Overview of Hygrothermal Modeling
and Demonstration of WUFI®
Presented to: The Atlanta Building Enclosure Council
July 11th, 2012
Bryce McQueen
Continuing Education Quality Assurance
“Wiss, Janney, Elstner Associates” is a Registered Provider with The American
Institute of Architects Continuing Education System (AIA/CES). Credit(s) earned on
completion of this program will be reported to AIA/CES for AIA members.
Certificates of Completion for both AIA members and non-AIA members are
available upon request.
This program is registered with AIA/CES for continuing professional education. As
such, it does not include content that may be deemed or construed to be an
approval or endorsement by the AIA of any material of construction or any method
or manner of handling, using, distributing, or dealing in any material or product.
Questions related to specific materials, methods, and services will be addressed at
the conclusion of this presentation.
Learning Objectives
This presentation will provide an introductory level discussion of concepts
associated with heat and moisture transfer through building enclosure assemblies
and provide a demonstration of the use of one-dimensional hygrothermal analysis
software known as WUFI® (Wärme und Feuchte instationär). After attending this
presentation participants will be able to:
1. Discuss the basics of hygrothermal modeling concepts and techniques
2. Discuss moisture transport and storage fundamentals and hygrothermal
phenomena in building materials and assemblies
3. Discuss moisture control performance criteria for building facades, including
ASHRAE Standard 160-2009 Criteria for Moisture Control Design Analysis in
Buildings.
4. Describe a case study demonstration of WUFI®
5. Discuss the basic general capabilities and limitations of hygrothermal modeling
software such as WUFI®
Hygrothermal Modeling
Hygrothermal
Change in properties due to moisture absorption and temperature change
Hygrothermal Modeling
Dynamic simulation of moisture and heat transport through multi-layer building
components over a period of time
Benefits
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Provides opportunity to simulate exterior wall hygrothermal performance
(without air flow) for comparative purposes
Can be used as a tool to confirm design decisions (within the limits of the
software) for wall assemblies in various climatic conditions
Does not require destructive investigation or significant resources to perform,
however, confirmation of simulation results are recommended
Environmental Data Monitoring
Actual hygrothermal conditions of an assembly using environmental data loggers
Environmental Data Analysis
Analysis of hygrothermal conditions using environmental data loggers
Benefits
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Actual results (not predicted)
that can be verified
Continual wireless monitoring
Drawbacks
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Significant Cost
Time for Analysis
Interruption of facility
operations
Destructive methods required
for installation
Possible equipment failure
Sudden environment changes
(i.e. opening windows or
ceiling tiles) can affect results
Hygrothermal Analysis
Benefits
• Dynamic view of modeled
results depicting
temperature, relative
humidity, dew point, and
water content values over
time
• Analysis time is relatively
short
• Non-destructive analysis
(except in instances of asbuilt verification)
Drawbacks
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Results are not actual
Numerous limitations apply
(discussed later)
Verification of as-built
conditions and results is still
recommended in most
instances
Hygrothermal Model Inputs
Assembly Data
Exterior Macro Climate
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Materials
– Material Data
– Hygrothermal Functions
•
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–
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Additional Data
– Heat Sources
– Moisture Sources
– Air Changes
Surface Transfer Properties
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Orientation
Inclination
Height
Permeance
Absorptivity
Heat Resistance
Initial Moisture Content
Historical Climate Data
Air temperature
Air humidity
Short wave radiation
Long wave radiation
Precipitation
Wind velocity and direction
Optional: measured data
Interior Climate
•
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Air temperature
Air humidity
Time
•
Simulation Duration
Fundamentals of Moisture Transport/Storage
• Moisture is present in the air both inside and outside the building
envelope and may be in the form of solid ice, liquid, or vapor
• Moisture is driven through the building envelope by natural forces such as:
– Temperature
– Partial vapor pressures (sometimes expressed in terms of relative humidity)
– Liquid pressure (suction forces) caused by rain water, water leaks, or surface
condensation.
• If moisture accumulates above a critical material-dependent threshold,
the building components begin to:
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Stain
Rot or degrade
Crack or spall
Corrode
Accumulate mold
Damage is in many cases related to the inability of the building owner
to control moisture within acceptable limits
Fundamentals of Moisture Transport/Storage
Basic Material Properties
•
Bulk Density (lb/ft3)
– Easy to Measure, known for most
materials
– Needed for internal conversions
(heat capacity, water content)
•
Porosity (ft3/ft3)
– Describes the maximum water
content of the material
– Needed when a material can absorb
vapor or water in its pore structure
•
•
Permeability (Perm in)
Specific Heat Capacity (BTU/lb degF)
– Sum of the dry material and the
amount of bounded water
•
Thermal Conductivity (BTU/h ft degF)
– Dependent on density, temperature,
and moisture
Hygrothermal Phenomena in Building Practice
Optical degradation
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Soiling, Staining
Microbial Growth
Efflorescence
Hygrothermal Phenomena in Building Practice
Damage caused by elevated water
content and/or condensation
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Freeze-Thaw Cycle
Trapped Moisture
Corrosion
Rust Jacking
Hygrothermal Phenomena in Building Practice
Damage by hygrothermal stress
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Shrinkage cracking
Damage from hygrothermal
dilatation (i.e. spalling ,
delamination, etc.)
