This detail shows how we take a more advanced wall assembly with higher R-value, larger thermal break, and more insulation than a conventional wall and combine it with a standard band joist. The second-floor band joist is the point where the first-floor above-grade wall, the second-floor framing, and the second-floor wall all intersect. This complexity makes it susceptible to incorrect detailing that can result in water seepage and air leakage. The concern is how the top of the double wall below and the bottom of the double wall above maintain continuity of the control layers.
We use a two-part water management system. The cement-board lap siding is the primary weather barrier; it is installed shingle style, so every consecutive layer sheds water above the layer below. A rainscreen provides the secondary defense. The 1×3 wood furring is spaced 24 in. o.c. to align with the stud frame below, providing a clear 3/4-in. drainage/ventilation plane. The rainscreen system is installed over 1/2-in. plywood sheathing, which is fully weatherized with a self-adhered weather-resistive barrier (WRB), specifically Siga Majvest 500 SA, which also serves as an air barrier. The air-sealing strategy includes sealing the exterior sheathing to the wood frame below. With this assembly, the air barrier stays close to the same plane across the exterior wall frame, making for an easy install.
Vapor is managed with the Majvest-wrapped sheathing to the exterior and Siga Majrex smart vapor retarder on the interior of the wall. The former limits the amount of moisture entering the wall assembly; the latter slows moisture migration. Should a higher level of moisture make it into the wall cavity, the Majrex’s ability to dry is increased, hence the term “smart” vapor retarder. As the plywood sheathing’s moisture content increases, so does its permeance, allowing the rainscreen system to assist in…
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