Learning Objectives

After reading this article, you should be able to:

• Describe thermal and moisture control performance for exterior metal walls.

• Define the components of an exterior wall system assembly.

• Analyze how different climates affect design of exterior wall systems.

• Review the advantages of using rainscreens in building design.

• Evaluate design criteria for superior performance exterior wall systems.

• Corrosion of metal structural elements in the wall cavity.

• Reduction of thermal values of some insulation.

• Deterioration of internal components, such as tapes and wraps.

• The potential for the growth of mold within the wall system.

While the first three of these concerns are common and long standing, mold has emerged as a major concern, particularly in multi-component walls. The three elements required for mold to grow are water (in vapor or liquid form), moderate temperature, and an organic food source that applies to many wood-based or paper-covered building materials. Moderate temperature inside a wall cavity is common, which means that controlling mold requires eliminating either the food source or moisture, or both.

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.

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. 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.

Moisture Control Designs for Cold to Very Cold Climates

Key elements for controlling moisture in a typical multi-component wall assembly for cold climates (Figure 4) are:

• The vapor barrier—retarder: Airflow and vapor move from warm high pressure to cold lower pressures. In northern regions, winter is the critical time, when outside temperatures will average 8 degrees Fahrenheit, with a 20 percent RH and the interior ambient is 70 degrees Fahrenheit, with a 40 percent RH. Given these conditions, the dew point of the wall is 45 degrees Fahrenheit. If the vapor barrier-retarder were ineffective, vapors that enter the wall cavity would condense at a rate of as high as three pints of water per 100 square feet of wall area per week.

• Air and water barriers: An ineffective air barrier causing air leakage is considerably worse, because 80 pints of water could condense in the same one-week time frame. The comparison depicts how critical the air barrier is for moisture control.

Using the previous temperatures, consider the temperatures of the metal studs (Figure 5). These range from 17.5 degrees Fahrenheit on the exterior side to 37 degrees Fahrenheit on the interior. This is significant, indicating a problem with either the air or vapor barriers entering the cavity. Any surface that is below the dew point of 45 degrees Fahrenheit and in contact with the moisture laden air will cause condensation. Condensation in the cavity will cause corrosion of the stud and reduction in the thermal value of fiberglass insulation (Figure 5).

Section detail of a typical multi-component wall assembly for cold climate construction.
Plan detail of insulated metal stud wall and dew point issues that cause moisture condensation.

Moisture Control Designs for Hot and Humid Climates

In hot, humid southern climates, where moisture is a concern during the summer, the vapor barrier is installed at the

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