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Envelope Moisture Control Techniques
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introduction
Moisture causes billions of dollars of reported damage to building
envelopes in North America each year. Moisture-related damage not only
reduces the service life of buildings, but also influences the indoor air quality,
the health and safety of the inhabitants, and the energy efficiency of walls,
roofs, and foundations. If all these additional influences are included
in the estimate of actual damage, several tens of billions of dollars in losses
may be attributed to moisture-related damage in buildings every year.
Effective moisture control in the building envelope is essential if acceptable
service life is to be achieved for the built environment.
issues
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,
the partial vapor pressures (sometimes expressed in terms of relative humidity)
on the inside and outside of the building, and the amount of liquid pressure
(suction forces) caused by rain water, water leaks, or surface condensation.
Building materials, like all other materials around us, have
different affinities toward storing moisture. As moisture accumulates
within the building envelope assembly, the energy efficiency can be reduced
by up to a factor of three, or even more if evaporation and condensation occur
inside the envelope. If moisture accumulates above a critical material-dependent
threshold, the building components begin to rot, corrode, or otherwise degrade
in structural or functional integrity. Damage induced by moisture includes
rotting of wood studs and other components, corrosion of steel frame members,
salt transport, mold growth, and efflorescence. Such damage is related
to the inability of the building owner to control moisture within acceptable
limits.
objectives
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Develop an advanced theoretical understanding of the
complex interaction in hygrothermal transport
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Develop multi-dimensional heat, air, and moisture transport
models |
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Develop models equipped with dynamic aging capabilities
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Develop field and laboratory capabilities to evaluate the
hygrothermal performance of and validate models of building envelope systems
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Use models to assist in resolving numerous moisture-related
problems created by energy efficiency measures and generate building envelope design
guidelines
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research
ORNL has launched a major new research initiative to assess
hygrothermal performance. This new initiative aims at setting the future
direction for the design of building envelope systems and sub-systems in the
United States. The focus will be on energy efficiency, moisture-performance
and system and sub-system durability. We are developing advanced hygrothermal
and damage models to predict envelope responses to a wide range of climatic
conditions. Environmental loads will be characterized for all major
North American cities, and weather data will be obtained and analyzed.
WUFI-ORNL/IBP is a menu-driven PC program which allows realistic calculation
of the transient hygrothermal behavior of multi-layer building components
exposed to natural climate conditions.
A method will be developed to characterize the dryability
of wall envelope systems as it relates to moisture control. The analysis
of moisture management performance will include systematic investigation of
the properties (e.g., moisture storage capacity, liquid and vapor permeance)
and of all key elements (e.g., cladding, air barrier, vapor barrier, second
line of defense) of wall systems. The purpose is to establish objective
criteria for materials and systems that will ensure acceptable long-term performance
in a given climate. Finally, an experimental prototype will be developed
and tested to determine the influence that the repetitive wetting and drying
cycles of construction systems have on durability.
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Revised: February 15, 2007 by Teresa Williams.
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