I've been searching through posts on how best to insulate the scissor truss roof system.  There doesn't seem to be anything conclusive as to best method and it seems to vary greatly depending on climate. 

The total span is 29'.  The roof pitch is 12/12 and the ceiling pitch is 8/12.  They have a 12" energy heel.  I've attached a section view to help illustrate.  How would you insulate it for climate zone 6a, southern edge?

I'd do a standard vented attic with vent baffles (and no hvac) and > R50 cellulose towards the center for an average exceeding your R50 spec. If you really want R50+ everywhere, then you could add some rigid foam on the interior side.

What jonr said. Plus you might want to add some foam insulation "baffles" every 4' along the slope so the insulation doesn't all slide down to the bottom. Cheap enough to do an R60.

The sliding down is exactly what I was worried about. I can't seem to find the source now but I thought I had read the recommend a max slope of 4/12 for loose cellulose? Some simple XPS baffles seem like a easy enough fix as long as the don't hamper getting a proper fill height. They would also have a good gage for fill height...

Would it be worth an inch of EPS between the trusses and drywall to give them a slight thermal break? Maybe 1/2" would be enough?

Glad to hear that cellulose is a viable option it's much more economical. After reading some other threads I was concerned that it couldn't handle the ceiling pitch. Will need to figure out how to gain access to the attic space after it's dry walled.

We've done two scissors trusses like that; not hard to make work. Forget the 1/2" of EPS.

You ALWAYS need to provide access to the attic!!! Build an insulated hatch, or come in from the garage or outside. We install a catwalk (2x10's) in the trusses so we can get around, check the insulation & do whatever.

With enough adhesive & water, cellulose can even be sprayed on the underside of ceilings. So it can be done, one way or another, on top of a 8:12 surface. I see one source that says 5:12 max for stabilized cellulose. So I'd expect a little more moisture than that, but not as much as is used in walls (∞ :12).

Don't need to wet spray. We've done barrel vaults in churches with greater slope with no problems. With your set up use some insulation baffles from JR Products The best ones are the cardboard units that can folded in various heights. Staple them up before you drywall. Also figure on furring every 16" or even 12" for needed support for the dry wall.