Re:

[dave.williams@chaos.lrk.ar.us (Dave Williams)]

-> Styrofoam meets most if not all of those criteria. While it is
-> flammable and puts off toxic gases,  it  is difficult to burn in the
-> horizontal plane. If you paint it with latex, it's even better.  It's
-> cheap, excelent insulation, light to work with overhead.   Glue it
-> up.

 Note most foam insulation sheets are self-extinguishing once the source
of flame is removed.