Now there are places where I'd prefer to leave the old sheathing, like
around windows and doors so I can leave the trim in place. The new
buildup will be 15/32" + 1/4" = 23/32" thick, or 5/32" less than the
old sheathing.

If that difference is too great, I could either use 5/8" CDX (20-40%
more expensive), or use horizontal furring strips (a hassle). Any
other ideas? It's too thin for 1/4" foam board.


Thin layer of low-expansion spray-foam, scraped to flush.