A vapor open wall/roof assembly uses materials that are most im-permeable to water vapor at the interior of the wall cavity, and as we transition through the cavity the materials become more permeable. This creates a "path of least resistance" for any water vapor to exit the wall cavity and evaporate to the dryer outdoor air:
The materials used to achieve this vapor-open assembly includes Roxul Mineral Wool Insulation board insulation (continuous exterior insulation) and a combination of SIGA tapes and membranes that provide the required vapor, air, and rain protection.
* Air Barrier Membrane
- SIGA Majvest 9.8'x164'/roll (1614-SF/roll) (Rainproof/Windtight/Vapor Open Membrane)
- SIGA Wigluv 60mm (2.4"x131'/roll) (Air Sealing Tape for seams of Majvest)
* Exterior Continuous Insulation
- Roxul Drainboard R10 2.38"x48"x72"
- Roxul Drainboard R5 1.25"x48"x72"
* Cavity Batt Insulation 2x6 Framing
- Roxul ComfortBatt R23 15"x47" (39.82-SF per bag)
* Vapor Control Layer
- SIGA Majpell 4.9' x164'/roll (807-SF) Vapor control minimizes vapor infiltration into the cavity from the interior)
The primary concern with vapor-closed wall assemblies is that because of the closed-cell nature of EPS foam board insulation, there is potential to have a condensation point for water vapor on the inside of the OSB sheathing. Water vapor from the humid interior has the tendency to diffuse through the assembly and get trapped at the OSB sheathing layer where it cools down and condenses leading to rot and mildew.
If you ever do need more ammo to convince homeowners of the long-term value of the extra upfront cost for this vapor-open wall assembly, there is robust Moisture Analysis software able to visualize any potential condensation points in a given wall assembly for particular climate conditions.