I have a ranch house in Westchester County, N.Y., Zone 5, built in the late 60’s.  I will be replacing the single pane windows and replacing the siding with Craneboard insulated vinyl.  The house is 2×4 construction with fiberglass batt insulation and plywood sheathing.  My question is that I would like to add additional insulation, and am looking for suggestions?  I’ve looked at the Zip R sheathing alternatives as they seem like the easiest solution with the exterior siding and windows being removed, but am confused regarding moisture concerns.