"Mike Marlow" wrote in message

I wouldn't use Styrofoam period. Not enough insulation value to offset
the
fire issues. Not enough insulation value period .

It is R4 per inch. There are no fire issues if installed properly. The
material is treated with a fire retardant and will not support combustion on
its own. Foam drop out ceiling tiles are REQUIRED when a dropped ceiling is
installed below a sprinkler system.