Exterior siding on cob walls
My wife and I live in a hand built cob house in Iowa. http://www.pbase.com/hapm/ourhouse
While we have no desire to change our home, I would like to have an exterior insulation layer in the next cob house I build. I envision a 4-6″ cob wall just inside a standard stud wall. I would like to do the stud wall with diagonal bracing and no plywood or osb. Cellulose insulation would be inside the studs and housewrap on the outside. Fiber cement lap siding would be the exterior.
The fiber cement siding would resist the mechanical weathering from rain which is a real issue for natural builders here in the Mid-west. The housewrap resists air and water infiltration. The insulation keeps the heat in. The cob is thermal mass and helps moderate the interior humidity. There would be a clay based plaster inside the cob.
First, are there any flaws in this system?
Second, which housewrap? Tyvek has a perm rating of 50 while Typar is about 11. That seems like a major differences and yet they are considered interchangeable. I would be inclined toward more permeability.
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