I don't picture how the side walls fit in the picture.
1. Are they between a heated area and the outside? If not then insulating them will not do you any good.
2. Is the attic floored?
3. If so, how much insutlation is there between the flooring and the ceiling of the heated area below?
4. Do you use the attic for storage?
5. Is the whole area floored?

Any areas unfloored should have about an R-40. If your floored area has less but still has a reasonable amount of insulation you may want to leave it alone.

A typical house loses more heat through the ceiling than any of the walls but all radiant loss, if well insulated, will probably not equal the loss from windows and doors, the biggest portion being infiltration which is what comes in thruogh any cracks around the doors and windows and when you open and close the doors.

My preferred insulation is fiberglass or mineral wool. It is non toxic and the critters won't build nests in it. Also if it gets wet when it dries out it is unaffected by having been wet. I like the loose wool version best when using on in a horizontal placement.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.