I chose a different method
I used Rmax ->> http://www.rmaxinc.com/wall-r-matte-plus3.asp
".......R-Matte Plus-3 is a rigid foam plastic thermal insulation board composed of environmentally sound, closed cell, polyisocyanurate foam bonded to a durable white-matte (non-glare) aluminum facer and a reflective reinforced aluminum facer. This product is suitable for use in wall sheathing applications in new residential, commercial and agricultural buildings and for thermal retrofit construction to existing buildings. R-Matte Plus-3 is ideal for use in both frame and masonry construction. R-Matte Plus-3 is available in ½", 5/8" and ¾" thicknesses, as well as, several different lengths....." Reflective foil one one side - non-glare alum on the other side
I just keep ALL THE DOOR AND WINDOWS OPEN - and used SOLVENT based contact cement - with a roller I just applied cement to the roof and to aluminum surface of the foam - I used muliple smaller pieces to make it easier - wait 60 second - then just TOUCHED to two glued surfaces together - tap them with a mallet and a small 2 x 4 - You would need a stick of dynamite to get them down
I framed my ceiling with 2 x 3 and left chanels for my "H" pattern "DUCTED A/C VENTS" - have pictures if you need to see what I did - I used multiple layers of foam to a total of 2.5" in most areas - I used the silver A/C duct tape on the seams (just to be sure ??)
The results were absolutely amazing - My RTS with it's huge windows (they are tinted) will HOLD TEMP a long long time - It's like driving in a thermos bottle
JUST MY WAY -
Just exactly how many layers or inches thick of foamboard did you end up with after it was all installed? Also, which facing (The reflective or Non-glare)is turned inwards towards the interior of the bus? Thanks...