the back half of the roof only has a very shallow slope, so any depth would mean filling up the roof rafters on at least the lower say 1/3-rd. Will this be a problem? The house was built in the 50's, had probably R-9 fiberglass, and some additional was installed some time ago, but I do not know how much. I figure I'd add at least 6" of blown-in stuff, and maybe more depending on what's there. So, will filling things up to the rafters be an issue?
I'm quite knowledgeable about insulation, among other things. Here's the thing - it's a balancing act of what you wish to invest, what a reasonable ROI might be for you, what is Practical in a situation, etc. So we can all offer advice but you need to weigh the costs and what is the best fit for you.
First - as others have pointed out, you can't have spray-in cellulose or fiberglass touching the roof - air must be able to circulate around. Granted you may not have "proper soffits" but ... it's a good bet that there are some cracks around the edge and some air infiltration. So even if you DO go with blown-in - install some of those spacers that create gaps at the edges. A simple wind-powered attic fan can be installed for $50. They work remarkably well (I'm in PA and have a low pitch mansard roof - Victorian).
I would really be hesitant to recommend closed cell insulation directly against the roof for this situation. THAT would make the entire attic part of the heated envelope. Were that attic a CATHEDRAL structure (and it's not - Cape, right?) then sure - that's a way to go. But with flat ceilings and an attic above - you do NOT want to spray foam on the inside of the roof.
Here are some ideas for you to consider - and feel free to contact me with any questions, or to bounce ideas back and forth.
IDEALLY ... you would vacuum out or remove what is currently in those bays up there. Then, SEAL (fire-rated foam only please) ALL penetrations (lights, holes, etc). Air infiltration is the #1 thing that costs the most. More than a lack of insulation. Air that goes up just carries the hot air with it in the winter - it's the "chimney effect". And be sure to seal whatever the attic entry is. Use foam tape, etc - whatever will do a really good job to ensure that air does NOT pass with the hatch closed. You may need to add weight if it's just a hatch panel - so that it pushes down on the foam, etc.
After making the attic as air tight as you can from the house underneath ... THEN decide what the best insulation options are. Ideally, you would want say 3" of closed cell foam sprayed between those joists on the attic FLOOR - not the ceiling. But ... it sounds like this is only possible in PART of the attic and there isn't roof to do it in the other part? Closed cell isn't cheap but it is the best. You will expect to pay $1 per board foot per inch.
Note: prices vary WIDELY for closed cell spray foam. You'll get guys that quote $1350-2400 for the SAME EXACT JOB. I would personally only hire an outfit that had a big spray rig (truck-based), and not some tanks that they carry around. The big truck guys invested a lot of money in equipment and training, and they will have the best prices. The tank guys ... who knows if they know what they are doing - it's hardly a regulated field!
Technically, this solution this isn't compliant with R-38 code - but you don't HAVE the room for R-38 in places anyway. And insulation code hasn't caught up to the benefits of foam vs traditional insulation. After 3" of foam there is little additional benefit. BUT even a few inches of closed cell foam is VASTLY superior to filling up every inch you can w/ cellulose or fiberglass blown-in. Think of how a cooler is constructed ... pretty darn air-tight and a solid layer of foam ... like your fridge ... like your water heater! They don't need 12" of fiberglass ... why should your house?
Another option is ... get a bunch of 2" thick 4x8 foam board panels from a big box store - foil side pointed toward the heated space (if only 1 side is foil). Carefully cut to fit slightly snugly between the joists and push it in - then put caulk or spray foam around the edges, so there is no air that can pass. Hopefully, there aren't too many wires in the way
Think of it as poor-mans closed cell spray foam. It will be VERY effective - if you are willing to do the work or hire some guys to do it. Then you can (if you wish) put more insulation on top of that. Even 2" of foam, with edges sealed will be incredibly effective.
Remember - first seal ALL penetrations. That is the most important thing. Then ... insulate the best you can, given the structure, your options, and your budget.
Please contact me if you wish! Thanks! I hope this helps!