[Bill Douglas]

Steve, I had an old house like that. If I pulled the shiplap off to insulate it properly the place would have fallen to bits. As per yours I pulled the bottom one or two shiplap boards off and stuffed pink batts (ordinary in wall insulating) in there to stop the air flow from under the house racing up the walls and into the ceiling space then out. It created a static air pocket that really did work. I put up sheets of drywall which insulated too. It was quite a warm house after that.

Not a perfect solution, but one that worked.