I finally took a look and it is as expected.  The entry, living & dining have cathedral ceilings with no access and the rest has just under 24".  The rafters and joists are both 2x8,16"oc. 

The blown cellulose is just below the joists and is down to nothing at the walls.  I had considered a conditioned attic however there is only one return duct. 

I would like to add 2" of insulation board over the cathedral areas when I re-roof.  Since the  decking is 1x8, I was thinking I could detach it, add the insulation board and renail the decking.

I do need to know how much support I should add between the rafters and insulation board, I was thinking a 4" wide strip of 1/2" ply on top of each rafter or sistering a 2x2 to each.

In addition;  I would need to add something to catch the blow-in at the top of the walls, then I could add the additional material.

I used to blow attics all the time when I was younger, but I sure dread it now.  Anything else I need to think of while I am at it? 

Thanks

Over the cathedral areas it's a lot less labor to just strip the shingles and add the foam on top with a seam-staggered nailer deck of OSB on top, long-screwed to the old deck (with screws long enough to completely penetrate the old deck, for maxiumum mechanical strength.)

This will perform better too, since you won't have the thermal short-circuiting by the rafters. There are several vendors out there that have insulation pre-laminated to an OSB nailer deck (Atlas, Hunter, et al). Anything much over 4" thick gets to be a lot of work with long screws, but 2-4" goes pretty quick. (I've read of people going nuts with 6", but that takes 8 inch screws, which gets to be pretty unwieldy working with a drill on a roof deck.)

There are several vendors of pre-formed XPS and cardboard chutes designed to be stapled to the roof decking to keep the soffits clear & venting. If you're trying to maintain a high R-value out to the edges, scooping out the cellulose and cutting in some stacked ISO or spraying some layers 2lb foam prior to the cellulose overblow will give you ~R7/inch of depth, whereas low density cellulose is good for about half that. (6" of iso is worth a foot of celluose, more or less.)

If the 2x8" joists are spanning a large distance unsupported by partition walls, you may have loading issues with a super-insulating depth of cellulose. Dry weight will be ~1.5lbs per cubic foot, and a 24" depth (~R85) weighs something.

Thanks for the input; I will probably have more labor than budget.

Another consideration is that the front slope is continuous over both cathedral and attic areas. Therefore I will have to add spacers to the rafters over the attic area to prevent having a step-down in the roof.

As for the spans I will be adding bracing between the rafters and joists as there is none currently.

And as for the weight I will probably not go 24", it is pretty mild in Memphis.

The answer to your insulation problems is the R-ETRO System.

www.r-etro.com

Here's some pictures:


http://img198.imageshack.us/img198/2378/retrom.png

http://img94.imageshack.us/img94/3443/shakefinish2.png