Secondary air barrier at attic – ventilated roof deck vs ventilated attic
In climate zone 6
I have seen many articles referencing the goal of many builders to add a secondary air barrier/wind-tight layer, but often those details end at the walls, especially when it comes to a standard truss roof assembly. Most plans I have seen for a truss roof include soffit vents, and only rely on a single air barrier at the ceiling as either drywall or a membrane such as Intello. I know the reason for this is partly due to the fact that a soffit ventilated truss roof, as described by Joe Lstiburek, is one of the most robust roof system you can have.
If the goal is to have a secondary air barrier on all sides of the house, and you are already relying on an air tight (strapped with 2×3 service cavity) ceiling with moisture management membranes such as Intello at the ceilings, would it be safe to wrap the WRB/Secondary air barrier of the exterior walls up the sides of the house onto the roof, creating a sealed open truss attic? This would leave all sides of the exterior air-tight. The same assembly would have plywood sheathing attached to trusses, then a smart membrane (as either peel and stick or taped membrane, then strapping for to create a ventilation space and roof overhang, then metal roof. In my case it would be a typical 8 pitch truss with r80 loose fill insulation.
While my initial skepticism leads me to worry that if any moisture found its way into the attic, it may want to condensate on the cold sheathing and will not be dried out by the ventilation you would normally have in that space. I have read that the moisture can be removed via diffusion in warmer climates with vapor diffusion ports, but that cant be done with insulation on the attic floor. The reason I want to have the plywood layer first, is because it just makes everything easier to install.
I came across this exact plan (minus the plywood against the trusses) which is advocated by Four Seven Five. I have attached an image that they created showing this exact assembly.
Does anyone have any thoughts on this idea.
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