8. Details on the Importance of Superior Air Barrier and Vapor Barrier Continuity

The following are important when designing and specifying exterior metal walls with superior thermal and moisture control:

Vapor drive and vapor barrier location according to climate.
Vapor diffusion causes moisture to penetrate wall materials. In certain climates, vapor barriers (perm ratings less than 1.0) are used to prevent vapor diffusion. Vapor pressure is greatest at high temperatures and high relative humidity. The greater the
vapor pressure difference from inside to outside, the greater the vapor diffusion concern. In climates like the hot and humid southeast U.S., the logical location for a vapor barrier is outside the insulation and in cold climates, a vapor barrier is required on the inside of the insulation. In moderate climates, where the vapor drive is bi-directional−outside to inside during summer, inside to outside during winter− a standard vapor barrier is not recommended. Instead, a special vapor barrier that varies its perm rating−low during the dry winter, high during the humid summer−is recommended on the interior of the wall assembly.

Multi-component wall systems combine barriers.
Creating vapor barrier and retarder continuity is very difficult to achieve on the interior of stud framing due to electrical penetrations and discontinuities at the floor and ceiling. To improve vapor barrier and retarder continuity, the Massachusetts Building Code's suggested details combine the function of the vapor barrier and retarder with the air and water barrier in a rubberized asphaltic barrier located outboard of the studs between rigid insulation and exterior gypsum sheathing. In the hot and humid climates the same rubberized asphaltic layer is placed over exterior gypsum sheathing located outboard of the studs, with the insulation located within the stud cavity.

Air and water barrier continuity is the key.
The volume of moisture transported by airflow is 50 to 200 times greater than the volume of moisture transported by vapor diffusion. Air from any small leak can travel throughout the inside of a wall, dropping condensation, whenever the temperature falls below the dew point. This moisture usually becomes trapped and can lead to wall system degradation. Therefore it's imperative that the air and vapor barriers be continuous. Relocating the air and vapor barriers outboard of the stud line is a big step toward achieving this continuity.

Superior wall performance should always be sought.
Avoid the many hazards of thin wraps and rubberized membranes by specifying and designing wall systems that provide superior air, water, and vapor control performance. Further, use systems that provide superior thermal performance with continuous insulation that is not compromised by metal studs, penetrations, or difficult details.

9. Wall Systems with Superior Performance

There are several different types of wall systems that are worth special attention.

Superior Wall System 1:
Outboard insulated panel and pressure equalized rainscreen
System 1 is a multi-component system with only two elements: a pressure equalized exterior rainscreen and an insulated composite foam panel backup (Figure 18). The exterior element provides ventilation to the cavity, while limiting water infiltration. It is tested at 15 psf and an imperfect air barrier. The insulated composite foam panel provides:

• Integral vapor retarder
• Integral air barrier
• Integral water barrier
• ASHRAE rated insulation R values as high as 26.5. Key benefits of the System 1 insulated wall include:
• One company is responsible for the wall.
• Composite foam panels have 35 years or more of performance as an exterior wall panel.
• Functioning behind an excellent rainscreen will provide lengthy service.
• Studs are on the warm side of the insulation and less subject to corrosion.
• Continuity of vapor barriers.
• Functions in all climatic conditions.

Figure 16 Section view of superior wall system 1 showing insulation outbound of metal studs and a pressure equalized rainscreen.