Spray foam backing requirements

[jives]

Hi folks;
Gathering here all options for insulting my scissor trusses. The top chord is insulated with double bubble reflective insulation, just under the metal roof. This if for condensation control. I'd like the real insulation to be placed between the bottom chord, which is a 6/12 pitch. Trusses are 4' OC, and about 18' from wall top plate to center peak.

Here are my options and questions. Keep in mind that the interior finish would hopefully be steel panels.

1. Use wide batt insulation, such as:
http://www.steelbuildinginsulation.com/polebuilding.html

This could also allow a finished surface, though it looks like a big baggie.The disadvantage is getting about 12" for a proper R value of about R40. This will require a double layer and a PIA to install.

2. Use rigid foam board between and on the chords. Stack the foam boards or have batts or blow in to fill the rafter space. Seems even more a PIA to install than #1. However, some rigid foam can be used as the interior finish surface (Thermax), and if screwed over the trusses it could make for a reasonable finish.
http://www.atlasroofing.com/continuous-wall-insulation/infinish-es

3. Use spray foam, but there is no backer. To enable spray foam between the truss bottom chords, there would need to be a backer that spans from truss to truss, like a wire frame. Is this possible? Would it work, or is there too much distance for the foam to span, even with, say, hardware cloth spanning the entire ceiling?

Here are the trusses:







Thoughts?

 

[73RR]

Spray foam expands/cures in seconds so it may be possible to spray against a screen and stay in place. How about fiberglass window screen material? Light weight and cheap.
To support the screen during spraying you could pull some string lines across the bottom chords, above the screen, after stapling the screen down.
Interesting project.