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I have a shorty school bus. I had to remove the ceiling and insulation. I now have an aluminum skin stretched over 1 1/2 inch square supports. The supports are further divided by 1 1/2  tall U brackets.

My original plan was to affix Reflectix to the inside of the aluminum skin and then layer 1 1/2 inch foam and then tongue and groove flooring for the ceiling.

It seems that this might not be the best way to go about things and I may be just as well off suspending the Reflectix between the outer edges of the supports leaving a significant air gap and then bending sheet paneling along the curve of the ceiling.

Ultimately this would be aluminum then an air gap, then Reflectix, then the ceiling paneling.

Is this a bad idea? Is this a good idea? Will it insulate well? 

If I understand correctly Reflectix against the skin of the vehicle is sort of pointless. The foam board insulation will require a lot cutting.

There are holes every 2-3 inches in the support beams. I'm considering filling the beams with Good Stuff. 

I would use Reflectix tape to fit the panels in place and would use the same tape to hold them in place.
Hey! I took pictures.

The spaces above the windows and across the ceiling are anywhere from 24x38 to 27x32. I'm trying not to use up too much of my head room.

On my van I removed the oem insulation then , I hotglue the rtech foam insulation. First I did 1/2 inch, then I added another 1/2 inch, about 1 inch total. There is no air gap or anything else but it works better the the oem by itself. Eventually I might add some plywood to get an air gap, but it hasn't got that hot in the 3 years since I install the foam.

Hotglue works great on metal and foam, drys quickly. Just make sure you cut the foam in small squares 1 feet x 1 feet, like a jigsaw puzzle, anything bigger and the glue will dry before you put it on the roof. In 3 years none of the foam has come off my roof. If you try to glue giant sections of foam it won't work, the foam will be too heavy and not stick properly. 

From my experience I recommend you put that 1 1/2 inch foam like your originally plan. Then you can do an air gap later. I notice the more foam I use, the cooler It got inside my van. 

Get an IR thermometer (about 15 dollars on ebay), that way you know where the heat is coming from and add more foam to the hotspots.
I'd suggest giving this a read and then taking a serious look at the different types of insulation and what they are used for.
Basically, 90% of the reflectix use you'll see on here is being used incorrectly.
You also have to consider which direction of thermal transfer you are trying to avoid.
Your environment plays a large roll too. Here in Arizona it's all about keeping the conduction of heat from the roof to the interior to a minimum. If I lived in Alaska I'd be trying to keep heat from the interior from radiating off the roof. Apples to oranges are far as design and best material choices.
I initially thought "Spray foam" insulation.

But that stuff is annoyingly permanent and it also makes it hard to isolate a roof leak if one should occur.

I also have concerns about air quality with on site spray in jobs. If the mixture is off on the two parts the foam can outgas for a LONG time.

I would opt for a ridgid closed cell foam glued to the Aluminum roof. Probably multilayers of thinner stock building up to 1 1/2" between the roof supports. You won't get thick foam to curve as you need it to.

Filling the tubes with expanding foam? I'm not a fan of that idea.......

Screw a nice ceiling board to the roof supports, a textured masonite product? You can get aluminum seam joiners to make it look professional and neat. The plastic ones will deteriorate far too quickly.

Another idea for the ceiling is to use a 1/4" plywood and then screw wood lath to it which creates a very attractive ceiling. Someone built out an old chevy van using wood lath extensively, it has a "designer" look to it.

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Just my 2¢ here. I'd cut 1/2" foam board and bend it to conform to the roof curve. 3 or more layers, whatever fills the space. Then add the finished ceiling. I'd recommend a stain/sealer (Minwax brand) or a spar varnish applied to both sides of whatever wood you choose. Keeps it from absorbing moisture.
Reflectix is not insulation.
DO NOT fill the tubes with expanding foam. The stuff can create A LOT of pressure if you try to contain it. It will bulge the skin of the body no problem.

You can bend foam insulation pretty easily actually by cutting grooves every inch or so. Obviously you wont be able to do sharp bends but accommodating a bus roof is doable no problem.

I think the best setup would be half thickness pink insulating foam grooved to accommodate the curve, then a layer of radiant barrier, then another half thickness layer of foam, cover that by a radiant barrier backed fascia. - Simple space blankets function as a radiant barrier and are thin enough to go in between anything. Layering your radiant barrier material with foam is better than just an empty air space.