Please help me make an informed decision on how to construct and insulate my addition’s walls.
I am adding onto my home here in south Louisiana (25 miles west of New Orleans) and am also retrofitting the existing structure. The original structure is a 2×4 wall with blackboard sheathing on the outside between the bricks. I would like to remove the entire outside envelope (not 2×4’s), spray the stud bays w/ closed-cell foam, then cover that with 1/2″ foam board, then 3/4″ plywood, housewrap, 1/2″ foam board and James Hardie lap siding.
On the new walls starting from inside: 1/2″ sheetrock, 1/2″ plywood, 2×6″(with closed-cell spray foam between stud bays), 1/2″ foam board, 3/4″ plywood, housewrap, 1/2″ foam board and James Hardie lap siding. This new addition is a master bedroom (17×17), master bath (12×17), and dining room (15×17) with double-pane low-E glass vinyl windows.
Let me explain my design; the plywood on both inside and outside is to essentially make the whole wall a shear wall (remember that I live in prime hurricane country and our new code requires shear walls at corners and windows). The closed-cell foam is to stop humidity transfer into the house (this is a high humidity area most of the year). The foam board is just to get some extra insulating properties and help with noise issues (apt. complex behind new addition).
I live in a hot, humid, rainy and dangerous area.
A few builders I talked to said they thought the extra 1/2″ plywood and foam board was overkill, and a couple thought it made sense and would be a negligible cost addition. I like them both for the ability to hang cabinets, and pictures on the inside and peace of mind. Also, I have upgraded my attic insulation to 8″ thick paper-faced batts in the existing attic. Should I use the same in the new addition our do something different?
I would like to know on scientific basis if you all agree with my design, and if not what would you change and why. I do not care about the cost factors as much as I do about heat and humidity transfer, the potential for mold growth and the structural aspects this system entails.
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