HUMID AIR
Locations with Plastic Wall CoveringATTIC
½" GYP. BOARD 2×4 STUD (TYP.)
ROOM
KRAFT PAPER FACED FIBERGLASS BATT
INSULATION
PLASTIC COVERED
WALL PAPER
AIR FLOW
SPOTS OF MOLD GROWTH ON PAPER FACE OF DRYWALL
HUMID AIR
GYPSUM CORE SATURATED WITH LIQUID WATER
PATCHES OF MOLD INFRINGING ON GYPSUM CORE
BATT INSULATION
KRAFT PAPER FACING
PLASTIC COVERED WALL PAPER
Wall Open to AtticObserved and Predicted Locations of Condensation
Surface
Painted Interior Vinyl Wall Covering
Field Rule of Thumb
Dew Point
Hour by Hour Field Rule of
Thumb Dew Point
Hour by Hour
Outside Face of Kraft × × × × × × Outside Face of Gypsum Board
Light Mold × × × ×
Gypsum Board × × × × Paint × × ×
Ducts Compress Insulation
Mold at Compressed Insulation
Mold at Wall Penetrations
AIR FLOW
CONDENSATION & MOLD GROWTH
½" GYP. BOARD 2×4 STUD (TYP.)
KRAFT PAPER FACED FIBERGLASS BATT
INSULATION
ATTIC
ROOM
OUTLET BOX
COVER PLATE
OUTLET OR SWITCH
Condensation at Penetrations in Wall
Test House # 2
½" PLYWOOD
2×4 STUD (TYP.)
KRAFT PAPER FACED FIBERGLASS BATT
INSULATION
½" GYP. BOARD
VENTILATED ATTIC
AIR CONDITIONED ROOM
OUTLET BOX
Insulated Attic
Attic Ventilated
Pour Insulation – No Vapor Retarder
Plywood Sheathing on Attic Wall
Kraft Paper Toward Inside
Painted Interior SurfaceATTIC ½" PLYWOOD
½" GYP. BOARD 2×4 STUD KRAFT PAPER FACED (TYP.) FIBERGLASS BATT
ROOM INSULATION
AIR FLOW STOPPED
NO MOLD ON ANY SURFACE
Wall with Plywood Air BarrierObserved and Predicted Locations of Condensation
Surface
Painted Interior Vinyl Wall Covering
Field Rule of Thumb
Dew Point
Hour by Hour Field Rule of
Thumb Dew Point
Hour by Hour
Outside Face of Kraft None × None None NA × × × Outside Face of Gypsum Board None None None NA × × Gypsum Board None None None NA × × Paint None None None NA × ×
Air Barriers vs.
Vapor Barriers Roger G. Morse AIA, Stephen M. Lattanzio PE
Paul Haas CIH, Gordon A. Brandon Brian Crowder
Morse Zehnter Associates Rensselaer Technology Park
165 Jordan Road Troy, NY 12180
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