Nailbase options over fiberboard – GreenBuildingAdvisor

I am working on a 1970s home in Zone 5A with the following exterior wall assembly: interior 1/2” gyp bd, 2×4 framing 16”oc with either original kraft faced batts or new R13 batts with Certainteed Membrain, 3/4” brown fiberboard sheathing, 3/4” polyiso foil faced ci and vinyl lap siding. I’d like to demo the siding, increase the CI to meet WI residential prescriptive code, and get a ventilated drainable cavity in place – all within a 6 9/16” overall wall thickness for new windows with extension jambs (not including siding depth). I believe the fiberboard is fairly permeable and about R2. I don’t want to remove the fiberboard as it is functioning just fine with let-in structural bracing and I’d probably pop screws and nails on the interior gyp bd. I am reviewing options below, looking for second opinions…

$ Option 1: Leave existing 3/4” polyiso in place, add 1/2” polyiso then 7/16” OSB for a site built nail base. Install BO Hydrogap over the OSB as the WRB and drainage plane between air barrier and siding. Window bucks optional. Install LP Smartside as the horizontal or vertical cladding.

$$ Option 2: Remove 3/4” polyiso, touch up any voids in the fiberboard, then add back 1” foil faced polyiso, taping seams for WRB. Fur out vertically over studs with 1x4s or 3” rips of 3/4” exterior grade ply. Buck out windows to depth. Screen top, bottom and opening perimeters of ventilated drainable cavity and install LP Smartside as the horizontal cladding.