According to BZ :
Chris Lewis wrote:

How warm do you want it to be? About the cheapest, but still pretty
good, you could install 15" unfaced batts, and line the inside with
vapor barrier. That's better than Kraft faced batts. You'll
lose a small amount of R having to compress the batts slightly, but the
loss would be minimal.

I called a radio call-in show about home repairs, and the host said to
just buy the wider Kraft paper batt insulation and cut it with a utility
knife. He said it cuts very easily once compressed, and gave me a
simple method for doing so. Yeah, it's a hassle, but it's not a big
one, and it'll end up being the least expensive way to get the job done.
I've got a few places where I can use the cut pieces, so it won't all
go to waste.

Why would you use kraft paper batts? Just use unfaced batts and
plastic vapor barrier and use a stapler/tuck tape. You don't really
need to cut the fiberglass at all, and the plastic does a better
job at sealing.

Or go with rockwool. The batts are much easier/less messy to work
with/cut.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.