I want to insulate the bottom of the floor (inside crawlspace). The problem
is the floor joists are nowhere near 16" on center. My single story 1000SF
house is built on a deck like subfloor. It is made up of 4x8 beams running
the length of the house set 4 feet apart with 2x8 tongue and grooved planks
set perpendicular. The result is a thick solid floor with few beams
underneith.

To put it another way, the floor joists are 4 feet apart and the crawlspace
access is a 2'x2" hole in the closet floor.

I was considering 4x8 sheets of solid core insulating panels cut in half the
long way but this would be expensive and would not have as good R value as
fiberglass. I would need to double or triple up on them to get the same R

Has anyone seen a product that sounds like it would work well here or am I
stuck using 16" wide rolls and stapling in strips to the underside of the
subfloor instead of the joists. This is not ideal because it would compress
the edges of the insulation.

Do you think installing paper faced insulation with the paper up so I can
staple through it is a good solution?

Fortunately, I am in northern CA and the climate is mild.