It can be done, but it will have its challenges. Dunno if celotex has any effect on FLIR, but insulation definitely will....FLIR looks for heat, or heat leaks actually, and insulation will keep it from "seeing" any heat, if you have enough. You'll no doubt have issues with cooling in the summer and heating in the winter without lots of insulation as well. If you have a big enough attic, and some old homes surely do, you can build two very insulated rooms inside the attic, and then insulate the attic also, if your really worried about FLIR. This would make temp and humidity easier to control in the rooms and you could have one be for veggin and one for flowering, so you could do a perpetual grow. Keep working on a plan and you'll find a way!