I have simply Styrofoam it came in 4x8 sheets and I cut it to fit. sandwiched between to walls and ribs.
any other ways will help those building now.
Ron Dickey
...... Inside almost done--Trolly top has opening windows & roof.doors need assembling--pictured above waley windows..galley 1/3 done Cross Bow in Build Journals....http://www.tnttt.com/viewtopic.php?f=50&t=54108
I went with 1" blue styrofoam inside 1/2" MDO and the brown paper bag treatment for inside finish. Roof is luan then 1" foam then another luan. Seems nice and cozy but I haven't had a chance to use it too much yet. Just over the past few days I've been thinking more and more about the floor. I suspect I will put some 1" down there too.