I am replacing a failed ceiling in an 85 year old flat roofed home. When I took out the original ceiling a ton of black particles were in the small attic space. I assume this is years of roofing tar that has fallen through spaces between roof boards. Anyhow, I am considering putting rigid insulation up against the ceiling of the attic (underside of roof boards). Then I plan to put regular rolled insulation between the ceiling rafters before I hang drywall. There is about 18-24 inches of attic space with one vent out the side of the attic.
I am thinking that the rigid insulation will prevent the old black tar from falling and smashing the rolled insulation below and provide and extra layer of heat/cool loss protection.
Is this overkill? Will the rigid insulation actually do me any good when there is an attic vent?
You absolutely should not insulate the underside of your roof decking. The reason being; it requires exposure to insure that adequate ventilation is taking place. Without proper ventilation, condensation can occur resulting in mold, rot, and potential damage to your ceilings. An attic is not meant to be enveloped. It should be hot in the summer and cold in the winter. The best place to put your money is in insulation between and above the ceiling joists.
As far as the debris falling out of the decking... put up chicken wire or something else to restrain it. Whatever you do make sure the decking is exposed and has ventilation.