Coating failures
Hygrothermal Phenomena in Building Practice
Health aspects
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Hygrothermal comfort
Air quality
Mold growth caused by elevated
surface humidity
ASHRAE Standard 160-2009 Criteria for Moisture
Control Design Analysis in Buildings
Design Criteria for Moisture Control in
Buildings
•
Purpose
To specify performance-based design
criteria for predicting, mitigating or
reducing moisture damage to the
building envelope, materials,
components, systems and furnishings,
depending on climate, construction
type, and HVAC system operation
•
Scope
– Design of new buildings and retrofit
and renovation of existing buildings
– All types of buildings, building
components and materials
– All interior and exterior zones and
building envelope cavities
– Does not address thermal comfort,
acceptable indoor air quality, or the
design of building components to
resist liquid water leakage (reference
ASHRAE Standard 55 Thermal
Environmental Conditions for Human
Occupancy
Moisture Performance Criteria
Mold (according to ASHRAE Standard 160-2009, with 2011 Addendum)
30-day running average surface RH > 80%
or
7-day running average surface RH > 98%
or
24 hours at 100%
and
Surface temperature between 50° and 104°F
Surface Condensation
Determined using Dew Point Analysis (which can be calculated through WUFI)
What is WUFI?
WUFI-ORNL/IBP is a menu-driven PC program which allows realistic calculation of the
transient coupled one-dimensional heat and moisture transport in multi-layer building
components exposed to natural weather. It is based on the newest findings regarding
vapor diffusion and liquid transport in building materials and has been validated by
detailed comparison with measurements obtained in the laboratory and on outdoor
testing fields.
WUFI Case Study
Wall Assembly Case Study
Atlanta, Georgia
Building Background
Initial Investigation
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Building operates as a hotel
Construction completed in 1987
Building contains 12 stories above
grade
Typical construction consists of
cold-formed metal stud infill walls
and brick veneer with aluminum
storefront windows in punched
openings
Water leakage and interior mold
growth reported prior to project
Exterior and Interior Visual Surveys
Roof Condition Assessment
Water Leakage Testing
Exterior and Interior Inspection
Openings
Supplemental Analysis
•
Exterior Wall Vapor and Heat
Transmission Analysis
WUFI Case Study
Original Wall Assembly
1.
2.
3.
4.
5.
6.
3-5/8” red modular brick veneer
1-1/2” air cavity (non-ventilated)
1/2“ exterior gypsum sheathing
6” cellulose fiber insulation
5/8” interior gypsum wallboard
Vinyl wall covering
Interior Design Conditions
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Summer
– 72 degrees F
– 60% R.H.
Winter
– 68 degrees F
– 40% R.H.
WUFI Limitations
Some Limitations
•
WUFI is most commonly one-dimensional. Two-dimensional WUFI software is available but
requires significantly more setup and processing time. Three-dimensional simulations using
WUFI are not available
•
Simulations do not account for additional materials or thermal bridges that may be present in
an assembly (i.e. metal studs, structural framing, etc.) or material discontinuities (i.e. vapor
or air barriers)
•
WUFI assumes an ideal wall construction. Factors that may introduce additional moisture
and/or air movement into the assembly (open sealant joints, open penetrations, deteriorated
mortar joints, spalled brickwork, etc.) may change the performance of the wall assembly
•
Moisture laden air flow cannot be simulated. Significant building problems may exist due
solely to air leakage problems, and WUFI does not account for this
•
Better simulations require accurate material data (certain materials may not be available in
the database and some materials may not contain detailed material properties). Composite
materials (i.e. OSB, Stucco) may require some manipulation or simplification. Additionally,
accurate interior conditions may be hard to determine if design conditions are not provided
WUFI Limitations
Additional Limitations
• WUFI does not take into account liquid transport via seepage flow through gravitation or
hydraulic flow through pressure differentials (exacerbated through air leakage sources)
• WUFI is a good tool for studying the use of vapor barriers within an assembly, but introducing a
double vapor barrier may not give accurate results
• Accuracy when introducing large air cavities is decreased due to the limitations of the software
in dealing with convective vapor transport
• Modeling vertical and horizontal surfaces is fairly straightforward, but modeling soffit
conditions can be tricky (must eliminate wind driven rain, solar exposure, and other conditions)
• Studying the application of coatings on exterior surfaces is useful and popular, but detailed
material data for proprietary coatings can be difficult to come by and are not readily available
in the database
• Air changes within air cavities and additional heat or moisture sources can be introduced into
the model, but can be hard to accurately determine.
Questions / Discussion