calculating BTU loss

I need to clean up my attic and put new insulation. I have an old colonial house and the roof has 4 gable sides. There is one window for each gable side. My attic is 1/3 finished (paneling and carpeted). The other space is just the rafters and the insulation underneath it. The current insulation looks to be partially rip out, maybe by squirrels, and I obviously want to replace that. The space between the floor rafters is 8- 10” wide (old construction), and is covered with 2 X 8 wood beams. The wood beams make up the floor in the attic. I can easily lift the wood beams. Once I clean and disinfect I will lay the new insulation between the rafters, and underneath the wood beams. I found some dead animals and some droppings in the unfinished area. I want to cleanup, vacuum and reinstall new insulation between the floor rafters underneath the wood beams. What type of insulation is appropriate? I checked home improvement stores; it seems that the standard is 15” wide rolls. Can I use these rolls by cutting them by half? Is there functionality in the plastic where the insulation is contained? Is this plastic the vapor barrier? If so, how will it work if I cut open the insulation from the plastic? I will greatly appreciate if you can advice me on how to take care of this project in my home. Thank you for your consideration.

I have an eight inch attic which is under-insulated. If I blow insulation in the ventilation will be insufficient and I'll get condensation and rot (confirmed by a professional). Someone mentioned that I should remove the current insulation and replace it with a couple layers of rigid board (with appropriate vapour barrier). I have no access to the attic so I'll have to cut holes in my ceiling and patch them up when I'm done so I want to make sure this is worth it and that I do it right. Any comments? Is this worth the cost and effort?