Helena Habitat for Humanity Adopts Double Studded Walls.

Double studded walls. Photo: NCAT

In past newsletters we have covered double-studied walls and the various ways of constructing them. As you may recall, constructing double studded walls is somewhat controversial, especially in wetter climates due to the concern that moisture vapor entering the walls will not have opportunity to dry either to the inside or outside depending upon the climate. Because of this concern, Building Science Corporation recommends adding a vapor-control layer in the center of the assembly to reduce the risk of wetting to the cold outside sheathing. They recommend that plywood be used for this layer, however, some designers and architects have had good results using polyethylene sheeting in this location. While there is a cost savings to this approach, the disadvantage is maintaining the integrity of the vapor-control layer, as polyethylene film is less durable than plywood.

Helena Habitat for Humanity took this into account when designing their double studded walls and adopted a newer technology for the vapor-control area. In their system, two identical 2×4 studded walls are joined at the top plate with a 2×10, and each wall having its own bottom plate 2×4 directly affixed to the concrete slab foundation. The outside wall acts as the load bearing surface.  Three layers of R-15 fiberglass bat are contained within the wall assembling with a total R-value of R-45 and greatly reduced thermal bridging due to the middle framing gap.  This is more than double the current Montana Energy Code requirement for wall assemblies.

The vapor-control layer is added to the inside of the outside wall after fiberglass batt insulation is installed in the stud bays. Mark Leland, Construction Director, with Helena Habitat explains that this system is much more volunteer friendly, and the insulation is more easily installed, especially because of the adoption of the Rothoblaas Clima Control 105 membrane. This product is tougher than poly sheeting, so more resistant to accidental punctures. The membrane is also considered a smart membrane. It is breathable when internal relative humidity is too high and serves as a vapor control layer when internal humidity is at suitable levels. Clima Control 105 is highly variable vapor diffusion resistance (0.1 – 20 m) that gives the product the ability to adapt to the hygrometric conditions of the structure.