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+1 to the previous comments. Depends on what the foam manufacturer specifies as well as what the exterior sheathing is going to be. I would also recommend a thicker foam for additional insulation and thermal break if possible. Depends on what climate you are in and what the wall construction is when it comes to optimal foam depth.
The manufacturer's installation specifications should specify the type and length of fasteners that should be used with installing the 5 inch foam board. You may want to contact the local store where you purchased the material for the manufacturer representative in your area. Hope this advice helps!
AROCON Roofing and Construction, LLC
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