Thermal and Moisture Control in Exterior Metal Walls
2. Performance Requirements for Outer Wall Materials
The weathering element of a multi-component wall system is the outer wall material. In addition to being the aesthetic wrap for the building, it is very important in determining the rest of the wall system design. Moisture management begins with the selection of this outer material.
Porous Materials
Materials like masonry, precast concrete, Exterior Insulation
Finish Systems (EIFS),and Glass Fiber Reinforced Concrete
(GFRC) are porous materials that will absorb and retain moisture.
Wind-driven rain in particular can be an issue for these porous
building materials that challenge designers to address the
conditions that arise after a storm. When the sun heats up
the outer wall, the absorbed moisture is changed to water
vapor. The vapors move from the warm, high-RH area to the
colder, often air-conditioned interior. A problem can occur
in cold or moderate climates where the vapors can pass through
the wall system components, enter the wall cavity, and condense.
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Non-Porous Materials
Other materials, like metal, glass, and polymer-based walls
are non-porous and do not retain moisture. They eliminate
a large portion of the moisture or water vapor problem through
their characteristic of not absorbing water. Metal cladding
systems are such materials and they can further be designed
to act as rainscreens, to minimize water entry and to ventilate
wall cavities where moisture can collect.
3. Climatic Zone Considerations
Each of the four U.S. climatic zones raise varying degrees of moisture concerns. (Figure 2) In the southeast, during the summer months, the hot and humid ambient conditions can lead to entrapment problems in the wall system. For the northern states, moisture control is more moderate during the summer, while controlling moisture from interior conditions during the winter is critical. Arid areas are considered low risk and do not have moisture problems.
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